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

引張変形したTRIP鋼において、応力、塑性ひずみおよび加工硬化率に対するオーステナイトおよび変形誘起マルテンサイトの寄与を定量的に評価

Mao, W.; Gao, S.*; Gong, W.   ; Bai, Y.*; Harjo, S.   ; Park, M.-H.*; 柴田 曉伸*; 辻 伸泰*

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

変形誘起塑性(TRIP)鋼は、変形誘起マルテンサイト変態(DIMT)に伴う加工硬化率の向上により、強度と延性の優れた組み合わせを示す。TRIP鋼や合金の加工硬化挙動におけるDIMTの役割を定量的に評価することは、強度と延性の両立を可能にする先進材料を設計するための指針を与えるが、変形中に相組成が変化し続け、応力と塑性ひずみの両方が構成相間で動的に分配されるため、その評価は困難である。本研究では、Fe-24Ni-0.3C(wt.%)TRIPオーステナイト鋼の引張変形とその場中性子回折測定を行った。中性子回折測定による応力分割と相分割に基づく解析手法を提案し、試験片の引張流動応力と加工硬化率を、オーステナイト母相,変形誘起マルテンサイト、DIMT変態速度に関連する因子に分解し、試料の加工硬化挙動における各因子の役割を考察した。さらに、回折プロファイル解析により測定した転位密度を用いてオーステナイトとマルテンサイト間の塑性ひずみ分配を間接的に推定し、材料中のオーステナイトとマルテンサイト間の応力・ひずみ分配の全体像を構築した。その結果、変形誘起マルテンサイト変態速度とマルテンサイトが負担する相応力の両方が、材料の全体的な引張特性に重要な役割を果たしていることが示唆された。提案した分解解析法は、TRIP現象を示す多相合金の機械的挙動を調べるために広く適用できる可能性がある。

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.