Ultrasonic Study of Polycrystalline Ti-40Nb Alloys at Cryogenic Temperatures

Ultrasonic velocity and attenuation measurements of hot-forged beta-type Ti-40Nb (%wt) alloy were realized in cooling-heating cycles between 130 and 300 K. The anelastic spectra evidence a complex nature of structural evolution mainly associated with martensitic-like phase transitions. In particular, evident anomalies at around 180 K and 160 K were not previously reported using ultrasonic measurements. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used at room temperature to characterize the sample previously to ultrasonic measurements. Differential scanning calorimetry (DSC) was used to add complementary results at low temperatures. The results detected the coexistence of beta and martensite alpha '' phases at room temperature, with traces of omega phase. The frequency-independent ultrasonic anomalies at around 190 K and 160 K suggest the occurrence of the reversible two-stage martensitic transitions involving beta -> omega and alpha '' -> omega phases and suppression of the beta -> alpha transition in the hot-forged sample.