Effects of Pre-Tension and Thermal Exposure on the Microstructures and Mechanical Properties of a Near-Alpha High Temperature Titanium Alloy

Abstract

Near-α titanium alloys play a pivotal role in aerospace applications, as they offer a unique combination of high strength derived from α+β alloys and exceptional creep resistance from α alloys. This research delves into the impact of pre-tension on TA29 titanium blisks with a bimodal structure, focusing on thermal stability and mechanical properties. Upon initial thermal exposure, the precipitation of α2 and silicides occurred, with α2 precipitates being more prominent in the αp phase compared to the αs phase after 8 h. Additionally, silicides were found to preferentially form at αs interfaces. This increased yield strength by 40 MPa but reduced ductility by 75%. Subsequent exposure for 100 h intensified silicide precipitation in the αp phase, increased precipitate density, and enlarged α2 in both phases. As a result, there were minor strength gains and a slight reduction in ductility. The application of plastic pre-tension at 950 MPa induced dislocations, particularly impacting the αp and αs phases, with enhanced Si diffusion and α2 ordering after 100 h. Conversely, alloys pre-tensioned to 880 MPa exhibited minimal microstructural changes post-exposure. Notably, plastic pre-tension significantly altered mechanical properties by increasing both yield and tensile strength after 8 h while decreasing ductility by 72.3%. Fracture analysis revealed mixed mode fracture for initially and elastically pre-tensioned alloys, whereas plastic pre-tensioned alloys exhibited cleavage after 100 h. The study underscores the pivotal role of pre-tension in modulating precipitation and mechanical behaviors, providing valuable insights for optimizing titanium alloys for aerospace applications.

Keywords: Titanium alloys, Pre-Tension, Thermal Stability, Tensile Properties, Bimodal Structure, Precipitation Behavior