Atomistic simulation of martensitic phase transformation at the crack tip in B2 NiAl

Mechanisms of low-temperature deformation at the crack tip in B2 NiAl are studied by molecular dynamics simulations. The stress-induced martensitic transformation is found to occur at the crack tip when a sufficiently high stress concentration exists. For cracks with 〈1 0 0〉 crack fronts, the layered structures of martensites are formed at the crack tip, which is caused by the atoms’ relative displacement on a basal plane due to the shear stress at the crack tip. The mechanism of the martensitic transformation from the B2 to the L1₀ structures occurs along the Bain path. For cracks with 〈1 1 0〉 crack fronts, the martensitic transformation occurs without any layered structures existing. The phase transformation is caused by the atoms’ relative displacements at different atoms layers in the entire martensite formed region.     

Orientation selection process during the early stage of cubic dendrite growth: A phase-field crystal study

Using the phase-field crystal model, we investigate the orientation selection of the cubic dendrite growth at the atomic scale. Our simulation results reproduce how a face-centered cubic (fcc) octahedral nucleus and a body-centered cubic (bcc) truncated-rhombic dodecahedral nucleus choose the preferred growth direction and then evolve into the dendrite pattern. The interface energy anisotropy inherent in the fcc crystal structure leads to the fastest growth velocity in the 〈1 0 0〉 directions. New {1 1 1} atomic layers prefer to nucleate at positions near the tips of the fcc octahedron, which leads to the directed growth of the fcc dendrite tips in the 〈1 0 0〉 directions. A similar orientation selection process is also found during the early stage of bcc dendrite growth. The orientation selection regime obtained by phase-field crystal simulation is helpful for understanding the orientation selection processes of real dendrite growth.     

On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi

We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important.     

Mechanical properties and hardening mechanism of Fe–Al–Ni single crystals containing NiAl precipitates

The microstructures and mechanical properties of Fe–23.0 Al–6.0 Ni (at.%) single crystals containing NiAl precipitates were investigated and the hardening mechanism due to the precipitates was discussed, focusing on the activated slip systems. When these alloys were slowly cooled to room temperature after homogenization at 1373 K, the NiAl phase with the B2 structure precipitated in the body-centered cubic (bcc) Fe–Al matrix, satisfying the cube-on-cube relationship with a small misfit strain. The single crystals containing the NiAl precipitates exhibited a high yield stress above 1 GPa at room temperature. In addition, the activated slip system and deformation behavior depended strongly on the loading axis. For instance, 〈1 1 1〉 slip, which is the primary slip for the bcc matrix, occurred at 〈1 4 9〉 and 〈0 0 1〉 orientations and the NiAl precipitates were sheared by the slip. A critical resolved shear stress of 〈1 1 1〉 slip in the NiAl phase was known to be extremely high, which led to strong precipitation hardening. On the other hand, at 〈5 5 7〉 and 〈0 1 1〉 orientations, 〈0 0 1〉 slip, which is the primary slip system for the NiAl precipitates, forcibly sheared the bcc Fe–Al matrix, also leading to strong hardening. Thus, in the Fe–Al–Ni alloys, the difference in the primary slip system between the bcc Fe–Al matrix and the NiAl precipitates resulted in extreme hardening. This hardening mechanism caused by the NiAl precipitates effectively increased the yield stress even at high temperatures. In fact, the crystals exhibited a high yield stress at ～1 GPa up to 823 K.     

Anomalous transformation-induced deformation in 〈1 1 0〉 textured Gum Metal

Tensile tests on single crystals of Gum Metal (Ti–36Nb–2Ta–3Zr–0.3O (wt.%)) showed, anomalously, that while a stress-induced β(bcc) → α″(orthorhombic) transformation occurred in a crystal pulled in the 〈1 1 0〉 direction, (1) no transformation was observed in crystals pulled in the 〈1 0 0〉 or 〈1 1 1〉 directions and (2) little or no transformation occurred in severely worked rods, which are polycrystals with very strong 〈1 1 0〉 texture. Analysis of the energetics of the β → α″ transformation offers straightforward explanations: (1) an α″ precipitate has zero elastic energy if it forms as a thin plate with the habit {1 1 1.5}; a 〈1 1 0〉 tensile load significantly decreases the energy of this plate; loading along 〈1 0 0〉 or 〈1 1 1〉 is less effective; (2) while worked rods have a strong 〈1 1 0〉 axial texture, the perpendicular planes are severely distorted, increasing the elastic energy of α″ and inhibiting the transformation.     

A molecular dynamics simulation study of the velocities,mobility and activation energy of an austenite–ferrite interface in pure Fe

Molecular dynamics simulations have been used to obtain the mobility, in pure Fe, of a face-centered cubic (fcc)–body-centered cubic (bcc) interphase boundary with an orientation given by (1 1 0)_bcc//(7 7 6)_fcc and [0 0 1]_bcc//[?1 1 0]_fcc. The interface is best described by a 4.04° rotation, about an axis lying in the boundary plane, from the Nishiyama–Wasserman orientation and the boundary consists of a parallel array of steps (disconnections). An embedded atom method interatomic potential was employed to model Fe, and the free energy difference as a function of temperature between the fcc and bcc phases, which provided the driving force for boundary motion, was determined by a thermodynamic integration procedure. Although the boundary was found to be very mobile, the transformation did not proceed by a martensite mechanism. The boundary mobility was obtained for several temperatures in the range 600–1400 K and Arrhenius behavior was found with an activation energy of 16.5 ± 2.7 kJ mol^?1 and a pre-exponential factor equal to 7.8( ± 0.9) × 10^?3 mmol J^?1 s^?1. The activation energy is much lower than that extracted from experiments on the massive transformation in Fe alloys and possible reasons for the discrepancy are discussed.     

Contribution of non-octahedral slip to texture evolution of fcc polycrystals in simple shear

The contribution of non-octahedral {1 0 0}〈1 1 0〉 slip to texture evolution under simple shear in face-centred cubic (fcc) polycrystals was studied. It was found that, by adding the {1 0 0}〈1 1 0〉 slip system family to the usual {1 1 1}〈1 1 0〉, the ideal orientations remain the same. However, the stability of the ideal orientations, the rotation field and the rate of change of the orientation density function were affected by the non-octahedral slip activity. The stress state, the slip distribution and the form of the equipotential functions were also examined along the ideal fibres. Finally, the texture evolution in pure aluminium during equal channel angular extrusion was simulated and analysed.     

Phase transformation of Cu precipitates from bcc to fcc in Fe–3Si–2Cu alloy

Phase transition of Cu precipitates during aging of an Fe–3Si–2Cu alloy was studied by transmission electron microscopy. The precipitation of 3–5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of ±a/3 [1 0 0]_9R Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moiré fringes in the correct identification of Cu structure is discussed in detail.     

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Electric fatigue tests have been conducted on pure and manganese-modified Pb(In_0.5Nb_0.5)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) single crystals along different crystallographic directions. Polarization degradation was observed to suddenly occur above 50–100 bipolar cycles in 〈1 1 0〉 oriented samples, while 〈0 0 1〉 oriented samples exhibited almost fatigue free characteristics. The fatigue behavior was investigated as a function of orientation, magnitude of the electric field and manganese dopant. It was found that 〈0 0 1〉 oriented PIN–PMN–PT crystals were fatigue free, due to its small domain size, being on the order of 1 μm. The 〈1 1 0〉 direction exhibited a strong electrical fatigue behavior due to mechanical degradation. Micro/macro cracks developed in fatigued 〈1 1 0〉 oriented single crystals. Fatigue and cracks were the result of strong anisotropic piezoelectric stress and non-180° domain switching, which completely locked the non-180° domains. Furthermore, manganese-modified PIN–PMN–PT crystals were found to show improved fatigue behavior due to an enhanced coercive field.     

Kinetic stages in the crystallization of deeply undercooled body-centered-cubic and face-centered-cubic metals

Crystallization velocities in several face-centered-cubic (fcc) and body-centered-cubic (bcc) metals are calculated using molecular dynamics computer simulations for the (1 0 0) and densely packed (1 1 1) or (1 1 0) planar interfaces. We show that the crystallization kinetics can be divided into high- and low-temperature regimes, separated at a crossover temperature, T_c, which is associated with kinetic arrest. In the high-temperature regime, the velocity in both fcc and bcc metals initially increases with the degree of undercooling before reaching a maximum somewhat above the glass temperature. The kinetics is characterized by a thermally activated process. In the low-temperature regime, stresses develop in the interface and reduce the apparent activation energies for interface mobility. For the fcc metals (Cu, Ni, Ag and Pt) the activation energies fall essentially to zero, indicating an athermal process. For bcc metals (Fe, Mo, V, Ta) the activation energies remain finite, varying from ≈0.013 eV (Ta) to ≈0.2 eV (Mo).     

Effects of temperature on structure and mobility of the 〈1 0 0〉 edge dislocation in body-centred cubic iron

Dislocation segments with Burgers vector b = 〈1 0 0〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2〈1 1 1〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈1 0 0〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈1 1 1〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈1 0 0〉 dislocation. It is concluded that the response of the 〈1 0 0〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈1 1 1〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 °C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.     

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160 μm. Quasi-static tensile tests at an engineering strain rate of 10^?3 s^?1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ～2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (～20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.     

Deformation mechanisms of a 20Mn TWIP steel investigated by in situ neutron diffraction and TEM

The deformation mechanisms and associated microstructure changes during tensile loading of an annealed twinning-induced plasticity steel with chemical composition Fe–20Mn–3Si–3Al–0.045C (wt.%) were systematically investigated using in situ time-of-flight neutron diffraction in combination with post mortem transmission electron microscopy (TEM). The initial microstructure of the investigated alloy consists of equiaxed γ grains with the initial α′-phase of ～7% in volume. In addition to dislocation slip, twinning and two types of martensitic transformations from the austenite to α′- and ε-martensites were observed as the main deformation modes during the tensile deformation. In situ neutron diffraction provides a powerful tool for establishing the deformation mode map for elucidating the role of different deformation modes in different strain regions. The critical stress is 520 MPa for the martensitic transformation from austenite to α′-martensite, whereas a higher stress (>600 MPa) is required for actuating the deformation twin and/or the martensitic transformation from austenite to ε-martensite. Both ε- and α′-martensites act as hard phases, whereas mechanical twinning contributes to both the strength and the ductility of the studied steel. TEM observations confirmed that the twinning process was facilitated by the parent grains oriented with 〈1 1 1〉 or 〈1 1 0〉 parallel to the loading direction. The nucleation and growth of twins are attributed to the pole and self-generation formation mechanisms, as well as the stair-rod cross-slip mechanism.     

Micromechanisms of fracture in NiAl studied by in situ straining experiments in an HVEM

The room temperature brittleness of NiAl constitutes a major problem for technical applications. In order to investigate the micromechanisms of fracture in NiAl, we have carried out in situ tensile straining experiments on stoichiometric NiAl single crystals in a high-voltage electron microscope. According to our observations, crack propagation always involves dislocation activity around the crack tip, even in the hard orientation at room temperature. The Burgers vectors and the typical arrangements of the dislocations, as well as the extension of the corresponding plastic zone vary with the loading direction and the orientation of the microcrack versus potential glide systems. We observe that local concentrations of slip leads to irregular deviation of the cleavage plane from the {1 1 0} facets one usually observes at the macroscopic level. The results of our experiments help to understand why the mode I fracture toughness of NiAl is significantly larger for 〈1 0 0〉 loading directions than for non-〈1 0 0〉 directions.     

Microstructure and texture evolution in a twinning-induced-plasticity steel during uniaxial tension

A combination of electron back-scattering diffraction and X-ray diffraction was used to track the evolution of the microstructure and texture of a fully recrystallized Fe–24 Mn–3 Al–2 Si–1 Ni–0.06 C twinning-induced-plasticity steel during interrupted uniaxial tensile testing. Texture measurements returned the characteristic double fibre texture for face-centred cubic materials, with a relatively stronger 〈1 1 1〉 and a weaker 〈1 0 0〉 partial fibre parallel to the tensile axis. The interaction with the stable 〈1 1 1〉 oriented grains results in preferential plastic flow in the unstable 〈1 1 0〉 oriented grains. Consequently, the grains oriented along the 〈1 1 0〉 and 〈1 0 0〉 fibres record the highest and lowest values of intragranular local misorientation, respectively. The viscoplastic self-consistent model was used to simulate the macroscopic stress–strain response as well as track the evolution of bulk crystallographic texture by detailing the contributions of perfect and/or partial slip, twinning and latent hardening. The simulations revealed the dominant role of perfect slip and the limited volume effect of twinning on the texture development. The effects of initial orientation and grain interaction on the overall orientation stability during uniaxial tension showed that while the 〈1 0 0〉 fibre remains stable and does not affect the unstable orientations along the 〈1 1 0〉 fibre, the orientations along the stable 〈1 1 1〉 fibre strongly affect the unstable 〈1 1 0〉 orientations.