Precipitation Behavior in Fe–Ni–Mn Martensitic Alloys

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  • Yodogawa Masayoshi
    Research Laboratory of Precision Machinery and Electronics, Tokyo Institute of Technology

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  • Precipitation Behavior in Fe–Ni–Mn Martensitic Alloys

Abstract

The object of this study is to investigate precipitation behavior during aging of age-hardenable Fe–Ni–Mn martensitic alloys by means of specific heat measurement, thermal differential analysis, electron microscopic observation, and X-ray and electron diffractions. The as-quenched structure before aging is lath martensite. In the initial stage of aging, very fine zones with high solute concentration are homogeneously formed due to the two-phase decomposition. Subsequently the zone acts as a nucleus of the fct NiMn phase. As the zone changes to the fct NiMn particle the hardness of the alloys increases up to its maximum value. The main cause of the age-hardening is considered to be the strain hardening due to the coherency strain between zone or NiMn particle and matrix. In the over-aging and softening processes, NiMn particles grow into disc-like platelets on the (100) planes of the matrix, and the crystallographic orientation relationship with respect to the matrix is determined to be (011)α′4°(111)NiMn, [\bar1\bar11]α′8°[\bar101]NiMn. In the final equilibrium state, NiMn particles disappear and the structure is microduplex lamellae consisting of austenite and ferrite (or martensite) phases. The Kurdjumov-Sachs relationship holds between the austenite and ferrite.

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