A Dislocation Core Transition Model for the Peculiar Temperature and Orientation Dependence of the Critical Resolved Shear Stress in L1-Type Co74Ni3Ti23 Single Crystals
Yi Liu
Center for Materials Research and Analysis and
Department of Mechanical Engineering
University of Nebraska - Lincoln
Date: Thursday, March 13, 1997
Time: 3:30 p.m.
Place: 306 Bancroft Hall
The deformation behavior of L12-type Co74Ni3Ti23 single crystals has been investigated in compression and tensile tests
as a function of orientation, test temperature and strain rate. The
measured critical resolved shear stress (CRSS) shows a minimum around 473K
(denoted as Tb) and a maximum around 1000K (denoted as Tp).
A negative temperature dependence of the CRSS is observed below Tb and Tp, and a positive temperature dependence of the CRSS is
observed between Tb and Tp. Slip trace analysis
shows that the (111) and (001)slip planes are activated around and above
Tp. The CRSS is almost strain-rate independent for (111)slip
and has a positive dependence for mixed (111) and (001)slip.
A model involving a thermally activated process of superdislocation
core transition from sessile segments to glissile segments is proposed
to explain the temperature and orientation dependence of the CRSS at low
temperatures. A cross-slip mechanism of the screw dislocations is
responsible for the anomalous increase of the CRSS with increasing temperature
between Tb and Tp. The CRSS for (111)[101]
slip is then expressed as t =tG + t n + tp where tG is a temperature-insensitive stress component and represents the CRSS required
to move the glissile segment of the superdislocation, and tn and tp are stress components attributed
to two thermally activated processes at low and high temperatures, respectively.
Very good agreement is obtained between the calculated and measured CRSS-temperature
curves. Evidence of the dislocation core transition process observed
by transmission electron microscopy (TEM) weak beam imaging technique will
be shown.
