Fatigue Resistance of Laminated and Non-laminated TRIP-maraging Steels: Crack Roughness vs Tensile Strength
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We recently found that high-strength metastable laminated steels, i.e., transformation-induced plasticity (TRIP) maraging steels show extraordinary fatigue resistance.[1] In particular, fine laminates comprising metastable austenite and precipitation-hardened martensite simultaneously induce martensitic transformation, tortuous crack growth, and high yield/tensile strength. The martensitic transformation from austenite to a¢-martensite causes crystalline volume expansion, which is known to assist crack closure.[2,3] Tortuous crack growth also assists crack closure by inducing crack roughness.[4,5] Furthermore, it is generally accepted that the fatigue strength increases with the
ZHAO ZHANG, MOTOMICHI KOYAMA and HIROSHI NOGUCHI are with the Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Contact e-mail: [email protected] MEIMEI WANG is with the Max-Planck-Institut fu¨r Eisenforschung, MaxPlanck-Straße 1, 40237 Du¨sseldorf, Germany. CEMAL CEM TASAN is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. Manuscript submitted September 8, 2018. Article published online January 1, 2019 1142—VOLUME 50A, MARCH 2019
tensile strength.[6] In other words, metastability, crack path, and strength level are expected to be critical in designing unconventional fatigue-resistant steels. Recently, pre-cold-rolling was reported to induce microstructural refinement and to affect associated martensitic transformability in TRIP-maraging steel.[7] Specifically, the microstructural morphology changed from laminates to equiaxed grains. The microstructural change improved the tensile strength markedly.[7] The increase in tensile strength was expected to increase the fatigue strength, but also note that microstructural changes associated with strengthening can decrease fatigue resistance because of variations in defect size,[8] plastic strain distribution,[9] and crack path.[10] In terms of grain refinement strengthening, the refinement of the non-laminated microstructure could decrease the fatigue crack resistance, because ultrafine-grained steels show lower fatigue crack resistances from their reduced crack roughness.[10] Since pre-cold-rolling may exert both negative and positive effects on fatigue resistance, a comparative study on fatigue resistance of TRIP-maraging steels with and without pre-cold rolling is highly interesting. To be specific, the aim of this work is to examine the effects of microstructural constituents on the fatigue properties associated with mechanically small crack growth. Fatigue crack growth resistance is strongly size-dependent for mechanically small initial crack lengths, but shows no dependence with a large initial crack, as shown in Figure 1.[11–13] Therefore, mechanically small crack growth is suitable for the purpose of this study. As an extreme case of mechanically small cracks, here we present the microstructural evolution and crack path in plain specimens of
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