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1. Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

   https://doi.org/10.1007/s11661-020-05666-8  

   Finfrock, Christopher B.; Clarke, Amy J.; Thomas, Grant A.; Clarke, Kester D. (February 2020, Metallurgical and Materials Transactions A)

   Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades. 

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2. Martensitic transformation induced strength-ductility synergy in additively manufactured maraging 250 steel by thermal history engineering

   https://doi.org/10.1016/j.jmst.2024.05.062  

   Mooraj, Shahryar; Feng, Shuai; Luebbe, Matthew; Register, Matthew; Liu, Jian; Li, Tianyi; Yavas, Baris; Schmidt, David P; Priddy, Matthew W; Nicholas, Michael B; et al (March 2025, Journal of Materials Science & Technology)

   Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility. Here, we report an intermittent printing approach to tailor the microstructure and mechanical properties of maraging 250 steel via engineering of the thermal history during plasma arc additive manufacturing (PAAM). Through introducing a dwell time between adjacent layers, the maraging 250 steel is cooled below the martensite start temperature, triggering a thermally driven, in-situ martensitic transformation during the printing process. Re-heating or thermal cycling during subsequent layer deposition impedes complete martensitic transformation, enabling coexistence of martensite and retained austenite phases with elemental segregation. The enrichment of Ni in the austenite phase promotes stabilization of the retained austenite upon cooling down to room temperature. The retained austenite is yet metastable during deformation, leading to stress-induced martensitic transformation under loading. Specifically, a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% retained austenite, resulting in improved work hardening via martensitic transformation induced plasticity (TRIP) during deformation. Meanwhile, the higher cooling rate induced by the dwell time results in substantially refined grain structures with an increased dislocation density, leading to a simultaneously improved yield strength. Notably, the yield strength increases from 836 MPa (0 min dwell) to 990 MPa (3 min dwell), and the uniform elongation increases from 3.2% (0 min dwell) to 6.5% (3 min dwell). This intermittent deposition strategy demonstrates the potential to tune the microstructure and mechanical properties of maraging steels through engineering the thermal history during additive manufacturing. 

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3. Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

   https://doi.org/10.1016/j.msea.2023.145754  

   Weiss, Jacob; Savage, Daniel J.; Vogel, Sven C.; McWilliams, Brandon A.; Mishra, Rajiv S.; Knezevic, Marko (November 2023, Materials Science and Engineering: A)

   In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage. 

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4. Assessing strength of phases in a quadruplex high entropy alloy via high-throughput nanoindentation to clarify origins of strain hardening

   https://doi.org/10.1016/j.matchar.2023.113594  

   Weiss, Jacob; Vasilev, Evgenii; Knezevic, Marko (January 2024, Materials Characterization)

   This paper describes the main findings from an experimental investigation into local and overall strength and fracture behavior of a microstructurally flexible, quadruplex, high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%). The alloy consists of metastable face-centered cubic austenite (g), stable hexagonal epsilon martensite (ε), stable body-centered cubic ferrite (a), and stable tetragonal sigma (σ) phases. The overall behavior of the alloy in compression features a great deal of plasticity and strain hardening before fracture. While the contents of diffusion created a and σ phases remain constant during deformation, the fraction of ε increases at the expanse of g due to the diffusionless strain induced γ→ε phase transformation. High-throughput nanoindentation mapping is used to assess the mechanical hardness of individual phases contributing to the plasticity and hardening of the alloy. Increasing the fraction of the dislocated ε phase during deformation due to the transformation is found to act as a secondary source of hardening because g and ε exhibit similar hardness at a given strain level. While these two phases exhibit moderate hardening during plasticity, significant softening is observed in σ owing to the phase fragmentation. While the phase transformation mechanism facilitates accommodation of the plasticity, the primary source of strain hardening in the alloy is the refinement of the structure during the transformation inducing a dynamic Hall-Petch-type barrier effect. Results pertaining to the evolution of microstructure and local behavior of the alloy under compression are presented and discussed clarifying the origins of strain hardening. While good under compression, the alloy poorly behaves under tension. Fracture surfaces after tension feature brittle micromechanisms of fracture. Such behavior is attributed to the presence of the brittle σ phase. 

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5. Resolving puzzles of the phase-transformation-based mechanism of the deep-focus earthquake

   Levitas, Valery I. (October 2021, ArXivorg)

   Deep-focus earthquakes that occur at 350–660 km, where pressures p =12-23 GPa and temperature T =1800-2000 K, are generally assumed to be caused by olivine→spinel phase transformation, see pioneering works [1–10]. However, there are many existing puzzles: (a) What are the mechanisms for jump from geological 10−17−10−15 s−1 to seismic 10−103s−1(see [3]) strain rates? Is it possible without phase transformation? (b) How does metastable olivine, which does not completely transform to spinel at high temperature and deeply in the region of stability of spinel for over the million years, suddenly transforms during seconds and generates seismic strain rates 10−103s−1 that produce strong seismic waves? (c) How to connect deviatorically dominated seismic signals with volume-change dominated transformation strain during phase transformations [9,11]? Here we introduce a combination of several novel concepts that allow us to resolve the above puzzles quantitatively. We treat the transformation in olivine like plastic strain-induced (instead of pressure/stress-induced) and find an analytical 3D solution for coupled deformation-transformation-heating processes in a shear band. This solution predicts conditions for severe (singular) transformation-induced plasticity (TRIP) and self-blown-up deformation-transformation-heating process due to positive thermomechanochemical feedback between TRIP and strain-induced transformation. In nature, this process leads to temperature in a band exceeding the unstable stationary temperature, above which the self-blown-up shear-heating process in the shear band occurs after finishing the phase transformation. Without phase transformation and TRIP, significant temperature and strain rate increase is impossible. Due to the much smaller band thickness in the laboratory, heating within the band does not occur, and plastic flow after the transformation is very limited. Our findings change the main concepts in studying the initiation of the deep-focus earthquakes and phase transformations during plastic flow in geophysics in general. The latter may change the interpretation of different geological phenomena, e.g., the possibility of the appearance of microdiamond directly in the cold Earth crust within shear-bands [12] during tectonic activities without subduction to the mantle and uplifting. Developed theory of the self-blown-up transformation-TRIP-heating process is applicable outside geophysics for various processes in materials under pressure and shear, e.g., for new routes of material synthesis [12,13], friction and wear, surface treatment, penetration of the projectiles and meteorites, and severe plastic deformation and mechanochemical technologies. 

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