High Schmid Factor Research Articles

Tensile twin evolution of Mg–3Al–1Zn magnesium alloy during high-strain rate deformation

In order to investigate the evolution of tensile twin in Mg–3Al–1Zn (AZ31) magnesium alloy under high-strain rate impaction, the cylindrical specimens were compressed along the rolling direction under the strain rate of 1600 s−1 by use of split Hopkinson pressure bars equipped with stop rings with different thicknesses. The microstructure was observed by electron back-scatter diffraction. The results show that tensile twinning dominates the early plastic deformation and the area fraction of tensile twins is increased with the increase of deformation strain. Furthermore, when the deformation strain is beyond ∼10%, plastic deformation is mainly dominated by dislocation slips. In addition, most of tensile twins activated in a single grain belong to a variant or a variant pair, which has the highest Schmid factor among all the variants.

Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

We present the microscale fracture mechanics aspects of LCF cracks non-propagating till failure in the cylindrical specimens of Ni based superalloy Haynes 282 and attempt to unravel the underlying crystallographic factors. Two key parameters, Crack Tip Opening Angle (CTOA) and maximum tangential stress (θMTS) have been opted for characterization. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime and the fraction of the stabilized regime increases in lower strain amplitude where the fatigue life is more. Mixity of local KI and KII fields is directly proportional to θMTS and that has been assessed by measuring local deflections. The modal mixity has been found to be microstructure sensitive and an entirely statistical parameter. Different growth modes of these non-propagating cracks have been identified from the microstructural observations. The role of elasto-plastic incompatibility of neighboring grains has been addressed by conducting crystallographic analyses. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favoring subsurface crack propagation below which grains (mostly with high elastic compliance and low SF) are not participating in crack propagation. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. From the results of a detailed slip transfer analysis based on the Luster-Morris parameter (LMP), it is observed that there exists microstructurally controlled lower bounds that inhibits slip transfer (LMP value <0.1). Dislocation debris analysis using TEM has also been carried out to understand micromechanisms clearly. Both non coplanar and planar mode of dislocation activity has been observed and twin boundaries has been see to act as barrier to the planar dislocation motion.

Overcoming the trade-off between stretch formability and heat resistance in magnesium via alloying dilute neodymium

This work aims to provide a reliable reference and a low-cost strategy for overcoming the trade-off between stretch formability and heat resistance in Mg alloys. A dilute Mg-0.8 wt%Nd alloy sheet was prepared for this purpose, and hot compression was conducted in order to regulate the microstructure and texture. For comparison, the samples with and without hot compression were labeled as fine-grained (FG) and coarse-grained (CG) samples respectively, corresponding to the occurrence of dynamic recrystallization. Tensile creep tests were carried out at 473 K and Erichsen tests were conducted at room temperature. Both the CG and the FG samples had higher stretch formability and stronger creep resistance than commercial Mg alloys tested for comparison. Especially, the FG sample performed the best in overcoming the trade-off. Pyramidal<c+a> slip dominated the creep behavior of the FG sample and enhanced the heat resistance, avoiding the deterioration caused by cross-slip, twinning and grain boundary sliding. High Schmid factors for basal slip of the fine grains enhanced the stretch formability of the FG sample, and compensated the adverse effect of the relatively low Schmid factors of the residual coarse grains. Thus, the dilute Mg-0.8 wt%Nd alloy successfully overcame the trade-off between stretch formability and heat resistance.

On the microstructure and RE-texture evolution during hot tensile deformation of Mg-Gd-Y-Zn-Zr alloy

This study was conducted to investigate the correlation between the microstructure/texture evolutions and the high temperature flow behavior of Mg-8.3Gd-3.6Y-1.6Zn-0.5Zr (wt. %) alloy containing long period stacking order (LPSO) phases. Toward this end, the hot tensile tests were conducted at 350 and 400 °C on two different starting microstructures: (i) as-extruded containing blocky LPSO, and (ii) as-annealed, which contained both blocky and lamellar LPSO phases. The peak stresses of the annealed microstructures at 350 and 400 °C, i.e. 200 and 80 MPa, were almost twice that of extruded ones at the same temperatures, i.e. 117 and 39 MPa, respectively. The annealing treatment effectively strengthened the alloy and decreased the ductility value. This was justified relying on the grain growth, deformation twinning and texture evolution, but a special emphasize was paid on the effect of lamellar and blocky LPSO phases. It was found that the blocky LPSO could assist the dynamic recrystallization through activation of particle stimulated nucleation (PSN) mechanism and effectively contributed in the formation of rare earth (RE)-texture component. This provided high Schmid factors for operation of basal slip and improved the ductility values. On the contrary, the fine lamellar LPSO effectively increased the strength of the alloy via texture hardening but reduced the ductility. The lamellar LPSO played a key role in the formation and strengthening of the <10-10> fiber texture, retarded the recrystallization process and suppressed the formation of RE-texture component.

Strength-ductility synergy in Mg98.3Y1.3Ni0.4 alloy processed by high temperature homogenization and rolling

A new Mg98.3Y1.3Ni0.4 alloy containing long period stacking ordered (LPSO) phases with good strength-ductility synergy was prepared by high temperature homogenization and rolling, which possesses a high ultimate tensile strength of 320 MPa and elongation of 17.0% and low work hardening rate during tensile deformation. Bulk LPSO phases with lamellar morphology exist in original grains after homogenization. Refined grains, broken LPSO phases and high-density geometrically necessary dislocations provide a good strength for as-rolled alloy with the strength increment of 146.3 MPa, 51.7 MPa and 68.3 MPa, respectively. Good ductility and low work hardening rate are attributed to weak basal texture, activation of non-basal slips with high Schmid factors and annihilation of high-density dislocations. In situ tensile testing result shows deformation coordination by non-basal slips, deformation twinning, grain rotation and absorption of plastic deformation energy by LPSO phases, which suggests a good ductility as well.

{10–12} extension twinning activity and compression behavior of pure Mg and Mg-0.5Ca alloy at cryogenic temperature

This study explored the deformation behavior of rolled pure Mg and Mg-0.5Ca alloy at room temperature (RT) and cryogenic temperature (CT) of −150°C. The samples were compressed at a strain rate of 10-3s-1 up to 5% strain along the rolling direction (RD). The deformed samples were examined via EBSD and analyzed concerning the loading condition and initial microstructure. The compression tests showed the temperature-insensitive hardening behavior in Mg-0.5Ca alloy. The twinning activity increased with an increase in grain size, and this was pronounced when the pure Mg samples were compressed at CT. The kernel average misorientation (KAM) analysis revealed that the twinning parent areas tend to have higher KAM values under CT deformation than the RT deformation, which links to a higher twinning activity at CT. The predominant low angle grain boundaries in the initial microstructure of pure Mg further helps to understand the profuse twinning activity in the context of existing literature. It was suggested that the higher twinned area fraction in the pure Mg sample than Mg-0.5Ca could be responsible for the temperature-dependent hardening behavior. This is presumably because the Ca element in the matrix results in a homogeneous microstructure with smaller grain size and higher Schmid factor (SF) for basal slip, leading to lower twinning dependent deformation behavior in Mg-0.5Ca than pure Mg.

Revealing the effect of local stresses on twin growth mechanisms in titanium using synchrotron X-ray diffraction

Deformation twinning has a significant impact on the evolution of microstructure and mechanical response in hexagonal close packed (hcp) metals. Understanding the physical mechanisms associated with twin nucleation and growth processes is important to enable the broad use of hcp metals. While both nucleation and growth are conditioned by the local stress state, few studies have quantified it in bulk deformed samples. In this study, we reveal the effects of local stresses on twin thickening using in-situ synchrotron experiments with differential aperture X-ray microscopy. High purity Ti is deformed under four-point bending to activate {1012} tensile twins. 3D stress fields with a spatial resolution of 0.5μm are mapped in the vicinity of low and high macroscopic Schmid factor (MSF) twins growing inside a grain. Reconciling this experimental analysis with recent twin growth models reveals that the growth of low MSF twins is limited by the nucleation of twin growth defects in the high-purity Ti sample. While growth-inducing defect nucleation may occur in many places on the interface of the high resolved shear stress (RSS)/MSF twin, the nucleation rate of twinning disconnections for the low MSF twin is high only at the end of the twin near a stress concentration. Once nucleated, these defects easily propagate into lower RSS regions of the grain resulting in twin growth. This work highlights the importance of stress concentrations not only for twin (embryo) nucleation, but also to the growth process of twins, particularly in low MSF twins.

Laser powder bed fusion of a Fe–Mn–Si shape memory alloy

The iron-based Fe–17Mn–5Si–10Cr–4Ni shape memory alloy (SMA) has been manufactured by laser powder bed fusion (LPBF) for the first time. High strength, elongation and ductility are observed after annealing. In addition, pronounced shape memory effect (SME) and pseudo-elasticity (PE), which exceed those measured for the conventionally fabricated Fe–17Mn–5Si–10Cr–4Ni-1(V,C) alloy containing VC precipitates, are achieved. The texture generated during the LPBF process brings to SME and PE highly dependent on the orientation of the loading direction with respect to the build direction. Improved strain recovery is observed when the loading direction is parallel to the build direction due to an increased amount of (101) orientated grains with high Schmid factor and low yield strength for phase transformation. Pronounced shape recovery is also obtained in more complex geometries successfully fabricated with the same manufacturing technique.

Microstructure evolution and grain refinement behavior during hot deformation of Fe micro-alloyed Ti-6Al-4V

In this study, the effect of Fe addition on microstructure evolution and grain refinement mechanism during hot deformation of new Ti-6Al-4V-xFe (x = 0–0.9 wt%) alloys were investigated. The peak stress showed a decreasing trend, and the microstructure presented obvious grain refinement with the increase of Fe content. The flow softening behavior was contributed by dynamic recovery (DRV) and dynamic recrystallization (DRX). The average grain size decreased from 52.2 μm to 11.3 μm, and the fraction of the high angle grain boundary (HAGB) increased from 66.49% to 76.33% when Fe concentration increased from 0 wt% to 0.9 wt%. Fe addition also facilitated in the annihilation of the dislocations, which resulted in a decrease in the average ρGND density. The high Schmid factor (>0.45) content increased with the Fe content in the basal slip (0001) <112¯0> and prismatic slip {101¯1} <112¯3> of the α phase, as well as in the {112¯} <111> and {123¯} <111> slip systems of the β phase. All evidence indicated that Fe addition promoted the hot workability of the Ti-6Al-4V-xFe (x = 0–0.9 wt%) alloys and refined the grain size during hot deformation.

Study of the ductility enhancement of 5A90 Al–Mg–Li alloy sheets with stress relaxation

ABSTRACT Stress relaxation tests were conducted on 5A90 (Al–Mg–Li) sheets using in-plane tension along various loading directions with different pre-strain levels to investigate the effects of stress relaxation on the mechanical properties. It is found that considerable ductility enhancement is achieved via the proposed stress relaxation approach. To investigate the mechanisms of ductility enhancement, in-situ X-ray diffractometer (XRD) and electron backscatter diffraction (EBSD) were conducted. The evolution of statistically stored dislocation (SSD) density and geometrically necessary dislocation (GND) density was investigated. A decrease in GND and SSD densities is observed, which is driven by internal stress homogenization and annihilation of dislocations after stress relaxation. Such decrease in dislocation density allows the rearrangement of GNDs and SSDs, and therefore improves the ductility with possible shifts of the strain localization during subsequent reloading. Meanwhile, the ductility enhancement is found to be strongly dependent on loading direction, which is related to the distinctive Schmid factor distribution. For instance, for loading direction with more ‘soft grains’ (i.e. crystal orientation with high Schmid factors), only a slight increase of ductility was observed owing to a lower possibility of shifting strain localization caused by much larger proportion of soft grain boundaries.

Effects of texture and stress sequence on twinning, detwinning and fatigue crack initiation in extruded magnesium alloy AZ31

Several types of cyclic stress were applied to specimens made of an extruded magnesium alloy, AZ31, to elucidate twinning, detwinning, and fatigue crack initiation mechanisms by electron backscattered diffraction analysis. In both the texture and the random orientation, twinning occurred under compressive stress exceeding the compressive yield strength. Detwinning occurred only in the texture upon the subsequent application of tensile stress less than the tensile yield strength of monotonic loading, whereas detwinning did not occur in the structure with random orientation. Under compression–compression fatigue test, successive twinning occurred, which changed the orientation of the basal plane. As a result, the random orientation of grains changed to the texture in which the normal of the basal plane was parallel to the loading direction. Cracks were formed along the boundary of grains with a high Schmid factor of the basal slip system and the misfit of grain on both sides was large. Near the crack initiation site, twin bands were observed; however, detwinning did not occur under the tensile stress after the cyclic compression loading.

Thermal deformation mechanism of TC11/TC17 linear friction welded joint during isothermal compression

Thermal deformation behavior has a significant effect on the microstructure evolution, crystallographic orientation and microhardness of two-phase titanium alloy. In the paper, the evolution rules of microstructure, texture and microhardness of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) and Ti–4Mo–4Cr–5Al–2Sn–2Zr (TC17) linear friction welded (LFWed) joint were systematically discussed during the isothermal compression. Comparing with the high microhardness on the welded zone (WZ) after welding, the microhardness value of the whole joint decreases significantly after the pretreatment and isothermal compressions. There is a close relationship between microtexture and plastic deformation. The electron backscattered diffraction (EBSD) analysis method indicates that the distribution of Schmid factor is very uneven, and most of the crystals are aligned to hard orientation on the TC11 WZ; whereas the high Schmid factors occur, and most of the crystals turn to soft orientation on the TC17 WZ, which lead to the plastic deformation of TC17 side more easily than that of TC11 side. With the increasing compression strains, the ratio of low angle grain boundaries (LAGBs) rises, meanwhile, the mean geometrically necessary dislocations (GND) density increases from 2.79 × 10 14 m −2 to 4.30 × 10 14 m −2 , and the fraction of high Schmid factor also goes up slightly. Those can be attributed to the increased deformation energy with the higher compression, which leads to the accumulated driven force and promotes the rotation of grains toward a specific favourable orientation. The relationship between the microhardness and the crystal texture of the TC11/TC17 LFWed joint. • Rules of microstructure, texture and microhardness of LFWed joint were discussed. • Plastic deformation mechanism of the joint was discussed by the Schmid factors. • The ratio of LAGBs and GND density rises with the increasing compression strains. • Microhardness at the joint is more uniform with the higher isothermal compression.

Effect of microstructure on short fatigue crack growth of wire arc additive manufactured Ti-6Al-4V

In the present paper, the short fatigue crack behavior in wire arc additive manufactured (WAAMed) Ti-6Al-4V was investigated. It was found that the crack in the colony microstructure showed the highest fluctuation range (213 μm) because the crack deflected at the colony boundary. While the crack in the duplex microstructure showed the lowest fluctuation range (48.3 μm) because the crack cut through the αp (primary α) and αs (secondary α) directly. In addition, the short crack growth behavior was determined by the highest Schmid factor (M) and the burgers orientation relationship (BOR), which indicated that the short crack tended to pass through the α/β interface along with the basal slip systems within the shortest path distance. It was suggested that the short crack behavior with colony microstructure was determined by the α/β interface while the crack growth behavior with duplex microstructure was determined by the critical resolved stress (τCRSS) and the αs/β interface.

Coordinated control of preferred orientation and uniformity of AZ31 in accumulative alternating back extrusion

This paper discusses how the accumulative alternating back extrusion (AABE) process was improved by the addition of water-quench and preheating steps to mitigate the of temperature-drop problem caused by long process flows and the problem of die gap feeding. The effect of the number of extrusion cycles on grain refinement was further investigated to elucidate and rationalize the preferred orientation in AABE. Compression experiments were carried out in tandem to explore the effect of AABE on the mechanical properties of the materials. After eight cycles of AABE, the average grain size reached 5.58 μm, and the fine-grain size reached <1 μm. Microstructure results showed that the grain size was significantly refined via discontinuous dynamic recrystallization (DDRX) and uniformity increased with subsequent extrusion cycles. Analysis of the deformation behavior of AABE showed that there was a preferred orientation in the microstructure after alternating, and the orientation of the basal plane was related to the flow velocity field that was simulated by the finite element method. A high Schmid factor and high deformation uniformity was observed after alternating, which was favorable for subsequent processing. Increasing the number of extrusion cycles decreased the material strength and plasticity up to a certain point. Subsequently, an increase was observed, with eight cycles giving the most strength and plasticity. Moreover, the grain size and crystal orientation affected the mechanical properties of the structure. These results provide an important experimental basis for creating AABE methods and fine-tuning the microstructure properties in magnesium alloy.

Crystallography and microstructure of the deformation bands formed in a metastable β titanium alloy during isothermal compression

The plastic deformation of crystals accompanied by the formation of deformation bands or slip bands has been observed in previous studies; however, this phenomenon has not been examined in sufficient detail. In this study, the crystallography and microstructure of the deformation bands formed in a metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al during isothermal compression were investigated. The obtained results revealed that numerous deformation bands were formed inside the β grains and along the initial β grain boundaries. The majority of the produced bands were located in the grain interiors, and their formation was accompanied by the fragmentation and spheroidisation of the lamellar α phase. According to the trace analysis results, these bands belonged to the {110} <11¯1> slip band type and exhibited high Schmid factors. The crystallographic calculation data indicated that the rotation axis of the misorientation between the β phase in the lamellar area and that in the band area passed through the deformation bands. Furthermore, the origin of the formation of bands was caused by the relative slip of the lamellar β and α phases due to the coordinated shear under compression. Thus, the findings of this study can help achieve a better understanding of the hot deformation mechanisms of titanium alloys.

Modelling of ultra-thin steel sheet in two-stage tensile deformation considering strain path change and grain size effect and application in multi-stage microforming

Multi-stage plastic deformation of metallic sheets for fabrication of complex micro structures with high aspect ratio features in different industrial clusters has been prevailing due to product miniaturization and integrated manufacturing. Owning to the varying strain path and the miniaturized scale of work pieces, both the strain path change (SPC) and size effect (SE) significantly affect the micro-scale deformation behaviors. To have a scientific insight and understanding of the influences of SPC and SE, two-stage tensile tests were conducted using the 0.1 mm thick SS 316L sheets with different grain sizes and EBSD was employed to characterize the microstructure evolutions. The results showed that the yield stress and elongation rate in the second tensile stage were decreased with the increase of pre-strain and the intersection angle between two tensile directions, while the hardening rate was found to be solely dependent on pre-strain. Changing the tensile direction in the second stage reversed the orientation distribution preference and raises the percentage of high Schmid factors, resulting in lowering the yield stress and hardening rate. On the other hand, more intra-granular mismatching boundaries accumulate in the coarsened grains, which impedes the dislocation movement and increases the deformation resistance. These two confronted mechanisms of SPC and SE interactively influence the deformation behaviors. A constitutive model for describing the flow stress affected by SPC and SE was established based on the micro-mechanism, which provided a basis to support the multi-stage microforming.

The effect of temperature on slip in microtextured Ti-6Al-2Sn-4Zr-2Mo under dwell fatigue

Slip activity in dwell fatigued Ti-6Al-2Sn-4Zr-2Mo, and its relation to the microstructure, was investigated using digital image correlation and electron backscatter diffraction at room temperature, 120 °C, and 200 °C to span a range of dwell sensitivities. The grains through which slip traces traversed were examined to identify critical grain characteristics in the percolation of long-range slip. It was found that slip accumulated more rapidly and in greater amounts at temperatures that exhibit dwell sensitivity (room temperature and 120 °C) versus temperatures that do not (200 °C). At all temperatures, plasticity occurred primarily by long-range basal slip through co-located grains with a high basal Schmid factor.

Analysis of the twin variant selection in polycrystalline cobalt

The samples of polycrystalline cobalt were strained in tension in the second stage of work hardening in order to activate twinning. The identification of twins and their orientation was analyzed using electron backscatter diffraction. The study of twin variant selection was carried out based on two parameters, namely the Schmid factor (SF) and the geometric compatibility factor (m'). The results indicate that the grains contained the $$\left\{ {10\overline{1}2} \right\}$$ extension twins in most cases while the $$\left\{ {11\overline{2}1} \right\}$$ extension twin mode was present in few cases, as well. In the grains containing only one twin variant, the SF analysis correctly predicted the activated twins in most cases. In the grains with variant pairs in para-position, nearly half of the studied twins had the highest SF of all possible variants, which means that they followed the Schmid law. On the other hand, for twin pairs in meta- or ortho-position, a different behavior was observed. Twins in these configurations were selected rather by the strain compatibility criterion than by the Schmid law, as SF values of the activated twins were significantly lower than those of the twins in para-position. Of these two configurations, the ortho-position one is highly preferred. The combined analysis using SF and m' parameter, predicted the occurrence of specific twin modes and variants only partially. A local analysis considering the effect of neighboring grains should be developed to account for the cases where the two parameters fail to explain the occurrence of particular twins, which seems to be related to the presence of a high fraction of high-angle grain boundaries.

Transmission electron microscopy investigation of the dislocation structure in a Ti-6Al-4V alloy subjected to an early stage of cyclic deformation

A detailed investigation was conducted of the dislocation structure formed within both the α and β phase of a Ti-6Al-4V alloy subjected to an early stage of cyclic deformation. The cyclic loading-unloading was carried out in tension between applied stress levels of 1100 and 100 MPa, the macroscopic yield stress (σ02) being about 1205 MPa, at a strain rate of 2×10−3 s−1 to 300 cycles, as part of an in-situ synchrotron experiment. The dislocation type, arrangement and density were examined post-mortem using transmission electron microscopy, complemented by the automatic determination of individual crystallite orientations through precession-enhanced nanobeam diffraction. The deformation took place entirely through dislocation glide and no evidence of deformation twinning was found within either of the constituent phases. The dislocation structure within the α phase was fairly similar to that formed during monotonic straining and no presence of periodic planar dislocation networks suggested in the literature was detected. Prismatic and basal slip were the dominant observed deformation modes, based on the calculated Schmid factor and critical resolved shear stress values reported in the literature. The <a>−type dislocations present after straining typically displayed a large screw component, with a majority of them having a pure screw character. There was also limited presence of <c+a> dislocations within the α phase and these were largely of a mixed type. Burgers vectors of the dislocations remaining after deformation within the β phase were also largely associated with the potential slip systems having high Schmid factor values and these dislocations mainly displayed a large screw component.

Analysis of the early stage of stress corrosion cracking in austenitic stainless steel by EBSD and XRD

This study examined nucleation behavior during the early stage of stress corrosion cracking (SCC) in sensitized austenitic stainless steel based on microstructural assessments, using X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD). The SCC experiment was conducted while applying a constant load corresponding to an initial strain of 2% in a tetrathionate solution. The crystal orientations on the smooth surface of a thermally sensitized specimen were determined via EBSD before and after testing, and diffraction spots were intermittently examined using XRD. Some grains having a high Schmid factor were found to be locally deformed near grain boundaries (GBs) due to constant tensile loading in the corrosive solution, such that intergranular stress corrosion cracks occurred at these GBs. The results of this work demonstrate that the passivation film on the metal first fractures due to localized deformation, after which newly created surfaces near GBs are exposed to the corrosive solution. Localized crack-like corrosion subsequently proceeds in chromium-depleted zones near GBs to generate intergranular SCC.