MICROSTRUCTURE OF BETA-SOLUTION-TREATED, QUENCHED AND AGED ALPHA+BETA-TITANIUM ALLOY TI-6AL-1.6ZR-3.30MO-0.30SI

Silicide precipitation in the (alpha + beta)-type titanium alloy containing (in weight per cent) 6Al-2Zr-3.30Mo-0.30Si (alloy VT9) has been investigated. The beta-transus temperature for this alloy is approximately 1243 K. Solution treatment in the beta-phase field at 1323 K followed by quenching in water at room temperature resulted in the formation of a single-phase martensitic structure. The martensitic structure was confirmed to be orthorhombic (alpha") using the X-ray diffraction technique. The water-quenched specimen was subjected to aging treatment over a temperature range 823-1073 K for 24 h. Both X-ray and electron diffraction results of the material aged at 823 K showed the structure to be single-phase h.c.p. martensite. However, aging in the temperature range 873-1073 K for 24 h caused heterogeneous precipitation of silicides. Electron diffraction analysis confirmed that the silicides are of hexagonal structure with a = 0.70(2) nm and c = 0.36(8) nm. Silicide precipitation is predominantly at the interplatelet boundaries of alpha'-martensite in the samples aged at 873 and 923 K. However, aging at 973 and 1023 K resulted in precipitation of thin films of beta-phase sandwiched between the interplatelet boundaries of the martensite. Silicides were mainly at the alpha-beta-interfaces or inside the beta-phase. In the sample aged at 1073 K, apart from thin films of beta and silicides, islands of transformed-beta were observed. The kinetics of silicide precipitation was faster than in near-alpha alloys. This is attributed to the role of increased molybdenum in the alloy of the present investigation.