Mechanistic origin of oxygen-induced twin suppression in titanium

Chong, Y.*; Tsuru, Tomohito; Gholizadeh, R.*; Minor, A. M.*; Tsuji, Nobuhiro*

Acta Materialia, 301, p.121523_1 - 121523_12, 2025/12

https://doi.org/10.1016/j.actamat.2025.121523

Twinning is essential for achieving large ductility in hexagonal close-packed (HCP) titanium alloys that inherently lack independent slip systems due to an asymmetrical HCP crystal structure. Unfortunately, twinning is in principle suppressed by a trace amount of interstitial oxygen, resulting in a substantially deteriorated ductility in titanium. However, the underlying mechanism remains in dispute so far. Here, we report a systematic multiscale study on the twinning/detwinning behaviors of Ti-O alloys that provides a clear mechanistic view of how interstitial oxygen inhibits twinning. We reveal for the first time that oxygen atoms segregate to both {102}2} compression twin boundaries using atom probe tomography. Combined with theoretical simulations that unravel a strong pinning effect of oxygen atoms on twin boundary due to an oxygen shuffling mechanism, we explain the distinctive migration abilities of twin boundaries in Ti-O alloys at different temperatures. The insights from our experimental and computational work provide a rationale for the design of titanium alloys with increased tolerance to variations in interstitial impurity content, with significant implications for more widespread use of this high strength, light weight material.

Mechanistic origin of grain size and oxygen interstitial effects on strain-induced martensitic transformation in Ti-12Mo alloy

Chong, Y.*; Tsuru, Tomohito; Mitsuhara, Masatoshi*; Guo, B.*; Gholizadeh, R.*; Inoue, Koji*; Godfrey, A.*; Tsuji, Nobuhiro*

Communications Materials (Internet), 6, p.50_1 - 50_11, 2025/03

https://doi.org/10.1038/s43246-025-00777-4

Strain-induced martensitic phase transformation (SIMT) critically affects the mechanical properties of metastable titanium alloys. In this study, the effects of grain size and oxygen content on SIMT in a Ti-12wt.%Mo alloy were systematically investigated. It is found that SIMT is promoted by a decrease in grain size and in oxygen content. The mechanistic origins of the anomalous grain size dependency and the acute oxygen content dependency of SIMT are discussed based on multi-scale microstructural characterization and state-of-the-art simulations. Grain refinement does not raise the energy barrier for SIMT but rather provides more nucleation sites for strain-induced martensite, thereby promoting SIMT in fine-grained Ti-12wt.%Mo alloy. In contrast, for the Ti-12wt.%Mo-0.3 wt.%O alloy, oxygen atoms substantially increase the energy barrier for SIMT, due to a change in the local configuration of oxygen atoms during the phase transformation. In addition, atom probe tomography reveals for the first time that oxygen atoms segregate at phase boundaries, thereby further restricting the growth of martensite.

Effect of carbon segregation at prior austenite grain boundary on hydrogen-related crack propagation behavior in 3Mn-0.2C martensitic steels

Okada, Kazuho*; Shibata, Akinobu*; Kimura, Yuji*; Yamaguchi, Masatake; Ebihara, Kenichi; Tsuji, Nobuhiro*

Acta Materialia, 280, p.120288_1 - 120288_14, 2024/11

https://doi.org/10.1016/j.actamat.2024.120288

Direct observations of dynamic and reverse transformation of Ti-6Al-4V alloy and pure titanium

Guo, B.*; Chen, H.*; Chong, Y.*; Mao, W.; Harjo, S.; Gong, W.; Zhang, Z.*; Jonas, J. J.*; Tsuji, Nobuhiro*

Acta Materialia, 268, p.119780_1 - 119780_11, 2024/04

https://doi.org/10.1016/j.actamat.2024.119780

Quantitatively evaluating the huge Lders band deformation in an ultrafine grain stainless steel by combining neutron diffraction and digital image correlation analysis

Mao, W.; Gao, S.*; Gong, W.; Harjo, S.; Kawasaki, Takuro; Tsuji, Nobuhiro*

Scripta Materialia, 235, p.115642_1 - 115642_6, 2023/10

https://doi.org/10.1016/j.scriptamat.2023.115642

In the present study, a hybrid neutron diffraction and digital image correlation measurement was performed on tensile deformation of an ultrafine grain (UFG) stainless steel exhibiting a huge Lders band deformation to evaluate the individual contribution of the austenite matrix and the deformation-induced martensite to the strain hardening during the propagation of the band. Quantitative analysis revealed that the strain hardening of the austenite matrix was insufficient to maintain a uniform deformation when the flow stress was greatly enhanced by the UFG structure. The strain hardening required for the Lders band to propagate was mostly provided by the formation of martensite and the high internal stress within it.

Quantitatively evaluating respective contribution of austenite and deformation-induced martensite to flow stress, plastic strain, and strain hardening rate in tensile deformed TRIP steel

Mao, W.; Gao, S.*; Gong, W.; Bai, Y.*; Harjo, S.; Park, M.-H.*; Shibata, Akinobu*; Tsuji, Nobuhiro*

Acta Materialia, 256, p.119139_1 - 119139_16, 2023/09

https://doi.org/10.1016/j.actamat.2023.119139

Transformation-induced plasticity (TRIP)-assisted steels exhibit an excellent combination of strength and ductility due to enhanced strain hardening rate associated with deformation-induced martensitic transformation (DIMT). Quantitative evaluation on the role of DIMT in strain hardening behavior of TRIP-assisted steels and alloys can provide guidance for designing advanced materials with strength and ductility synergy, which is, however, difficult since the phase composition keeps changing and both stress and plastic strain are dynamically partitioned among constituent phases during deformation. In the present study, tensile deformation with neutron diffraction measurement was performed on an Fe-24Ni-0.3C (wt.%) TRIP-assisted austenitic steel. The analysis method based on stress partitioning and phase fractions measured by neutron diffraction was proposed, by which the tensile flow stress and the strain hardening rate of the specimen were resolved into factors associated with each phase, i.e., the austenite matrix, deformation-induced martensite, and the transformation rate of DIMT after differentiation, and then the role of each factor in the global strain hardening behavior was discussed. In addition, the plastic strain partitioning between austenite and martensite was indirectly estimated using the dislocation density measured by diffraction profile analysis, which constructed the full picture of stress and strain partitioning between austenite and martensite in the material. The results suggested that both the transformation rate and the phase stress borne by the deformation-induced martensite played important roles in the global tensile properties of the material. The proposed decomposition analysis method could be widely applied to investigating mechanical behavior of multi-phase alloys exhibiting the TRIP phenomenon.

Compressive deformation behavior of AZ31 alloy at 21K; An neutron diffraction study

Gong, W.; Kawasaki, Takuro; Zheng, R.*; Mayama, Tsuyoshi*; Sun, B.*; Aizawa, Kazuya; Harjo, S.; Tsuji, Nobuhiro*

Scripta Materialia, 225, p.115161_1 - 115161_5, 2023/03

https://doi.org/10.1016/j.scriptamat.2022.115161

Unexpected dynamic transformation from phase to phase in zirconium alloy revealed by in-situ neutron diffraction during high temperature deformation

Guo, B.*; Mao, W.; Chong, Y.*; Shibata, Akinobu*; Harjo, S.; Gong, W.; Chen, H.*; Jonas, J. J.*; Tsuji, Nobuhiro*

Acta Materialia, 242, p.118427_1 - 118427_11, 2023/01

https://doi.org/10.1016/j.actamat.2022.118427

Ultrahigh yield strength and large uniform elongation achieved in ultrafine-grained titanium containing nitrogen

Chong, Y.*; Tsuru, Tomohito; Guo, B.*; Gholizadeh, R.*; Inoue, Koji*; Tsuji, Nobuhiro*

Acta Materialia, 240, p.118356_1 - 118356_15, 2022/11

https://doi.org/10.1016/j.actamat.2022.118356

In this study, we systematically investigated the influences of nitrogen content and grain size on the tensile properties and deformation behaviors of titanium at room temperature. By high-pressure torsion and annealing, we obtained ultrafine-grained (UFG) Ti-0.3wt.%N alloy with a fully recrystallized microstructure, which combined an unprecedented synergy of ultrahigh yield strength (1.04 GPa) and large uniform elongation (10%). The hardening and strain-hardening mechanisms of Ti-0.3wt.%N alloy were comprehensively studied via deformation substructure observation and first-principles calculations. It is revealed that the contributions of nitrogen to the excellent strength/ductility balance in UFG Ti-0.3wt.%N were twofold. On one hand, nitrogen atoms inside the grains strongly impeded the motion of dislocations on prismatic plane due the shuffling of nitrogen from octahedral to hexahedral site, giving rise to a six-fold increase in the friction stress than pure Ti. Moreover, the greatly reduced stacking fault energy difference between prismatic and pyramidal planes in Ti-0.3wt.%N alloy facilitated an easier activation of dislocations, which contributed to an enhanced strain-hardening rate. On the other hand, some nitrogen atoms segregated near the grain boundaries, a phenomenon discovered in -titanium for the first time. These segregated nitrogen atoms served as an additional contributor to the yield strength of UFG Ti-0.3wt.%N, by raising the barrier against dislocation slip transfer between grains. Our experimental and theoretical calculation work provide insights for the design of affordable high strength titanium without a large sacrifice of ductility, shedding lights on a more widespread use of this high strength to weight material.

Grain orientation dependence of deformation microstructure evolution and mechanical properties in face-centered cubic high/medium entropy alloys

Yoshida, Shuhei*; Fu, R.*; Gong, W.; Ikeuchi, Takuto*; Bai, Y.*; Feng, Z.*; Wu, G.*; Shibata, Akinobu*; Hansen, N.*; Huang, X.*; et al.

IOP Conference Series; Materials Science and Engineering, 1249, p.012027_1 - 012027_6, 2022/08

https://doi.org/10.1088/1757-899X/1249/1/012027