Transformation-induced Plasticity Research Articles

The design of appropriate Si content overcomes the strength-ductility trade-off in dual-phase high entropy alloy

To reduce the stacking fault energy (SFE) of the alloys, silicon (Si) was introduced into Fe40Mn35Co20Cr5 dual-phase high entropy alloys (HEAs). We propose a novel mechanism supported by first-principles calculations that predicts Si atoms may preferentially substitute Cr atoms, resulting in enhanced lattice distortion and reduced SFE. Experimental results indicate that incorporating 0.2 wt % Si enhances the ductility of the alloy while maintaining its strength through a synergistic effect involving dislocations and phase transformation. Furthermore, in the Si0.2 alloy, the integrity of 9 R structures were preserved, thereby enhancing the transformation-induced plasticity (TRIP) effect and facilitating attainment of an optimal balance between strength and ductility. However, incorporation of 0.1 wt % and 0.3 wt % Si content leads to a decline in ultimate tensile strength due to improper Si content causing grain growth and premature phase transition. Therefore, meticulous selection of an appropriate Si content is crucial for simultaneously enhancing both strength and ductility.

Achieving high ductility in Fe45Mn35Co10Cr10 metastable high entropy alloy by constructing chemical boundary with stacking fault energy heterogeneity

Additively-manufactured metastable high entropy alloys are emerging as promising alternatives for energy absorption in protective equipment due to their exceptional ductility. In this study, an elongation of over 50 % was achieved in a directed energy deposition fabricated Fe45Mn35Co10Cr10 metastable high entropy alloys (MHEAs). The inherent high cooling rate and repeated heat treatment in the directed energy deposition process were harnessed to deliberately induce an uneven distribution of Mn elements, forming a cellular structure with chemical boundary. The unique heterogeneity stacking fault energy results in a synergistic interplay of multiple deformation mechanisms such as, dislocation slip, transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). In the initial stage of deformation, dislocation slip and TRIP contribute to the main plasticity. Stacking faults/martensite are confined within the cellular structure with low stacking fault energy (SFE). As strain increases, TWIP and TRIP are activated in the high SFE regions. Twins and martensite across the cell wall were observed. As strains exceed 40 %, primary martensite variant and twinning with a particular intersection angle take place to further enhance the deformation ability. The chemical boundary with stacking fault energy heterogeneity not only simultaneously activates both TRIP and TWIP, but also maintains the martensite and twins at the nanoscale.

Understanding plasticity in multiphase quenching & partitioning steels: Insights from crystal plasticity with stress state-dependent martensitic transformation

This study develops a novel crystal plasticity (CP) model incorporating deformation-induced martensitic transformation (DIMT) and transformation-induced plasticity (TRIP) effect to predict the complex interplay between microstructural evolution and mechanical behavior in a third-generation advanced high-strength steel QP980. This model introduces phenomenological theory of martensite crystallography (PTMC) based TRIP theory and DIMT kinetics grounded on nucleation-controlled phenomenon. Notably, the DIMT model is improved by utilizing a geometric approach for calculating shear band intersections. A virtual multiphase representative volume element (RVE) based on the Voronoi tessellation is generated for the QP980 steel, which comprises ferrite, martensite, and retained austenite (RA). The study highlights how phase transformation affects mechanical properties, notably the strengthening from transformed martensite and the mechanical alterations in RA due to the TRIP effect. The DIMT kinetics dependent on stress states such as uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST), and equi-biaxial tension (EBT) are analyzed using the developed model. The role of microstructural surroundings on martensitic transformation is also examined. Furthermore, analysis under biaxial loading angles using the model reveals an asymmetric yield surface, with more pronounced changes in yield stress in the tensile region due to accelerated transformation behaviors, as opposed to the more gradual transformations in the compressive region. This study provides valuable insights into the deformation mechanisms of the third-generation advanced high-strength steels including relationship between plastic anisotropy, transformation kinetics, and microstructural evolution.

Interpass temperature strategies for compressive residual stresses in cladding low-transformation-temperature material 16Cr8Ni via wire arc additive manufacturing

Low-transformation-temperature (LTT) materials induce compressive residual stresses within an effective depth in wire arc additive manufacturing (WAAM), which is well suited for cladding applications. However, this role was found to be sensitive to the phase transformation sequence. We developed an LTT material—16Cr8Ni (16Cr8Ni-LTT)—and to explore its potential, it was clad onto a KA36 substrate via WAAM. Three different interpass temperature strategies were designed as follows: (1) no interpass temperature control; (2) interpass temperature controlled at approximately 270 °C above the martensite transformation start temperature (Ms = 200 °C) for simultaneous phase transformation; and (3) interpass temperature controlled at approximately 50 °C below the martensite transformation finish temperature (Mf = 60 °C) for sequential phase transformation. A thermal elastic-plastic numerical model considering the phase transformation-induced plasticity (TRIP) was developed using in-house software, JWRAIN-Hybrid, to reproduce the temperature field and residual stresses of 16Cr8Ni-LTT cladding. X-ray diffraction (XRD) and contour methods were employed for residual stress measurements. The high accuracy of the model was validated in terms of the temperature, deformation, and residual stress. The reproduced maximum historical temperature distributions were combined with hardness tests to identify the depths and temperatures of the heat-affected zone (HAZ). The measured maximum compressive residual stresses induced by the three interpass temperature strategies for cladding 16Cr8Ni-LTT were −503, −420, and −720 MPa. Moreover, irrespective of the adopted interpass temperature strategy, both longitudinal and transverse residual stresses were compressive in the 16Cr8Ni-LTT cladding. This study provides scientific insights into LLT-induced compressive residual stresses and highlights the superiority and flexibility of cladding LTT materials via WAAM for improving the resistance of large metal components to fatigue and corrosion.

Mechanical evaluation of DIW-printed carbon nanofibers - alumina-toughened zirconia composites

Direct ink writing (DIW) offers cost-effective fabrication of intricate ceramic structures. However, the mechanical properties (reliability) of DIW-obtained components have been inadequate for structural applications. This study aimed to improve mechanical properties of DIW alumina-toughened zirconia (ATZ) composite filaments with and without carbon nanofibers (CNFs). Optimized debinding and sintering conditions were determined for Conventional Sintering (CS) and Spark Plasma Sintering (SPS). The influence of nozzle diameter, CNF content and sintering method on the fracture strain, Young’s modulus, flexural strength and Weibull modulus was carefully assessed via correlation analyses. CNFs, although intended to enhance properties, were often agglomerated, which negatively affected mechanical properties. Smaller nozzle diameters enhanced Young’s modulus, with CS-sintered ATZ filaments showing transformation-induced plasticity (TRIP). Mechanical properties rivaled or surpassed those of conventional techniques (e.g., cold isostatic pressing, slip casting) with a good level of mechanical reliability achieved. Overall, DIW showed promise for ceramic materials, particularly ductile Ce-TZP-based composites.

In-situ alloying of maraging steel with enhanced mechanical properties and corrosion resistance by laser directed energy deposition

Maraging steels are known for their exceptional strength derived from the martensite matrix and nano-precipitate strengthening. However, the trade-off to the high strength is often a loss of ductility and minimal strain hardening. Introducing metastable austenite to the matrix to activate transformation-induced plasticity (TRIP) is an effective approach to achieve high strength and ductility synergy. Existing methods to introduce TRIP into additively manufactured maraging steels are limited by the compositions of pre-alloyed powder or additional heat treatment steps. In-situ alloying via the laser directed energy deposition (L-DED) process allows flexibility in tailoring the alloying composition to achieve an austenite–martensite microstructure. Herein, we develop a TRIP–maraging steel by in-situ alloying of M789 with 316L (4–8 wt%) during the L-DED process. The addition of 316L facilitates austenite reversion in solution-treated maraging steel. As a result, the TRIP–maraging steel exhibits a 76 % increase in uniform elongation at the expense of a minimal sacrifice of strength. Furthermore, the corrosion resistance of the TRIP–maraging steel is enhanced. This work showcases a pathway to developing superior alloys by in-situ alloying via additive manufacturing.

Transformation pattern of shape-memory NiTi alloy during stress-biased thermal cycling

Despite the superelastic deformation of NiTi has been documented and analyzed elaborately, its shape-memory behavior during stress-biased thermal cycling has not been thoroughly unveiled. This paper examines the evolution of transformation pattern in NiTi upon thermal cycling over a wide range of biasing stress. For the present shape-memory NiTi, both forward and reverse transformations proceed via the growth of localized deformation band (LDB) under low biasing stress (σbias ≤ 150 MPa), while LDB only appears during forward transformation and reverse transformation is uniform under high biasing stress (σbias ≥ 200 MPa). This is the first time that delocalization (conversion of deformation mode from localization to homogeneity) is observed during stress-biased thermal cycling. We clarify that the intrinsic undercooling/overheating of martensitic transformation results in unstable thermomechanical response, and it is the origin of localization in shape-memory NiTi. It is found that transformation-induced plasticity (TRIP), which is dominated by dislocation slip and deformation twining, not only causes irreversibility in the present shape-memory NiTi but also leads to delocalization through stabilization of intrinsic thermomechanical response of reverse transformation.

Remarkable contribution of stress-induced martensitic transformation to strain-hardening behavior in Ti−Mo-based metastable β-titanium alloys

The role of stress-induced β → α″ martensitic transformation in contributing to the exceptional strain-hardening behavior of a Ti-11Mo (wt. %) model alloy was studied. The results reveal that the α″-martensite plates, undergoing relatively high transformation strains upon formation, act as effective barriers against dislocation motion. As deformation proceeds, the progressive generation of these plates dynamically reduces the dislocation mean free path (dynamic Hall-Petch effect), while simultaneously encouraging the accumulation of dislocations. These dual effects substantially enhance the flow stress with increasing strain, thereby resulting in remarkable strain-hardenability, a crucial factor for maintaining excellent ductility. This study presents innovative experimental evidence elucidating the mechanism of the transformation-induced plasticity effect in metastable β-titanium alloys, while also offering insights for designing novel alloys with superior strain-hardenability and ductility.

Research on deformation behaviors at different temperatures of lean duplex stainless steel S32101 produced by direct cold rolling process

In this paper, the mechanical properties, work hardening characteristics and deformation mechanism of lean duplex stainless steel (LDSS) S32101 produced by direct cold rolling were studied under different temperature conditions. The yield strength (YS) and tensile strength (TS) of S32101 decreased with the increase of deformation temperature, while the elongation increased first and then decreased. The TS of S32101 at −70 °C was 1052.6 MPa, the elongation remained 57.2%, and the product of strength and elongation (PSE) was as high as 60.2 GPa·%, which was mainly due to the synergistic strengthening of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects in austenite at cryogenic temperatures. The efficiency of grain refinement at cryogenic temperature was improved by TRIP effect and deformation twins (DTs), indicating that it had the potential to be applied at low temperature (−70 ∼ −30°C). Moreover, the high density nanotwins and the strong interaction between the dislocation and nanotwin bundles also contributed to strain hardening at low temperatures and thus better comprehensive mechanical properties could be achieved.

Manipulating TWIP/TRIP via oxygen-doping to synergistically enhance strength and ductility of metastable beta titanium alloys

Metastable β-Ti alloys exhibiting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) generally have excellent ductility, but typically at the expense of relatively low yield strengths which has restricted their widespread use. Our work shows that interstitial oxygen can be employed to regulate β phase stability to significantly enhance both strength and ductility of TWIP/TRIP alloys. For a Ti-32Nb wt.% base alloy, inclusion of 0.3 wt.% O enhanced ductility by more than 140 %, reaching up to 54 % strain, and improved the tensile yield strength by over 95 % to 632 MPa. Compared to other common engineering alloys such as Ti-45Nb, elongation was increased by 29 %, and the yield strength increased by 182 MPa, respectively. Here, we elucidate on impacts of oxygen doping on TWIP/TRIP behaviors in the Ti-32Nb alloy. We reveal that oxygen regulates the critical stress for martensitic transformation, twinning, and dislocation slip. At lower oxygen doping concentrations (≤0.3 wt.% O), multi-stage martensitic transformation and martensitic twinning resulted in high ductility. In higher oxygen content alloys (≥0.5 wt.% O), deformation occurred initially via twinning, while strain induced martensite was subsequently induced in retained β phase regions. Oxygen concentrations control the deformation mechanisms, providing a flexible means to synergistically balance an alloy's strength and ductility. The use of oxygen to enhance stability of the β phase and regulate deformation behaviors is a promising new approach for creating high-performance TWIP/TRIP metastable β-Ti alloys with outstanding mechanical properties.

Probing phase transformation and dislocation evolution in high-entropy alloy under cyclic loadings

Stress-induced FCC-BCC phase transformation plays a crucial role in the mechanical behaviors of high-entropy alloys (HEAs). While there has been extensive research on this transformation during monotonic deformation, studies on fatigue behavior are extremely limited. Here, we use molecular dynamics simulations to investigate phase transformation and dislocation evolution in HEAs under strain-controlled symmetric tension–compression cycles. Our results show that cyclic deformation behavior is sensitive to strain amplitude, revealing three distinct cyclic responses. Notably, a progressive FCC-BCC phase transformation process occurs at a high strain amplitude of 4.8%. Grain boundaries and their triple junctions are identified as preferred sites for phase transformation under cyclic loading conditions. These findings provide valuable atomic-scale insights for understanding fatigue deformation in HEAs with transformation-induced plasticity.

Enhancing ductility of the TRIP aided bainitic ferrite steel by Mn heterogeneity introduced via reversion: Towards the 3rd generation

A novel approach to enhance the ductility of the transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel was proposed in this work. Lamellar lath Mn-heterogeneity was introduced into the single austenite grain through austenite-reversion from martensite by slow-heating or intercritical annealing. Acicular austenite is formed accompanied with apparent Mn enrichment occurring during reversion, and such a lamellar lath Mn-inhomogeneity is remained even after full-austenitization. Highly stable retained austenite (RA) is formed at the original Mn-enriched region after austempering with higher Mn and carbon contents than those of traditional austempering from Mn-homogenized austenite. In addition, higher total fraction and larger ratio of film-like RA is obtained in the heterogeneous-austenite than homogeneous one. These contribute to the ductility improvement of the TBF steel.

Change in deformation mechanism to change TRIP to TWIP strengthening in the class of 304 austenitic stainless steel sheets by trapezoidal wavy-rolling process

ABSTRACT Strengthening of steels through twinning-induced plasticity (TWIP) has several advantages like high-strength and high-ductility combination over the transformation-induced plasticity (TRIP) mechanism which gives high strength, low ductility and product phase of martensite in the single phase solid solution of austenitic matrix. But, development of TWIP effect will be more difficult unless the steel contains high stacking fault energy (SFE) (>20mJ/M2), fine grain structure and high manganese alloying (15–30 wt.%). These are very expensive processes and susceptible to metallurgical complexity in the austenitic stainless steels. Therefore, new processing technology was developed for strengthening of the austenitic stainless steel sheets by trapezoidal wavy rolling (TWR). Type 304 austenitic stainless steel was selected for this study which is known as martensite transformation, a dominant hardening mechanism due to low SFE and coarse grained structure. Wavy deformation-induced results have been studied through strain hardening behaviour by flow stress-strain curves and microstructural changes by atomic force microscope (AFM), and phase transformations by using X-ray diffractometer (XRD) analysis. It is found that this process introduced changes from TRIP to TWIP mechanism by forming plenty of mechanical twins and shear bands type structure at initial stage of deformations and partial martensite is observed in advanced stage.

Enhanced yield stress and reduced elastic modulus of metastable β Ti–Nb alloys deformed by equal channel angular pressing attributed to the Synergistic effect of deformation mechanisms

This work aims to obtain Ti–Nb alloys with a high yield stress, good plasticity, and low elastic modulus. To this end, metastable β Ti–Nb alloys were subjected to multipass equal channel angular pressing (ECAP) deformation at room temperature, and a thorough investigation of their microstructure evolution, deformation mechanisms, and strengthening/toughening mechanisms was conducted through X-ray diffraction, transmission electron microscopy, electron backscatter diffraction, and tensile tests. In the initial deformation stages, multiple deformation mechanisms are observed, including dislocation slip, {332} <113> twinning, stress-induced martensite (SIM) α″-phase, {112} <111> twinning, and stress-induced ω-phase. However, as the strain accumulates, the ω-phase disappears, the volume fraction of the SIM α″-phase gradually decreases, and the dominant deformation mechanism changes to dislocation slip and {332} <113> twinning. The ECAP deformation results in a complex-network microstructure and the precipitation of the nanoscale SIM α″-phase, which leads to the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects. As a consequence, the Ti–Nb alloy simultaneously exhibits a high yield stress and a good plasticity. Furthermore, the high-density dislocations and interfaces generated by the ECAP deformation reduce the stability of the β-phase, and the introduced nanopores cause a decrease in density, resulting in a decrease in the elastic modulus of the alloy.

Heterostructure mediated high strength and large ductility in novel medium-Mn steels with low Mn content

Medium-Mn steels (MMnS) exhibit superior strength and ductility via the transformation-induced plasticity (TRIP) of retained austenite. However, to realize the partitioning of Mn atoms into retained austenite for thermal stability optimization, long-term intercritical annealing is required. In addition, the high Mn content of MMnS also causes other issues, e.g., high cost, welding, etc. In this work, instead of utilizing the TRIP effect of retained austenite, heterostructure consisting of alternatively distributed lamellar martensite zones and lamellar ultrafine dual-phase (martensite + ferrite) zones was engineered in a MMnS with low Mn content by a conventional rolling and short-term intercritical annealing process. The formation of the heterostructure was attributed to the heterogeneous distribution of Mn, the formation of highly dispersed martensite-austenite (MA) islands in Mn-poor zones and the low diffusion rate of Mn atoms at the intercritical annealing temperature. Tensile test results show that the steel exhibits superb synergy of a high ultimate tensile strength of 1452 MPa, a large ductility of 17 % and a high work hardening capability. The high work hardening capability and large ductility were mainly attributed to the apparent microhardness difference between the lamellar martensite zones and the ultrafine dual-phase zones that caused a strong hetero-deformation induced (HDI) strengthening effect. This study proposes a novel approach to developing high-performance steels, which provides an effective paradigm for addressing the long-established conflicts between mechanical performance, high Mn content and long intercritical annealing period of MMnS.

High-performance room temperature quenching and partitioning (RT Q&P) steel fabricated through laser powder bed fusion

The present investigation focuses on analyzing the microstructure and tensile properties of room temperature quenching and partitioning (RT Q&P) steel manufactured using laser powder bed fusion (LPBF). The as-built specimen reveals a dual-phase microstructure composed of retained austenite (RA) grains within the martensite matrix, with the absence of the conventional Mn banded microstructure. However, the as-built sample demonstrates poor tensile properties, exhibiting only a 2.5% total elongation, largely due to significant residual stress. To mitigate this issue, the as-built sample undergoes tempering at low temperatures (300, 350, and 400 °C) for 10 min. This treatment results in a reduction of the average compressive residual stresses from −214.4 to −132.1 MPa, and a transformation of the residual stresses in RA from a compressive state of −52.4 MPa to a tensile state of 136.4 MPa after tempering at 350 °C. Furthermore, the volume fraction of RA increases by approximately 10%, and there is a slight increase in the C content after tempering. The tempered RT Q&P steel demonstrates an ultra-high tensile strength ranging from 1.3 to 1.5 GPa, coupled with a substantial total elongation of 10–15%. This exceptional combination of strength and ductility is attributed to the transformation-induced plasticity (TRIP) effect and the reduction of residual stresses. Notably, this study marks the first confirmation that strong and ductile RT Q&P steel can be produced through the LPBF process.

Understanding the effect of refractory metal chemistry on the stacking fault energy and mechanical property of Cantor-based multi-principal element alloys

Multi-principal-element alloys (MPEAs) based on 3d-transition metals show remarkable mechanical properties. The stacking fault energy (SFE) in face-centered cubic (fcc) alloys is a critical property that controls underlying deformation mechanisms and mechanical response. Here, we present an exhaustive density-functional theory study on refractory- and copper-reinforced Cantor-based systems to ascertain the effects of refractory metal chemistry on SFE. We find that even a small percent change in refractory metal composition significantly changes SFEs, which correlates favorably with features like electronegativity variance, size effect, and heat of fusion. For fcc MPEAs, we also detail the changes in mechanical properties, such as bulk, Young's, and shear moduli, as well as yield strength. A Labusch-type solute-solution-strengthening model was used to evaluate the temperature-dependent yield strength, which, combined with SFE, provides a design guide for high-performance alloys. We also analyzed the electronic structures of two down-selected alloys to reveal the underlying origin of optimal SFE and strength range in refractory-reinforced fcc MPEAs. These new insights on tuning SFEs and modifying composition-structure-property correlation in refractory- and copper-reinforced MPEAs by chemical disorder, provide a chemical route to tune twinning- and transformation-induced plasticity behavior.

Overcoming strength-ductility trade-off in Si-containing transformation-induced plasticity high-entropy alloys via metastability engineering

Benefiting from the transformation-induced plasticity effect has been recognized as a new approach to develop high-performance Si-containing high-entropy alloys with outstanding room-temperature strength-ductility balance. However, closely controlling the kinetics of deformation-induced α΄-martensite formation is essential to further boost ductility in addition to ultrahigh strength. In the present work, this aim was fulfilled by manipulating the chemical composition of the promising FeCoCrVMnSi system with the replacement of Mn with Ni and adjusting the Si content to fine-tune the stacking fault energy and the driving force of austenite to martensite transformation. The experimental results were obtained by EBSD, TEM, XRD, and tensile testing, and the results were supported by thermodynamic calculations, work-hardening analysis, and assessment of α΄-martensite formation kinetics. Accordingly, the single-phase Fe47Co30Cr10Ni5V8-xSix system was designed. In this system, the Fe47Co30Cr10Ni5V2Si6 alloy exhibited exceptional tensile strength (∼1.1 GPa) and total elongation (72 %), outperforming the competitive dual-phase Fe46Co30Cr10Mn5V2Si7 alloy. High strength was attributed to the high solid-solution strengthening effect as well as the volume fraction of α΄-martensite, while its ductility benefited from fine-tuned kinetics of martensitic phase transformation.

Designing the heterogeneous microstructure via a direct warm rolling process to synergistically enhance the strength-ductility in a medium Mn steel

This work introduced a facile method for fabricating heterostructures in a medium Mn steel via a direct warm rolling process. The heavy warm rolling promoted the transformation of metastable austenite (∼12.2 vol%) into hard domain α’ martensite, which formed a heterogeneous microstructure with the untransformed γ (soft domain). After 600 °C re-annealing, the hard domain α’ re-reversed to α + γ, and the heterostructure disappeared. Compared to the homogeneous structure, the heterostructure exhibited the synergistic enhancement of strength-ductility (yield strength (YS): 782 → 1113 MPa; ultimate tensile strength (UTS): 1099 → 1304 MPa; total elongation (TEL): 20.8 % → 21.7 %). Quantitative analysis showed that the heterogeneous deformation induced (HDI) strengthening (∼261 MPa) was the primary contributor to the increased ΔYS (782 → 1113 MPa) value, rather than the traditional strengthening mechanisms. Transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effect were the main deformation mechanisms of both homogeneous/heterogeneous structures. However, compared to the homogeneous microstructure, the heterogeneous structure possessed the more pronounced TRIP and HDI hardening effects as well as L-C locks, the cooperative effect of these three factors further improved the strain hardening ability to break through the strength-ductility trade-off dilemma.

Cyclic quenching treatment doubles the Charpy V-notch impact energy of a 2.3 GPa maraging steel

A cyclic quenching treatment (CQT) succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments. The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure, rather than the refinement of prior austenite grain (PAG), as well as the Transformation-Induced Plasticity (TRIP) effect facilitated by small austenite grains. The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains, Ni-rich martensite laths and Ni-poor martensite laths, was investigated and verified by DICTRA simulations. This study highlights the important influence of Ni partitioning behavior during CQT, providing insights into microstructural evolution and mechanical properties.