The microstructure of Ti-Ni-Cu shape memory thin films: A review

Abstract

Ti-Ni-Cu shape memory thin films have attracted more attention as a result of the perfect combination of various advantages such as narrower thermal hysteresis, superior thermal cycling stability, higher strength and long-term functional fatigue. The prominent functional properties were closely related to their specific crystal structure and microstructural features. In the present paper, the microstructure in equiatomic, Ti-rich and (Ni,Cu)-rich shape memory thin films were reviewed. In addition, the corresponding results including martensitic transformation, microstructure and strain recovery characteristics of Ti-Ni-Cu thin films without precipitates and containing different types of precipitates were summarised. Besides, the mechanisms for obtaining the outstanding functional properties are discussed and reviewed, which can offer the theoretical basis to the design of Ti-Ni-Cu shape memory alloy thin film micro actuators with excellent performances.

Keywords

Ti-Ni-Cu thin films shape memory alloy martensite structure precipitates

Introduction

Shape memory alloys (SMAs) are kinds of intelligent materials that have desirable performances such as shape memory effect and superelasticity [1,2]. Such superior properties derive from the reversible solid-state phase transformation between the higher temperature parent phase and lower temperature martensite phase [3,4]. Based on the unique properties, SMAs have been paid more attention, and have been becoming a most potential material in various applications fields including aerospace, aeroplane, automobile and biomedicine industries, etc. [5,6]. With the rapid development of electronic chips, micro-sensors, micro-machinery and micro-electro-mechanical systems (MEMS), it is necessary to develop the SMAs with smaller volume and outstanding properties [7 -9]. In order to meet the above requirement, the shape memory thin films with higher functional performances have been investigated.

Compared with other shape memory thin films, Ti-Ni based shape memory thin films are the most promising ones [10 -13]. Herein, binary Ti-Ni shape memory thin film is an attractive candidate for powerful micro actuators due to their high work output, large recoverable strain, excellent fatigue resistance and good biocompatibility [14 -18]. Nevertheless, the binary Ti-Ni shape memory thin films is featured with the slower respond speed, which severely limits its extensive applications. The solution to the low response frequency is to reduce the transformation temperature hysteresis. Doping Cu is an effective way to decrease the transformation temperature hysteresis [19]. Moreover, ternary Cu addition can reduce the compositional sensitivity of the transformation temperature [20]. The researches have revealed that the transformation temperature hysteresis of Ti-Ni-Cu alloys decreases with the increase of Cu content. As the content of Cu addition is up to 20at.-%, the transformation temperature hysteresis can be reduced to approximately 4 K [21]. At the same time, the response frequency can be enhanced significantly, ∼100 Hz [22]. However, the excessive Cu addition causes the deterioration of mechanical and functional properties [23 -26]. For one thing, the theoretical maximum recoverable strain of Ti-Ni-Cu shape memory alloy decreases with Cu content increasing. For another thing, the brittleness of Ti-Ni-Cu alloy increases due to the addition of excessive Cu, which makes the deformation difficult [23 -26]. Hence, it is a challenge to obtain simultaneously the narrower transformation temperature hysteresis and the larger recoverable strain. The recent studies revealed that the preparation of amorphous Ti-Ni-Cu thin films and the following annealing treatment can make a trade-off between the maximum recoverable strain and the thermal hysteresis [27 -29]. The perfect performances such as shape memory effect, superelasticity and thermal hysteresis of Ti-Ni-Cu shape memory thin films are closely related to the microstructure [1,2]. However, the microstructure of Ti-Ni-Cu shape memory thin films can be tailored by controlling the chemical compositions and annealing process [24 -26,28 -38].

In the present study, the evolution of microstructure in Ti-Ni-Cu thin films with different Cu contents and annealing processes is reviewed. Meanwhile, the relationship between the microstructure and functional characteristics of Ti-Ni-Cu thin films are clarified, which provides guidance to obtain the Ti-Ni-Cu thin films with both narrow hysteresis and large recoverable strain.

A brief introduction of martensitic transformation and structure of bulk Ti-Ni-Cu alloy

The martensitic transformation of Ti-Ni-Cu alloys is largely dependent on Cu content. As shown in Figure 1, Cu≤ 7.5at.-%: B2⇌B19′;7.5at.-%<Cu≤ 15at.-%:B2⇌B19⇌B19′; Cu ≥ 15at.-%:B2⇌B19 [30]. Moreover, the martensitic temperature of B2→B19 increases with the increased Cu content, while the Cu addition results in a slight decrease of martensitic transformation temperature for B2→B19′. Compared with B2⇌B19′ transformation, B2⇌B19 transformation is characterised by the smaller temperature hysteresis. In addition, the temperature hysteresis of Ti-Ni-Cu shape memory alloy becomes smaller and smaller with the increasing of Cu content. As shown in Figure 2, when the Cu content increases from 5 to 20at.-%, the temperature hysteresis of Ti-Ni-Cu SMAs continuously decreases from 17 to 4 K [31]. This means that the Ti-Ni-Cu SMAs with the smaller temperature hysteresis are suitable for actuator application.

Figure 3 displays the structural relationship between B2 austenite phase, B19 martensite phase and B19′ martensite phase [32]. It is obvious that {110}< 10> shear is necessary to produce B19 orthorhombic martensite structure from B2 austenite structure. However, in addition to {110}< 10> shear, {001}< 10> shear is also important for the transformation between B2 austenite ⇌ B19′monoclinic martensite.

Nevertheless, the Ti-Ni-Cu alloys with the higher Cu content are featured with the narrower temperature hysteresis. The exceeding Cu addition can result in the deterioration of ability of deformation and reduction of recoverable strain [23 -26]. Hence, how to obtain the higher performance Ti-Ni-Cu SMAs with the narrower temperature hysteresis and larger recoverable strain is an urgent problem to be solved. To date, the effective measure is the fabrication of amorphous Ti-Ni-Cu thin film and the following crystalline treatment. The microstructure of Ti-Ni-Cu thin films is largely dependent on the chemical composition, parameters of annealing treatment, which further contributes to the martensitic transformation and functional performances.

Microstructure and strain recovery characteristics of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film

Microstructure of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin films

Figure 4 represents the typical microstructure of the slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film. The annealed Ti-Ni-Cu thin film is in B19 martensite state. In addition, no precipitates can be observed in the present thin films [33]. The majority of grains contain single-pair (011) compound twins, as shown in Figure 4(a,b). In some grains, there are (011) compound twins with two perpendicular directions. Every part is a black of alternate (011) compound twinning platelets. Besides, the grains containing the single-pair twinned variants showing a (111) type I twins relationship also can be observed, which can be verified by Figure 4(d,e).

Besides, martensite variants with twinless were also found in slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film, as shown in Figure 5. The microstructure of twinless martensite shows a self-accommodating configuration with a triangular morphology of B19 martensite in the Ti_50.2Ni₃₀Cu_19.8 film annealed at 773 K for 1 h [34]. The triangle B19 martensite consists of three pairs of martensite variants, marked as A, B and C, respectively. Each pair of these variants has a {111} twin-relation, as shown in the SAED patterns in Figure 5.

The different morphologies of martensite structure in Ti-Ni-Cu thin films can be explained by the phenomenological theoretical calculation [34]. The results reveal that the total shape change of triangular self-accommodation morphology is smaller, compared with single-pair morphology [34]. This indicates that the single-pair morphology of martensite cannot release the transformation strain efficiently in one grain. In this case, many grains with various orientations cooperatively reduce the total transformation strain in the whole sample to maintain the integrity of the material.

Strain recovery characteristics of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film

Figure 6 shows the room temperature stress–strain curves of Ti_50.2Ni₃₀Cu_19.8 thin films annealed at 973 K for 1 h [33]. The present thin film is in martensite state at room temperature. In addition, the present thin film can recovery the original shape during the heating processing, when a deformation strain of 2% is employed. Even then the larger deformation strain of 5.5% is applied, no obvious plastic strain is observed. The larger recoverable strain of 5.5% is far more than the strain (2.8%) that stemmed from the stress plateau. This means that the recoverable strain of Ti-Ni-Cu thin film is larger than that of the bulk Ti-Ni-Cu SMAs.

Figure 7 illustrates the reasons for achieving the larger recoverable strain in the Ti_50.2Ni₃₀Cu_19.8 thin films annealed at 973 K for 1 h. It is obvious that the elastic deformation of B19 martensite phase contributes to the initial elastic deformation stage in the stress–strain curve. The stress plateau in stress–strain curve is related to the reorientation of {011}_B19 twins. {011}_B19 twins with two reorientations gradually evolve into single-orientated {011}_B19 twins. The strain in this stage can recover by heating. Upon beyond the stress plateau, multiple deformation behaviours are included. For example detwinning process of (011)_B19 martensite twin; the formation of new (011)_B19 twins in the original (111)_B19 twins as well as the stress-induced B19→B19′ martensitic transformation. Thus, the excellent recoverable strain is attributed to the elastic deformation, the recoverable of the de-twinned B19 martensite and stress-induced B19′′ martensite.

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films

As the annealing temperatures is ranging from 773 to 973 K, the grain size of annealed Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films almost keeps unchanged, as shown in Figure 8 [20]. However, the types of precipitates are determined by the annealing temperatures. The precipitate in Ti-Ni-Cu thin films annealed at 773 K is plate-like GP zones; while the plate Ti₂Cu precipitates are dominated in Ti-Ni-Cu thin films annealed at the temperature of 873 K. Besides, a few of Ti₂Ni spherical precipitates are also found. When the annealing temperature is raised to 973 K, the precipitate is evolved into spherical Ti₂Ni phase, which can be seen in the interior of grains and along grain boundaries [20,36]. In contrast, the Ti₂Ni precipitate distributing along the grain boundary becomes prominent in Ti-Ni-Cu thin film annealed at 973 K.

Figure 9 shows the structure characteristics of GP zone and Ti₂Cu in the Ti-Ni-Cu thin films [36]. It can be seen that GP zone is coherent with the matrix. GP zone is comprised of five layers and has a thickness of about 1nm, as shown in Figure 9(a). Formation of GP zone results in the bend of (110) planes of B2 phase at the (010) habit plane, which suggests that GP zone has a body-centred tetragonal structure. The tetragonal structure of GP zones is closely related to the excess Ti atoms clustered on {100}_B2 planes. The difference is that Ti₂Cu precipitate has C_11b-type Ti₂Cu precipitates and its composition is rich in Ni. The bright spots are corresponding to the Cu atoms and the dark spots correspond to the Ti atoms, as displayed in Figure 9(b). Moreover, the tetragonalities of GP zone and Ti₂Cu precipitate are calculated to be 1.19 and 1.11, respectively [33,35]. For comparison, the Ti₂Ni precipitates in Ti-Ni-Cu thin film are semi-coherent with the matrix [23].

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with GP zones

Figure 10 shows a typical martensite structure in the Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films annealed at 773 K for 1 h [37]. The martensite twins in two perpendicular orientations are frequently observed. The SAED pattern in Figure 10(b) confirms that the twinning platelets are (011) compound twin related in every block. The (001) compound twin plates are parallel to each other and tend to pass through the whole grain unless they meet the grain boundary or other martensite variants with different orientations, as indicated in Figure 10(a,d). However, compared with Ti-Ni-Cu thin films without precipitates, the amount of the (011) compound twin showing a single-pair morphology in annealed slightly Ti-rich Ti-Ni-Cu thin film containing GP zone is less. The width of (011) compound twins varies over a wide range from 10 to 400 nm. The average width of twin bands is approximately 60 nm.

Figure 11 shows the morphologies of coexistence of (011) compound and (111) type I twins as well as the corresponding SAED patterns [37]. In Figure 11(a), the (111) type I twins pass through the whole grain with the same orientation, while (011) compound twins grow in the (111) twinning region in a zigzagged configuration. Figure 11(b,c) represent the corresponding SAED patterns taken from the A and B areas in Figure 11(a), respectively. In contrast, the amount of (111) type I twin is less than that of the (011) compound twin in the Ti-rich Ti-Ni-Cu thin films containing GP zones. Meanwhile, the (111) type I twins always simultaneously appear with the (011) compound twins.

Among, GP zone has a slight effect on the growth of the B19 martensite plates in Ti-Ni-Cu thin films. When the martensite variant meets the GP zones in the width direction, such a part connected to the GP zones will be temporarily stopped as a result of the elastic deformation of the GP zones, whereas the other part will continuously grow. As a consequence, the tip of the (011) martensite variant shows a wavy twin boundary.

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Cu + Ti₂Ni precipitates

The morphology of the martensite in Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with many Ti₂Cu and Ti₂Ni precipitates is shown in Figure 12 [37]. In the present thin film containing Ti₂Cu and Ti₂Ni precipitates, (011) compound twins are frequently observed, as shown in Figure 12(a). Unlike the martensite structure in Ti-Ni-Cu thin films with GP zones, most of the grains consist of many martensite domains, each of which is a block of alternate (011) twinning platelets, for Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Cu and Ti₂Ni precipitates. The average width of (011) twin in Ti-Ni-Cu thin film with Ti₂Cu and Ti₂Ni precipitates is approximately 40 nm, which is smaller than that in the films with GP zones. Nevertheless, some grains containing the single-pair martensite are still observed. The Ti₂Cu precipitates and the twinning boundaries of (011) compound twin interact with each other. Moreover, the twinning boundaries are always stopped by the precipitates when the martensite plates pass through the Ti₂Cu precipitates in the width direction, as shown in Figure 12(b). Figure 12(c) is an enlarged bright field image of the interaction between the martensite and Ti₂Cu precipitates (A–A, B–B and C–C) in the length direction. After being cut through, the straight Ti₂Cu precipitates become zigzag when they meet the martensite, as indicated by the dotted lines in Figure 12(c). It can be deduced that the Ti₂Cu precipitates are coherent with the martensite matrix, and can be elastically deformed for accommodating the shear deformation of the (011) twinning. Similarly, Ti₂Cu phase has a slight effect on the growth of martensite.

The typical martensite morphology of (111) type I twin martensite coexisting with (011) compound twins is that (111) type I twin martensite and (011) compound twins tend to run through the whole grain. Martensite plates with this morphology often possess different widths at various positions. The (111) twin bands and (011) compound twins intersect with each other. And the (111) martensite variant is sometimes divided into several branches by (011) compound twins. The quantity of (111) type I twin is smaller in Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Cu and Ti₂Cu precipitates, compared to the Ti-Ni-Cu thin films with GP zones. Moreover, the width of (111) type I twin in the present film is smaller than that in the Ti-Ni-Cu film containing GP zone

In most cases, the martensite variant can pass through Ti₂Cu precipitates. For instance, the boundary of martensite variant becomes curved and the direction of the martensite plate changes, the width of the martensite plate also changes. Sometimes, Ti₂Cu precipitates restrain the growth of the martensite variant. Under this condition, the width and directions of martensite plates may alter due to the presence of Ti₂Cu precipitate.

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Ni precipitates

Figure 13 displays the morphology of martensite structure in Ti-Ni-Cu thin film with many Ti₂Ni precipitates [37]. It is obvious that a larger number of smaller Ti₂Ni precipitates are distributed in the interior of the grain, while large ones mainly appear at the grain boundaries. The diameter of these precipitates in the grain interior is estimated to be about 20∼50 nm and the size of Ti₂Ni precipitate at the grain boundaries is about 200∼300 nm. Similarly, the martensite morphology is characterised by the (011) compound twins with a single-pair morphology showing one or two perpendicular directions, as shown in Figure 13(a,b). Moreover, the martensite plates pass through the whole grain until they meet the grain boundary. The average width of (011) compound twin is ranging from 30 to 200 nm in the area without Ti₂Ni precipitates. While the martensite variants at the vicinity of Ti₂Ni precipitate is about 30 nm, which is slightly smaller than that in the Ti-Ni-Cu thin films with GP zones and Ti₂Cu precipitates. This indicates that the width of (011) compound twins decreases with the Ti₂Ni particles precipitate. Sometimes, (111) type I twin also can be observed except from (011) compound twin in the Ti-Ni-Cu thin film containing Ti₂Ni precipitates. The larger Ti₂Ni precipitate has a significant influence on the growth of martensite, as illustrated in Figure 13(c). Martensite plates stop to grow or change its original growing directions upon they meet the Ti₂Ni particles.

Evolution of martensite structure in Ti-rich thin films with different precipitates

Based on the above results, the evolution of B19 martensite microstructure in Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films annealed at different temperatures is listed in Table 1. Both (011) compound twins and (111) type I twins are observed in the Ti-Ni-Cu thin films, irrespective of the types of precipitates and annealing temperatures. When the annealing temperature increases, the evolution of precipitation is as follows: GP zone→ larger numbers of Ti₂Cu precipitate+ smaller amount of Ti₂Ni precipitate→Ti2Ni precipitate. In turn, the amount of (011) compound twins increases, while the amount of (111) type I twins decreases. Among, the width of (011) compound twins is decreased. Moreover, the amount of a single-pair martensite is smaller in Ti-Ni-Cu thin films with precipitate, compared to Ti-Ni-Cu thin films without precipitates. The number of martensite variants with a single-pair morphology firstly decreases and then increases, with increasing the annealing temperature. In the Ti-Ni-Cu thin films with Ti₂Cu and Ti₂Ni precipitate, the single-pair martensite variants can be hardly observed. The resistance to the growth of martensite caused by GP zones, Ti₂Cu and Ti₂Ni gradually increases.

Table 1

Summary of martensite structure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin film containing different precipitates.

Ti-rich Ti-Ni-Cu films	773 K (GP zones)	973 K (Ti₂Ni)
The amount of (011) compound twin	Increasing↑
Average width of (011) twin	Decreasing slowly↓
The amount of (111) type I twin	Deceasing↓
The amount of single-pair morphology	Decrease↓	Increase↑
Resistance to the growth of martensite	Increasing↑

Relationships between the strain recovery behaviour and structure features in Ti-rich Ti-Ni-Cu thin films

Figure 14 shows the strain-temperature curves of Ti-Ni-Cu thin films annealed at different temperatures under the constant stress of 360 MPa. It can be concluded from Figure 14 that the martensitic transformation temperatures gradually increase with the increased annealing temperature. The above results reveal that the types of precipitates are largely dependent on the annealing temperatures. The precipitate is GP zone in Ti-Ni-Cu annealed at 773 K; while the coexistence of Ti₂Cu and Ti₂Ni precipitate can be found under the condition of annealing temperature of 873 K. As the annealing temperature is 973 K, the precipitate is evolved into single Ti₂Ni phase. In proportion, the resistance of GP zone on the growth and moving of martensite is smallest, while the resistance of Ti₂Ni precipitate on the growth of martensite is largest. Hence, the excellent strain recovery behaviour is obtained in Ti-Ni-Cu thin films at the annealing temperature of 773 K, which is featured with the GP zone.

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films

For (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films, the Ti(Ni,Cu)₂ phase precipitates during the crystalline process. Moreover, the martensite structure in the Ti-Ni-Cu thin films varies with the changing of size and density of Ti(Ni,Cu)₂ precipitates. Nevertheless, the (011) compound and (111) type I twinning are also the predominant, regardless of the annealing temperatures.

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 773 K

Figure 15 represents the bright-field TEM images and corresponding SAED patterns of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 773 K for 1 h [38]. Larger numbers of disk-like precipitates are observed. The fine precipitate lies in two approximately mutually perpendicular (011) and (0 1) planes. The thickness and average diameter of the fine precipitate are about 1∼2 and 40 nm, respectively. The distance between the adjacent precipitates varies in the range of 10∼40 nm.

In the present Ti_48.6Ni_35.9Cu_15.5 thin film annealed at 773 K, the single-pair martensite with (011) compound twinning is more frequently observed, as shown in Figure 15(b,d) [38]. The width of (011) twin bands is varying over a wide range from 10 nm to 1 μm, the average value is estimated to be approximate 100 nm.

In addition to (011) compound twin, the (111) type I twins is also found in the Ti_48.6Ni_35.9Cu_15.5 thin film annealed at 773 K. The morphology of (111) type I type is composed of many martensite domains with three orientations, which is quite different from the single-pair morphology in Ti-Ni-Cu thin films without precipitates or Ti-Ni-Cu thin films with GP zones. One type of domain is obviously different from the other domains, which can be confirmed by the direction of their inside martensite plates. Compared with the dense distribution of fine Ti(Ni,Cu)₂ precipitates, the size of martensite plates related to (111) twins is very large. It can be assumed that the fine Ti(Ni,Cu)₂ precipitates have no remarkable effect on the growth of (111) martensite twins.

However, the shear of (011) compound twinning can be obstructed to some extent, when the (001) compound twins meet the fine Ti(Ni,Cu)₂ precipitates. As shown in Figure 16(a) [38], the (011) compound twins will meet the fine Ti(Ni,Cu)₂ precipitate in two directions. One is the length direction, in which the precipitates are approximately perpendicular to the (011) twin planes. The bright-field TEM image and schematic diagram reveal that the precipitates become zigzag, when the (011) compound twins meet the fine Ti(Ni,Cu)₂ precipitates in the length direction, owing to the shear deformation (Figure 16(b)). It can be deduced that the (011) compound twins can shear through the fine Ti(Ni,Cu)₂ precipitates in the length direction, as illustrated in Figure 16(b) [39]. The other is the width direction, in which the precipitates are parallel to the (011) twin plane. As shown in the illustration in Figure 16(c), The movement of the (011) compound twin boundary is usually limited to some extent by the fine Ti(Ni,Cu)₂ precipitates in the width direction [39].

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 873 K

Figure 17 shows the bright-field TEM images of the film annealed at 873 K for 1 h [38]. The Ti(Ni,Cu)₂ precipitates are coarsened when the annealing temperature is up to 873 K. The average length and thickness of Ti(Ni,Cu)₂ precipitates are about 330 and 15 nm, respectively. In addition, the distance between neighbouring precipitates increases to about 300 nm. Accordingly, the density of the precipitates decreases sharply. The coarse precipitates seriously affect the morphology of martensite in the Ti-Ni-Cu thin film. It is observed that the whole grain consists of fine martensite plates with two perpendicular orientations. This morphology is similar to rectangular cell other than the single-pair martensite. The SAED patterns in Figure 17(c,d) reveal that the martensite plates are (011) compound twin related. The width of (011) compound twin is about 30 nm. In addition, it can be seen that the Ti(Ni,Cu)₂ precipitates act as the boundary of the (011) compound twin. In other words, the coarse Ti(Ni,Cu)₂ precipitates have an influence on the nucleation and limit the growth of martensite plates to some extent.

The (111) type I twins with a morphology of many domains in the three directions is also observed in the Ti_48.6Ni_35.9Cu_15.5 thin film annealed at 873 K. However, in the present thin film, it is very difficult to distinguish the precipitates from the fine (111) martensite plates, owing to their similar size and the strain field around the Ti(Ni,Cu)₂ precipitates. It is also interesting to note that the average size of the martensite domains is similar to the average interval of the Ti(Ni,Cu)₂ precipitates. Thus, one can imagine that the coarse Ti(Ni,Cu)₂ precipitates separate these martensite domains containing (111) twins.

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 973 K

With further increasing annealing temperature to 973 K, the size of Ti(Ni,Cu)₂ precipitates increases. The average thickness and diameter of the precipitates are about 50 and 500 nm, respectively. However, the density of Ti(Ni,Cu)₂ decreases obviously. A grain usually consists of several domains including a block of alternate (011) twinning platelets. Additionally, the martensite shows a single-pair martensite variant morphology in some grains with a few precipitates. The growth of the (011) martensite twin and (111) type I twins is limited by the coarse Ti(Ni,Cu)₂ precipitates, which is quite similar to the effect of Ti₂Ni precipitates on the growth of martensite [39,40].

In Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 873 and 973 K, B19′ martensite with a monoclinic structure is also frequently observed. The presence of B19′ martensite can be ascribed to the local stress concentration. Moreover, B19′ martensite plates are (001)_B19′ compound twin related.

Evolution of (Ni,Cu)-rich Ti-Ni-Cu thin films with different precipitates

As stated above, the evolution of microstructure in annealed (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films with the changing of annealing temperatures is summarised in Table 2. The type of precipitates in the Ti_48.6Ni_35.9Cu_15.5 thin films is Ti(Ni,Cu)₂, no matter how the annealing temperature changes. However, with increasing of annealing temperatures, the thickness and diameter of Ti(Ni,Cu)₂ precipitates increase, while the density of precipitates decreases. In turn, the resistance of Ti(Ni,Cu)₂ precipitates on the growth of martensite plates is becoming larger and larger. And the larger Ti(Ni,Cu)₂ precipitates have an influence on the morphology of martensite variants. In addition, the local stress concentration at the nearby of coarse Ti(Ni,Cu)₂ precipitates and grain boundaries results in the presence of B19′ martensite with (001) compound twinning in Ti-Ni-Cu thin films annealed at 873 and 973 K.

Table 2

Summary of martensite structure of (Ni, Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin film containing different precipitates.

(Ni, Cu)-rich Ti-Ni-Cu thin film		773 K (fine Ti(Ni, Cu)₂)	873 K (coarse Ti(Ni, Cu)₂)	973 K (further coarse Ti(Ni, Cu)₂)
Precipitates size	Thickness	1∼2 nm	15 nm	50 nm
Precipitates size	diameter	40 nm	330 nm	500 nm
Density		Decrease
morphology	(011) twin	Single-pair	Rectangular cell	Several domains
morphology	(111) twin	Many fine domains in three directions		Self-accommodation
Resistance to growth of martensite	(011) twin	Cutting through	Stop
Resistance to growth of martensite	(111) twin	Cutting through	Stop
B19′ martensite		Not observed	Accommodation twin

Relationships between the microstructure and strain recovery characteristics of (Ni,Cu)-rich Ti-Ni-Cu thin films

Figure 18 displays the strain-temperature curves of Ti-Ni-Cu thin films with the different compositions annealed at various temperatures [25]. It can be seen that the annealing temperatures have a similar effect on shape memory behaviours of Ti-Ni-Cu thin films, irrespective of the chemical compositions. With the increasing of annealing temperatures, both the martensitic transformation temperatures and residual strain increase for all Ti-Ni-Cu thin films. The rising of martensitic transformation temperatures can be attributed to the variation of chemical composition caused by precipitation of Ti(Ni,Cu)₂ phase. In addition, the increased annealing temperatures result in the growth of Ti(Ni,Cu)₂ precipitates and the strengthening effect stemmed from coarse Ti(Ni,Cu)₂ precipitates is weaken. Meanwhile, the resistance of coarse Ti(Ni,Cu)₂ precipitates on the growth and moving of martensite is larger. Hence, the residual strain of Ti-Ni-Cu thin films gradually increases with the increased annealing temperatures.

Effect of Ti content on microstructure and functional performances of Ti-Ni-Cu thin films

Effect of Ti content on microstructure of Ti-Ni-Cu thin films

Figure 19 shows the bright-field TEM images of Ti-Ni-Cu thin films with different Ti contents and annealed at various annealing temperatures [41]. It is found that the grain size of annealed (Ni,Cu)-rich Ti-Ni-Cu thin film is reduced from 3.5 to 1.4 μm, as the Ti content decreases from 48.6 to 44.6at.-%. However, the grain size of Ti-rich Ti-Ni-Cu thin films decreases from 1.6μm to 130nm with the Ti content increasing from 50.2 to 55.4at.-%. It is well known that the precipitates distributing along the grain boundary restrict the movement of grain boundaries and further reduce the grain size [42,43]. For (Ni,Cu)-rich Ti-Ni-Cu thin film, the amount of Ti(Ni,Cu)₂ precipitates distributing along the grain boundary would become more and more with the Ti content decreasing from 48.6 to 44.6at.-%, further leading to the reduction of grain size. Similarly, the amount of Ti₂(Ni,Cu) precipitates distributing along the grain boundary would become more and more, as the Ti content increases from 50.2 to 55.4at.-% in Ti-rich Ti-Ni-Cu thin films, causing the decrease of grain size. In contrast, the grain size of (Ni,Cu)-rich Ti-Ni-Cu thin films is larger than that of Ti-rich Ti-Ni-Cu thin films. For both (Ni,Cu)-rich and Ti-rich Ti-Ni-Cu thin films, the grain size is not affected by the annealing temperatures. Nevertheless, the precipitates at the grain boundary become dominated by the rising of annealing temperatures.

Figure 20 illustrates the equilibrium phase diagram of the Ti-Ni-Cu ternary alloy [41,44]. It can be seen that the precipitates are Ti₂Ni and Ti₂Cu phases in Ti-rich Ti-Ni-Cu thin films. While only Ti(Ni,Cu)₂ precipitates can be observed in (Ni,Cu)-rich Ti-Ni-Cu thin films. Among, Ti(Ni,Cu)₂ precipitate is formed on the {100}B2 planes with [100]Ti(Ni,Cu)₂//<100> B2 and (001)Ti(Ni,Cu)₂//{001}B2 orientation relationship. The orientation relationships between the Ti₂Ni (or Ti₂Cu) precipitate and B2 phase are as follows: [100]precipitates//<100> B2 and (001)precipitates//{001}B2. With the increasing of annealing temperatures, the Ti(Ni,Cu)₂ precipitates gradually grow up for (Ni,Cu)-rich Ti-Ni-Cu thin films; whereas the evolution of precipitates with the rising of annealing temperatures is as follows: GP zone→Ti₂Cu + Ti₂Ni→Ti₂Ni.

Effect of Ti content on performances of Ti-Ni-Cu thin films

Figure 21 displays the effect of Ti content and annealing temperature on the critical stress for slip, maximum recoverable strain and martensitic transformation temperature of Ti-Ni-Cu thin films. The results in Figure 21(a) reveal that the deviation of Ti content from 50at.-% and the decrease of the annealing temperatures result in the rising of critical stress for slip. The higher critical stress for slip is closely related to the grain refinement and the increase of coherent precipitates. Both the maximum recoverable strain and martensitic transformation temperature (Ms) firstly increase and then decrease, as shown in Figure 21(b,c). The evolution of maximum recoverable strain is largely dependent on the volume fraction of B2 phase. With the increasing of Ti content, the volume fraction of B2 phase in the annealed Ti-Ni-Cu thin films firstly increased and then decrease, further leading to the first increase and then decrease of maximum recoverable strain. For Ti-rich Ti-Ni-Cu shape memory thin films annealed at different temperatures, the precipitates are GP zone, Ti₂Cu + Ti₂Ni phase, respectively. The precipitation of the second phase causes the decrease of Ti content in matrix. While the precipitate in annealed (Ni,Cu)-rich Ti-Ni-Cu thin films is Ti(NIi,Cu)₂ phase, which leads to the rising of Ti content in matrix. Thus, the martensitic transformation temperature shows a decreasing trend with deviation from 50.0at.-%Ti.

Effect of Cu content on the microstructure and functional performances of Ti-Ni-Cu thin films

Effect of Cu content on the microstructural evolution of Ti-Ni-Cu thin films

Figure 22 represents TEM images of Ti_44.5Ni_55.5-XCu_X thin films annealed at the different annealing temperatures [28]. TEM results reveal that the grain size of Ti-Ni-Cu thin films increases from 1.4 to 3.7 μm, as the Cu content increases from 15.3 to 27.3at.-%. Upon the Cu content is 32.8at.-%, the grain size of Ti-Ni-Cu thin film shows a downward trend. The decrease of grain size in annealed Ti-Ni-Cu thin film with higher Cu content can be attributed to the suppressing effect of many precipitates at grain boundary on the grain growth. Similarly, the grain size slightly decreases with the annealing temperature increasing owing to the inhibiting effect of coarsening of the precipitates. However, the opposite phenomenon is also found in Ti-Ni-Cu thin films with 48.5at.-%Ti and 6.2–27.8at.-%Cu [25]. The precipitates can be observed in all Ti-Ni-Cu thin films, irrespective of the annealing temperatures. The difference is that the precipitate gradually becomes larger and larger. In addition, the precipitate is Ti(Ni,Cu)₂ phase in the present (Ni,Cu)-rich Ti-Ni-Cu thin films. When the Cu content is 32.8at.-%, another TiCu precipitate start to appear in Ti-Ni-Cu thin films. The distinctions between the Ti(Ni,Cu)₂ and TiCu precipitate is that the misfit strains along the c-axis are estimated to be −12.3% and −2.5%, respectively. Ti(Ni-Cu)₂ precipitates grow up in the grain interiors as well as along the grain boundaries, with increasing of annealing temperature.

Effect of Cu content on the performance of Ti-Ni-Cu thin films

Figure 23 shows the effect of Cu content on the critical stress, maximum recoverable strain and the martensitic transformation temperature (Ms) of Ti-Ni-Cu thin films. With the increase of Cu content, both the martensitic transformation temperature (Ms) and the critical stress also increase, while the maximum recoverable strain decreases. It has been reported that the grain size is reduced with the decreasing of Cu content. The grain size is not the main factor in determining the critical stress for slip, because the grain size is larger than 1μm. In the present Ti-Ni-Cu thin films, the solution strengthening of Cu addition should be responsible for the increase of critical stress. In addition, the presence of TiCu precipitate and the grain refinement in Ti-Ni-Cu thin films with higher Cu content contributes to the significant improvement of critical stress. The larger maximum recoverable strain in the Ti-Ni-Cu thin film can be attributed to an additional strain stemmed from B19→B19′ martensitic transformation. The precipitation of TiCu phase results in the lower volume fraction of B2 phase in annealed Ti-Ni-Cu thin film with higher Cu content, which further leads to the relatively smaller maximum recoverable strain. The martensitic transformation temperature (Ms) is closely related to the variation of chemical composition in matrix. The solubility limit of Ni in the B2 phase decreases with increasing Cu content. However, the martensitic transformation temperature of Ti-Ni based SMAs decreases with the increase of Ni content. Therefore, the annealed Ti-Ni-Cu thin films with the higher content of Cu addition possess lower martensitic transformation temperatures.

Insights

The above researches revealed that the Ti-Ni-Cu thin films with the narrower thermal hysteresis, higher strength and larger recoverable strain by tailoring the chemical composition and annealing treatments. The present high-performance Ti-Ni-Cu thin films can be applied to various micro-mechanic and micro-electro-mechanical systems (MEMS). In practice, MEMS are often subjected to severe vibrations and shocks in many occasions such as aviation, aerospace, transportation, and construction machinery. For instance, the acceleration peak of impact can reach more than 10⁵g during the launch of rockets and missiles, which can decrease the accuracy of micro-nano sensors (such as speedometers, accelerometers, etc.) [44]. Hence, there is an urgent need for large localised damping (Large localised damping). This indicates that the shape memory thin films with excellent damping properties are necessary. Additionally, the simultaneous achievement of high strength, toughness, larger recoverable strain or good fatigue resistance in shape memory thin film materials is rather difficult. Meanwhile, the current studies are mainly focused on the microstructure, martensitic transformation and shape memory effect as well as the superelasticity of shape memory thin films. Therefore, the damping properties of Ti-Ni-Cu thin films should be paid more attention and may be the hot topic of further studies. The Ti-Ni-Cu thin-film materials with high damping performance, high strength, and high fatigue resistance can reduce the vibration at the micro-nano scale, and further improve the accuracy and reliability of MEMS operations.

Besides, with the rapid development of MEMS, effective refrigeration under micro conditions has become a difficulty of current researches [45]. Conventional refrigeration technologies have many kinds of disadvantages, such as high energy consumption, difficult miniaturisation and environmental pollution. Solid-state refrigeration can stem from elastocaloric effect of shape memory material, which regulates temperature by absorbing and releasing the latent heat of phase transformation under stress [46,47]. Solid-state refrigeration has many advantages including environmental protection, energy-saving, high efficiency, stability and reliability. To date, the elastocaloric effect of Ti-Ni-Cu shape memory alloy primarily focuses on the bulk materials [48]. However, the investigations of the elastocaloric effect in Ti-Ni-Cu shape memory thin film are absent. In addition, the Ti-Ni-Cu shape memory thin films with the narrower thermal hysteresis and stable cycling properties lay a good foundation for obtaining the rapid response and long-term life elastocaloric effect. Compared with the Ti-Ni shape memory alloy, the adiabatic temperature of Ti-Ni-Cu thin films is lower, which would affect the refrigerating efficiency. In conclusion, seeking new ideas and methods to improve the elastocaloric effect of Ti-Ni-Cu shape memory thin films will be another research focus in the future.

Conclusions

The microstructure, martensitic transformation and strain recovery characteristics of the Ti-Ni-Cu thin films were summarised. It can be concluded that:

The types of precipitates (Such as GP zone, Ti₂Ni and TiCu₂, etc.) in Ti-Ni-Cu shape memory thin films can be obtained by tailoring the chemical compositions, annealing parameters;

The type, size and distribution of precipitates have a significant influence on the martensitic transformation behaviour, martensite structure and functional properties of Ti-Ni-Cu thin films;

Controlling the precipitate is an effective method to improve the mechanical and functional properties of the Ti-Ni-Cu shape memory alloy thin film as a promising actuator materials.

Footnotes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51801023, 51871080 and 52101231) and the Science Fund of Shandong Laboratory of Advanced Materials and Green Manufacturing (Yantai), AMGM2021F09 as well as the Natural Science Foundation of Shandong Province (No. ZR2021QE044).

Disclosure statement

No potential conflict of interest was reported by the author(s).

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The microstructure of Ti-Ni-Cu shape memory thin films: A review

Abstract

Keywords

Introduction

A brief introduction of martensitic transformation and structure of bulk Ti-Ni-Cu alloy

Microstructure and strain recovery characteristics of slightly Ti-rich Ti50.2Ni30Cu19.8 thin film

Microstructure of slightly Ti-rich Ti50.2Ni30Cu19.8 thin films

Strain recovery characteristics of slightly Ti-rich Ti50.2Ni30Cu19.8 thin film

Microstructure of Ti-rich Ti51.5Ni33.1Cu15.4 thin films

Microstructure of Ti-rich Ti51.5Ni33.1Cu15.4 thin films with GP zones

Microstructure of Ti-rich Ti51.5Ni33.1Cu15.4 thin films with Ti2Cu + Ti2Ni precipitates

Microstructure of Ti-rich Ti51.5Ni33.1Cu15.4 thin films with Ti2Ni precipitates

Evolution of martensite structure in Ti-rich thin films with different precipitates

Relationships between the strain recovery behaviour and structure features in Ti-rich Ti-Ni-Cu thin films

Microstructure of (Ni,Cu)-rich Ti48.6Ni35.9Cu15.5 thin films

Microstructure of (Ni,Cu)-rich Ti48.6Ni35.9Cu15.5 thin films annealed at 773 K

Microstructure of (Ni,Cu)-rich Ti48.6Ni35.9Cu15.5 thin films annealed at 873 K

Microstructure of (Ni,Cu)-rich Ti48.6Ni35.9Cu15.5 thin films annealed at 973 K

Evolution of (Ni,Cu)-rich Ti-Ni-Cu thin films with different precipitates

Relationships between the microstructure and strain recovery characteristics of (Ni,Cu)-rich Ti-Ni-Cu thin films

Effect of Ti content on microstructure and functional performances of Ti-Ni-Cu thin films

Effect of Ti content on microstructure of Ti-Ni-Cu thin films

Effect of Ti content on performances of Ti-Ni-Cu thin films

Effect of Cu content on the microstructure and functional performances of Ti-Ni-Cu thin films

Effect of Cu content on the microstructural evolution of Ti-Ni-Cu thin films

Effect of Cu content on the performance of Ti-Ni-Cu thin films

Insights

Conclusions

Footnotes

Acknowledgements

References

Microstructure and strain recovery characteristics of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film

Microstructure of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin films

Strain recovery characteristics of slightly Ti-rich Ti_50.2Ni₃₀Cu_19.8 thin film

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with GP zones

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Cu + Ti₂Ni precipitates

Microstructure of Ti-rich Ti_51.5Ni_33.1Cu_15.4 thin films with Ti₂Ni precipitates

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 773 K

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 873 K

Microstructure of (Ni,Cu)-rich Ti_48.6Ni_35.9Cu_15.5 thin films annealed at 973 K