The Yield Anomaly in CoTi

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The Yield Anomaly in CoTi 1



M. Wittmann , I. Baker and N.D. Evans 1

Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, U.S.A


Oak Ridge National Laboratory, Metals and Ceramics Division, Bldg. 5500, MS 6376, Oak Ridge, TN 37831-6376, U.S.A.

ABSTRACT Compression tests performed on both stoichiometric and cobalt-rich CoTi over a range of temperatures show a positive temperature dependence of the yield stress with increasing temperature, before a decline occurs at high temperatures. In the region of the peak yield stress, serrated yielding and a negative rate sensitivity of the yield stress were observed. Static strainaging also occurs. These observations are consistent with strong solute-dislocation interactions. Results from quenching experiments and strain rate change tests are presented, together with transmission electron microscope observations of the dislocation structures below, at, and slightly above the peak temperature. The results suggest that the yield anomaly in CoTi can be accounted for by a classical dynamic strain aging mechanism. INTRODUCTION The cobalt–transition metal B2 compounds CoTi, CoHf and CoZr remain ordered up to their melting points, deform by movement of a dislocations [1,2], and exhibit a positive flow stress dependence with temperature [3,4,5]. Since the a dislocations are not dissociated into partials [1,6,7], the yield anomaly cannot be explained by a mechanism that relies on cross slip (locking) to a plane on which the APB energy is lower [8]. The mechanical properties and dislocation behavior of CoTi have been investigated most [1,3,6,7,9]. Based on their experimental observations, Takasugi and Hanada [10] proposed that the yield anomaly arises from increased frictional force due to the spreading of the screw dislocation core, the socalled core dissociation mechanism. There is circumstantial evidence to support this mechanism. There is an apparent violation of Schmid’s law in CoTi [2], suggesting that the core spreading is stress induced; transmission electron microscope (TEM) observations of specimens strained at intermediate temperatures revealed some bowing of the screw dislocations from apparent pinning points [1]; and high-resolution electron microscopy showed that some screw dislocations had indeed undergone a core-dissociation on a non-slip {100} plane [9]. However, a shortcoming of the core dissociation mechanism is that it is unclear why there should be a yield stress peak, and not merely increasing yield strength with increasing temperature since increased temperatures would presumably cause more core spreading and greater dislocation pinning. The vacancy-hardening model was developed to describe the yield anomaly in FeAl [11], and it is possible that this other mechanism is responsible for the yield anomaly in CoTi. The vacancy-hardening model is not dependent on a specific dislocation mechanism, but associates the increased yield strength at intermediate temperatures with “solid solution strengthening” by vacancies. A peak in the yield st

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