The Effects and Mechanisms of Pre-Deformation with Low Strain on Temperature-Induced Martensitic Transformation
WANG Jinliang, WANG Chenchong, HUANG Minghao, HU Jun, XU Wei()
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
WANG Jinliang, WANG Chenchong, HUANG Minghao, HU Jun, XU Wei. The Effects and Mechanisms of Pre-Deformation with Low Strain on Temperature-Induced Martensitic Transformation. Acta Metall Sin, 2021, 57(5): 575-585.
Pre-deformation with low strain can effectively control the thermal stability of metastable austenite. Till now, research has mainly focused on the effect of pre-deformation on martensitic transformation at one or more temperatures. However, research is still lacking on the effect of pre-deformation on the temperature at which martensite is formed (Ms), the final martensite content, and the transformational kinetics during continuous cooling. Furthermore, the mechanism underlying how pre-deformation affects martensitic transformation has not been reported. In this work, the influence rule and the corresponding effect of pre-deformation with low strain on martensitic transformation induced by temperature under continuous cooling from 300 K to 4 K was studied with 321 stainless steel samples by using the quasi-in-situ observation technique. The results show that Ms and the final amount of martensite increased under pre-deformation with low strain, and the martensitic transformation during continuous cooling was simultaneously accelerated. The quasi-in-situ observation demonstrated that the slip bands introduced by pre-deformation effectively provided nucleation sites for ε-martensite transformation. Accordingly, the formed ε-martensite increased the number of α'-martensite nucleation sites during continuous cooling, and finally promoted α'-martensite transformation. This builds on the theory proposed by other researchers that the dislocation defects introduced by pre-deformation directly provide the nucleation sites for α'-martensite transformation, and thus, promote martensitic transformation. In addition, by analyzing the nucleation behavior and nucleation priority at slip band defects, it is shown that the nucleation behavior of slip bands introduced by the pre-deformation was similar to that of faulted austenite induced by temperature. However, it is worth noting that the slip bands introduced by pre-deformation had a relatively higher nucleation priority. The crystallography of α'-martensite in the pre-deformed samples was analyzed, and it was found that the slip bands effectively changed the variant selection of α'-martensite so that the texture of α'-martensite was modified. This study advances the existing theory of martensitic transformation and provides theoretical guidance for the proactive control of temperature-induced martensitic transformation.
Fig.1 ECCI-SEM images of the undeformed (a) and 5% pre-deformed (b) 321 stainless steel samples
Fig.2 Morphology and crystal structure of defects produced by pre-deformation
Fig.3 Volume fraction (a) and transformation rate (b) of α'-martensite versus temperature in pre-deformation and undeformed 321 stainless steel samples during cryogenic treatment (Ms1 and Ms2 are the martensite transformation start temperature(Ms) of undeformed and pre-deformation samples, respectively. Stages I, II, and III represent the transformation start, fast transformation, and transformation saturation, respectively)
Fig.4 Microstructure evolutions of annealed 321 stainless steel sample subjected to pre-deformation and then cryogenic treatment
Fig.5 Crystallographic characteristics of ε-martensite of pre-deformation 321 stainless steel sample
Fig.6 Crystallographic characteristics of α'-martensite of pre-deformation 321 stainless steel sample (white areas represent ε-martensite)
Fig.7 Pole figure comparisons between experiment (Red points, corresponding to the data obtained from square areas in Fig.6) and calculation (other symbols) for γ→ε→α' transformation (N.D. and R.D. refer to normal direction and rolling direction of sample, respectively)
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