Development of nondestructive evaluation methods to characterize anomalous microstructures in titanium-6aluminum-4vanadium

The main objective of this dissertation is to confirm through research the following hypothesis: "The use of nondestructive evaluation tools allows the detection of different microstructure types and allows the identification of microstructure anomalies (interior and surface) in metals and alloys." The work was conducted on Ti-6A1-4V forged bar stock, presenting a case study for a high performance structural alloy. Ti-6A1-4V is a good model material, which cannot tolerate microstructure anomalies in demanding applications. The alloy is well established with extensive documentation on physical, chemical, and mechanical properties. It is also available in many different microstructures, readily generated by heat treating. This dissertation addresses issues concerning microstructure characterization and the identification of microstructural anomalies. Specifically, this work includes (i) background research on the identification of ultrasonic and electrical characteristics of five different TP6A1-4V microstructures; (ii) an application of ultrasonic backscattering measurements to detect diffusion bonded Ti-6A1-4V microstructure changes, to simulate locally isolated remnant cast structure for billet NDI; (iii) original research on laser interferometric detection for ultrasonic phase mapping to characterize macroscopic texture in Ti-6A1-4V; and (iv) original research on eddy current electrical conductivity mapping in titanium alloys.; Three original NDE methods were developed to evaluate microstructure and microstructure anomalies in Ti-6A1-4V. First, a forward scattering measurement technique was developed to spatially map the incoherent grain scattering in the forward propagation direction. These results showed, for the first time, that mapping of the forward scatter provides a basis for characterization of texture in polycrystalline titanium alloys. Second, a laser interferometric system was developed to map the signal amplitude and phase of the transmitted acoustic field in samples with different preferred crystallographic orientations. This new development allows microscopic variations in phase (and amplitude) to be experimentally measured, which is vital to understanding statistical characteristics of NDE data due to microstructure. The method is original as this the first time the signal phase has been mapped with a sub-wavelength aperture for materials characterization. The third method was discovered serendipitously, as a result of applying an eddy current probe to the surface of different microstructures of titanium alloy samples. As a result, a new and original noncontacting surface characterization approach was developed. The method is based on the effects of grain anisotropy on electrical conductivity in titanium alloys. This electrical property imaging method allows for characterization of near-surface microstructure and processing anomalies.