EFFECT OF MICROSTRUCTURE ON THE ROOM TEMPERATURE TENSILE AND CREEP DEFORMATION MECHANISMS OF ALPHA-BETA TITANIUM ALLOYS

Two-phase alpha-beta titanium alloys are used in many applications because of their high specific strength, corrosion resistance, processability, and biocompatibility. The room temperature tensile and creep deformation mechanisms of alpha-beta alloys must be understood in order to design alloys with desired properties and improved creep resistance. There is a lack of understanding in this regard. The aim of this investigation is to systematically study the effects of microstructure, stability of the beta phase, and alloying elements on the deformation mechanisms of alpha-beta titanium alloys using Ti-6.0wt%Mn and Ti-8.1wt%V as the model systems. The tensile and creep deformation mechanisms and microstructure were studied using SEM, TEM, HREM, and optical microscopy. In addition, theoretical modeling was performed in terms of crystallographic principles and stress analysis. It was found for the first time in an alpha-beta titanium alloy (Ti- 8.1wt%V) that the alpha phase deforms by twinning and the beta phase deforms by stress induced martensite, different mechanisms than the single-phase alpha and beta alloys with similar grain size. Single-phase alpha deforms predominantly by slip, and single-phase beta deforms predominately by twinning. This is also the first time that stress induced martensite has been observed in a creep deformed alpha-beta titanium alloy. However in the case of Ti-6.0wt%Mn, where the beta phase stability is higher, stress induced martensite was not observed. The deformation mechanisms are modeled in terms of the beta phase stability and interactions between phases, including elastic interaction stresses, alpha phase templating, interactions of deformation products, and alpha-omega interactions. A model is also proposed which explains anisotropic interface sliding based on locking of growth ledges. These results are extremely valuable when designing new alloys with improved resistance to creep and other failure modes. The observed deformation mechanisms can directly affect the mechanical reliability of systems. For instance, increased creep strain can alter the dimensional tolerances of components and the observed stress induced products can act as nucleation sites for fracture initiation and stress corrosion cracking. This work was supported by the National Science Foundation under grant number DMR-0102320