NUCLEATIGN AND GROWTH OF beta PHASE PRECIPITATED ALONG THE PATHWAY OF alpha- > beta+gamma IN gamma-TiAl ALLOYS

摘要

The formation mechanism of the beta-Ti (bcc) phase precipitated within the lamellar microstructure under the sequential mode of the decomposition of the a phase (alpha- > beta+gamma- >beta+gamma) in Ti-47A1-2.5Mo has been investigated utilizing transmission electron microscopy. The (3 phase nucleates at gamma/gamma interfaces with Nishiyama's orientation relationship:<11-bar 0>_(beta)//<12-bar 1>_(gamma), {110}_(beta)//{111}_(gamma). As aging proceeds, the (3 phase preferentially grows along the direction of <111>_(beta)//<11-bar 0]_(gamma) on the interfaces with bar shape, and the orientation relationship changes to Kurdjumov-Sachs: <11 l>_(beta)//<11-bar0>_(gamma), {110}_(beta)//{111}_(gamma). In addition, the lattice parameter of the beta phase remains almost unchanged whereas the c/a ratio of y phase increases from 1.0 to 1.02 with aging. Therefore, the supersaturated Mo content in the prior y phase is responsible for the nucleation and growth of beta phase.