Dislocation activities at the martensite phase transformation interface in metastable austenitic stainless steel: An in-situ TEM study

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 703; pp. 236 - 243
• Main Authors: Liu, Jiabin, Chen, Chenxu, Feng, Qiong, Fang, Xiaoyang, Wang, Hongtao, Liu, Feng, Lu, Jian, Raabe, Dierk
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 04.08.2017; Elsevier BV
• Subjects: Austenite; Austenitic stainless steel; Austenitic stainless steels; Damage accumulation; Damage prevention; Deformation; Deformation mechanisms; Dislocation density; Dislocations; Edge dislocations; Electron microscopy; Emission; Grain boundaries; Hardening rate; High strength steel; High strength steels; In-situ transmission electron microscopy; Martensite; Martensitic stainless steels; Martensitic transformation; Martensitic transformations; Necking; Notches; Phase transitions; Planes; Plastic deformation; Slip; Slip planes; Strain hardening; Transmission electron microscopy; TRIP steels; Martensitic transformation; Deformation; In-situ transmission electron microscopy; Dislocations
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2017.06.107

Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and α′ martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for α′ martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels.