Microstructure, thermo-mechanical properties and Portevin-Le Chatelier effect in metastable β Ti-xMo alloys

The microstructure, mechanical properties and Portevin-Le Chatelier (PLC) effect in Ti-xMo alloys (x = 10, 12, 15 and 18 wt%), in the temperature range of 250-350 degrees C with strain rates from 10(-3) s(-1) to 10(-4) s(-1), are systematically investigated in tension by using transmission electron microscopy and Gleeble 3500 testing machine combined with a digital image correlation technique. Results show that Young modulus decreases with increasing Mo contents, which is related to a more stable beta phase matrix and a decrease of omega phase fraction. Moreover, the values of Young modulus and 0.2% offset yield strength at elevated temperature are higher than the ones at room temperature in all the Ti-xMo alloys, except the Ti-18Mo alloy which shows an opposite result. These macroscopic features are consistent with the omega phase precipitation in deformed Ti-xMo alloys, due to the combined effects of omega phase strengthening and temperature softening. Furthermore, the serration type transforms from A to A + B, then to B and eventually to C as increasing temperature and decreasing strain rate as well as Mo contents, which mainly depends on the spatial cohesion of PLC bands influenced by the intensity of omega precipitate-dislocation interactions. Finally, the peak value of maximum stress drop magnitude appears in Ti-12Mo alloy and increases with decreasing the strain rate, which is attributed to a stronger intensity of omega precipitate-dislocation interactions caused by reducing dislocations movement and providing a longer ageing time for the precipitation of omega phase particles. Besides, the average stress drop magnitude increases in Ti-18Mo alloy and decreases in the other compositions as increasing engineering strain, which is related to the variation of omega phase fraction.