In situ neutron diffraction study of residual stress development in MgO/SiC ceramic nanocomposites during thermal cycling

Ceramic nanocomposites often contain large residual stresses due to differing thermal contraction between phases upon cooling from processing temperatures. Their role in affecting the mechanical properties is not fully understood, but is certainly of importance. This investigation used neutron diffraction to quantify the residual stresses in MgO/SiC nanocomposites throughout a thermal cycle to 1550 °C. The results showed that average stresses in 10 vol.% SiC samples at 100 °C approached −4 GPa in the particles and were +560 MPa in the matrix. The stresses showed good agreement with an elastic model with a stress-free temperature of 1600 °C. A small amount of inelastic relaxation (15%) was observed after cooling back to room temperature. Modelling suggested that this was due to relaxation of the stresses in grain boundary particles at a rate limited by diffusional processes in the MgO/SiC interface. The effect of particle size on stress level is discussed.     

A modified die for equal channel angular pressing

Tensile stress occurs in the vicinity of upper surface of the specimen in the severe plastic deformation zone, which increases the cracking and fracture tendency of the specimen and impedes the further ECAP processing. In this paper, the conventional ECAP die (Ψ = 16° and Φ = 90°) was modified to eliminate the tensile stress and enhance the compressive stress in the severe plastic deformation zone, therefore reducing the cracking and fracture tendency of the specimen. Finite element analysis demonstrated that the stress state changes from tensile to strongly compressive when using the modified die. A modified die was made and employed to extrude the commercially pure aluminum to verify its effectiveness experimentally. The billet was successfully extruded for 20 passes without obvious surface defects with the modified die, compared to 13–14 passes at most for the conventional die. Consequently, much more fine and uniform microstructure was obtained with the average grain size of 200–300 nm, while the average grain size is ～500 nm in the case of using the conventional die.     

High temperature micropillar compression of Al/SiC nanolaminates

The effect of the temperature on the compressive stress–strain behavior of Al/SiC nanoscale multilayers was studied by means of micropillar compression tests at 23 °C and 100 °C. The multilayers (composed of alternating layers of 60 nm in thickness of nanocrystalline Al and amorphous SiC) showed a very large hardening rate at 23 °C, which led to a flow stress of 3.1 ± 0.2 GPa at 8% strain. However, the flow stress (and the hardening rate) was reduced by 50% at 100 °C. Plastic deformation of the Al layers was the dominant deformation mechanism at both temperatures, but the Al layers were extruded out of the micropillar at 100 °C, while Al plastic flow was constrained by the SiC elastic layers at 23 °C. Finite element simulations of the micropillar compression test indicated the role played by different factors (flow stress of Al, interface strength and friction coefficient) on the mechanical behavior and were able to rationalize the differences in the stress–strain curves between 23 °C and 100 °C.     

Comparative study of the deformation behavior of hexagonal magnesium–lithium alloys and a conventional magnesium AZ31 alloy

Specimens of a conventional magnesium AZ31 alloy and a binary α-solid solution Mg4Li alloy with similar starting textures and microstructure were subjected to plane strain deformation under various deformation temperatures ranging from 298 K to 673 K. Lithium addition to magnesium exhibited remarkable room temperature ductility improvement owing to enhanced activity of non-basal slip, particularly, 〈c + a〉-slip mode. Furthermore, the addition of lithium to magnesium seemed to reduce the plastic anisotropy, typical for commercial magnesium alloys. This was evident in the flow curves and texture development obtained at 200 °C and 400 °C. At 400 °C prismatic slip gains strong influence in accommodating the imposed deformation. In terms of thermal stability against microstructure coarsening at elevated temperatures, the lithium containing alloy undergoes significant grain growth following recrystallization.     

On plasticity and fracture of nanostructured Cu/X (X = Au,Cr) multilayers: The effects of length scale and interface/boundary

Plastic deformation and fracture behavior of two different types of Cu/X (X = Au, Cr) multilayers subjected to tensile stress were investigated via three-point bending experiments. It was found that the plastic deformation ability and fracture mode depended on layer thickness and interface/boundary. The Cu/Au multilayer showed significant features of plastic flow before fracture, and such plasticity was gradually suppressed by premature unstable shearing across the layer interface with decreasing layer thickness. In comparison, Cu/Cr multilayers were prone to a quasi-brittle normal fracture with decreasing layer thickness. Both experimental observations and theoretical analyses revealed differences in plasticity and fracture mode between the two types of metallic multilayers and the relevant physical mechanism transition due to length scale constraint and interface/boundary blocking of dislocation motion.     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

Constitutive equations of flow stress of magnesium AZ31 under dynamically recrystallizing conditions

Constitutive equations for the relationship between flow stress, strain, strain rate and temperature for magnesium AZ31 alloy under hot working conditions where dynamic recrystallization is prevalent have been developed. Equation development data were obtained using isothermal plane strain compression (PSC) tests carried out at 300–500 °C with strain rates ranging from 0.5 to 50 s⁻¹, to an equivalent strain of 0.7. The predicted flow stress curves show good comparison with the experimental isothermal flow curves in terms of peak, steady state stress and flow softening behaviour but at higher Zener–Hollomon (Z) values (>10¹¹ s⁻¹) the predicted peak stress deviates from the isothermal value in the range of 14–25 MPa suggesting a breakdown in the hyperbolic sine equation at those Z values. The developed constitutive equations for the valid thermomechanical conditions were adopted in a finite element model to simulate the PSC conditions. The distributions of strain, strain rate and temperature qualitatively suggest higher strain rate at the centre of the sample which agrees well with that of the quantitative analysis of the dynamically recrystallized grain size.     

Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts

The plastic deformation behavior and the effects of the impact time on the LY2 aluminum (Al) alloy during multiple laser shock processing (LSP) impacts were investigated. The residual stress in the near-surface region was determined by X-ray diffraction. In addition, the micro-structural features of the hardening layer were characterized by scanning electron microscopy, optical microscopy and transmission electron microscopy. It was found that the micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface after multiple LSP impacts was about 100–200 nm. The grain refinement process after multiple LSP impacts can be described as follows: (i) the formation and development of dislocation lines in original grains; (ii) dislocation tangles (DTs) and the formation of dense dislocation walls (DDWs); (iii) transformation of DTs and DDWs into subgrain boundaries; and (iv) evolution of the continuous dynamic recrystallization in subgrain boundaries to refined grain boundaries.     

Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol.% SiC composites

Microstructure evolutions of ZrB₂–30 vol.% SiC composites, prepared by hot pressing at different processing temperatures (1700, 1850 and 2000 °C) for 30 min under 10 MPa, were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The microstructures of the fabricated composites were compared with and the effects of the processing temperature on the sintering process and densification behavior during the hot pressing were found. The amount and the orientation of dislocations which were indicated by TEM analysis in the sample hot pressed at 1700 °C showed that no plastic deformation and atomic diffusion occurred. But the presence of amorphous phases and rearrangement of particles are signs of the fact that liquid phase sintering and particle fragmentation/rearrangement is the main densification mechanism. On the other hand, in the sample hot pressed at 1850 °C, aggregation of dislocations behind the grain boundaries and the presence of twinnings addressed wide plastic deformations which were introduced as the main densification mechanism at 1850 °C. Finally in the sample hot pressed at 2000 °C, lower amounts of un-oriented dislocations and also some annealing twinnings were observed in TEM micrographs together with fractographical SEM analysis and showed that the atomic diffusion is the dominant densification mechanism of hot pressed ZrB₂–30 vol.% SiC composite.     

Development of intergranular thermal residual stresses in beryllium during cooling from processing temperatures

The intergranular thermal residual stresses in texture-free solid polycrystalline beryllium were determined by comparison of crystallographic lattice parameters in solid and powder samples measured by neutron diffraction during cooling from 800 °C. The internal stresses are not significantly different from zero >575 °C and increase nearly linearly <525 °C. At room temperature, the c axis of an average grain is under ～200 MPa of compressive internal stress, and the a axis is under 100 MPa of tensile stress. For comparison, the stresses have also been calculated using an Eshelby-type polycrystalline model. The measurements and calculations agree very well when temperature dependence of elastic constants is accounted for, and no plastic relaxation is allowed in the model.     

Thermomechanical fatigue behaviours of a third generation γ-TiAl based alloy

In order to clarify the behaviours of thermomechanical fatigue (TMF) of a third generation γ-TiAl based alloy, the influence of related microstructural instability during TMF process on stress–strain response, fatigue life and fracture way under in-phase (IP) and out-of-phase (OP) loading mode was investigated. Cyclic softening at high temperature (>700 °C) arises from the dissolution of α₂ lamellae and recrystallization of γ phase. Cyclic hardening at low temperature (<550 °C) is caused by strong interaction between dislocations. As temperature increases, the mean stress and remained plastic strain range increase, leading to severe TMF damage. Owing to the formation of superfine γ grains in IP condition, a superimposed effect of creep and fatigue damage contributes to the TMF failure. OP loading mode brings about the coarsening of primary equiaxed γ grains. Fatigue damage displays the intergranular fracture and transgranular cleavage fracture ways of coarse γ grains.     

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10 wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25 °C to 725 °C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500 °C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500 °C. Under uniaxial tension at temperatures higher than 650 °C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.     

High temperature deformation behaviors of a high Nb containing TiAl alloy

In the present paper, high temperature tensile and creep behaviors of Ti–45Al–9(Nb,W,B,Y) alloy with duplex (DP) microstructure were investigated. In addition to tensile tests at 815 °C and a strain rate range of 1 × 10⁻⁴ s⁻¹−1 × 10⁻³ s⁻¹ and tensile, creep tests at 760 °C and 815 °C under the stress of 180 MPa, the microstructure evolutions during tensile and creep tests were studied. The results show that high temperature high Nb containing TiAl alloy with DP microstructure has a good balance between ductility and strength and intermediate creep resistance. The tensile properties have the strain rate dependence, and ultimate tensile strength (UTS) and yield strength (YS) vs. strain rate obey a single-logarithm linear relationship. Minimum creep rate is affected by the test temperature and stress. Using loading change experiment a stress exponent of 4.3 is determined. DP microstructure is unstable after long-term exposure at high temperatures, and the spheroidization of lamella and recrystallization along grain boundaries occur during the high temperature deformation. It is assumed that the diffusion-assisted climb of dislocations might be the controlling mechanism at the minimum creep rate stage.     

An alternative approach in ceramic shell investment casting of AZ91D magnesium alloy: In situ melting technique

In this research, the possibility of ceramic shell investment casting of a magnesium alloy using in situ melting technique was explored. AZ91D granules were charged into shell investment mould and in situ melted under various processing parameters including heating temperature, flux application, shell mould thickness and permeability. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to characterise the cast samples. Thermal analysis was employed to further investigate the effect of mould thickness on the solidification behaviour of the metal. It was found that mixing flux with the granules not only reduced the temperature at which melting can be achieved, but it also contributed to produce castings with acceptable surface quality. The use of thinner mould provided higher solidification rate, which is believed to favour in situ melting of the granules. It enabled melting of the granules at 650 °C, which in turn helped to suppress the mould–metal reaction and produce castings with good surface quality. Shell mould permeability showed no influence on suppressing the mould–metal reaction at 650 °C.     

The influence of rolling temperature on texture evolution and variant selection during α → β → α phase transformation in Ti–6Al–4V

The role of starting texture in variant selection has been studied during α → β → α transformation in Ti–6Al–4V. By hot rolling at different temperatures followed by recrystallization, material with either a strong basal texture or a strong transverse texture was generated. Subsequently, both conditions were heat-treated above the β transus followed by slow cooling. The degree of variant selection was assessed by comparing the strength of the measured and predicted α texture from high temperature β texture, assuming equal occurrence of all possible variants during β → α transformation. It was found that, even though the material rolled originally at 800 °C displayed a stronger α texture after β heat treatment, it was the material rolled originally at 950 °C that showed greater variant selection. The variant selection mechanism is discussed in terms of the generated β texture and common 〈1 1 0〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈1 1 0〉 pole misorientation distributions for the two investigated β textures showed that the combination of texture components generated during rolling Ti–6Al–4V at 950 °C increases the likelihood of having β grain pairs with closely aligned (1 1 0) planes compared to rolling at 800 °C. Therefore, it can be proposed that avoiding the generation of certain combinations of β texture components during thermomechanical processing has the potential for reducing variant selection during subsequent β heat treatment.     

Texture memory effect in heavily cold-rolled Ni3Al single crystals

In Ni₃Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.     

A multiscale coupled Monte Carlo model to characterize microstructure evolution during hot rolling of Mo-TRIP steel

A multiscale modelling framework has been proposed to characterize microstructure evolution during hot strip rolling of transformation-induced plasticity (TRIP) steel. The modelling methodology encompasses a continuum dislocation density evolution model coupled with a lumped parameter heat transfer model which has been seamlessly integrated with a mesoscale Monte Carlo (MC) simulation technique. The dislocation density model computes the evolution of dislocation density and subsequently constitutive flow stress behaviour has been predicted and successfully validated with the published data. A lumped-parameter transient heat transfer model has been developed to calculate the average strip temperature in the time domain. The heat transfer model incorporates the effect of plastic work for different strain rates in the energy conservation formulation. A coupled initial value problem solver has been developed to integrate the system of stiff ordinary differential equations in the time domain to predict dislocation density and temperature profiles simultaneously. The temporal evolution of microstructure during hot rolling of TRIP steel is simulated by the MC method incorporating thermal and dislocation density data from the continuum models. Simulated microstructural maps, kinetics of recrystallization and grain size evolution have been generated in a 200 × 200 lattice system at different strain rates and temperatures. The simulation code has been implemented in a high-performance grid computing network. The predicted temporal evolution of grain size, recrystallized fractions and flow stress have been validated with the published literature and found to be in good agreement, confirming the predictive capability of the integrated model.     

Effect of filtration on the morphology and mechanical properties of Mg molten alloy entering the mould cavity

In this research, the experiments were carried out in two methods. In the first, the foam filter has been placed at the gate or runner of the mould, and in the second, no filter was used. In both methods, the casting temperatures were 680 °C and 750 °C. A pyrex glass plate was used on one side of the mould cavity in order to evaluate the morphological behaviour of entering melt into the cavity. The height of filtered molten Mg–Al–Zn alloy in mould was increased step-by-step with the filling time. This showed that the filtered molten alloy flows in mould cavity with different velocities. In case of no filtered molten alloy this was not observed. The reliability of the results of bending tests has been evaluated by Weibull analysis. The filters had absorbed the inclusions very well. A fracture toughness of 260 MPa was obtained in specimen poured at 680 °C with ceramic filters. The Weibull plots showed that the castings filtered at the gate at temperature of 680 °C had the best reliability.     

Effects of heat treatment on microstructure and mechanical properties of Cr–V–Mo steel processed by recrystallization and partial melting method

The phase segregation of semisolid processed products resulted in an inhomogeneous microstructure and poor mechanical properties of such products. Optimal subsequent heat treatments including quenching and tempering with various processing parameters were conducted to improve the quality of RAP (recrystallization and partial melting) processed Cr–V–Mo steel. The microstructure characteristics and mechanical properties, such as hardness, tensile strength, elongation, impact toughness, and resistance to high-temperature wear, of specimens subjected to various heat treatments were investigated. When the RAP-processed specimen was quenched from 1050 °C after isothermal holding for 480 s and then tempered twice at 560 °C for 2 h, microstructural evolution took placed in both former solid-phase and liquid-phase regions. The weakening of phase segregation, the redistribution of carbides, and the release of residual stress occurred during this heat treatment strategy caused a good combination of mechanical properties.     

Microscale-calibrated modeling of the deformation response of low-carbon martensite

Micropillar compression tests were used to determine the uniaxial compressive stress–strain response of martensite blocks extracted from a low-carbon, fully lath martensitic sheet steel, M190, with the nominal composition C = 0.18, Mn = 0.47, P = 0.007, S = 0.006, Si = 0.18, Al = 0.06, Ti = 0.045, B = 0.0014 and balance Fe (all in wt.%). Specimens with a diameter exceeding ～1 μm and consisting of a single martensite block showed elastic–nearly perfectly plastic behavior with a yield stress of the order 1200 MPa. Similar specimens which contained multiple martensite blocks showed pronounced strain hardening, arising from the geometrical constraint produced by the interface(s). No size dependence of flow stress was observed in micropillars with diameters exceeding 1.0 μm, but a significant scatter in strength and hardening rate was observed in micropillars with smaller diameters. Flow data for micropillars in the size-independent regime were used to determine parameters in a crystal-plasticity-based model of martensite. Full three-dimensional crystal plasticity simulations, with material properties determined from micropillar tests, were then used to predict the macroscopic uniaxial stress–strain behavior of a representative volume element of martensite. The predicted stress–strain behavior was in excellent agreement with experimental measurements, and demonstrates the potential for micropillar tests to determine material parameters for individual phases of a complex microstructure.