Materials Science and Technology

Plastic flow is never a continuous process either in time or in space, yet it continues to be modelled in this way. In this review, plasticity is discussed from the perspective of self-organised criticality (SOC). The following topics are discussed: slip bands and models for them; how crystallographic is slip?; distributed sources and collective motion of dislocations; the triumph and the failure of the Taylor model; long range stress does not contribute directly to the flow stress, so the forest rules; the perils of coarse-graining and the relationship between smoothed engineering variables and intermittently fluctuating physical variables; work-hardening of a slip band; what controls the structure of slip bands?; interpretation of precursor and exhaustion phases; relationship to other recent theories; consequences which follow from a sound theory of plastic flow. Brief appendices present algebraic results for slip bands modelled as ellipsoids.

Aluminium matrix composites reinforced with submicrometre and nanosize Al₂O₃ particles were successfully manufactured in the form of sheets through eight cycles of accumulative roll bonding process. The mechanical properties of the produced composite are compared with accumulative roll bonded commercially pure aluminium. It is shown that only 1 vol.- of submicrometre or nanosize alumina particles as reinforcement in the structure can significantly improve the yield and ultimate tensile strengths. Scanning electron microscopy revealed that particles have a random and uniform distribution in the matrix especially in the less volume fraction of alumina particles, and strong mechanical bonding occurs at the interface of the particle matrix. According to the results of the tensile tests, it is observed that with less alumina content, the composite reinforced by nanosize particles has higher strength than that by submicrometre size particles. However, more reinforcement up to 3 vol.- of submicrometre particles, as a result of including fewer microstructural defects, leads to better mechanical properties in comparison to the nanoparticle composite.

A theoretical model has been developed to investigate the effects of fluid flow on the dendrite coarsening. The accelerated convective solute transport due to convection is considered by incorporating the new apparent diffusivity parameter into the classical coarsening model for stagnant melts. The model predicted that the dendrite arm coarsening followed the and relationships under laminar and turbulent fluid flow conditions respectively. The model predictions are found to be in good quantitative agreement with experimental data in literature.

The relative performance of coatings for furan resin sand (FRS)–P-toluol sulphonic acid (PTSA) mould (FRS–PTSA), compared with no S, Novolak resin sand coated mould (NRS) was evaluated to determine the occurrence of a degenerated graphite surface layer. The FRS–PTSA moulds evidently incited graphite degeneration in the surface layer, as its thickness increased up to 10 times that with NRS moulds. Applying a mould coating containing S aggravated graphite degeneration. Conversely a desulphurising coating could protect the nodular graphite and limit the surface layer thickness. Independently of the S source at the metal/mould interface, sulphur affected graphite degeneration drastically. The coatings incorporating desulphurisation materials, [MgO; (CaO+MgO+Talc); Mg–FeSi] protected the iron from graphite degeneration, with the average layer thickness decreasing from 3·8 to 4·2: a factor of 7–10 reduction in FRS–PTSA moulds. If the first two coatings acted mainly as desulphurisers, the Mg–FeSi coating had an additional role to desulphurisation and provided Mg for the molten iron, to restore some nodularising potential. It is important to combine desulphurisation with supplementary Mg to remove migrating S and restore lost Mg. This complex behaviour could be realised by the application of coatings with active Mg from fine sized Mg–FeSi.

Corner cracking in medium C electric arc steel was experienced after continuous casting of square billets. This led to the investigation of the influence of high N levels on hot ductility. Steels microalloyed with V, Ti and B were subjected to in situ melting followed by cooling according to typical off-corner temperature profiles. Steels containing moderate amounts of V resulted in good ductility at the estimated off-corner temperature during unbending of 900°C due to the suppression of precipitation. Ti additions to V, Al, Si and B containing steels, although beneficial at low N levels, are detrimental to hot ductility when the N content is high due to a higher precipitate volume fraction. High Ti–B steels produced the worst hot ductility in medium C steels containing high N.

The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of the M-A constituents.

We report the impact of room temperature forging on microstructure evolution and mechanical properties of hot extruded Mg pipes with a special focus on twinning behaviour and texture. The twinning is identified as an important deformation mechanism. The amount of twins are first increased with the increase in forging strain and then reduced remarkably, followed by a sudden increase until the saturation state. Different types of twins are identified, and the role they play in the deformation process is discussed. The basal planes are found to be rotated by a right angle from the extrusion direction during the room temperature forging. The Vickers hardness is increased drastically once imposing the room temperature forging, thereby demonstrating that this technique is effective in modifying microstructures and mechanical properties of metals.

Additions of Sc and Zr were introduced into Al–15 vol.-B₄C composites, and eight experimental composites with different Sc and Zr levels were prepared via a conventional cast process. Optical microscopy, SEM and TEM were applied for observing the as cast microstructures, including the interfaces between the Al matrix and the B₄C as well as the evolution of the precipitates. It was found that Sc involved the interfacial reactions with B₄C that partially consumed the Sc. On the other hand, no major Zr reaction products were found in the interfaces, and the major part of Zr remained in the matrix for precipitation strengthening. The Sc addition yielded considerable precipitation strengthening in the as cast and peak aged conditions. The combination of Sc and Zr significantly enhanced the precipitation strengthening. Nanoscale precipitates Al₃Sc and Al₃(Sc,Zr) were found in the as cast microstructure and contributed to the significant increase of matrix hardness.

Semisolid extrusion with twin screw extruder has been successfully developed for eutectic alloy. In this process, the eutectic melt is sheared and cooled down inside the twin screw extruder to a semisolid state and simultaneously extruded through an open die at a temperature close to solidus. The accurate control of heat balance in the extruder results in the formation of two solid phases and one liquid phase in the Zn–5 wt-Al eutectic alloy. A little plastic deformation in the extruded alloy can be introduced by twin screw extrusion. In semisolid extrusion, the particle size in Zn–5 wt-Al eutectic alloy is close to 40 μm for Zn rich particles and 25 μm for Al rich particles. Two solid particles are at the similar size in longitudinal and transverse directions and distribute uniformly and independently on the whole cross-section of the extruded bar. The remnant liquid can act as lubricant for reducing extrusion force during extrusion and solidify in lamellar morphology between Al rich and Zn rich particles.

The melts of aluminium alloys are very sensitive to oxidation during casting, and the surface oxide film formed during casting can be folded and entrained into the melt due to melt surface turbulence. In this research, sandwiches of oxide–metal–oxide (OMO) formed in a very short time within the cast during solidification were investigated in order to see the effect of magnesium content (i.e. 1 and 2 wt-) on the oxide film thickness. To form OMO sandwiches within the cast, a certain amount of air was blown into the melt every 0·5 s during casting time by means of a compressor at 0·5 atm pressure. Where bubbles of air collided, they formed a sandwich which later was used for investigating purpose. Both the thickness and the surface of oxide films were studied via SEM. The results showed that the thickness of the short time oxide film varies in the range of 150–250 and 200–300 nm for Al–1Mg and Al–2Mg alloys respectively.

In the present work, the effect of manganese addition to ZA8 alloy on thermal analysis parameters, heat transfer and microstructure was investigated. The thermal analysis parameters were found to be significantly affected by chemical modification of ZA8 alloy. Cooling curve and differential scanning calorimetry analyses of modified alloy showed nucleation of new phase other than β dendrites. Chilling of modified alloy resulted in decreased liquidus temperature and enhanced eutectoid transformation. Further, chilling avoids the formation of intermetallic compounds in modified alloy. The heat flux transients were estimated using inverse modelling during solidification of unmodified and modified alloys against different chills. The peak heat flux decreased on addition of Mn to ZA8 alloy. Differential scanning calorimetry analysis indicated that the addition of Mn to ZA8 alloy decreases the heat of solidification. The addition of Mn to ZA8 alloy increased the contact angle, indicating decreased wettability of the modified alloy on the chill surface. The microstructure of ZA8 with Mn showed an increased amount of β phase and a decreased amount of eutectic. X-ray diffraction analysis confirmed the formation of MnAl₆ intermetallics in Mn added ZA8 alloy. Chilling with chemical modification resulted in enhanced decomposition of β phase.

In this work first principles calculations are performed to study the elastic moduli and energy band gap of PbTe under equilibrium and strained condition. Hydrostatic pressures are applied to bulk PbTe in rocksalt structure, and the results show that as the pressure increases, the bulk modulus, shear modulus and the C₁₁ component demonstrate a decreasing trend, whereas the C₁₂ component shows an improving trend. Energy band gap presents an unusual improving degrading trend with respect to pressure, with a band gap closure and reopening being observed. The predicted elastic moduli and band gaps can change over a wide range, which suggests that strain and pressure can be used as an efficient way to modulate the elastic and electronic properties. Furthermore, these findings are helpful in predicting the structural stability and band gap associated performance in PbTe based thermoelectric materials under pressure.

Nanostructured martensite–austenite microstructure was achieved by a quenching–partitioning–tempering (Q–P–T) treatment of high carbon low temperature bainitic steel. Microstructure observations showed that the nanostructured steel consisted of fine martensite (about 25 nm in thickness), retained austenite and carbides. X-ray diffraction analysis indicated that carbon was partitioned into austenite after martensite transformation. High hardness (about 655 HV1) and relatively high retained austenite fraction (0·36–0·41) was attained. The heat treatment times were greatly reduced, nearly equal or higher hardness and retained austensite fraction were achieved by Q–P–T treatment, compared with low temperature super bainite process.

Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.

A Heusler Ni_45·4Mn_39·5In_13·1Gd₂ alloy with high transformation temperature has been obtained by substituting 2 at-Gd for Mn in a ternary Ni_45·4Mn_41·5In_13·1 ferromagnetic shape memory alloy. It is shown that the microstructure of Ni_45·4Mn_39·5In_13·1Gd₂ alloy consists of a matrix and a Gd rich phase. The Ni_45·4Mn_39·5In_13·1Gd₂ alloy exhibits a martensitic transformation start temperature of 726 K, and the transformation hysteresis is A_f−M_s = 148°C. At room temperature, non-modulated martensite of tetragonal L1₀ structure is observed in Ni_45·4Mn_39·5In_13·1Gd₂ alloy. In addition, it is revealed that the addition of Gd significantly enhances the compressive strength and improves the ductility of Ni_45·4Mn_39·5In_13·1Gd₂ alloy.

The influence of substitutional alloying elements (Al, Nb and W) on the mechanical properties of MoSi₂ with both C11_b and C40 structures was explored by first principles calculations. Using the shear modulus/bulk modulus ratio (G/B), Poisson's ratio and Peierls stress, the change in the mechanical properties of the C40 and C11_b structured MoSi₂ due to the addition of the solute atom has been investigated. The results showed that the ductility of C11_b MoSi₂ was increased by alloying with the elements (Al and Nb) that form a C40 structured disilicide but decreased by alloying with the elements (W) that form a C11_b structured disilicide. In contrast, the effect of alloying elements on the ductility of C40 MoSi₂ showed a reverse trend to that of C11_b MoSi₂.

Carbon nanotubes (CNTs) filled with metals can be used in capacitors, sensors, rechargeable batteries and so on. Atomic arrangement of the metals plays an important role in the function of the composites. Here, we show that Ni and NiO nanoparticles in CNTs are crystalline, which results in the occurrence of lattice shrinkage. Interplanar spacing of (111)_Ni, (200)_Ni, (220)_Ni and (311)_Ni lattice plane decreases by 0·08–0·35. The crystal lattice constant of the Ni diminishes by 0·35. Lattice shrinkage causes a misfit of the lattice constant between the nickel and the internal CNT surface: from theoretical 1·21 to actual 0·86. This variation is beneficial to heterogeneous nucleation of Ni crystal nucleus on the surface at a point with the lowest interfacial energy at coherent interface, where the match is (111)_Ni//(0001)_C. According to our findings, one-dimensional nanostructures can be changed and controlled during synthesis by use of CNTs as a template.

The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s⁻¹ have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s⁻¹, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s⁻¹. The liquid density difference increased the most for rates from 20 to 40 μm s⁻¹ as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s⁻¹, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.

Further correspondence on ‘Development of rapid heating and cooling (flash processing) process to produce advanced high strength steel microstructures’ by Lolla et al.

Correspondence regarding the role of white etching cracks and related features in the failure processes of bearing steels, with responses from the authors

Comments on ‘On microstructure and properties of Si modified 100Cr6 bearing steels’ by Kim and Lee, with a response by the authors