Cavitation erosion characteristics of a NiTi alloy
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At 450 °C
At 500 °C
Fcc MnNi Mn3Ni MnNi2 Mn2Ni MnNi3 FeNi3
0.739 0.598 0.380 0.305 0.214
0.168
0.720
0.638 0.431 0.054 0.146 0.084
0.311
0.857
Mn and Ni low at the grain boundaries. The driving force for the precipitation of the -MnNi intermetallic is low at 500 °C, suggesting a very low possibility of the precipitation of this phase at grain or lath boundaries.
REFERENCES Fig. 3—CALPHAD calculated equilibrium phase diagram of Fe-Mn-Ni ternary alloy at 450 °C.
ThermoCalc version L with the Kaufman database.[9] For an Fe-8Mn-7Ni alloy, the equilibrium phases at 450 °C are -ferrite and -austenite. Precipitation of the -austenite phase after prolonged aging was reported in the literature.[10,11,12] Our article[8] reported the precipitation of austenite at grain or lath boundaries at the very early stage of tempering. When austenite forms at the grain boundaries, it is expected that grain-boundary ferrite will be in equilibrium with the grainboundary austenite, because grain-boundary diffusion of Mn and Ni is much faster than matrix diffusion. CALPHAD calculations give the equilibrium composition value of ferrite of 1.6 wt pct Mn and 0.8 wt pct Ni. After the precipitation of austenite at grain boundaries, the segregation level of Mn or Ni at grain boundaries may be reduced and was, in fact, undetected.[8] The driving forces for the precipitation of the phases shown in Figure 3 were calculated by the CALPHAD method and are given in Table I. The driving force for the precipitation of austenite is greater than that of the -MnNi at 450 °C and 500 °C, but the difference increases at 500 °C. Austenite and -MnNi may precipitate simultaneously at the grain boundaries at 450 °C or at lower temperatures, but, at 500 °C or higher, only austenite precipitation may be seen. Indeed, precipitation of -MnNi intermetallic phase was observed, in addition to the precipitation of austenite at the grain and lath boundaries in the Fe-10Ni-5Mn alloy at the early stages of aging at 480 °C.[13] The major source of grain-boundary embrittlement during aging of age-hardenable Fe-Mn-Ni alloys at temperatures below 500 °C is thus believed to be due to the grain-boundary precipitation of -MnNi intermetallic particles. The nonequilibrium segregation calculation conducted by Heo,[6] as shown in Figure 1, suggests a higher level of segregation of Mn at 500 °C than at 450 °C. However, after again at 500 °C, grain-boundary embrittlement does not occur. Again, segregation alone cannot explain this result. Precipitation of austenite can scavenge the segregating elements such as Mn and Ni, keeping the concentration of 356—VOLUME 35A, JANUARY 2004
1. M. Guttman: Surf. Sci., 1975, vol. 53, pp. 213-27. 2. E.D. Hondros and M.P. Seah: Int. Metall. Rev., 1977, vol. 222, pp. 262-301. 3. M.P. Seah: Acta Metall., 1980, vol. 28, pp. 955-62. 4. H.C. Feng, E.A. Wilson, and C.J. McMahon, Jr.: 3rd Int. Conf. on the Strength of Metals and Alloys, Cambridge, United Kingdom, 1973, p. 129. 5. N.-H. Heo and H.-C. Lee: Metall. Mater. Trans. A, 1996, vol. 27
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