THE STUDY OF RE-CRYSTALLISATION IN BETA TITANIUM ALLOY PREPARED VIA POWDER METALLURGY PROCESS

摘要

Beta titanium alloys attract attention in last decade, because they are promising materials for bio-applications. They contain only fully biocompatible elements exhibit low Young's modulus. Their preparation through arc melting is quite difficult, because alloying elements (Ta, Nb, Zr, ...) have different densities and melting points. Preparation via powder metallurgy processes can solve these problems. In this paper beta titanium alloy with nominal chemical composition Ti35Nb6Ta (all compositions are in wt%) was prepared via powder metallurgy process. Blend of elemental powders was filled in rubber moulds and cold isostatically pressed at 400 MPa for 5 s. Pressed samples were sintered in a vacuum furnace at 1,300 ℃ for 20 h. These specimens were hot forged in order to remove residual porosity and then solution treated (850 ℃ / 0.5 h). Solution treated specimen were cold swaged into the shape of wires with 5 mm diameter. These wires have high tensile strength (above 1,000 MPa) and low Young's modulus (～50 GPa). Their disadvantage is lower elongation. Because of that annealing for 8 hours at various temperatures (i.e., 550, 600, 650 and 700 ℃) was performed on these specimens and re-crystallisation was studied using EBSD technique. Specimens were polished with SiC papers up to #P4000 and than electropolished. Electropolishing was carried out on Elyana 230 electrolytic polisher at room temperature by using 5 vol% HClO_4 and 1.5 vol% HNO_3 solution in C_2H_5OH at 50 V. EBSD observation was performed on Jeol JSM 7600F microscope (at 20 kV) equipped with a Nordlys EBSD detector. Results were processed by HKL Channel 5 software equipment. The microstructure of cold swaged alloy consists of deformed β-Ti grains. These grains have highly deformed boundaries where the Kikuchi lines were diffused and indexing of these lines generally was not possible. Inside these grains fine precipitates identified as α-Ti phase. This is caused by relatively high oxygen content in alloy (up to 1 wt%). After annealing at 550 ℃ for 8 hours the microstructure is similar. Only the width of areas where low contrast of Kikuchi lines did not allow indexing is much smaller. So probably recovery takes place at this temperature. After annealing 600 ℃ for 8 hours very small grains on grain boundaries of large original deformed grains emerged however there are still areas which are so strongly deformed that indexing is not possible. So it seems that the re-crystallisation begins at this temperature. In specimen annealed at 650 ℃ can be seen fine grains along grain boundaries of large deformed grains and areas with poor Kikuchi lines contrast almost disappeared. Similar situation is also in specimen annealed at 700 ℃ where high fraction of fine re-crystallized grains can be observed. So it can be concluded that the re-crystallisation for this alloy starts between 600 and 650 ℃.