Metallurgical and crack-tip mechanics effects on environment-assisted cracking of beta-Ti alloys in aqueous chloride

The objective of this research is twofold. First, determine the critical metallurgical factor(s) and associated embrittlement mechanism that govern aqueous environment-assisted cracking (EAC) in {dollar}beta{dollar}-Ti alloys that are solution heat treated, then isothermally aged to precipitate a (ST/A condition). Second, characterize and understand the crack tip mechanics variables that affect time-dependent aqueous EAC of ST/A {dollar}beta{dollar}-Ti alloys.; The metallurgical variable(s) that govern intergranular (IG) EAC of ST/A Beta-C in aqueous NaCl are examined with fracture mechanics experiments on both immune and susceptible microstructures, coupled with high resolution imaging and analytical electron microscopy. Segregated Si at grain boundaries is the likely requisite for IG EAC of ST/A Beta-C. The EAC data and fractographic results for isothermally aged and double-aged Beta-C microstructures are consistent with effects of heat treatment time and temperature on segregation-induced embrittlement, and both small-probe TEM EDS and Auger measurements of grain boundary composition reveal elevated Si concentrations.; The hydrogen embrittlement mechanism that operates at {dollar}beta{dollar} grain boundaries in EAC-susceptible Beta-C is probed by a high-magnification analysis of IG fracture surfaces. Intergranular cracking of Beta-C in aqueous NaCl is likely from hydrogen-assisted decohesion of grain boundaries, based on the presence of contiguous featureless areas on all fracture surface facets that suggest cracking along a continuous {dollar}beta/beta{dollar} interface path. Neither fine dimples nor slip offsets are observed on the surface of most IG facets, indicating that hydrogen-enhanced plasticity does not operate in Beta-C.; Crack tip mechanics variables that affect EAC of ST/A {dollar}beta{dollar}-Ti are studied with fracture mechanics experiments in aqueous NaCl using programmed loading and high-resolution crack length measurements, as well as stereoimaging measurements of crack tip strain. Crack tip strain rate is a critical variable for EAC of ST/A {dollar}beta{dollar}-Ti alloys and is affected by loading rate, crack growth rate, cracking mode and strain vs time history. Subcritical EAC growth rates depend interactively on stress intensity (K) and crack tip strain rate, although this function is not known quantitatively. This hypothesized da/dt-K-strain rate relationship is supported by novel alternating slow-rapid crack growth kinetics, where da/dt is not a unique function of K.