Deformation-induced Transformation Research Articles

A superhard incompressible carbon allotrope with deformation-induced transformation to diamond

A superhard incompressible carbon allotrope with deformation-induced transformation to diamond

Effects of intercritical annealing time on microstructure, tensile properties, and cryogenic toughness of newly designed ultra-low C low Ni Medium-Mn steel

An ultra-low carbon and low nickel medium manganese heavy steel plates was fabricated by quenching and intercritical annealing process. The effect of annealing time on the tensile properties and impact toughness were studied in relation to the formation of reversed transformation. The results show that excellent mechanical properties with yield strength of 680 MPa, tensile strength of 871 MPa, total elongation of 38.2% and high impact energy of 187 J at −60 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite (RA) together with a small amount of martensite after intercritical annealing. Increasing the intercritical annealing time resulted in coarser matrix microstructures, larger and more voluminous retained austenite, but with notably reduced stability. Highly stable RA enhances the work hardening capacity of the steel via sustained deformation-induced transformation (TRIP effect), significantly contributing to the toughening of the multiphase system. In addition, Austenite stability, rather than large-angle grain boundaries and the amount of austenite transformation, is the primary factor determining the low-temperature toughness of medium manganese steel.

Nanosized austenite and coherent nanoprecipitates making maraging steel strong and ductile

Here we report a microstructural design in maraging steel, which renders superior strength-ductility synergy by combining the advantages of transformation induced plasticity and dislocation cutting mechanisms, via an economic thermomechanical route suitable for industrial scale. It aims at innovating ductile maraging steel without conceding too much strength. Specifically, a high-strength maraging steel is first treated by cold-rolling and intercritical annealing, which leads to partial retransformation and precipitation of supersaturated needlelike/near-spherical B2-structured nanoparticles occurring simultaneously in dislocated α′-martensite matrix. The material is further optimized by peak-aging, yielding a desired microstructure, with nanosized γ-austenite (d‾γ ∼150 nm) dispersed in lath α′-martensite (d‾α′ ∼130 nm) with dense spherical B2-nanoprecipitates (d‾B2 ∼6.5 nm), i.e., an austenite-maraging structure with hierarchical heterogeneity at nanoscale. The yield strength is measured to be ∼1.65 GPa, which is largely retained as it combines boundary strengthening, dislocation strengthening and ordering strengthening of nanoprecipitates. Surprisingly, significant work hardening renders an ultimate strength even high up to ∼1.9 GPa and a decent ductility of ∼6–7%, twice that of conventional maraging steels. Interrupted microstructure examination unravels that it is the hierarchical microstructure heterogeneity-induced sequential activation of plasticity-enhancing mechanisms that confers such excellent deformability. At the stage soon after yielding (ε∼1.5–3%), deformation-induced transformation from metastable γ-austenite to α′-martensite imparts a significant strain hardening rate (Θ) up-turning. At the high strain stage (ε>3 %), multiple dislocations cutting through B2-nanoprecipitates promotes the interaction and accumulation of dislocations, thereby postponing Θ attenuation and prolonging ductility.

A critical review on deformation-induced transformation kinetics of austenitic stainless steels

Deformation-induced austenite to martensite transformation is an important phenomenon during the deformation of metastable fully austenitic transformation induced plasticity (TRIP) steels. The kinetics of austenite to martensite transformation influence the deformation behaviour of austenitic stainless steels but is often neglected by the materials community. In this paper, after an initial discussion on thermodynamics and mechanism, the importance of deformation-induced transformation kinetics is briefly outlined and the influence of various parameters like grain size, deformation temperature, strain rate, stress state and prior austenite deformation on the transformation kinetics is critically assessed. Variation of transformation kinetics with various parameters has been identified and justified. These variations could act as a ready reference for designing austenitic stainless steels with the desired set of mechanical properties and deformation behaviour. Further in this paper, selected models that can predict the transformation kinetics are reviewed and compared. In the end, research fronts related to transformation kinetics that can be further explored have been identified.

Influence of microstructure on slurry erosion resistance of medium carbon high silicon steels

The present study compares slurry erosion resistance of austempered and tempered medium carbon high silicon steels. A 0.62C-1.91Si-0.85Mn-0.23Cr steel was subjected to austempering at 300, 350 and 400 °C for 10 min to develop a microstructure comprising of bainite and retained austenite. With the increase in the austempering temperature, the mechanical stability of retained austenite decreased, and the size of bainitic ferrite laths increased. As a result, strain hardening ability and toughness of the steels decreased with austempering temperature. Another set of steel with the same composition was quenched and tempered at 300, 450 and 550 °C for 60 min. An increase in the tempering temperature led to an increase in the size and fraction of carbides and a decrease in the mean free path and dislocation density. Strain hardening ability and toughness of tempered steels was substantially small than austempered steels. Both the austempered and tempered steels were subjected to slurry erosion. Austempered steels exhibited higher slurry erosion resistance as compared to tempered steels. Slurry erosion resistance of austempered steels has been noted to depend on the mechanical stability of retained austenite and the size of bainitic ferrite laths. Due to the difference in the extent of deformation-induced transformation of retained austenite into martensite in the steels austempered at different temperatures, number of mobile dislocations generated (to accommodate the dilatation due to transformation) in bainitic ferrite laths differed. A large number of mobile dislocations in the specimens austempered at 300 °C blunted the propagating crack and delayed the material loss during erosion. The erosion resistance of tempered steels was observed to decrease with an increase in tempering temperature. A large fraction of coarse carbides led to the formation of many decohesion sites in steels tempered at 450 and 550 °C. Further, a small number of mobile dislocations present per unit volume in the steels tempered at high temperatures could not resist the propagation of cracks nucleated from the decohesion sites leading to a high erosion rate in these steels.

Microstructure evolution and corrosion behaviors of cold-rolled 304 stainless sheets of steel in 3.5% NaCl solution

Austenitic stainless steel (ASS) is widely used in engineering applications due to its good corrosion resistance and mechanical properties. Several studies have indicated that the deformation-induced transformation of martensite in ASS significantly affects its corrosion resistance. However, corrosion resistance behavior in chloride-rich environments is more complex, and different cold-working methods have distinct impacts on localized corrosion. This study investigated the structural and corrosion resistance changes induced by cold rolling in AISI 304 samples. The used samples were initially subjected to a cold-rolling process with a thickness reduction of up to 50%. The results demonstrate an increase in the deformation-induced martensitic transformation and micro-hardness as the level of cold deformation increases. However, higher levels of deformation lead to the fragmentation of the formed a’‑martensite lath structure into smaller laths, ultimately resulting in a predominantly refined and diffuse dislocation-cell-type structure. Potentiodynamic tests were conducted to analyze conventional electrochemical parameters, revealing a reduction in corrosion resistance with increasing cold deformation. This suggests that the formation, amount, and microstructure of a’‑martensite, under imposed strain conditions, induce changes in the studied electrochemical parameters. Additionally, the more deformed samples exhibited a higher current density during passive layer formation, and exhibited more intense metastable pits, suggesting decreased corrosion resistance with higher degrees of deformation.

Effect of retained austenite on the fatigue behavior of modified bainitic 100Cr6 steels considering local phase transformation

Since highly loaded components, such as roller bearings, tend to have relatively high lifetime scatter caused by microstructural defects, an increased defect tolerance of the material can improve the performance as well as the reliability of these highly stressed components. To increase the defect tolerance of the roller bearing steel 100Cr6, two different laboratory melts with either 1.5 wt.-% Al or 1.5 wt.-% Si in addition to the standard composition were developed. After an adapted heat treatment, both steels exhibited a bainitic microstructure with a relatively high content of retained austenite with more than 20 vol.-%. Considering the effects of the retained austenite, a defined mechanical stability is essential to avoid excessive deformation-induced transformation from the retained austenite into α′-martensite, which can lead to shape deviations in components. However, besides the increased deformability of the austenitic phase, local phase transformations in the vicinity of defects can increase the defect tolerance. The fatigue tests performed at ambient temperature (AT) and at T = 100 °C, show a higher fatigue strength of the Si-alloyed steel due to a refined microstructure and revealed for both steels a relatively high austenite stability, since no macroscopic phase transformation was observed. The high austenite stability can be explained by a relatively high carbon content of the austenitic phase. However, in contrast to the Al-alloyed steel, a decrease in the fatigue strength and in the defect tolerance due to the increased temperature were observed for the Si-alloyed steel, which indicates a more pronounced phase transformation at AT. The lower carbon content and the finer distribution of the retained austenite, when compared to the Al-alloyed steel, are supposed to be the reason. While a smaller carbon content decreases the phase stability, the refined austenite distribution results in a higher probability of retained austenite presence in the vicinity of a defect.

Deformation-Induced Transformation of the Lamellar Pearlite Structure in Rail Steel during Tension

Deformation-Induced Transformation of the Lamellar Pearlite Structure in Rail Steel during Tension

A Detailed Analysis of the Microstructural Changes in the Vicinity of a Crack-Initiating Defect in Additively Manufactured AISI 316L

The fatigue life of metals manufactured via laser-based powder bed fusion (L-PBF) highly depends on process-induced defects. In this context, not only the size and geometry of the defect, but also the properties and the microstructure of the surrounding material volume must be considered. In the presented work, the microstructural changes in the vicinity of a crack-initiating defect in a fatigue specimen produced via L-PBF and made of AISI 316L were analyzed in detail. Xenon plasma focused ion beam (Xe-FIB) technique, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were used to investigate the phase distribution, local misorientations, and grain structure, including the crystallographic orientations. These analyses revealed a fine grain structure in the vicinity of the defect, which is arranged in accordance with the melt pool geometry. Besides pronounced cyclic plastic deformation, a deformation-induced transformation of the initial austenitic phase into α’-martensite was observed. The plastic deformation as well as the phase transformation were more pronounced near the border between the defect and the surrounding material volume. However, the extent of the plastic deformation and the deformation-induced phase transformation varies locally in this border region. Although a beneficial effect of certain grain orientations on the phase transformation and plastic deformability was observed, the microstructural changes found cannot solely be explained by the respective crystallographic orientation. These changes are assumed to further depend on the inhomogeneous distribution of the multiaxial stresses beneath the defect as well as the grain morphology.

Effects of different intercritical annealing processes on microstructure and cryogenic toughness of newly designed medium-Mn and low-Ni steel

Through the idea of replacing Ni with Mn, a shortened process of ultra-fast cooling and two-step intercritical annealing for medium-Mn and low-Ni steel after rolling was designed. Microstructures were characterized through optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and X-ray diffractometer. Furthermore, the effects of different intercritical annealing temperatures on austenite including content, stability, and the impact toughness of experimental steel were investigated. The microstructures of the experimental steel comprised retained austenite (RA), fine-grained ferrite, and part of martensite. The results show that fine grain structure and austenite with high stability enriched by Mn and Ni provided excellent mechanical properties for experimental steel with yield strength of 645 MPa, tensile strength of 745 MPa, total elongation of 32%, and high impact energy of 133 J at −150 °C. As the intercritical annealing temperature increased, the microstructures of the matrix coarsened, and the volume fraction and size of RA increased, while the stability decreased significantly. RA with high stability provided long-lasting work hardening ability through continuous deformation-induced transformation (TRIP effect) during the deformation of steel, which played an important role in toughening the multiphase system.

Two-step deformation-induced martensitic transformation in additively manufactured High-Si stainless steel

Deformation-induced phase transformation in the additively manufactured (AM) Fe–18Cr–8Ni-5.5Si (in wt.%) alloy was analyzed using the in-situ electron backscattered diffraction (EBSD) record during uniaxial tensile testing in a scanning electron microscope (SEM). Analysis of EBSD data was focused to investigate phase transformation behavior during deformation with consideration of mechanical interactions among phases and grains in the ultrafine AM microstructure. The initial EBSD phase map illustrated that the as-built microstructure was a duplex structure consisting of major austenite (γ) and minor ferrite phases, some of which showed non-traditional orientation relationship since the rapid solidification in the AM process has produced non-equilibrium state in phase formation. In the deformed as-built AM alloy, the two-step martensitic transformation (γ→ε→α′) was identified as the main deformation-induced transformation mechanism. Martensitic transformation steps were found to be associated with glides of Shockley partial dislocations on closed packed (111) planes under the influence of atomic disturbance in the γ and ε phases, respectively. Using the closed-packed plane pole figures and unit cell orientations, their evolutions and mechanisms accompanying with the transformation were analyzed. The results indicate that one variant of ε-martensite forms from their parent γ phase, while two variants of α′ form from their parent ε phase.

Investigation of a novel sub-surface work hardening phenomenon in micro-turning of Zr-based bulk metallic glass

This study investigates the influence of the micro-turning operation on the subsurface deformation-induced transformation. The variations in nanohardness, shear transformation zone (STZ) volume, and free volume in the machined subsurface of the Zr-based bulk metallic glass (BMG) have been characterized. Nanoindentation studies reveal that the machined subsurface is strain hardened, a unique phenomenon that has not been reported in Zr-based BMG machining. The work-hardened zone extends until a depth of ∼20 μm from the surface. The quantitative evaluation of the deformation-induced hardening has been done experimentally by nanoindentation followed by the free volume characterization via Differential Scanning Calorimetry (DSC). The Shear Transformation Zone (STZ) volume approach has been used to understand the changes in the machined subsurface induced due to microturning. The STZ volume in the machined region is lower than that in the undeformed zone for all the machining conditions investigated in this work. The enthalpy change obtained using DSC analysis for estimating the change in free volume content of the machined regions correlates with the STZ volume.

Hot deformation behavior and microstructural evolution for dual-phase Mg–9Li–3Al alloys

The hot deformation behavior of dual-phase Mg–9Li–3Al alloys was investigated by the isothermal hot compression tests using the Gleeble-3500 thermal-mechanical simulation testing system over a temperature range from 473 to 623 K and a strain rate range of 0.001–1 s−1. The flow curves exhibited obvious serrations of periodic fluctuation at high strain rates, which can be considered as the Portevin-Le Chatelier effect. The relationship among flow stress, strain rate, and deformation temperature was analyzed. The deformation activation energy (Q) and some basic material factors (A, n, and α) were calculated based on the Zener–Hollomon equation. An approach of processing map composed of power dissipation and instability domains was established by the dynamic material model to reveal the hot workability. The flow instability domain only occurred at low temperatures and high strain rates. When the Mg–9Li–3Al alloy was deformed at 473 K and the strain rate of 1 s-1, numerous deformation twins were formed in α-Mg phases and, meanwhile, the β-Li phase was deformed and broken. When the temperature was increased to 573 K, the synergetic deformability between α-Mg and β-Li phases was improved due to the activation of more slip systems. However, the proportion of dynamic recrystallization was still low at the strain rate of 0.001 s-1. The needle-shaped α-Mg phase precipitated out in the β-Li matrix when the alloy was deformed at 623 K and the strain rate of 0.001 s-1. Its formation was attributed to the deformation-induced transformation. Moreover, the α-Mg phase can retard the dislocation movement and grain growth during deformation, leading to the precipitation/dispersion hardening.

Experimental investigation on kinetic behavior of the deformation-induced α→β transformation during hot working of Ti–6Al–2Zr–1Mo–1V alloy below the β-transus

Hot compression test was performed on Ti–6Al–2Zr–1Mo–1V alloy below the β-transus (Tβ) with the temperature range of 940–970 °C. Microstructural evolution, especially on the equiaxed primary α phase (αp), was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results showed that the αp could transform into β during hot deformation at α+β two-phase region, i.e., deformation-induced transformation of α→β (DIT). DIT, as an additional softening mechanism, caused the unusual stress response on the flow curves, which was reflected by the fact of the steady stress (σst)<initial yield stress (σ0.2). The fraction of transformed αp was influenced by various hot working parameters (such as temperature, strain and strain rate). Even complete dissolution of αp was present in some cases, indicating a premature Tβ during straining. The DIT is considered as a displacive nucleation and diffusion-controlled growth process. Its mechanism can currently be divided into two types: internal penetration of αp and α/β interface migration. The αp particles with higher favorable orientation preferentially undergo phase transformation. A kinetic model based on the transformed αp fraction (ωαp(t)) during DIT was developed by taking into account the thermal activation contribution (corresponding to heating duration) and mechanical driving force contribution (corresponding to deformation). The model was fitted to the ωαp(t) as a sigmoidal function of time (t) based on the classical Johnson–Mehl–Avrami (JMA) equation. Such a quantitative kinetic model for DIT can be used to accurately tailor the required balance between αp and β in Ti–6Al–2Zr–1Mo–1V alloy during hot working.

Model description of hydrogen-induced damage evolution in high-strength steel weld metal based on results of a novel test method

In this paper, the development of a novel method for testing welded tensile specimens under two-stage external tensile loading is presented. The test method is intended to investigate the susceptibility of submerged arc welded high-strength steel specimens of type S690QL-S3Ni2.5CrMo to hydrogen-induced damage. Multiple cracking events within one specimen prove that the locally prevailing boundary conditions decisively determine the hydrogen-induced damage sequences.By means of SEM and EBSD analyses on specimens after test completion, interrelationships about the microstructural damage behavior are being examined. The crack tip environments are of particular importance here, since complex stress and strain conditions tend to cause hydrogen accumulation and further propagation of inter- and transgranular cracks here. In the vicinity of crack tips, retained austenite is of special importance. This metastable phase might undergo a deformation-induced transformation into crack-sensitive martensite and simultaneously release high local hydrogen contents as a former hydrogen trap. Finally, a model description including various sections of the stress- and hydrogen-induced damage evolution is derived from the empirical-analytical results.

Effect of Intercritical Annealing Parameters and Starting Microstructure on the Microstructural Evolution and Mechanical Properties of a Medium-Mn Third Generation Advanced High Strength Steel

A prototype medium-Mn TRIP steel (0.2 C–6 Mn–1.7 Si–0.4 Al–0.5 Cr (wt %)) with a cold-rolled tempered martensite (CR) and martensitic (M) starting microstructures was subjected to continuous galvanizing line (CGL) compatible heat treatments. It was found that the M starting microstructures achieved greater than 0.30 volume fraction of retained austenite and target 3G properties (UTS × TE ≥ 24,000 MPa%) using an intercritical annealing temperature (IAT) of 675 °C with an IA holding time of 60–360 s, whereas the CR microstructure required an IAT of 710 °C and annealing times of 360 s or greater to achieve comparable fractions of retained austenite and target 3G properties. This was attributed to the rapid austenite reversion kinetics for the M starting microstructures and rapid C partitioning from the C supersaturated martensite, providing chemical and mechanical stability to the retained austenite, thereby allowing for a gradual deformation-induced transformation of retained austenite to martensite—the TRIP effect—and the formation of nano-scale planar faults in the retained austenite (TWIP effect), such that a high work-hardening rate was maintained to elongation of greater than 0.20. Overall, it was concluded that the prototype steel with the M starting microstructure is a promising candidate for CGL processing for 3G AHSS properties.

Microstructure evaluation of an advanced high strength steel with superior results in terms of strength-ductility trade-off

Major improvements in new advanced high strength steels, especially related to microstructural features, have been made by the steel sector to access the trade-off between strength and ductility. In this context, the present study provides a detailed analysis of the microstructure of a cold rolled steel with a minimum tensile strength of 980 MPa, which possesses superior elongation when compared to other conceptions of steels from the same strength level. The annealing process was simulated in a Gleeble machine, and the microstructural characterization was done using optical and scanning electron microscopy, EBSD and XRD analysis. Austenite decomposition, using dilatometric test, and mechanical properties were also evaluated. The steel characterization revealed a microstructure consisting of ferrite matrix with martensite islands and retained austenite particles, in a fraction equivalent to that of conventional TRIP steels, dispersed throughout. The carbon content in the austenite, however, was less than 1.0% w/w, which results in a relatively low stability. Therefore, the increase in strain hardening capacity enabled by the deformation-induced transformation of austenite to martensite produces increased ductility during straining, distinguishing the analyzed material from other steels of the same strength level.

Effects of CGL-compatible thermal processing, starting microstructure, and Sn micro-alloying on the mechanical properties of a medium-Mn third generation advanced high strength steel

Two prototype medium-Mn third generation advanced high strength steels (Fe-0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn wt%, x = 0 or 0.05) were annealed with continuous galvanizing line (CGL) compatible thermal processing parameters. It was found that the starting microstructure, intercritical annealing (IA) parameters and Sn micro-alloying had a significant effect on the volume fraction, stability, and deformation-induced transformation kinetics of the retained austenite and, therefore, the resultant mechanical properties of the steels. It was shown that Sn segregation at the carbide/matrix interface inhibited C diffusion and consequently, reduced carbide dissolution and precipitation kinetics during intercritical annealing. This resulted in lower volume fraction of retained austenite with lower stability after intercritical annealing of the as-received cold-rolled Sn-containing alloy. CGL-compatible IA treatments, therefore, were only able to produce target 3G-AHSS mechanical properties in the as-received cold-rolled alloy without Sn micro-alloying. However, the martensitic starting microstructures were more robust in achieving the 3G-AHSS target mechanical properties for both alloys. A martensitic starting microstructure resulted in higher volume fraction of stable retained austenite for 650 °C and 675 °C IA treatments. Gradual transformation of this stable retained austenite to martensite (TRIP effect) and mechanical twinning (TWIP effect) resulted in high sustained work hardening rates during plastic deformation. As a result, a favourable strength/ductility balance was achieved in the prototype steels. Moreover, Sn micro-alloying did not have a significant detrimental effect on the mechanical properties for the martensitic starting microstructure heat treatments which achieved target 3G-AHSS properties. This is promising in terms of developing a CGL-compatible processing route for these prototype steels.

Enhanced ductility in high performance polyamides due to strain-induced phase transitions

Copolymerizing poly(hexamethylene terephthalamide-co-isophthalamide) (PA6TI) with 40 wt% polyamide 66 (PA66) results in only minor decreases in modulus and yield stress owing to isomorphous substitution of PA6TI units by PA66 units in the PA6TI α phase, so that crystallinity is maintained in the resulting PA6TI-66 terpolymer. However, the ductility also significantly increases. This is attributed to a deformation-induced transformation of the PA6TI α phase to a disordered form of the PA66 α phase, where PA6TI units constitute defects and the chains adopt extended conformations. This phase is argued to be favored in initially amorphous PA66-rich regions and hence stabilizes interlamellar micro-necks that develop during yielding. By contrast, segregation of isophthaloyl units to the amorphous regions in PA6TI hinders strain-induced crystallization, leading to failure at significantly lower strains. Brittleness may therefore be a generic problem when comonomers are used to facilitate processing, but one that may be overcome through proper microstructural control.

Effect of a two-step annealing process on deformation-induced transformation mechanisms in cold-rolled medium manganese steel

In this work, the relationship between the mechanical properties and deformation-induced transformation mechanisms in transformation-induced plasticity (TRIP) medium manganese steel subjected to a two-step annealing process is comprehensively examined. Two-step annealed samples have bimodal microstructures, which are reversely transformed from partially recrystallized thermal martensite into lath- and globular-shaped grains of austenite (γ)–ferrite (α). This process widens the distribution of austenite stability, increasing the product of strength and elongation (PSE) from 35.5 GPa% to 46.9 GPa% at 600 °C annealing. When the annealing temperature is further increased to 640 °C, the difference of partitioning Mn content in austenite transforms the strain-induced TRIP effect into the stress-assisted TRIP effect. The yield strength and PSE are significantly reduced to 596 MPa and 30.5 GPa%, respectively. In addition, different degrees of stability in austenite occur in different martensite transformations (i.e., γ → ε martensite, γ → ε martensite → α′ martensite) at the same deformation stage, and the deformation mechanism of γ is generally based on the cooperation of the TRIP effect and twinning. For the coarse metastable austenite grains annealed at 640 °C, the α’ martensite transformation is assisted by the applied stress. An intergranular fracture occurs because of a high stress concentration mismatch between the deformed martensite and ferrite, leading to premature fracture.