YIELD SURFACES OF BCC CRYSTALS FOR SLIP ON THE (110) (111) AND (112) (111) SYSTEMS

The single crystal yield surfaces (SCYS) for a slip in b.c.c metals on either [112] or a mixed slip on [112] and [110] have been derived. In particular, for the general case of mixed slip, the SCYS is described in terms of Ε, the ratio of the critical resolved shear stresses for a slip on the [112] and [110] systems. All the possible types of yield vertices are tabulated and classified into crystallographically non-equivalent groups. It is confirmed that there is a limited range of Ε for mixed slip. Within this range, there are only two different SCYS configurations, depending on whether Ε is greater or smaller than 5/3√3. Each of these configurations has 15 groups of vertices and 216 stress states (plus their negatives). The two yield surfaces only differ with respect to 3 of the groups of vertices (72 stress states). The evolution of these vertices near the critical Ε value of 5/3√3 is described in detail.