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THE YIELD STRESS ANOMALY IN L12 ALLOYS

PETER B. HIRSCH

University of Oxford, Department of Materials, Parks Road, Oxford OXI 3PH, England.

ABSTRACT

The implications of the basic assumptions of the local pinning theories for the yield stress anomaly in Li 2 alloys are discussed. An alternative theory is presented in which the superpartials on (111) cross-slip on to (010) to form long locks lying partially on (11) and (010) or completely on (010) planes (Kear-Wilsdorf locks). The ends of the locks are joined by glissile superkinks. The yield stress is controlled by superkinks bypassing the screw dislocation locks, and the increase of the yield stress with increasing temperature is due to the decrease of the lengths of superkinks. The theory accounts satisfactorily for the mechanical properties including the small strain-rate dependence of the yield stress and is consistent with electron microscope observations. INTRODUCTION

Several detailed models have been developed to explain the anomalous yield stress observed in some L1 2 ordered alloys in which the yield stress for slip on (111) increases with increasing temperature [1, 2, 3, 4, 5, 6, 7 8]. In all these models, the [101] dislocations on (111) are dissociated into two 1/2[16t] superpartials bounding on APB and the strengthening results from the pinning of screw dislocations, which, owing to a lower APB energy on {001} than on {111} [9] and a torque between the superpartials due to elastic anisotropy [10] can reduce their energy by cross-slipping from the primary glide plane (111) to (010). The activation energy for cross-slip from (111) to (010) is assumed by all the models (except [1]) to be that derived by Paidar, Pope and Vitek (PPV) [2]. In this model short screw segments of the leading partial cross-slip on (010) by b/2, where b is the Burger's vector of the superpartial (Fig la). This process involves the constriction of the superpartial, which is initially dissociated on (11 ) into two Shockley partials bounding a complex stacking fault. A double jog of height b/2 is formed and the process is completed by redissociation on the (1T1) cross-slip plane (Fig. lb). In all the theories the cross-slipped dislocations act as obstacles to further slip on (111); in broad terms the yield stress increases with increasing temperature because the number of cross-slip events, and therefore of obstacles, increases with increasing temperature. Furthermore, in all the theories the orientation dependence and tension/compression asymmetry of the yield stress are predicted correctly from the PPV activation energy.

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Fig. 1 Pinning configuration in the PPV model after cross-slip of b/2 on (010) However, the models differ in respect of the evolution of the barriers after the first cross-slip step takes place, the nature of the unlocking process, and that of the the slip which is producing the strain. There are two types of model; in the first, following Takeuchi and Kuramoto [1], the screws are pinned locally, the screws between the pinning points advance Mat. Res. Soc. S