A STUDY OF GRAIN-BOUNDARY NUCLEATED WIDMANSTATTEN PRECIPITATES IN A 2-PHASE STAINLESS-STEEL

The factors which control the nucleation and growth of Widmanstatten precipitates at grain boundaries in a two-phase stainless steel, have been studied by transmission electron microscopy. Widmanstatten precipitation usually involves a separate nucleation and growth event at prior austenite films or allotriomorphs, which are the first phase to form at grain boundaries. The Widmanstatten precipitates have a preferred orientation relationship with both the prior grain boundary gamma phase and with the alpha matrix. The particular gamma variant which forms has a near Kurdjumov-Sachs relationship with the alpha grain and the invariant line associated with the variant is nearly normal to the grain boundary plane. The orientation relationship changes during growth, towards an exact K-S relationship, with the growth direction (which is parallel to the invariant line) also rotating to be nearly parallel to the common close-packed direction associated with the transformation.