| The transformation behavior, aging response and mechanical properties of some Fe-Ni-Mn alloys have been studied. Transmission electron microscopy observations revealed that retained(, )austenite films appeared between the martensite laths. Many irregular 112 (,b) (,b) microtwins were observed within the martensite laths as well as four sets of screw dislocations whose(, )Burgers vectors were identified to be a(,b)/2 . A remarkable maraging-hardening response was observed upon aging between 300 C ('(TURN)) 550 C.(, )A two-step aging process was found to improve the ductility without a loss in strength.; Precipitation sequences during aging were studied in detail using optical microscopy and transmission electron microscopy along with energy dispersive X-ray emission spectroscopy. In addition to the reported 100 (,b) (theta)-NiMn precipitates, three other different phases were observed as a result of aging: 110 (,b) (theta)-NiMn, twinned Widmanstatten austenite and spherical ferrite.; It is suggested that the formation of lath-like reversed austenite may be shear dominated although is probably aided by a prior or simultaneous diffusion-controlled process. After prolonged aging at 550 C the precipitates disappeared and a remarkably fine grained austenite was formed and essentially retained to room temperature. Specimens aged to maximum hardness were seriously brittle, and fractured intergranularly along prior austenite grain boundaries. There were no precipitate particles observed along prior austenite grain boundaries, and Auger electron spectroscopy did not reveal decisive evidence of grain boundary segregation of Mn.; Theoretical calculations showed the feasibility of directional alignment in that twenty four variants of lath martensite could be reduced to four aligned variants by applying a strain along a specific direction in a supposedly textured austenite. |
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