Creep deformation of TiAl-Si alloys with aligned γ/α2 lamellar microstructures

Creep tests were conducted on directionally solidified TiAl-Si alloys to discern the effect of lamellar spacing and lamellar orientation on the high-temperature strength. Directional solidification of a Ti-43Al-3Si alloy was used to produce a model microstructure consisting of a single gamma/alpha(2) lamellar colony of controlled orientation for creep testing. Different heat treatments were used to vary the lamellar spacing and tensile creep tests were performed at 750 degrees C using applied stresses of 180, 210, and 240 MPa. In addition, ingots with large columnar grains of various lamellar orientation were also tested. The results clearly show the beneficial effect of microstructural control with the aligned microstructure having a much superior creep resistance. Furthermore, for specimens with aligned lamellar microstructure, decreasing the lamellar spacing greatly improved creep resistance, as both the primary creep strain and secondary creep rate are much smaller for materials with the fine lamellar spacing.