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 and the structure is microduplex lamellae consisting of austenite and ferrite (or martensite) phases . The Kurdjumov-Sachs relationship holds between the austenite and ferrite.

Electron microscopy study on the grain-boundary precipitation during aging of Fe-10Ni-5Mn steel

“…The increase in the hardness of these Fe-Mn-Ni age-hardened alloys is well known to be associated with the precipitation of coherent MnNi intermetallic particles during annealing. [11,12,13] Figure 2 shows the variation in the tensile strength and elongation of a Fe-10Ni-5Mn alloy as a function of the aging time at 480°C. Note that the Fe-10Ni-5Mn alloy shows no tensile ductility after aging for more than 5 minutes.…”

“…[1][2][3][4][5][6][7][8][9][10] This hardening is believed to result from the precipitation of a coherent face-centered tetragonal (fct) -MnNi intermetallic phase. [11,12,13] Unfortunately, alloys in the age-hardened state have been found to undergo severe grain-boundary embrittlement and exhibit premature intergranular fracture in tensile tests. [6,[14][15][16][17] Embrittlement occurs after only a very short aging treatment, but it recovers after prolonged aging.…”

Development of pseudoelasticity in Fe–10Ni–7Mn (wt%) high strength martensitic steel by intercritical heat treatment and subsequent ageing

“…2b, the precipitation is suppressed and only the reverse martensite transformation can take place. Yodogawa [24] studied the specific heat of Fe-9Ni-5Mn alloy at the heating rate of 0.04 °C/s. He found that fct-NiMn precipitates formation is occurred between 370 ~ 490 °C.…”

Ductility enhancement in ultrafine-grained Fe–Ni–Mn martensitic steel by stress-induced reverse transformation

“…Fe-Ni-Mn martensitic steels have excellent age-hardenability by formation of nanometer-sized intermetallic precipitates such as f.c.t. -NiMn [5,6] but they suffer from serious intergranular embrittlement after aging. The source of intergranular embrittlement and ductility improvement of this alloy was the subject of many papers [7][8][9][10].…”