Texture and Stress Determination in Metals by Using Ultrasonic Rayleigh Waves and Neutron Diffraction

Polycrystalline metals often contain significant levels of texture and internal stress. Plastic anisotropy originates from the texture or crystallographic alignment and can affect the formability of the metal, whereas the presence of internal stress can seriously limit the lifetime of a component. In the presence of either texture or stress, the ultrasonic velocity in the material depends on the propagation and polarization directions, and there is considerable interest in exploiting this anisotropy for non-destructive texture and stress determination. In this paper an analytical expression for the angular dependence of the Rayleigh-wave velocity in polycrystalline metals with small anisotropy typical of rolled plate is given. It is shown that surface-wave velocity measurements can be combined with the normal shear-wave birefringence technique to give a texture-independent determination of the difference in principal stresses in the plane of the plate. The theory is tested by comparing ultrasonic measurements on two aluminium plates with the behaviour predicted by using a neutron diffraction determination of the crystallite orientation distribution function. The possibility of obtaining the depth profile from the frequency dependence of the Rayleigh-wave velocity is considered.