Role of Misorientation in Assessing Fretting Damage

The primary focus of the current work is to develop quantifying measures that can describe the evolution of fretting damage at the microstructural scale in a dual phase Ti-6Al-4V as well as two single phase materials: commercially pure titanium (CP-Ti), which consists of pure alpha-phase titanium, and a near alpha Ti-5Al-2.5Sn. It is important to understand deformation behavior at the microstructural scale in heterogeneous materials because features at this scale, such as grain size, crystallographic orientation, and phase distribution, strongly influence crack development and are dimensionally of the same magnitude as the fretting damage volume. In Ti-6Al-4V, the size, distribution, and crystallographic orientation of the alpha-phase, which has an HCP crystalline structure, is particularly significant in fretting crack formation. Recent studies have linked an increase in average intra-grain misorientation (AMIS) measured using electron backscatter diffraction (EBSD) to increasing strain in medium to high stacking fault metals such as titanium, nickel, copper, and aluminum. A high AMIS value in the near surface layers of specimens subjected to procedures that may induce surface damage has been shown to correlate with a reduction in low-cycle fatigue life. Furthermore, AMIS may be used to estimate plastic strain accumulation when calibrated to specimens tested at a known plastic strain. In the current study, the effect of slip displacement amplitude and number of fretting cycles on the evolution of fretting damage is quantified using AMIS. Additional supporting evidence of significant plastic strain accumulation in the near surface layers of the fretted specimens obtained using nanoindentation and energy dispersive X-ray analysis (EDX) will also be presented. An opportunity exists to directly link microstructural damage measures such as AMIS with life prediction procedures, and therefore, remaining challenges in developing such methods will be discussed.