Materials Science and Technology

Al–Bi immiscible alloy is of particular interest as potential self-lubricating wear materials with a homogeneous distribution of minority phase. However, it is difficult to obtain a homogeneous microstructure by conventional casting methods due to liquid phase separation of Al–Bi immiscible alloy. We have developed a new strategy to restrain liquid phase separation and improve the properties of Al–Bi immiscible alloy by in situ phases. The in situ AlB₂ phase acts as heterogeneous nucleation site to accelerate the nucleation and slow down the velocity of the Bi-rich droplet, resulting in a significant size reduction and a homogeneous microstructure of Al–Bi immiscible alloy. The self-lubricating wear resistance of Al–Bi immiscible alloy can be further enhanced by in situ Al₂Cuphase.

Nanostructured semiconductor, copper sulphide, CuS prepared by a mechanochemical synthesis in a large-scale operation industrial mill was modified by surfactant-assisted milling in a planetary mill. Surface modified CuS with cetrimonium bromide, and sodium dodecyl sulphate surfactants were characterised by particle size distribution and compared to mechanochemically synthesised CuS. X-ray diffraction, Fourier-transform infrared spectroscopy and elemental analysis confirmed the binding of surfactants to the CuS structure. Transmission electron microscopy has revealed that the particles composed of nanocrystallites are embedded in the surfactant matrix. Their optical properties were studied using ultraviolet–visible and photoluminescence spectroscopies. The optical band gap energies were determined to be 1.70 and 1.63 eV for CuS modified with cetrimonium bromide and sodium dodecyl sulphate.

Electropulsing has been imposed on molten AISI 304 stainless steel and its effects on the inclusion size and corrosion properties have been investigated. The average size of inclusions in electropulsing-treated samples was finer than that in untreated ones. A theoretical analysis shows that the mobility of solute atoms increases with electropulsing treatment. This promotes the nucleation of inclusions. Owing to a smaller inclusion size, the electrochemical experiments reveal that the pitting corrosion resistance of the electropulsing-treated stainless steel is significantly improved. The beneficial effects of electropulsing on the refinement of inclusions and the resistance to pitting corrosion increase with an increase in current density. Electropulsing treatment, therefore, provides a promising means to improve pitting corrosion resistance.

In the present work, the possibilities of tailoring crystallographic texture via cross rolling are presented. It is shown that a strong rotated Brass texture develops upon cross rolling in aluminium alloys which also remains intact during the subsequent recrystallisation annealing treatment. The governing mechanisms behind the evolution of deformation and recrystallisation texture are discussed in terms of effect of strain path on stability of deformation texture components and strain-induced boundary migration mechanism, respectively. In addition, the likelihood of rotated Brass texture having a positive effect on formability is discussed in terms of sluggish cross-slip criteria as the rotated Brass texture presents a unique scenario where cross-slip is inhibited along all the three principal directions.

The current study focuses on the composition of the constituents that are obtained from ultra-fast heating and the composition of the parent austenite that was achieved at the peak temperature. In order to analyse the content and distribution of important elements such as carbon, manganese, chromium and molybdenum in the microstructure, simulation software was used. It is also found that the content of these elements shows a gradient in the microstructure in relation to the distance from the undissolved cementite. This leads to carbon-rich areas in the vicinity of cementite and carbon-depleted areas as the distance from the cementite grows. The former creates ideal conditions for the retainment of austenite due to the higher carbon content while the latter creates conditions that enable the formation of bainite in the microstructure.

High silicon ductile irons are being developed due to their advantages relating to pearlitic-ferritic grades (high ductility, fully ferritic structures, good machinability, etc.). Recent studies reported that silicon contents higher than 5.2 wt-% originates drastic embrittlement due to chemical ordering. For improving the mechanical properties, the addition of other alloying elements becomes an interesting way of work. This study focuses on the cobalt effect on as-cast microstructures and mechanical properties of ductile irons with silicon contents that maximise ultimate tensile strength. The results obtained show that addition of 4 wt-% cobalt increases the ultimate tensile strength by about 10% and decreases the silicon content at the maximum in this property respecting the unalloyed alloys because cobalt enhances ordering as does silicon.

An amorphous equiatomic FeCrMnNiC alloy thin film was synthesised on silica and silicon substrates by ion beam sputter deposition. Scanning electron microscopy energy-dispersive X-ray spectroscopy, X-ray diffraction, selected area electron diffraction and nano indentation were used to investigate the material. The produced film is amorphous and uniform on the atomic level without any detectable element segregation. The corrosion properties were assessed in 3.5% NaCl solution. The FeCrMnNiC alloy has corrosion resistance better than corrosion resistance of 304 SS. The thin film has hardness at around 12.3 ± 0.5 GPa and reduced Young's modulus at around 222 ± 12 GPa.

We study here the underlying factors that govern the stability of austenite in a medium Mn (Fe–0.18C–11Mn–3.8Al) (wt-%) steel. In this regard, a novel heat treatment involving intercritical quenching and tempering was designed to obtain high total elongation (TEL) and high ultimate tensile strength (UTS) in the cold-rolled steel. And the UTS and TEL approached 920–1150 MPa and 35–65%, respectively. The product of TEL and UTS (PSE) exceeded 40 GPa%, with a maximum value of 60 GPa%. A detailed analysis of microstructure before and after tensile deformation revealed that the TRIP effect occurred and the stability of austenite was predominantly governed by the grain sizes of austenite rather than the orientation of austenite grains. The theoretical analysis of work hardening data suggested that the superior elongation of medium Mn TRIP steel is related to the high stability of austenite and the cooperative deformation of ferrite.

High strength structural (HSS) steels are widely used in many construction equipment and infrastructures. Currently, there is a substantial demand of Non-Destructive Testing of these steel structures. In this research, the electromagnetic technique was utilised to assess the plastic damage in a typical HSS steel (i.e. Q690 steel). The results showed that the magnetic hysteresis loop of this material changed apparently after the tensile deformation, and the relevant magnetic parameters such as coercivity, remanence, and maximum permeability were sensitive to the cumulative plastic strain. The results demonstrated that there is a good coherence between the plastic damage evolution and the magnetic parameters variation, which indicated that the electromagnetic technique is an effective method to assess the plastic damage of HSS steel.

Effects of a long annealing up to 10 h were investigated on damping capacity and phase evolutions in FeMn-based alloys. The damping capacity in the Fe17.5Mn alloy rose with the annealing time. A highest damping capacity was obtained in the Fe16.5Mn10.5Cr alloy after 2 h annealing. The highest damping capacity was comparable in Fe17.5Mn and Fe16.5Mn10.5Cr alloys. A positive relationship between the damping capacity and the stacking faults existed in both the Fe17.5Mn and Fe16.5Mn10.5Cr alloys annealed for above 2 h. To reduce the amount of α′-martensites was favourable to the further improvement of damping capacity. The α′-martensites clustered locally in the Fe17.5Mn alloy, but they distributed inside the ϵ-martensite bands in the Fe16.5Mn10.5Cr alloy.

The present work describes the preparation of near-full density Cu–Al–Ni shape memory alloy (SMA) strips via a novel processing route consisting of ‘spray deposition’ of atomised liquid Cu–Al–Ni alloy with a jet of argon gas followed by hot-rolling densification of the deposited preform. The subsequent homogenisation of the hot rolled Cu–Al–Ni strips resulted in complete martensitic structure in the finished strip, consisting of self-accommodated plates of β₁′ and γ₁′ martensites. The characteristic transformation temperatures and shape memory effects of Cu–Al–Ni strips were studied. It has been demonstrated that the Cu–Al–Ni SMA strips, prepared in the present work, resulted in relatively finer grain size with better combination of strength and ductility compared to other techniques based on conventional casting method.

Research aimed at enhancing the surface properties of carbon steels by incorporating fine silicon carbide particulates has had limited success because the dissolution of the ceramic occurred. This research considers a method of reducing SiC dissolution by generating a high Fe–Si liquid which protects the ceramic. Three particulate groups were investigated, (1) ∼ 5 µm SiC, (2) ∼45 µm Si +∼ 5 µm SiC, and (3) ∼45 µm Si, all incorporated into a microalloyed steel using a tungsten inert gas process. Detailed microhardness of the melt zones together with microstructural analysis showed that the addition of Si resulted in a cracked hard layer containing SiC. However, in Specimen 1, a thicker, hardcrack-free layer resulted from the microstructure developed by the dissolution of SiC.

Spark plasma sintering (SPS) has been used to synthesise samples of a model dual-phase (DP) system, consisting of copper and AISI420 martensitic steel. Mixed powders were sintered at different temperatures in the range from 850°C to 1000°C using a loading pressure of 60 MPa. Tensile testing revealed dimpled fracture surfaces in samples sintered at higher temperatures, indicating ductile failure and good interfacial bonding between the phases, with the best results overall obtained for samples sintered from wet-mixed powders. The results show that by control of the mixing, sintering and post-sintering heat treatment conditions, the fabrication of dense samples covering a range of DP microstructures is possible using SPS, with independent control of the microstructure and properties of the two phases.

The present study investigated the sintering behaviour of Nb–16Si–25Ti–8Hf–2Cr–2Al alloy powders. The alloy powders were produced using ahydrogenation–dehydrogenation method and featured an irregular morphology. Powders sintered at 1500°C and 1600°C exhibited pores in their microstructures, while powders sintered at 1700°C for 4 h were fully densified and poreless. The cast ingot and powders were composed of three phases: Nb_ss, Nb₅Si₃, and Nb₃Si. However, the Nb₃Si phase was not observed, while HfO₂ oxides formed in the sintered compact. The hafnium and oxygen reacted to form an HfO₂ oxide during the high-temperature sintering process. From the result of the thermodynamic calculation, Hf oxide formed after sintering because Hf has the highest driving force for oxidation among the elements constituting the alloy.