Visualizing micro structural features and imaging local stiffness of alpha and beta phases in titanium Ti-6Al-4V alloy by atomic force acoustic microscopy

Atomic force acoustic microscopy (AFAM) with a spatial resolution of some tens of nanometers has been used to characterize Ti-6Al-4V alloy. The aim was to qualitatively visualize the microstructure and quantitatively determine the elastic properties of Alpha- and Beta-phases for different heat treatments. The qualitative AFAM images were compared with backscattered electron images (BSE). Energy-dispersive X-ray spectroscopy (EDX) was carried out in order to distinguish between Alpha- and Beta- phase. The results of the AFAM images were equivalent with the images obtained with the BSE method. However in contrast to BSE, AFAM was able to visualize the very fine needle-like Alpha'-martensite structure. EDX analysis showed that the chemical composition of Alpha was almost constant for all heat treatments whereas the amount of vanadium (V) in the Beta-phase decreased with increasing temperature. Quantitative AFAM evaluation by using only one resonance mode of the cantilever was attempted but did not yield any reasonable values, which is believed to arise from difficulties to control the force on the cantilever. Semi-quantitative evaluation indicated an increasing modulus of Beta with decreasing amount of V. However ultrasonic bulk measurements showed decreasing modulus with increasing temperature which clearly indicates that the predominant factor attributing to the change in modulus of Ti-6Al-4V is the higher volume fraction and not the chemical composition of Beta-phase.