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.     

Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.     

Kinetics of austenitization under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Differential dilatometry has been employed to study the kinetics of the massive ferrite (α) → austenite (γ) transformation upon isochronal heating (i.e. austenitization) of the substitutional Fe–2.96 at.% Ni alloy subjected to a range of applied constant uniaxial compressive stresses. A phase-transformation model, involving site saturation, interface-controlled (continuous) growth and incorporating an impingement correction for an intermediate of the cases of ideally periodically and of ideally randomly dispersed growing particles, has been employed to extract the interface-migration velocity of the α/γ interface and the transformation-induced deformation energy taken up by the specimen. The value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the α/γ volume misfit depends on the austenite fraction and increases distinctly with an increase in the applied uniaxial compressive stress, which is compensated by, in particular, an increase in the chemical driving force corresponding to an increase in the onset temperature. The opposite effects of an applied uniaxial compressive stress on the α → γ transformation and on the γ → α transformation can be discussed as the outcome of constrained plastic deformation due to transformation-induced strain.     

The axial compressive failure of titanium reinforced with silicon carbide monofilaments

Monofilament-reinforced titanium has been subjected to compressive loading, with a range of angles between the fibre axis and the loading direction. Under axial loading, the failure stress is about 4 GPa, which is well below levels predicted for kink band formation. It is proposed that compressive failure occurs under these circumstances by the crushing of individual fibres. A model is proposed for prediction of the composite strength as controlled by this mechanism. Observed strengths are consistent with monofilament crushing stresses of about 8–10 GPa. Composites were also studied after a post-consolidation heat treatment and with weak fibre–matrix interfacial bonding. In both cases, slightly higher compressive strengths were recorded than for the standard material. These increases are attributed to an enhanced matrix yield stress and to a higher monofilament compressive strength, respectively. Under off-axis loading, strengths fell from about 4 GPa at low misalignment angles to just above 1 GPa at an angle of 16°. A transition occurs between fibre crushing at low angles and kink band formation at higher angles. The transition range is around 3–4°, which is consistent with model predictions. Microstructural studies confirmed that the expected failure modes were operative in these two regimes.     

Tensile strength of 〈1 0 0〉 and 〈1 1 0〉 tilt bicrystal copper interfaces

Molecular dynamics (MD) simulations are used to model dislocation nucleation at or near symmetric tilt bicrystal copper interfaces with 〈1 0 0〉 or 〈1 1 0〉 misorientation axes. MD simulations indicate that orientation of the opposing lattice regions and the presence of certain structural units are two critical attributes of the interface structure that affect the stress required for dislocation nucleation. Boundaries that contain the E structural unit are found to emit dislocations at comparatively low tensile stress magnitudes. A simple model is proposed to illustrate the impact of interfacial porosity and stresses acting on the slip-plane in non-glide directions on tensile interface strength. Accounting for interfacial porosity through an average measure is found to be sufficient to model the tensile strength of boundaries with a 〈1 0 0〉 misorientation axis and many boundaries with a 〈1 1 0〉 misorientation axis.     

Internal stress and mechanical deformation of Al and Al/Ni multilayered nanowires

Isothermal molecular dynamics is used to study the correlation between the spatial distribution of internal stress and mechanical deformation of a 6.7-nm-diameter Al nanowire with <1 0 0> axis is subjected to an external uniaxial stress. The stress–strain relationship is asymmetrical. In the case of a tensile load, the internal stress distribution is found to result from the interplay between structure and morphology. As a general rule, yielding nucleates where the internal stress gradient is the highest. If the Al wire is interfaced with a harder material—Ni in this study—the highest gradients occur at the interface, where a characteristic interfacial stress pattern is induced. Remarkably, compressive and tensile yield strengths are found to be unaffected by the hard/soft interfaces. The structure of the stress–strain relationship is found to correlate with identified discrete plastic events. These may be complex, involving interactions between partial dislocations, stacking faults, surfaces and interfaces, internal stress localization and release.     

Molecular dynamics simulations of grain boundary sliding: The effect of stress and boundary misorientation

Molecular dynamics simulations were used to study the effect of applied force and grain boundary misorientation on grain boundary sliding in aluminum at 750 K. Two grains were oriented with their 〈1 1 0〉 axes parallel to their boundary plane and one grain was rotated around its 〈1 1 0〉 axis to various misorientation angles. For any given misorientation, increasing the applied force leads to three sliding behaviors: no sliding, constant velocity sliding and a parabolic sliding over time. The last behavior is associated with disordering of atoms along the grain boundary. For the second sliding behavior, the constant sliding velocity varied linearly with the applied stress. A linear fit of this relationship did not intersect the stress axis at the origin, implying that a threshold stress for sliding exists. This threshold stress was found to decrease with increasing grain boundary energy. The ramifications of this finding for modeling grain boundary sliding in polycrystals are discussed.     

Microstructure and mechanical properties of MoSi2/TaSi2 two-phase alloys

Effects of pre-strain on the compressive mechanical behavior were investigated on the alloys with an aligned lamellar microstructure consisting of C11_b MoSi₂ and C40 TaSi₂ prepared through optical floating zone (OFZ) method and annealing in the two-phase MoSi₂/TaSi₂ region. Single crystals of C40 TaSi₂ were successfully grown at solidification rate of 5 or 10 mm/h, and well-aligned lamellar microstructure can be achieved by selecting an MoSi₂–17 mol% TaSi₂ alloy. Firstly, compression tests were conducted to examine the effect of the angle ϕ between the aligned lamellae and the loading axis on strength and ductility. Results indicate that ϕ = 0, lamellae parallel to the axis, is a hard-orientation and ϕ = 54 and 40 (identically 45) are soft orientations. The alloy with ϕ = 0 shows higher strength but lower ductility than those with ϕ = 54 and 40 where ductility is evaluated in terms of brittle-to-ductile transition temperature (BDTT). Then effects of pre-straining at 1773 K on the mechanical behavior at 1573 K were investigated using soft-oriented lamellar alloys with ϕ = 40 whose BDTT is determined at least lower than 1673 K. Pre-straining to a few to several percent at 1773 K improves ductility of the alloy at 1573 K and also raises 0.2% flow stress, compared with the absence of the pre-strain. We believe that dislocations can be generated and stored in the alloy at 1773 K, and these dislocations are mobile even at 1573 K, although the analyses of operative slip systems and characteristics of dislocations remain as future works.     

Influence of boron and oxygen on the microstructure and mechanical properties of high-strength Ti66Nb13Cu8Ni6.8Al6.2 alloys

The influence of boron additions and different oxygen contamination levels on the microstructure and the mechanical properties in the Ti_66?xNb₁₃Cu₈Ni_6.8Al_6.2B_x (0 ? x ? 1) system were investigated. The alloys were prepared by levitation copper mould casting as rods with a diameter of 5 mm using different grades of starting elements. The alloy without boron exhibits a maximum compressive stress of more than 2500 MPa, associated with a compressive strain of more than 30%. The ultimate tensile stress is ～1075 MPa with a maximum elongation of 1.6%. Increased oxygen content leads to a rise of yield strength due to solid solution hardening. Boron additions promote grain refinement and reinforce the interdendritic phase compound by forming needle-like TiB precipitates. This change in microstructure increases the yield stress and the Young’s modulus and lowers the plastic strain. The microstructure was analysed in terms of the boron content by means of scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy. The presented mechanical properties are compared with the compression and tensile properties of the commercially available Ti6Al4V ELI (ELI = extra low interstitial) alloy.     

Inter-martensitic transitions in Ni–Fe–Ga single crystals

The strain–temperature response of Ni–Fe–Ga single crystals underscores the role of the inter-martensitic transformation in creating intersecting heating and cooling segments; the separation of these segments occurs due to irreversibilities at high stresses and at high temperatures. An ultra-narrow tensile (1 °C) and compressive (<10 °C) thermal hysteresis are observed for the A ⇌ 10M ⇌ 14M case, accompanied by a small stress hysteresis (<30 MPa) in compressive and tensile stress–strain responses. The hysteresis levels increase and the intersecting segments disappear at high stresses and at high temperatures. This paper reports the use of a thermo-mechanical formulation to rationalize the role of inter-martensitic transformations. Plotting the transformation stress as a function of temperature indicates that inter-martensitic transformations enable a very wide pseudoelastic temperature range, as high as 425 °C. The measured Clausius–Clapeyron curve slope in compression (2.75 MPa °C⁻¹) is eight times the tensile slope (0.36 MPa °C⁻¹); the higher slope is attributed to the predominance of A ⇌ L1₀ at high temperatures.     

Oxidation-induced embrittlement of TiAl alloys

The ductility of oxidised TiAl-based alloys is reduced even when the oxygen-rich region is of the order 100 nm thickness; this loss in ductility is smallest in lamellar samples. Removal of this oxidised region restores ductility. Acoustic events are observed during tensile tests at stresses above 300 MPa and cracks at about 250 MPa. In-situ tensile tests on samples, with part of the oxygen-enriched region removed, have shown that cracks are formed only in regions where the oxygen-rich region is present. X-ray diffraction measurements have shown that the oxygen-rich surface generates a tensile stress in the top 1 or 2 μm of the alloy of about 250 MPa corresponding to a compressive stress in the oxygen-rich layer of 2000 MPa. It is concluded that embrittlement is caused by (i) the tensile stress induced by the oxygen-rich region and (ii) the corresponding ease of crack nucleation in this region. Subsequent propagation is controlled by the fracture toughness.     

Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn₃ (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 μm and lengths ranging from 50 nm to 500 μm were formed at a growth rate of 0.1–1.8 nm s^?1 on the surface of RESn₃ compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈1 0 0〉, 〈0 0 1〉, 〈1 0 1〉 and 〈1 0 3〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn₃ compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)₃ layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)₃ layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn₃ compounds.     

Variation of structure and mechanical properties of Ti–6Al–4V with isothermal hydrogenation treatment

Grain refinement of Ti–6Al–4V (Ti64) was achieved by means of nucleation and growth of hydride phases associated with repeated isothermal hydrogenation (RIH). With a hydrogen loading of 0.7 H/M at 600 °C, a refined nanostructure (～50 nm) in the α matrix resulted mainly from the formation of platelet β_H and δ by the RIH treatment. The first cycle of RIH significantly increased the hardness and yield compressive stress, but further RIH cycling became less effective. For RIH with a hydrogen loading of 0.5 H/M at 750 °C, the refined structure was eliminated, and the compressive yield stress and microhardness of Ti64 were lowered.     

Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10⁻⁶ to 1.18 × 10⁻⁴ ms⁻¹ and at a constant temperature gradient at the solid–liquid interface of G_L = 12 × 10³ K m⁻¹. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni₃Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q_a = 401 kJ/mol.     

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.     

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.     

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.     

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin²ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5 MPa) to 90.5 pores/μm² (135.9 MPa). However, the average pore size around 40 nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.     

The impact of peak shock stress on the microstructure and shear behavior of 1018 steel

As-received and shock-prestrained 1018 steel specimens were subjected to forced shear experiments in a split-Hopkinson pressure bar (SHPB) at room temperature and a strain rate of 3800 s⁻¹ to determine the influence of shock-prestraining on the shear behavior of ferrite. Shock-loading was performed below (12.5 GPa) and above (14 GPa) the pressure-induced epsilon phase transition occurring at 13 GPa. Using electron microscopy and electron backscatter diffraction, twinning and microbanding were observed only in the shock-prestrained specimens. Quasi-static compression tests showed an increase in yield and compressive strengths with increased peak shock stress. SHPB tests produced shear localization in all specimens, with shear banding occurring only in the shock-prestrained specimens. Transmission electron microscopy revealed that, at the shear band edge, elongated cells dominate the microstructure, with more shock-induced twins remaining intact in the 12.5 GPa specimen than in the 14 GPa specimen.