Low Schmid Factor Research Articles

Primary twinning selection with respect to orientation of deformed grains in ultra-rapidly compressed AZ31 alloy

Abstract The primary twinning selection with respect to orientation of deformed grains in AZ31 alloy ultra-rapidly compressed was investigated. It was found that { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 〉 tension twinning can be activated in the grains with a wide range of orientation, whereas { 1 0 1 ¯ 1 } 〈 1 0 1 ¯ 2 ¯ 〉 contraction twinning in the narrow range. The tension twin variant selection is not solely of preference of Schmid factor rank. Twinning with very low Schmid factor can be activated. It was believed that critical resolved shear stress (CRSS) and strain accommodation are very important for the primary twinning and its variant selection. The much lower CRSS value of the tension twinning compared with that of the contraction twinning is responsible for its activation in wide orientation range of grains.

Relationship between localized strain and irradiation assisted stress corrosion cracking in an austenitic alloy

Irradiation assisted stress corrosion cracking may be linked to the local slip behavior near grain boundaries that exhibit high susceptibility to cracking. Fe–13Cr–15Ni austenitic steel was irradiated with 2 MeV protons at 360 °C to 5 dpa and strained in 288 °C simulated BWR conditions. Clusters of grains from the experiment were created in an atomistic simulation and then virtually strained using molecular dynamic simulation techniques. Cracking and grain orientation data were characterized in both the experiment and the simulation. Random high angle boundaries with high surface trace angles with respect to the tensile direction were found to be the most susceptible to cracking. Grain boundary cracking susceptibility was also found to correlate strongly with slip continuity, indicating that the strain accommodation at the boundary is related to cracking resistance. Higher cracking susceptibility was also found at grain boundaries adjacent to grains with low Schmid factor or high Taylor factor. The basic trends reported here are supported by both the experiments and the simulations.

The role of strain accommodation during the variant selection of primary twins in magnesium

Samples of magnesium alloys AM30 and AZ31 were deformed in tension at room temperature and a strain rate of 0.1 s −1 to strains of 0.08 and 0.15. Of the numerous contraction twins that formed, the orientations of 977 were determined by electron backscatter diffraction techniques. The orientations of their host grains were also measured, so that the Schmid factors (SFs) applicable to each of the six contraction twins that could potentially form in each grain could also be calculated. About half of the observed twins were of the “high SF” (0.3–0.5) type, while nearly half had “low” SFs (0.15–0.30). Furthermore, 5% of the observed twins had associated Schmid factors of only 0.03–0.15, i.e. these were of the “very low SF” type. Of particular interest is the observation that many potential “high Schmid factor” twins did not form. The presence of the low and very low SF twins and the absence of many potential high SF twins are explained in terms of the accommodation strains that are or would be required to permit their formation. These were calculated by rotating the twinning shear displacement gradient tensor into the crystallographic reference frame of the neighboring grain. It is shown that the very high plastic anisotropy of Mg grains permits the “easy” accommodations to take place but conversely prevents accommodation of the potential twinning shears when these are “difficult” (when these involve high critical resolved shear stresses). The twins that appear require little or no “difficult” accommodation.

A model for the normal stress dependence of intergranular cracking of irradiated 316L stainless steel in supercritical water

The intergranular cracking behavior of irradiated 316L stainless steel in supercritical water (SCW) was found to be strongly dependent on the local grain boundary normal stress. Tensile specimens of 7 dpa proton irradiated 316L stainless steel were strained in 400 °C SCW to 2%, 5%, and 10% macroscopic plastic strain. The cracking behavior was characterized according to the distributions of cracked grain boundary plane surface trace inclinations to the tensile axis and the Schmid factors of the grains adjacent to the cracked boundaries. Cracks occurred preferentially along grain boundaries with trace inclinations perpendicular to the tensile axis and adjacent to grains with low Schmid factors. There was also a slight increase in cracking propensity for grain boundaries adjacent to grains with high Schmid factor mismatch. The normal stress dependence of intergranular cracking was confirmed by developing and applying the Schmid-Modified Grain Boundary Stress model which characterizes the normal stress acting on a grain boundary as a function of the inclination of the grain boundary plane to the tensile axis and the flow stress of the grain, as estimated from its Schmid factor. By applying this model, the Schmid factor dependence of intergranular cracking could be predicted from the distributions of cracked grain boundary trace inclinations to the tensile axis.

Room-temperature equal channel angular extrusion of pure magnesium

In this paper, we demonstrate a way to impart severe plastic deformation to magnesium at room temperature to produce ultrafine grain size of ∼250 nm through equal channel angular extrusion (ECAE). The strategy to deform magnesium at lower temperature or to achieve such grain sizes has been proposed as: (i) to obtain a suitable initial orientation with high Schmid factor for basal slip and low Schmid factor for pyramidal/prismatic slip; (ii) to take advantage of low stacking fault energy of basal and high stacking fault energies of prismatic/pyramidal planes in order to relatively work-harden the basal plane with respect to the pyramidal/prismatic plane; and (iii) to lower the temperature of deformation in steps, leading to continual refinement of grains, resulting in finer grain size. The experimental as well as simulated texture of ECAE-processed samples indicate that the deformation mechanism leading to ultrafine grain size is slip-dominated. The recrystallization mechanism during ECAE has been found to be orientation-dependent.

Twin Nucleation by Slip Transfer across Grain Boundaries in Commercial Purity Titanium

The role of strain transfer in the activation of deformation twinning at grain boundaries has been characterized in commercially pure titanium deformed in bending. Two different orientations of a textured polycrystal were deformed in bending and were analyzed using electron backscattered diffraction (EBSD) to determine the active slip and twinning systems in the surface tensile region. Prismatic slip and $$ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle $$ twinning were the most widely observed deformation modes in both orientations. Nonprismatic slip systems were also activated, most likely to accommodate local strain heterogeneities. A slip-stimulated twin nucleation mechanism was identified for soft/hard grain pairs: dislocation slip in a soft-oriented grain can stimulate twin nucleation in the neighboring hard grain when the slip system is well aligned with the twinning system. This alignment was described by a slip-transfer parameter m′.[24] Twins activated by this mechanism always had the highest m′ value among the six available $$ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle $$ twinning systems, while the Schmid factor, based on the global (uniaxial tensile) stress state, was a less significant indicator of twin activity. Through slip transfer, deformation twins sometimes formed despite having a very low global Schmid factor. The frequency of slip-stimulated twin nucleation depends strongly on the texture and loading direction in the material. For grain pairs having one grain with a large Schmid factor for twinning, nonparametric statistical analysis confirms that those with a larger m′ are more likely to display slip-stimulated twinning.

Determination of the minimum entropy configuration and velocity surfaces for elastic–plastic wave scattering at grain boundaries

A recent model based on full elastic anisotropy and crystal plasticity predicted the existence of multiple wave configurations during the interaction of stress waves with grain boundaries. Since the multiple wave configuration scenario cannot exist in nature, the principle of minimum entropy production is applied in the current work to find the most probable configuration. A large amplitude transmitted quasi-longitudinal wave is predicted for the given bicrystal orientation studied due to the wave propagating near a [001] direction and thus requiring large stress given the very low Schmid factor in this direction (for nickel aluminide (NiAl) as a model material). Anisotropic elastic–plastic velocity surfaces for quasi-longitudinal and quasi-shear waves in NiAl have also been constructed to gain an understanding of the general nature of plastic waves as a function of crystallographic direction.

Size effect on strength and strain hardening of small-scale [1 1 1] nickel compression pillars

This study investigates uniaxial compression behavior of focused ion beam (FIB) manufactured [1 1 1] nickel (Ni) small-scale pillars, ranging in diameter from approximately 25 μm to below 200 nm, in order to examine the effect of crystallographic orientation on the mechanical properties. This study is unique from other micro-pillar studies in that the [1 1 1] orientation has a considerably lower Schmid factor, and has multiple slip systems available. The [1 1 1] Ni pillars show a strong increase in yield stress and work hardening with decreasing diameter. The relationship between yield stress and diameter ( σ y ∝ d −0.69) matches well with previous small-scale pillar studies. Strain hardening, which has been inconsistently observed in other micro-pillar studies, is found to be a function of both diameter and orientation. Although the precise mechanism for hardening is unknown, transmission electron microscopy reveals dislocations throughout the pillar and into the base material suggesting that dislocation interactions and deformation below the pillar play a role in the observed strain hardening. Furthermore, a slight crystallographic rotation of the pillar is observed likely contributing to the observed mechanical properties. By exploring the role of crystallography on the plastic deformation behavior, this study provides additional insight into the nature of the size effect.

Effect of temperature on single asperity contact and separation in Au

We performed a series of molecular dynamics simulations of contact and separation of two Au surfaces at different temperatures. The pull-off force decreases monotonically with increasing temperature except for an abrupt rise that occurs between T1 ⩽ T ⩽ T2. This abrupt rise can be traced to a series of phase transformations in the sample during separation (face-centered cubic (fcc) → hexagonal close-packed → fcc) at elevated temperatures. The new fcc structure has a lower Schmid factor than the original fcc structure and consequently exhibits higher tensile strength.

Atomistic simulation of multicycle asperity contact

We performed molecular dynamics simulations of repetitive contact and separation of a surface containing an initially hemispherical asperity and a flat. We monitor the force between the two surfaces and the Q 6 order parameter together with the evolution of the structure/morphology. Typically, the force–displacement relation exhibits a sawtooth form during loading, associated with repetitive generation and motion of partial dislocations and the creation and annihilation of stacking faults. The unloading is characterized by elastic deformation followed by plastic deformation of a type that leads to stacking faults different from those seen in loading. During repetitive contact, the system accumulates stacking faults as a result of Shockley partial glide and transforms from face-centered cubic (fcc) to hexagonal close packed (hcp). On the next unloading step, additional Shockley partial glide occurs, transforming the system back from the metastable hcp to fcc. The new fcc structure has a twin relation to the original fcc structure. The new fcc structure has a lower Schmid factor than the original fcc (or hcp) structure and consequently exhibits larger pull-off forces.

Study of fatigue crack nucleation processes by crystalline FE analysis

In order to accumulate knowledge about how material compositions and manufacturing methods affect fatigue strength, this paper investigates the relationship between the swiftness of the changes in the macroscopic indices of the crack initiation process and the hardening properties of a material. This is done by calculating the cyclic deformation behavior of a f.c.c. single crystal. The relationship between the swiftness of the changes and the crystal geometries is also examined by calculating the deformation behavior of a f.c.c. crystal with a high Schmid factor buried in a large crystal with a low Schmid factor. In addition, a multiscale hardening rule based on forest theory is developed in order to examine the microscopic mechanisms of fatigue slip band (persistent slip bands, PSBs) formation. The validity of the theories presented is examined by comparing the changing nature of the measured and calculated hysteresis loop shapes of f.c.c. single crystals. Inhomogenous slip deformation through the crystal is also investigated, and inferences are then drawn about the microscopic mechanisms of cyclic hardening and PSB formation.

A scanning electron microscope study of the effects of dynamic recrystallization on lattice preferred orientation in olivine

Effects of dynamic recrystallization on lattice preferred orientation (LPO) in olivine were investigated through the combination of two SEM-based techniques, electron backscattered diffraction (EBSD) technique for crystallographic orientation measurement and backscattered electron imaging (BEI) for dislocation observation. Samples are experimentally deformed olivine aggregates in simple shear geometry. In the sample deformed at T=1473 K and high stresses (∼480 MPa), only incipient dynamic recrystallization is observed along grain-boundaries. Orientations of these small recrystallized grains are more random than that of relict grains, suggesting an important role of grain-boundary sliding at this stage of recrystallization. In the sample deformed at T=1573 K and low stress (∼160 MPa), dynamic recrystallization is nearly complete and the LPO is characterized by two [100] peaks. One peak is located at the orientation subparallel to the shear direction and is dominated by grains with high Schmid factor. The other occurs at high angles to the shear direction and is due to the contribution from grains with low Schmid factor. Grains with high Schmid factor tend to have higher dislocation densities than those with low Schmid factor. Based on these observations, we identify two mechanisms by which dynamic recrystallization affects LPO: (1) enhancement of grain-boundary sliding due to grain-size reduction, leading to the modification of LPO caused by the relaxation of constraint for deformation; (2) grain-boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities. Based on the deformation mechanism maps and stress versus recrystallized grain-size relation, we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions.

The response to nie and muddle’s comment about li and wawner’s paper

Interrupted bi-axial compressions are applied to a hot-rolled Mg alloy with a strong basal texture in order to activate {1 0 −1 2}-{1 0 −1 2} double extension twinning. Compound cross-grain boundary (cross-GB) extension twin structures form after the compressions, which consist of cross-GB secondary twin pairs within their host cross-GB primary twin pairs. Shear accommodation of a twin is proven to be proportional to a geometrical m′ factor and the magnitude of its twinning shear. It is found that 60% of cross-GB twin pairs, including both the primary and the secondary ones, have a value of m′>0.7, indicating twinning shear transmission over GB is a major mechanism. Multi-level twinning shear transmissions over GB contribute prominently to the formation of 40% of the observed compound cross-GB twin structures. About 98.3% of the twins are high Schmid factor (SF) ones, while low SF ones appear due to twinning shear transmission. About 81.7% of secondary twins have a misorientation of 〈0 14 −14 1〉60° with respect to their host grains, indicating a strong confining effect of primary twins. A cross-GB twin pair could form through a mechanism of associated nucleation or isolated nucleation. High m′ value is characteristic of associated nucleation through twin-to-twin accommodation mode. Depending on the process of microstructure evolution, SF, shear accommodation and the confining effect of the matrix if necessary should be synthetically considered to predict twin variant selection related to cross-GB twin pairs.

Deformation of single-crystal Mn 3Sn

Single-crystal Mn 3Sn was chosen to investigate the behavior of a DO 19 intermetallic compound. The alloy of composition 23.75 at.% Sn was made from the pure elemental materials and single crystals were grown using the Bridgman process. Compression samples were prepared using an electric discharge machine and tested at moderate strain rates in an air or argon environment, depending upon the test temperature. Testing temperatures ranged from 77 to 1200 K and various orientations within the standard unit triangle were chosen so as to produce slip on several slip systems. Preliminary results show that the dominant mode of deformation is basal slip accompanied by an anomalous flow stress vs. temperatures behavior. There was, however, also some prismatic and pyramidal slip observed. For most orientations slip traces were observed on the sample surfaces; however, for some orientations with low Schmid factors, local crushing or even brittle fracture were observed.

A study of (001) glide in [112] aluminium single crystals

Abstract [110](001) slip has been studied in [112] aluminium single crystals deformed in creep. Metallographic and microstructural observations were performed. At 150°C, the two primary {111} planes in a symmetrical orientation are activated. At 180-200°C, (001) starts being activated in some areas of the sample. The corresponding wavy slip traces indicate that slip is unstable on (001) and frequent cross-slip occurs between this plane and two {111} slip systems with low Schmid factors. In situ experiments in this temperature range show that screw dislocation motion on (001) is difficult. At 400°C and above, (001) slip becomes easy and straight traces are clearly visible. Two distinct dislocation substructures correspond to these different deformation modes. It is shown that (001) glide does not originate from the recombination of Lomer-Cottrell barriers, but dislocation multiplication takes place on this non-compact plane. The microscopic mechanism associated with (001) glide, and the importance of this ...

Anomalous slip induced by the surface effect in molybdenum single-crystal foils deformed in a high voltage electron microscope

A series of in situ deformation experiments in a high voltage electron microscope was carried out at room temperature and at 150 K with high purity molybdenum single-crystal foils. Several slip systems with low Schmid factors were observed to operate predominantly. Since the slip planes of these systems make large angles with the foil specimen surface, the choice of these slip systems are considered to be strongly influenced by the image force due to the free surface. The choice of the slip plane was well explained by considering the Schmid factor and a parameter which gives the strength of the surface effect (SE) in terms of the orientation of the slip direction and the slip plane with the foil surface. Thus, this result supports the SE mechanism of anomalous slip. Evidence for the SE mechanism is the observation of dislocation cross-slip from a “normal” slip plane to an “anomalous” slip plane with the aid of the surface image force which supplies kinks to the screw dislocation.

Cherries versus fish for gulls : Clarence Cottam

Hot rolled Mg–1 wt% Nd alloy with a weak {0001}<112¯0> basal texture was subjected to plane strain compression (PSC) at room temperature. Compressive loads were imposed parallel to the in-plane (IPC) and through-thickness (TTC) directions of the rolled plate in order to engage different types of deformation twins. PSC was conducted up to 6% and 12%, and the deformed specimens were subsequently annealed at different elevated temperatures to investigate recovery and recrystallization in the formerly developed twins. Deformation and annealing textures and microstructures were characterized using X-ray diffraction, optical microscopy and electron backscatter diffraction techniques. Deformation under IPC and TTC displayed competitive activation of extension and contraction twinning, supplemented by homogenous dislocation slip. Deformation in IPC up to 6% witnessed equal activation of both aforementioned twinning modes that gave rise to soft and hard texture components. TTC up to 12% exhibited a less regular basal-type deformation texture, characterized by a large outspread in the rolling direction, and an asymmetrical splitting of the basal poles around the transverse direction by about −5° and +25°. Texture evolution during annealing was associated with general texture weakening, preserving the primary deformation texture components at relatively low annealing temperatures with no recrystallization (300 °C). Presence of strong impurity drag and particle pinning delayed the onset of recrystallization till 325 °C for TTC and 350 °C for IPC specimens, which took place preferably inside compression and double twins. Related recrystallized grains possessed a high Schmid factor (SF) for basal slip, and were larger and more prevalent on average than competing low SF grains related to extension twins or the matrix. Re-dissolution of precipitates at 450 °C triggered grain growth that led to strong evolution of orientations associated with compression and double twins as a result of a grain size advantage that emerges during recrystallization.