Materials Science and Technology

There has long existed a controversy regarding the formation of the bainite phase in steels. Nonetheless, following the clear identification of bainite in 1930, several characteristic features associated with the bainite transformation became well established. These features include: the classification into upper and lower bainite; the existence of a bainite start temperature; and the incomplete reaction phenomenon. Accordingly, two competing theories have been developed to explain the bainite transformation. However, no overriding consensus has been reached as to which is correct. The first theory invokes a diffusion controlled transformation, describing the growth of bainite via the propagation of ledges – a series of steps on the transformation interface. The second interpretation favours a displacive, diffusionless transformation. Bainite growth occurs via the autocatalytic nucleation and growth of successive subunits or platelets. Over time, a wealth of techniques has been implemented in order to increase the understanding of the mechanism by which bainite forms. From the early approaches that involved thermodynamic and kinetic considerations; through detailed work on the crystallography of the transformation; to studies involving advanced characterisation techniques that focused on the distribution of atoms, etc. Some of the theories have been progressively adapted to new evidence and new concepts as they emerge. For example, the concepts of the T₀ curve and of paraequilibrium transformation. This work presents a chronological summary of the two theories of the bainite transformation, charting their progression and noting the variety of evidence both in support of and in contradiction to each viewpoint. The bainite controversy is still debated by steel metallurgists today, although it has mostly been reduced to the question of whether or not bainitic ferrite initially forms with a supersaturation of carbon. This review gathers and evaluates some of the accumulated work in the hope that this question may finally be answered.

Strain arrest during uniaxial tests on nickel–titanium polycrystalline wires indicates that beyond a certain stress level at which the microstructure is stable at a given temperature, there is an additional stress needed to sustain pseudoelasticity or reorientation. This additional stress converges asymptotically to a non-zero value with a decrease in deformation rate in case of reorientation as well as pseudoelasticity (in the latter case, the rates are low enough such that isothermal conditions are expected to prevail). The additional stress is sensitive to the value of the overall strain only when there is a change in crystallinity (during pseudoelasticity), but not when the crystallinity does not change (during reorientation). The effect is studied as a function of temperature, and a rate independent stress–temperature phase diagram is proposed. Noting that thermomechanical coupling is a significant feature of the phase transformation in shape memory alloys, a connection is made between the strain arrest results presented in the present paper and the body of work on thermal arrests reported in the literature.

The current study aimed at investigating the effect of fluidised sand bed quenching on the mechanical performance and quality index of A356·2 aluminium cast alloys. Traditional water and conventional hot air quenching media were used to establish a relevant comparison with fluidised sand bed quenching. Quality charts were generated using two models of quality indices to support the selection of material conditions on the basis of the proposed quality indices. The use of a fluidised sand bed for the direct quenching aging treatment of A356·2 casting alloys yields greater UTS and YS values compared to conventional furnace quenched alloys. For the same aging conditions (170°C/4 h), the fluidised bed quenched aged 356 alloys show nearly the same or better strength values than those quenched in water and then aged in a CF or an FB. Based on the quality charts developed for alloys subjected to different quenching media, higher quality index values are obtained by water quenched T6-tempered A356 alloys. Using hot sand as a quenching medium at different temperatures, namely 170, 190 and 210°C, reduces both the strength and the quality with increase in quenching temperature for the alloys investigated. The regression models indicate that the eutectic silicon modification factor has the most significant effect on the quality results of the alloys investigated, for all heat treatment cycles, as compared to other metallurgical parameters.

A study of the microstructural characteristics of laser arc hybrid welded nickel based IN738 superalloy was performed. Laser arc hybrid welding produced a desirable weld profile in the alloy, similar to what is usually obtained during laser beam welding, and no cracking occurred exclusively in the fusion zone. Elemental partitioning pattern in the fusion zone was studied by electron probe microanalysis and calculating the volume fraction of the weld metal that resulted from the consumption of the filler wire. The result showed that Ti, Ta, Nb, Mo, Al and Zr partitioned into the interdendritic regions of the fusion zone. SEM and TEM examination of the fusion zone showed the presence of secondary solidification reaction constituents, which consists of MC type carbides. The study further revealed that non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the heat affected zone (HAZ), which consequently resulted in extensive HAZ intergranular cracking during welding. Although laser arc hybrid welding appears promising for improving the weldability of nickel based IN738 superalloy, a suitable weldability improvement procedure is required in order to minimise HAZ intergranular cracking and thereby enhance the applicability of this technology to the joining of the superalloy.

In the present study, microstructural evolutions at the joint region of transient liquid phase bonded IN-738LC superalloy were examined. The experiments were carried out at 1100°C with different holding times using Ni–B–Si–Cr–Fe filler alloy, AMS 4776, with B and Si as the melting point depressant elements. It was found that insufficient isothermal solidification of liquated insert caused the formation of continuous centreline eutectic phases. The width of the eutectic decreased with the increase in holding time. Complete isothermal solidification, which precluded the formation of the centreline eutectic, occurred within 45 min of holding time. In addition to the centreline eutectic products, precipitation of boron rich particles was observed at the diffusion affected zone. Homogenisation of isothermally solidified joints at 1120°C for 300 min resulted in the elimination of intermetallic phases formed at the diffusion affected zone and formation of significant γ′ precipitates in the joint region.

The wetting properties of porous Cu nanowall arrays prepared by a facile replacement reaction are reported. The influences of various experimental parameters including the concentrations of Cu ions and reaction time on final products were investigated. It was found that a hierarchical structure containing micro- and nanoscale structure can be formed on zinc substrate. The measurements of water contact angle indicated that the hydrophobic nature of the hierarchical nanostructures can be tuned by reaction times. When reaction time reached 5 min, its wettability switched from hydrophilic to hydrophobic with a contact angle of >120°. By using modified Cassie–Baxter models, we investigated the relation between hierarchical structure and hydrophobic property. Moreover, Cu flower-like hierarchical nanostructures exhibits good hydrophobic properties.

The effect of grain space orientation and particle–matrix stress transfer on dislocation movement forces with difference of acting stress components in slip plane in climb direction is discussed. A modified equation for calculation of creep rate of creep resistant steel including M₂₃C₆ particles size and spacing is proposed. Small differences of experimental and calculated creep rates are obtained by stress exponent n = 3·65.

The nano/ultrafine grains with different orientations were obtained by reversion transformation for austenitic stainless steel, and their tensile properties were investigated at room temperature. Comparing with the transverse direction sample, the much slower transformation induced plasticity was responsible for excellent elongation of rolled direction sample and the much higher strength originated from the presence of a considerable number of nanotwin bundles and the finer dislocations inside twins. The lower strength and plasticity in transverse direction sample could be attributed to the forming of shear bands and lack of deformation nanotwins.

Fe_73·37Cu_0·92Nb_2·9Si_13·56B_9·24 amorphous alloy ribbon was prepared by melt spinning method. The influences of the annealing temperature on its microstructure and soft magnetic properties were investigated. Two crystallisation stages were found with temperature increasing by using differential scanning calorimetry. X-ray diffraction test showed that the first crystallisation phase is α-Fe(Si) with the average grain sizes ranging from 12 to 25 nm when annealing temperature is below 560°C and the secondary crystallisation phase with an average grain size of about 45 nm appears when annealing temperature is at 700°C. The soft magnetic property measurement showed that the B_s increases to 1·26 T and the H_c decreases to 0·002 Oe when annealing temperature increased from 520 to 560°C, while corresponding values for the alloy annealed at 700°C are 1·1 T and 0·079 Oe respectively. These results indicate that the Fe_73·37Cu_0·92Nb_2·9Si_13·56B_9·24 alloy annealed at 560°C has the relatively best magnetic properties.

In the present work, spreading behaviour and development of interfacial microstructure in Sn–0·7Cu, Sn–0·3Ag–0·7Cu and Sn–2·5Ag–0·5Cu lead free solder alloys on Fe–42Ni substrates having two different surface textures were investigated. Smooth textured surfaces yielded better wettability than rough surfaces particularly for Sn–0·7Cu solder alloy. Wettability of Sn–0·7Cu solder on rough textured surfaces was found to be poor compared to other solder alloys. Spreading of Sn–0·7Cu solder on substrate surface showed longer viscous regime, Sn–2·5Ag–0·5Cu solder exhibited shortest viscous regime. Sn–0·3Ag–0·7 solder showed intermediate behaviour. Sn–Cu solder alloy exhibited needle and coarse shaped (Cu,Ni)₆Sn₅ intermetallics at the interface and in the matrix of the solder alloy on smooth substrate, whereas on rough substrate, formation of only coarse shaped (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) was observed. For Sn–0·3Ag–0·7Cu solder alloy, Fe–Ni–Sn and FeSn₂ IMCs identified at the interface. (Cu,Ni)₆/Sn₅ IMCs were found to be less coarser than as observed at Sn–0·7Cu/substrate interface. Sn–2·5Ag–0·5Cu alloy exhibited (Cu,Ni)₃Sn₄ and (Cu,Ni)₆ Sn₅ IMCs at the interface and in the bulk of solder alloy.

Hot compression deformation behaviours of ultrafine grained (UFG) and coarse grained (CG) commercially pure (CP)-Ti were investigated and compared by isothermal simulation tests. Strains over 50 were achieved in these tests over a temperature range of 298–823 K and strain rate of 10⁻⁴–10⁻² s⁻¹. The results show that the peak stresses of UFG and CG CP-Ti fall with increasing deformation temperature and decreasing strain rate. Within the plastic deformation regime, the compressive stress–strain responses of UFG and CG CP-Ti exhibit quite different features, which strongly depend on the temperature and strain rate. The flow stress of CG CP-Ti monotonically increases with strain, while that of UFG CP-Ti increases first and then tends to be constant after a peak value below recrystallisation temperature. The stress–strain curves of UFG CP-Ti show similar characteristic with those of CG CP-Ti as the temperature is above recrystallisation temperature. Strain rate sensitivity values m of the UFG CP-Ti increase with the rise in temperature and are higher than those of CG CP-Ti. It can be concluded that deformation of UFG CP-Ti is controlled by grain boundary and dislocation activity.

In the present work, the effects of Zn content on the microstructures and mechanical properties of as cast Mg–xZn–5Y–0·6Zr alloys (x = 2, 5, 8 and 13 wt-) have been investigated. The results show that the ternary Mg–Zn–Y phase compositions change with Zn/Y ratios induced by the change in Zn content. It is found that the fracture is mainly decided by the characteristics and distribution of second phase rather than the grain size. The influences of these phases, especially the W phase, on the mechanical properties of the alloys have been discussed. Both ultimate tensile strength (UTS) and elongation decrease with the increase in Zn content, while the instance of yield strength (YS) is just the reverse. The W phase is easily cracked because of its brittleness and easy to result in decohesion from the matrix because of the weak atomic bonding, which greatly degrade the UTS and elongation. It can be concluded that the YS closely depends on the grain size, while UTS and elongation closely depend on the volume fraction of eutectic compound (α-Mg+W phase).

The present paper presents the influence of wear parameters on wear behaviour of Al–LM25–zircon metal matrix composites. The composite has been fabricated by novel stir casting process and heat treated. The wear behaviour of composites against EN32 steel was studied using pin on disc type wear test rig. The mathematical model to predict the wear rate of Al–zircon composites has been expressed in terms of reinforcement volume fraction, particle size, applied load, sliding speed and sliding distance using five factors, five level central composite design matrix approach by response surface method. The morphology and topography of worn out surface wear were studied using a scanning electron microscope. The effect of wear parameters on wear rate of composites was analysed by statistical method.

For comparing the enhancement degree of the thermal fatigue behaviour of different hot work tool steels processed by laser remelting, three kinds of hot work tool steels (HHD, H13 and HD steels) were selected to investigate the effect of biomimetic coupled laser remelting process on the thermal fatigue behaviour. The results showed that biomimetic non-smooth samples had better thermal fatigue behaviour compared to untreated samples, the biomimetic non-smooth sample of HHD had the optimum thermal fatigue behaviour. Moreover, before and after thermal fatigue testing, the microhardness and microstructure of biomimetic non-smooth units and parent materials have been investigated. The results showed that biomimetic non-smooth sample of HHD had the highest resistance of thermal cycles softening among the biomimetic non-smooth samples. The microstructures observation indicated that the microstructures of parent materials were a substantial amount of the carbides coarsening while the microstructures of biomimetic non-smooth units were coarse grains after thermal fatigue test.

Microstructure in the hot cracking mechanism was investigated via the Spot-Varestraint test to assess the sensitivity of hot cracking for three aluminium alloys, i.e. A2024-T351, A2219-T87 and A7050-T6, after gas tungsten arc welding. A stereomicroscope, an optical microscope and a scanning electron microscope were adapted to observe the microstructure of hot cracking mechanisms in the weld metal fusion zone (WMFZ), the weld metal heat affected zone (WMHAZ) and the partially melted zone (PMZ). The results indicate that the hot cracking structures differ: solidification cracking in the WMFZ and liquefaction cracking in the WMHAZ. An energy dispersive spectrometer was used to analyse the PMZ of WMHAZ to confirm the segregation of elemental Cu in the liquefaction cracking mechanism at grain boundaries.