Deformation-induced Phase Transformation Research Articles

Hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening of the micro-alloyed high entropy alloy

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 HEA. Three types of precipitates, σ-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present after cold-rolling and annealing at 600 °C. Such hierarchical precipitates could lead to sluggish recrystallization and grain growth upon annealing. The partially recrystallized microstructures and hierarchical precipitates could lead to a high yield strength even for prolonged annealing conditions. Deformation mechanisms change with annealing time. The materials annealed for short times (< 2 h) are deformed by dislocation glide, deformation twinning, and deformation-induced ε phase. A longer annealing time (> 10 h) triggers a multi-variant ε phase, reverse transformation from ε to γ, and the multi-step sequential transformation, γ → ε → reverse transformed γ from ε → ε transformed from the reverse transformed γ. Further, materials annealed for longer times shows a higher contribution of back stress strengthening, which could be attributed to the increase in γ/ε and γ/σ interfaces. The activation of various deformation mechanisms and high back stress strengthening could lead to a superior strain hardening capacity and strength-ductility combination (YS: 699 MPa, UTS: 1041 MPa, TE: 45%) of the material annealed for 10 h. The present work provides the novel microstructure design solution of the metastable high entropy alloys with exceptional mechanical properties, utilizing hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening.

Abnormal work hardening in a TRIP-assisted metastable β titanium alloy under high strain rate loading

In this study, the dynamic tensile deformation behavior and microstructural evolution of a metastable β titanium alloy (Ti–10 V–2Fe–3Al, Ti-1023) with a single β phase structure were investigated at ambient temperature and high strain rates ranging from 1000 s−1 to 4000 s−1 on a split Hopkinson tension bar (SHTB) system. The dynamic tensile properties and work hardening ability exhibited a significant strain rate dependence. The calculated strain rate sensitivity (0.026–0.090) showed a strong strain and strain rate hardening ability. Microstructural analyses indicated that the dynamic deformation mechanisms were dislocation slip and deformation-induced α" martensite at all strain rates, while deformation-induced ω phase was activated beyond a strain rate of 2000 s−1. Deformation-induced phase transformations during dynamic tensile deformation led to a significant transformation-induced plasticity (TRIP) effect and a double-yielding phenomenon. As strain rate increased, deformation-induced martensitic transformation and the corresponding TRIP effect were suppressed, while work hardening rate first increased and then rapidly decreased. This abnormal TRIP effect on work hardening behavior was mainly due to the deformation-induced ω phase, promoting the dislocation plasticity in the β matrix and contributing to extra work hardening ability.

Deformation-Induced Phase Transformations in Gold Nanoribbons with the 4H Phase.

The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.

How Austenitic Is a Martensitic Steel Produced by Laser Powder Bed Fusion? A Cautionary Tale

Accurate phase fraction analysis is an essential element of the microstructural characterization of alloys and often serves as a basis to quantify effects such as heat treatment or mechanical deformation. Additive manufacturing (AM) of metals, due to the intrinsic nonequilibrium solidification and spatial variability, creates additional challenges for the proper quantification of phase fraction. Such challenges are exacerbated when the alloy itself is prone to deformation-induced phase transformation. Using commonly available in-house X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) and less commonly used synchrotron-based high-energy X-ray diffraction, we characterized nitrogen-atomized 17-4 precipitation-hardening martensitic stainless steel, a class of AM alloy that has received broad attention within the AM research community. On the same build, our measurements recovered the entire range of reported values on the austenite phase fractions of as-built AM 17-4 in literature, from ≈100% martensite to ≈100% austenite. Aided by Calphad simulation, our experimental findings established that our as-built AM 17-4 is almost fully austenitic and that in-house XRD and EBSD measurements are subject to significant uncertainties created by the specimen’s surface finish. Hence, measurements made using these techniques must be understood in their correct context. Our results carry significant implications, not only to AM 17-4 but also to AM alloys that are susceptible to deformation-induced structure transformation and suggest that characterizations with less accessible but bulk sensitive techniques such as synchrotron-based high energy X-ray diffraction or neutron diffraction may be required for proper understanding of these materials.

CRYOGENIC MILLING OF METASTABLE AUSTENITIC STAINLESS STEEL AISI 347

The metastable austenitic stainless steel AISI 347 offers the possibility to induce a phase transformation from γ-austenite to ε- and α’-martensite when machining. This knowledge is well understood during cryogenic turning and was already applied to improve the surface morphology of metastable austenitic steel. However, the potential of this in-process hardening method is so far limited to rotationally symmetrical components. The aim of this study is to investigate deformation induced phase transformation during cryogenic milling, aiming at an improved surface morphology and at the resulting beneficial surface properties of the workpiece for parts with complex geometries.

Deformation-induced phase transformation and twinning in Fe and Fe–C alloys

The phase transformations and twinning in a nano-sized face-centred cubic (FCC) crystal of iron with different carbon contents are studied by molecular dynamic (MD) simulations. The simulation cells are stretched uniaxially along the direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCChexagonal close-packed (HCP)’ and ‘BCCFCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.

A novel approach to analyse combined pre and post strain rate dependent phase transformation in uniaxially deformed AISI 304 stainless steel using XRD and EBSD

ABSTRACT The effect of pre-strain, pre-strain rate and post-strain rates on the deformation-induced austenite to martensite phase transformation have been studied in AISI 304 stainless steel. The martensite content obtained from the X-ray diffraction analysis and complemented by the magnetisation measurements showed that the phase transformation is enhanced for a unique combination of pre-strain, pre-strain rate and post-strain rate. The microstructural characterisation using detailed X-ray diffraction line profile analysis in terms of the size of the coherently scattering region and the dislocations that cause the microstrain within this region could give evidence of the enhanced/reduced martensite formation. The results of the electron backscattered diffraction in terms of the angular distribution of the lattice correlation boundaries and the orientation relationship boundaries of the austenite-martensite phase substantiated this finding. Considering that the multiple combinations of deformation parameters control the mechanism of martensitic transformation, this study will also contribute to the understanding of deformation-induced phase transformation of other materials with improved plasticity.

Unraveling the effect of deformation-induced phase transformation on microstructure and micro-texture evolution of a multi-axially forged Mg-Gd-Y-Zn-Zr alloy containing the LPSO phase

The effect of blocky to lamellar phase transformation on the microstructure and micro-texture of a Magnesium-Rare earth alloy containing long-period stacking order (LPSO) phases were examined comprehensively at the isothermal temperature of 400 °C via multiaxial forging. A very fine-grained microstructure with an average grain size of 1 μm was achieved after applying three multi-axial forging (MAF) passes. Particle stimulated nucleation (PSN) along with continuous dynamic recrystallizations (CDRX) due to the blocky LPSO phases were realized to be the main reasons for the achievement of such fine microstructure. The deformation-induced blocky to lamellar phase transformation began at 1/3 pass and expanded through the whole microstructure after the third pass. Such phase transformation was found to be initiated by the fragmentation of blocky phases. As a result of PSN and CDRX mechanisms, two new rare earth (RE) texture components of <10-11> and <2-1-10> || Transverse Direction (TD) were formed at the second MAF pass that led to a highly randomized deformation texture. Nevertheless, the continuous breakdown of blocky phases and their subsequent phase transformation to lamellar LPSO suppressed PSN at the third deformation pass. Hence, the formed RE texture components were disappeared which in turn increased the texture intensity at this deformation pass.

Strengthening of ferrous medium entropy alloys by promoting phase transformation

The design of medium-entropy alloys (MEAs) with low-cost and high-performance is a hot topic in the field of metal materials. Here, low amount of Co and Ni element is employed to prepare Fe–Co–Ni–Cr MEA, resulting in a reduced material cost of MEA, which also lead to low stacking fault energy and subsequently promote phase transformation in the alloy system. The achieved cost-effective Fe65Co10Ni10Cr15 MEA has excellent cryogenic strength of 1.6 GPa and strain of 56%, which has the highest strength-price ratio among the reported MEAs so far. The significant strengthening of the alloy is attributed to the hetero-deformation and the accelerated deformation-induced phase transformation.

Влияние насыщения водородом на структуру и механические свойства аустенитной стали 01Х17Н13М3, формируемые в процессе прокатки при разных температурах

Introduction. The development of hydrogen energy implies a decrease in the dependence of various human activities on fossil energy sources and a significant reduction in carbon dioxide emission into the atmosphere. Therefore, the requirements for the quality of structural materials, which have the prospect of being used for storage and transportation of hydrogen, as well as for the creation of infrastructure facilities for hydrogen energy, are increasing. Therefore, the scientific researches on the hydrogen-assisted microstructure and mechanical behavior of structural materials in various loading schemes are of great importance. The aim of this work is to establish the effect of chemical-deformation treatment, including rolling combined with hydrogen saturation, on the microstructure, phase composition, and mechanical properties of 316L-type austenitic stainless steel. Methods. Transmission electron microscopy and backscattered electron diffraction, X-ray diffraction, X-ray phase and magnetic phase analysis, microindentation and uniaxial static tension are utilized. Results and Discussion. It is shown experimentally that after rolling with 25 and 50 % upset, the morphology of the defect structure and the phase composition of 316L steel substantially depends on the deformation temperature (at room temperature or with the cooling of the samples in the liquid nitrogen) and on hydrogen saturation rate (for 5 hours at a current density of 200 mA/cm2). The main deformation mechanisms of the steel in rolling are slip, twinning, and microlocalization of plastic flow, which all provide the formation of ultrafine grain-subgrain structure in the samples. In addition, deformation-induced ε and α' martensitic phases are formed in the structure of the rolled samples. Regardless of the regime of chemical-deformation processing, grain-subgrain structures with a high density of deformation defects are formed in steel, but its morphologies are dependent on the processing regime. The experimental data indicate that both preliminary hydrogen saturation and a decrease in the deformation temperature contribute to the more active development of mechanical twinning and deformation-induced phase transformations during rolling. Despite the discovered effects on the influence of hydrogen saturation on the deformation mechanisms and the morphology of a defective microstructure formed during rolling, preliminary hydrogenation has little effect on the mechanical properties of steel at a fixed degree and temperature of deformation. These data indicate that irrespective of the morphology of the defective grain-subgrain structure, grain refinement, accumulation of deformation defects and an increase in internal stresses lead to an increase in the strength characteristics of the steel.

Influence of compositions and heat treatment on metastability of structure and properties of wear-resistant cast irons of the Fe-Cr-Mn-C system

The influence of chromium, manganese, carbon content and quenching modes on the formation of the structure, metastability of austenite and properties wear-resistant cast iron alloying system Fe-Cr-Mn-C with small additions of titanium, copper, silicon in various variations was investigated. The metallographic, durometric research methods, tests for dynamic bending, impact-abrasive wear resistance were applied. The regularities of changes in the phase composition and microstructure under the influence of the heating temperature for quenching in the range from 920 to 1070 &deg;C with cooling in sand have been established. With an increase in the heating temperature for quenching in the structure of cast irons, the content of austenite increases and the content of quenching martensite decreases. The amount of carbide phases of compositions (Fe, Cr)3C, (Cr, Fe)7C3, (Cr, Fe)23C6 and titanium carbonitrides TiCN, depending on the content of carbon and alloying elements, as well as the heating temperature for quenching, varies within 20...35 %. A significant contribution of deformation induced phase transformations - martensitic, dynamic aging, occurring in the surface layers in the process of testing in the formation of impact-abrasive wear resistance of cast irons was revealed. It is characterized by the effects of deformation self-hardening, manifested in the increase in the hardness of the surface (working) layer within HRC = 2...9 and relaxation of microstresses. The formation of optimal martensitic-austenitic-carbide metastable structure could be provided at the optimal ratio of alloying elements and under the influence of heat treatment at which the effect of deformation self-hardening at wear that increases drastically the wear resistance of the investigated cast irons could be realized. The developed cast irons of optimal compositions have shown high economic efficiency in the production of working bodies of shot-blast machines, sand-throwers, protective lining plates of sintering-blast furnace equipment. They can be recommended for the manufacture of various high-wear machine parts of machines operating under conditions of impact-abrasive andabrasive wear.

Highly pressurized helium nanobubbles promote stacking-fault-mediated deformation in FeNiCoCr high-entropy alloy

Tailoring nanoscale defect structures for desirable deformation behaviors is crucial to designing and optimizing the mechanical properties of alloys. Distinguishing from the predominant toughening mechanisms (e.g., mechanical twinning and deformation-induced phase transformation), here we report an unusual stacking-fault-mediated deformation in equiatomic FeNiCoCr high-entropy alloy (HEA) by controllably introducing helium nanobubbles with high pressures of ~2.5-4.7 gigapascals. Using in situ transmission electron microscopy nanomechanical testing, we demonstrate that highly pressurized helium nanobubbles can not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Through atomistic simulations, we reveal that high helium pressures contribute to reducing the nucleation energy of partial dislocations at the nanobubbles surface, which enhances dislocation nucleation rates and offers sustainable stacking fault sources for retaining ductility. Our results provide a novel design strategy for tuning deformation mechanisms of HEAs via introducing highly pressurized helium nanobubbles, which may open up avenues towards the facile tailoring of mechanical responses in micro/nanoscale HEA components.

Enhanced mechanical performance of grain boundary precipitation-hardened high-entropy alloys via a phase transformation at grain boundaries

Grain-boundary (GB) precipitation has a significant adverse effect on plasticity of alloys, which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading. Herein, we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs. This in situ deformation-induced phase transformation at GBs introduces a well-known effect: transformation-induced plasticity (TRIP), which enables an exceptional elongation to fracture (above 38 %) at a high strength (above 1.5 GPa) in a GB precipitation-hardened high-entropy alloy (HEA). The present strategy in terms of “local stress concentration-induced phase transformations at GBs” may provide a fundamental approach by taking advantage of (rather than avoiding) the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

Dislocation hardening and phase transformation-induced high ductility in Ti-6Al-4V with a heterogeneous martensitic microstructure under tensile load

In this work, a kind of heterogeneous martensitic microstructure was successfully fabricated within the fine β grains of Ti-6Al-4V through a rapid heating and cooling processing (RHCP) method, resulting in excellent ductility during tensile deformation. A total elongation was reached at 25.1%, while a considerable tensile strength of 1118 MPa was maintained. The main reasons for achieving this good ductility were the occurrence of dislocation hardening and deformation-induced phase transformation during tensile deformation. We used transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to characterize and analyze the dislocation configurations and new face-centered cubic (FCC) phase during tensile deformation. We found that, to accommodate plastic deformation between the microcrystalline soft lamellae and ultrafine hard lamellae, many geometrically necessary dislocations (GNDs) were generated, which inevitably led to dislocation hardening, thereby greatly improving the ductility of the alloy. As a result of dislocation hardening, the improved ductility induced two types of FCC phase transformations, and the FCC phase quantity increased significantly during further tensile deformation. Plastic deformation can be effectively accommodated by the formation of a new FCC phase so that the ductility of the material can be further improved. These accommodating plastic deformation mechanisms reveal the nature of the good ductility of the material under a tensile load and provide a new strategy for fabricating strong and ductile titanium alloys.

Dislocation structure and phase composition of Ti13Nb13Zr after high-pressure torsion

The approach based on synchrotron X-ray diffraction was applied in this research to investigate the dislocation and phase structures of severely deformed disk of Ti13Nb13Zr alloy. It was found, that the final structure of the material is inhomogeneous along the disk diameter. The local area of highest dislocation density of α′-phase matches with the zone of passed deformation-induced phase transformations.

An in situ TEM nanoindentation-induced new nanostructure in cadmium zinc telluride.

Phase transformations occurring in a solid govern the structural and physical properties significantly. Nevertheless, deformation-induced phase transition in a soft-brittle solid has not been demonstrated yet. Soft-brittle cadmium zinc telluride (CZT) based instruments have produced technological breakthroughs in the semiconductor industry, and therefore their phase transformations have been widely investigated during the past 60 years. In this study, in situ transmission electron microscopy (TEM) nanoindentation was performed on CZT, and it was found that no brittle fracture occurred at a peak load of 41.9 μN, corresponding to a stress of 1.75 GPa. A new nanostructure induced by in situ TEM nanoindentation was observed, consisting of a single crystal, slip bands, stacking faults, a superlattice, a new tetragonal phase, and Moiré fringes. The new tetragonal phase was formed by partial Cd and Te atoms in the (111[combining macron]) plane slipping along the [1[combining macron]21[combining macron]] orientation, which was elucidated by ab initio simulations. It belongs to a tetragonal crystal system, and the lattice distances along the X and Y axes were 0.382 and 0.376 nm, respectively. Our findings provide new insights into the deformation-induced phase transformation for a soft-brittle solid, and have application potential in solar cells, radiation detectors, and medical imaging, quantum, flexible electronic and optoelectronic devices.

Oxygen addition for improving the strength and plasticity of TiZr-based amorphous alloy composites

The addition of a small amount of oxygen improves the mechanical properties, especially plasticity, of Ti45.7Zr33Ni3Cu5.8Be12.5 amorphous alloy composites (AACs) at room temperature (298 K). Compared to the plasticity of AACs without added O (5%), the plasticity of the composites with 0.73 at.% O (nominal composition) was much higher (11 %). Even at O content higher than 0.73 at.%, the AACs exhibited good plasticity. The highest plasticity of ∼12.3 % was observed with 2.87 at.% O. Two distinct mechanisms are proposed to explain the enhanced plasticity of the AACs. At low O content, although deformation-induced phase transformation was suppressed, a substantial amount of α’’ martensite was formed. The microstructural features of α’’ martensite, such as thinner laths and homogeneous distribution, induced the formation of multiple shear bands in the amorphous matrix. At high O content, deformation-induced phase transformation was seriously suppressed. A dispersed nano ω phase was formed during rapid solidification in AACs with O content higher than 1.45 at.%. This resulted in a weakening in the anisotropy of β dendrites and led to their homogenous deformation. Furthermore, multiple shear bands were formed in the amorphous matrix. Apart from plasticity, the strength of the AACs also increased with an increase in the O content. This phenomenon was explained in terms of three mechanisms, viz. the solid-solution-strengthening effect of O, fine-grain strengthening of β dendrites, and secondary phase strengthening by the nano ω phase.

Influence of Grit Blasting on Residual Stress Depth Profile and Dislocation Density in Different Metallic Substrates

This work deals with the effect of grit blasting on the surface and in-depth properties of a number of commercially available metals/alloys, i.e., low carbon steel, C45 steel, SS316, Ti-6Al-4V, Inconel 718, and Hastelloy X. The residual stress depth profile and dislocation density of the specimens were determined using X-ray diffraction. A substantial increase in dislocation density was observed after grit blasting. A transmission electron microscope was utilized to observe the dislocation structure in a grit-blasted specimen. Strain hardening was also observed at the blasted surface owing to dislocation interaction and entanglement. The hardness profile followed a trend similar to that of dislocation density along the depth of grit-blasted low carbon steel. Moreover, compressive residual stress is induced in the blasted surface having a maximum value at a certain depth. The maximum induced compressive residual stress was found to have a good correlation with the Johnson–Cook flow stress. Moreover, the depth of the affected layer and the increase in hardness were found to depend on the yield strength and the strain hardening exponent of the materials, respectively. A plastic deformation-induced phase transformation from austenite to martensite was also identified in the case of SS316.

Quality protection of technical products – Unique identification with a hidden fingerprint in smart materials

To ensure that technical products are traceable, they must be clearly identifiable. A novel concept for rendering components made of metastable austenitic stainless steel traceable and thus counterfeit-proof is presented. The characteristic deformation-induced phase transformation of this material group and the associated changes in permeability are utilized for this purpose. This material behavior allows to create hidden component inherent patterns with varying permeability. The possibility to manufacture these “hidden fingerprints” in the workpiece subsurface by machine hammer peening is discussed.

Автоволновое описание пластичности материалов с нестабильной фазовой структурой на макромасштабном уровне

The regularities of plastic flow in materials with deformation-induced phase transformations on the example of titanium nickelide and TRIP steel are studied. For experimental analysis of plastic deformation processes, the method of digital image correlation has been used, which allows us to quantitatively describe the behavior of plasticity fronts associated with the course of phase transformations in the materials under study. The mechanisms of formation of Lüders bands and Portevin-Le Chatelier bands at different stages of plastic flow are considered.