A NUMERICAL FORCE AND STRESS-ANALYSIS ON A THIN TWIN LAYER IN TIAL

Distributions of forces and stresses along a thin twin layer in TiAl t hat was qualitatively described in a related paper were analysed using the experimentally measured locations of twinning dislocations in the thin twin layer. The results show that both the forward stress and th e back stress are very large at the twin tip and they drop quickly as the distance from the twin tip increases. The forward stress and the b ack stress are equal to 1140 MPa and 1200 MPa respectively on the firs t twinning dislocation and 275 MPa and 300 MPa respectively on the six teenth twinning dislocation. The external stress is found to be the re sidual stress in the matrix and it is uniformly distributed along the thin twin layer. The magnitude of residual stress is about 50 MPa. The internal friction stress on the first twinning dislocation is very la rge, near 765 MPa, but the internal friction stress on the second twin ning dislocation is lower than on any twinning dislocations behind it. The high internal friction stress on the first twinning dislocation i s due to a compressive stress normal to the twinning plane and a tensi le stress in the twinning direction within the core of the first twinn ing dislocation besides the core structural change in the first twinni ng dislocation. A high stress concentration at a twin tip is an intrin sic property of a growing twin layer, but the stress concentration for an unblocked twin has an upper limit, which is equal to 1200 MPa in t his study. The results are discussed and compared with models for twin s in the literature.