Due to their high specific strength, good corrosion resistance, and the ability to withstand high temperature, titanium (Ti) alloys are often employed in structural applications, such as the aerospace industry. Ti and Ti alloys exhibit a lower crystallographic symmetry than cubic metals due to the presence of the hexagonal crystal structure of the α phase. The intrinsic deformation mechanisms of Ti alloys activated within the broad service conditions for targeted structural components have not been as well documented as those for cubic metals. A greater knowledge of the distribution of the active deformation mechanisms in Ti alloys would be useful for developing a robust crystal plasticity finite element (CPFE) model to simulate the damage nucleation process of polycrystalline Ti alloys, thereby advancing efforts to establish Ti alloys with enhanced mechanical performance. In this dissertation, a systematic investigation of the deformation behavior was conducted on commercially pure (CP) Ti, and the Ti-5Al-2.5Sn (wt.%), Ti-3Al-2.5V (wt.%), and Ti-6Al-4V (wt.%) alloys. These materials allowed a thorough study of the changes in deformation behavior due to alloying and microstructure, from the simple single α-phase CP Ti to the more complicated two-phase α+β Ti-6Al-4V. Experiments were performed in tension and tensile-creep at 296K-763K using an in-situ scanning electron microscopy (SEM) testing method, which allows for the observation of the surface deformation evolution. A technique combining backscattered electron (BSE) and secondary electron (SE) imaging and electron backscattered diffraction (EBSD) was used to identify the distribution of the active deformation systems observed under various testing conditions. For the majority of the testing condition/material combinations, over 100 active deformation systems were analyzed (in some cases over 250 deformation systems were characterized). These data were useful for establishing statistically significant data sets for describing the deformation behavior and were useful for analyzing the critical resolved shear stress (CRSS) ratios of each deformation mode for each testing condition.The main findings of this work were that the distribution of the deformation modes varied as a function of the material composition, texture, and the testing condition (i.e. temperature and stress). For the lower Al-containing materials, prismatic slip dominated the tensile deformation; a minimum of 48% of the deformation systems were prismatic slip and each of the other deformation modes (i.e. basal, pyramidal
, pyramidal , and T1 twin) comprised at most 25% of the deformation systems. However, the basal slip activity increased with increased Al content in the alloys. For example for the Ti-5Al-2.5Sn and Ti-6Al-4V alloys, basal slip was almost as equally active as prismatic slip. In fact, for some testing conditions of these alloys, such as the 728K tension tests of Ti-6Al-4V, basal slip was more active than prismatic slip. Basal slip activity was also enhanced by increasing temperature. The twinning behavior was also dependent on the Al content of the materials; less twinning occurred for the alloys containing more than 3wt.% Al. Moreover, increasing temperature usually resulted in a decreased twinning activity. Texture also influenced the distribution of the deformation modes by affecting the Schmid factors, which played an important role in the deformation behavior. Compared to the higher-stress tensile experiments, slip was observed to a significantly lesser extent during creep. Instead, grain boundary sliding (GBS) was dominant. In some cases, GBS eventually evolved into grain boundary cracks. For Ti-5Al-2.5Sn, the formation of grain boundary cracks was associated with grains which displayed hard orientations, where the c-axis was nearly perpendicular to the tensile direction. A novel methodology for calculating the CRSS ratios of the different deformation modes was developed. The calculated CRSS ratios indicated that the CRSS ratios of α-phase Ti changed as a function of alloy composition and test temperature. The methodology developed, which relied on the experimentally gathered data with no external parameter input, provides a flexible manner to assess the CRSS ratios for a variety of deformation systems in polycrystalline microstructures, and it has the potential to be applied to a wide range of materials and crystal systems.This work has established a thorough understanding of the deformation mechanisms of Ti and Ti alloys as a function of alloy composition and test temperature. The insights gained from these data have a broad impact on understanding the heterogeneous deformation processes in Ti and Ti alloys and also on development of CPFE modeling.
