TRANSFORMATION CHARACTERISTICS OF SINGLE CRYSTAL Mn-Ni-C ALLOYS

Abstract
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 (M_s) and although the (A_f) 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 M_S), 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 M_s is higher by 10-20K.