TRANSFORMATION CHARACTERISTICS OF SINGLE CRYSTAL Mn-Ni-C ALLOYS

Transformation of an Mn85Ni9C6 single crystal has been studied by neutron diffraction, ultrasonic techniques, coldstage microscopy and low temperature X-ray diffraction. An FCC→FCT transformation occurs at 174K (Ms) and although the (Af) is only some 12K (± 3) higher, both forward and reverse transformations are spread over some 70K despite marked lattice softening (C is only 0.27 x 10-10 Nm-2 and the Zenar anistropy parameter equals 36 at the MS), and there is a discontinuous change in both the 'c' axis and the volume, suggesting the presence of a first order transformation. However, in contrast to most alloys which show this transformation, the Mn85Ni9C6 alloy appears to show no discontinuity between the 'a' FCC and the 'a'FCT parameters throughout the region where the two phases co-exist. Two-surface analysis has confirmed that shear traces occur on (110) planes, and metallographic features in general are similar to those observed in in-Tl. The transformation as monitored at the surface shows the same characteristics as found in the bulk material, except that the Ms is higher by 10-20K