EFFECT OF DEFORMATIONHEAT TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A COMPOSITE BASED ON A TWOPHASE TI13NB13ZR ALLOY STRENGTHENED WITH BORIDES
Abstract
The Ti13Nb13Zr/(Ti,Nb)B metal-matrix composite was produced by vacuum arc remelting; the weight percentage of TiB2 in the initial charge before melting was 0.7 wt.%; an unreinforced Ti13Nb13Zr alloy was also prepared for comparison. The initial structure of the unreinforced Ti13Nb13Zr alloy was represented by finely dispersed acicular α martensite, consisting of thin laths of submicron thickness, in β-phase grains. The microstructure of the composite was represented by a similar two-phase α+β-matrix Ti13Nb13Zr and multiple randomly located fibers of complex (Ti,Nb)B borides with a volume fraction of fibers of ~ 2.0 %. A study of the microstructure evolution of composites during rolling at deformation degrees of 20, 50, and 80% revealed the alignment of (Ti,Nb)B fibers in the rolling direction and their minor fragmentation. After rolling to 80%, a lamellar microstructure was formed in the composite, in which layers of crushed (Ti,Nb)B fibers separated interlayers of the Ti-Nb-Zr matrix. Cold rolling was accompanied by a decrease in the length of (Ti,Nb)B fibers from ~ 20 μm in the initial state to ~ 4.5 after rolling to 80 %. A study of the effect of deformation and heat treatment (cold rolling and recrystallization annealing) on the evolution of the structure and microhardness of the unreinforced alloy and composite was carried out, and the patterns of static recrystallization were determined. It was found that boride particles inhibit grain growth during recrystallization annealing of the composite in a wide temperature range of 800-1100 °C and allow the formation of a homogeneous structure consisting of polygonal grains with a grain size of ~ 30-40 μm, in contrast to the unreinforced Ti13Nb13Zr alloy, in which there is a pronounced heterogeneity of the structure at the initial stages of annealing with intensive grain growth from ~ 50 to ~ 200 μm.