Method for measuring transformation energy and quantitative characterization of transformation-induced plasticity
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NTRODUCTION
THE strain-induced martensite (SIM) transformation is a very well-known phenomenon, which takes place during deformation of metastable austenitic stainless steel at temperatures below Md . The kinetics of M transformation was described by Olson and Cohen.[1] Microstructure study was performed by Murr et al.[2] However the two basic scientific problems have not yet been clarified for SIM transformation: (1) determination of M transformation energy; and (2) characterization of transformation-induced plasticity (TRIP). The first one is mainly related to explaining if the M transformation is difficult or easy; the latter is related to describing the characterization of plasticity increment and alloy development. Up to now, except estimating by homogeneous nucleation theory and so on,[3,4,5] the measurement of Mtransformation energy, whether quenching or SIM for different materials, has not yet been reported. The theory concerned has no method to test and verify either. The TRIP is still described qualitatively[6–9] and cannot be characterized quantitatively. In order to design high-performance materials, three problems must be solved, which include (1) the measuring method of M-transformation energy; (2) the plasticity increment induced by transformation of martensite per unit volume fraction; and (3) the selection of alloy elements, which make it easy to produce M transformation and to induce a larger plasticity. According to the preceding considerations, an attempt has been made in this article to measure the transformation energy and to quantitatively characterize
W.F. ZHANG, formerly Postdoctoral Student, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, is Postdoctoral Fellow, Beijing Institute of Aeronautical Materials, Beijing 100095, People’s Republic of China. Contact e-mail: [email protected] or wf [email protected] Y.M. CHEN, Associate Professor, and J.H. ZHU, Professor, are with the State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. Manuscript submitted July 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the TRIP based on the characteristics of the tensile curve of metastable austenitic stainless steels. II. EXPERIMENTAL PROCEDURE The tested materials were three austenitic stainless steels prepared by vacuum melting. After forging and rolling to produce round bars with a diameter of 12 mm, the materials were machined into tensile specimens with a gage length of 25 mm and a diameter of 5 mm. The chemical compositions of experimental steels and corresponding heat treatment are listed in Table I. The specimens were solution treated at temperature (Table I) for 10 minutes and then water quenched to room temperature to get a single-phase austenite. The tensile tests were carried out on an Instron 1342 testing machine and the specimens were soaked in liquid nitrogen at ⫺196 ⬚C. The relative contents of austenite and martensite in specimens with different tensile deformat
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