Annealing Twin Boundaries Research Articles

The Role of Grain Orientation and Grain Boundary Characteristics in the Mechanical Twinning Formation in a High Manganese Twinning-Induced Plasticity Steel

In the current study, the dependence of mechanical twinning on grain orientation and grain boundary characteristics was investigated using quasi in-situ tensile testing. The grains of three main orientations (i.e., 〈111〉, 〈110〉, and 〈100〉 parallel to the tensile axis (TA)) and certain characteristics of grain boundaries (i.e., the misorientation angle and the inclination angle between the grain boundary plane normal and the TA) were examined. Among the different orientations, 〈111〉 and 〈100〉 were the most and the least favored orientations for the formation of mechanical twins, respectively. The 〈110〉 orientation was intermediate for twinning. The annealing twin boundaries appeared to be the most favorable grain boundaries for the nucleation of mechanical twinning. No dependence was found for the inclination angle of annealing twin boundaries, but the orientation of grains on either side of the annealing twin boundary exhibited a pronounced effect on the propensity for mechanical twinning. Annealing twin boundaries adjacent to high Taylor factor grains exhibited a pronounced tendency for twinning regardless of their inclination angle. In general, grain orientation has a significant influence on twinning on a specific grain boundary.

In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy

The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility.

Microstructure development in a high-nickel austenitic stainless steel using EBSD during in situ tensile deformation

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with 〈111〉 or 〈001〉 parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only.

On recrystallization texture and magnetic property of Cu-Ni alloys

The crystallographic texture, micro-hardness and magnetic property have been investigated in Cu-57at.% Ni, Cu-53at.% Ni, and Cu-49at.% Ni alloy used as substrates for coated conductors. In this work, a similar rolling texture was observed in the cold rolled condition for the three alloys. For each material, annealing at higher temperature results in a much stronger cube texture but the fraction of annealing twin boundary was quiet low. When annealing at 700°C and 800°C, it was presumed that the increase of Cu element increased grain boundary mobility, which then lead to the enhancement of twin boundary and weakened the cube texture. In addition, magnetization measurements indicate that the increase of Cu content can effectively reduce the saturation magnetization in Cu-Ni alloys. The comparatively low ferromagnetism for both Cu-53at.% Ni and Cu-49at.% Ni alloys suggests that substrates produced using these materials will benefit from decreasing hysteresis losses for alternating current applications.

Effect of preferential orientation on the annealing twins during the low temperature treatment in nickel

The influence of preferential orientation (namely cube texture) on annealing twins in a cold rolled (95% thickness reduction) pure nickel has been investigated during isothermal annealing at low temperature. The results reveal that the cube oriented grains in cube related twinning (CT) regions have larger average boundary migration rate in comparison with grains in non-cube related twinning (NCT) regions. However, the twin density is always lower in CT regions than that in NCT regions. This is contrary to the growth accident model that suggests that twin density was proportional to the grain boundary velocity and grain boundary distance. It can be interpreted by the fact of the selective nucleation and growth mechanism for cube oriented grains which are less dependent on annealing twinning during recrystallization. Furthermore, during grain growth the average length of annealing twin boundary segments has a significant increment, which is probably attributed to lengthening of low energy twin boundaries along with the migration of other neighboring boundaries.

Development of microstructure and mechanical properties during annealing of a cold-swaged Co–Cr–Mo alloy rod

In this study, we investigated the evolution of the microstructure and mechanical properties during annealing of a cold-swaged Ni-free Co–Cr–Mo alloy for biomedical applications. A Co–28Cr–6Mo–0.14N–0.05C (mass%) alloy rod was processed by cold swaging, with a reduction in area of 27.7%, and then annealed at 1173–1423K for various periods up to 6h. The duplex microstructure of the cold-swaged rod consisted of a face-centered cubic γ-matrix and hexagonal closed-packed ε-martensite developed during cold swaging. This structure transformed nearly completely to the γ-phase after annealing and many annealing twin boundaries were observed as a result of the heat treatment. A small amount of the ε-phase was identified in specimens annealed at 1173K. Growth of the γ-grains occurred with increasing annealing time at temperatures ≥1273K. Interestingly, the grain sizes remained almost unchanged at 1173K and a very fine grain size of approximately 8μm was obtained. The precipitation that occurred during annealing was attributed to the limited grain coarsening during heat treatment. Consequently, the specimens treated at this temperature showed the highest tensile strength and lowest ductility among the specimens prepared. An elongation-to-failure value larger than 30% is sufficient for the proposed applications. The other specimens treated at higher temperatures possessed similar tensile properties and did not show any significant variations with different annealing times. Optimization of the present rod manufacturing process, including cold swaging and interval annealing heat treatment, is discussed.

Effects of Low Temperature on Hydrogen-Assisted Crack Growth in Forged 304L Austenitic Stainless Steel

The objective of this study was to evaluate effects of low temperature on hydrogen-assisted crack propagation in forged 304L austenitic stainless steel. Fracture initiation toughness and crack-growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 140 wppm hydrogen and tested at 293 K or 223 K (20 °C or −50 °C). Fracture initiation toughness for hydrogen-precharged forgings decreased by at least 50 to 80 pct relative to non-charged forgings. With hydrogen, low-temperature fracture initiation toughness decreased by 35 to 50 pct relative to room-temperature toughness. Crack growth without hydrogen at both temperatures was microstructure-independent and indistinguishable from blunting, while with hydrogen microcracks formed by growth and coalescence of microvoids. Initiation of microvoids in the presence of hydrogen occurred where localized deformation bands intersected grain boundaries and other deformation bands. Low temperature additionally promoted fracture initiation at annealing twin boundaries in the presence of hydrogen, which competed with deformation band intersections and grain boundaries as sites of microvoid formation and fracture initiation. A common ingredient for fracture initiation was stress concentration that arose from the intersection of deformation bands with these microstructural obstacles. The localized deformation responsible for producing stress concentrations at obstacles was intensified by low temperature and hydrogen. Crack orientation and forging strength were found to have a minor effect on fracture initiation toughness of hydrogen-supersaturated 304L forgings.

Hot Ductility Characterization of Sanicro-28 Super-Austenitic Stainless Steel

The hot ductility behavior of a super-austenitic stainless steel has been studied using tensile testing method in the temperature range from 1073 K to 1373 K (800 °C to 1100 °C) under the strain rates of 0.1, 0.01, and 0.001 s−1. The hot compression tests were also performed at the same deformation condition to identify the activated restoration mechanisms. At lower temperatures [i.e., 1073 K and 1173 K (800 °C and 900 °C)], the serration of initial grain boundaries confirms the occurrence of dynamic recovery as the predominant restoration process. However, in the course of applied deformation, the initial microstructure is recrystallized at higher temperatures [i.e., 1273 K and 1373 K (1000 °C and 1100 °C)]. In this respect, annealing the twin boundaries could well stimulate the recrystallization kinetic through initiation new annealing twins on prior annealing twin boundaries. The hot tensile results show that there is a general trend of increasing ductility by temperature. However, two regions of ductility drop are recognized at 1273 K and 1373 K (1000°C)/0.1s−1 and (1100°C)/0.01s−1. The ductility variations at different conditions of temperature and strain rate are discussed in terms of simultaneous activation of grain boundary sliding and restoration processes. The observed ductility troughs are attributed to the occurrence of grain boundary sliding and the resulting R-type and W-type cracks. The occurrence of dynamic recrystallization is also considered as the main factor increasing the ductility at higher temperatures. The enhanced ductility is primarily originated from the post-uniform elongation behavior, which is directly associated with the strain rate sensitivity of the experimental material.

Influence of Mo concentration on corrosion resistance to HF acid solution of Ni–Co–Cr–Mo alloys with and without Cu

The corrosion resistance of Ni–30Co–16Cr–6Fe–xMo and Ni–30Co–16Cr–6Fe–2Cu–xMo (x=7–15wt%) alloys was investigated by immersion tests in 5.2M HF solution at 100°C. The weight loss of the Ni–30Co–16Cr–6Fe–xMo alloys relative to Mo concentration after immersion followed a Boltzmann equation. With decreasing Mo concentration, the Ni–30Co–16Cr–6Fe–xMo alloys first lost the Mo protection of the grain boundaries, then the annealing twin boundaries, and finally the grain matrix. In contrast, decreasing Mo content did not cause a significant increase in Ni–30Co–16Cr–6Fe–2Cu–xMo alloy weight loss, but led to a slightly preferential dissolution of both grain and annealing twin boundaries.

Effect of starting grain size on the evolution of microstructure and texture during thermo-mechanical processing of CoCrFeMnNi high entropy alloy

The effect of starting grain size on the formation of microstructure and texture in heavily deformed equiatomic CoCrFeMnNi high entropy alloy (HEA) was investigated. For this purpose two alloys with average grain size ∼7 μm (fine grained starting material or FGSM) and 200 μm (coarse grained starting material or CGSM) were cold-rolled to 90% and 95% reduction in thickness and isochronally annealed for one hour at temperatures ranging from 700 °C to 1200 °C. Development of strong brass type texture, typical of low stacking fault energy (SFE) materials was noticed in both FGSM and CGSM. Near ultrafine microstructure formation was observed after annealing at 700 °C but extensive grain growth was observed only after annealing at 1200 °C. The FGSM showed finer grain size as compared to CGSM after heavy deformation and annealing. The annealing twin boundary fraction increased with increasing annealed grain size. The annealing textures of both FGSM and CGSM did not show strong dominance of the {236<385> or {113}<332> components unlike low SFE brass. Furthermore, the volume fractions of different annealing texture components of the HEA were not significantly affected by starting grain size or annealing temperature in stark contrast to low SFE brass. These differences could be explained on the basis of homogenously deformed matrix and sluggish diffusion in HEA which greatly reduced the preferential nucleation and growth of components, respectively.

Grain size dependence of the serrated flow in a nickel based alloy

In this paper, uniaxial tensile tests at a constant strain rate were performed on a nickel based alloy with different grain sizes, and the effect of grain size on serrated flow was studied. The results show that the obvious serration phenomena only occurs in the tensile curves for the large-grained samples (>1.3μm), and the critical plastic strain for the onset of serrations increases with the grain refinement. A large number of slip bands are observed in the deformed samples with relatively large grains rather than fine grains. The analysis reveals that the interaction between slip bands and boundaries is mainly responsible for the serration occurrence, and the mean serration amplitude is closely associated with the density of annealing twin boundary.

Grain boundary segregation in Fe–Mn–C twinning-induced plasticity steels studied by correlative electron backscatter diffraction and atom probe tomography

We report on the characterization of grain boundary (GB) segregation in an Fe–28Mn–0.3C (wt.%) twinning-induced plasticity (TWIP) steel. After recrystallization of this steel for 24h at 700°C, ∼50% general grain boundaries (GBs) and ∼35% Σ3 annealing twin boundaries were observed (others were high-order Σ and low-angle GBs). The segregation of B, C and P and traces of Si and Cu were detected at the general GB by atom probe tomography (APT) and quantified using ladder diagrams. In the case of the Σ3 coherent annealing twin, it was necessary to first locate the position of the boundary by density analysis of the atom probe data, then small amounts of B, Si and P segregation and, surprisingly, depletion of C were detected. The concentration of Mn was constant across the interface for both boundary types. The depletion of C at the annealing twin is explained by a local change in the stacking sequence at the boundary, creating a local hexagonal close-packed structure with low C solubility. This finding raises the question of whether segregation/depletion also occurs at Σ3 deformation twin boundaries in high-Mn TWIP steels. Consequently, a previously published APT dataset of the Fe–22Mn–0.6C alloy system, containing a high density of deformation twins due to 30% tensile deformation at room temperature, was reinvestigated using the same analysis routine as for the annealing twin. Although crystallographically identical to the annealing twin, no evidence of segregation or depletion was found at the deformation twins, owing to the lack of mobility of solutes during twin formation at room temperature.

Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries.

Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (<3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties.

Investigations of twin boundary fatigue cracking in nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Implant retrieval studies have indicated that the primary cause of failure in stainless steel devices is fatigue, and time or cycles required for fatigue crack initiation often consumes the majority of implant lifetime. Stainless steels with significant nitrogen additions have shown an improved fatigue response, but have also shown a peculiar preference for fatigue crack initiations at or along annealing twin boundaries in the face-centered cubic (FCC) materials. In a recent comparison study on cold-worked implant grade stainless steels, a number of fatigue crack initiations were found along former annealing twin boundaries on both nitrogen-stabilized austentitic (HNASS) and nickel-stabilized austenitic steels. Further investigations were warranted to determine the crystallographic conditions present around these annealing twin boundary cracks, since not every twin boundary showed crack initiation. The present study examined the crystallographic conditions present around each of the former annealing twin boundary cracks relative to the applied loading direction. It was determined that the former annealing twin boundary cracks showed the complete range of misorientation deviations allowed by the Brandon criterion. The textures of the cracked twin boundaries were found to be random relative to the overall global textures of the materials. Most of the cracked twin planes in the HNASS steel were shown to be high angles, and in many cases were nearly perpendicular to the material surface. The nickel-stabilized steel showed a preference for lower twin plane inclination angles relative to the material surface. High Schmid factors were shown for all grains surrounding the cracked twin boundaries indicating each grain was oriented favorably for slip relative to the applied loading direction. A high Taylor factor mismatch was also shown across most of the cracked twin boundaries in both steels indicating strong difference in expected yield response for each of the grains which suggest localized strain incompatibility was another important factor in twin boundary cracking.

An EBSD based comparison of the fatigue crack initiation mechanisms of nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Nitrogen additions are known to improve both the pitting potentials and fatigue response of high-nitrogen austenitic stainless steels (HNASS). HNASS alloys are of particular interest to the biomedical industry due to patient allergy concerns. Available HNASS fatigue studies in the literature were all performed on materials in the solution annealed condition, and not the 20–30% cold-worked condition often ordered for implant manufacturing. Previous studies in our laboratories evaluated the corrosion fatigue response of commercially available cold-worked 316L and HNASS implant grade steels, and found evidence of unusual cleavage-like facets in the crack initiation and early propagation regions of the fracture surfaces in the HNASS. The 316L steel showed a typical ductile fatigue striated fracture surface with no evidence of cleavage-like facets. These findings suggested the fatigue crack initiation mechanisms may be different between the two austenitic steels. The purpose of the present study was to compare and contrast fatigue crack initiation mechanisms in the cold-worked 316L and HNASS steels, using an SEM and EBSD based examination technique, under both low-cycle and high-cycle conditions. Under both fatigue conditions, cracks were shown to preferentially initiate along former annealing twin boundaries in the HNASS steel. In contrast, fatigue crack initiation was shown to occur primarily along extrusions or intrusions associated with trans-granular slip markings in the 316L steel.

Study on Cyclic Strain Localization and Fatigue Fracture Mechanism in High Manganese Twinning-Induced Plasticity Steels

The fatigue cracking mechanisms of two high Mn TWinning-Induced Plasticity (TWIP) steels are investigated in detail using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Furthermore, the fracture surfaces of the fatigued steels have been studied by employing a field emission gun scanning electron microscope (FEG-SEM). The fine details of the fatigued surface topography are verified using an atomic force microscope (AFM). The results indicate that the fatigue crack embryos nucleate at an early stage of the fatigue life as a result of local straining at grain and annealing twin boundaries at sites, where persistent slip bands create dislocation piled-ups that impinge on boundaries. The EBSD measurements showed that unlike in monotonic straining, the formation of deformation twins is not observed under cyclic straining.

Strain-induced martensitic transformation near twin boundaries in a biomedical Co–Cr–Mo alloy with negative stacking fault energy

Biomedical Co–Cr–Mo (CCM) alloys have been commonly used for artificial hip and knee joint prostheses, but a need to improve their biomedical inertness and wear resistance has become widely recognized. The mechanical behavior of CCM alloys is dominated by strain-induced martensitic transformation (SIMT), which causes crack initiation during plastic deformation but dramatically enhances the wear resistance in practical use. To develop more reliable CCM alloys it is essential to clarify the factors affecting the occurrence of SIMT. In the present study we have focused on the effect of annealing twin boundaries (ATBs) on SIMT behavior. We have analyzed in detail the substructures near a parallel pair of ATBs after deformation under a stress preferential for slip parallel to the ATBs. Preferential formation of ε-hexagonal close-packed (HCP) phase at ATBs in metastable γ-face-centered cubic (FCC) phase was found by both scanning electron microscopy with electron backscattered diffraction (EBSD) analysis and transmission electron microscopy (TEM). High resolution TEM images indicated that thickening of the ε-HCP phase does not proceed regularly on every second atomic plane, which would form perfect ε-phase HCP structure, but irregularly leaving a high density of stacking faults. Furthermore, the thickness of the ε-HCP phase was found to be different at ATBs on the two sides of the twin. The difference was attributed to the internal stress due to strain incompatibility at the ATBs on the basis of residual stress analysis by the EBSD–Wilkinson method and phase-field simulation of solute segregation at ATBs.

Optimum microstructure combination for maximizing tensile strength in a polycrystalline superalloy with a two-phase structure

Different microstructural features were obtained under various heat treatment conditions, which provided insight into the factors controlling the critical strength in a polycrystalline Ni–Co-based disk superalloy (TMW-4M3 alloy) with a two-phase structure. The contribution of each microstructural feature, namely, the grain size, annealing twin boundary and distribution of γ′ precipitates, to the total strength was analyzed quantitatively by measuring the Vickers hardness over the nanometer to micron size range. Grain boundary strengthening decreased to nearly zero with increasing solution heat treatment temperature, while the secondary and tertiary γ′ precipitation hardening increased. Therefore, there is an optimum combination of microstructural features for achieving the highest tensile strength in such superalloys, the key factors being the temperature and time used for the solution heat treatment and the subsequent aging treatment. A method for determining the optimum factors for TMW-4M3 is proposed.

Enhanced strength and ductility of friction stir processed Cu–Al alloys with abundant twin boundaries

Ultrafine-grained Cu–Al alloys were prepared via friction stir processing (FSP) with additional rapid cooling. These FSP Cu–Al alloys exhibited equiaxed recrystallized grains with relatively low dislocation density and a high fraction (96%) of high-angle grain boundaries. Abundant annealing twin boundaries were successfully introduced into the ultrafine grains in FSP Cu–15Al (at.%) alloy, resulting in superior strength–ductility synergy with a yield strength of ∼700 MPa and a uniform elongation of ∼13%.

Effect of annealing twins on crack initiation under high cycle fatigue conditions

Several experimental studies have shown that fatigue cracks in Ni-based superalloys preferentially form in the vicinity of favorably oriented annealing twin boundaries. However, this increase in the probability of crack nucleation has not been quantified in detail since 3 material modeling has not typically considered twins. The present work employs finite element simulations and a continuum crystal plasticity constitutive model for RR1000 Ni-based superalloy to assess the potential enhancement of nonlocal fatigue indicator parameters (FIPs) incurred by the presence of Σ3 twins. Two cases are analyzed: single crystals with twins of different widths subjected to single slip cyclic shear deformation aligned with the twin plane and polycrystals with 20 % of the grains half-twinned. Simulations with and without twins are compared in both cases. The results demonstrate the detrimental effect of twins in shifting the upper tail of the FIP distribution to higher values; this shift is more pronounced with an increase of twin thickness.