The effect of microalloying with yttrium and zirconium in the amount of 0.06 and 0.07 wt.% respectively on the formation of structure, phase composition and measured hardness characteristics of a two-phase titanium alloy of the Ti -Al -Mo -V -Cr -Fe system is studied. The laws of variation of the phase composition and structure of the alloy with microadditives in the temperature range of 700 -950°C and during subsequent cooling in water and air are determined. The diagram of variation of the phase composition of the alloy under water quenching is plotted.
INTRODUCTIONMircoalloying of titanium alloys with low additives of elements (up to 0.1 wt.%) including REM attracts researchers from both theoretical and practical standpoints [1 -4]. The most frequently used REM is yttrium and its oxides [1, 5 -11]. Yttrium, like cerium, lanthanum, and neodymium, occurs in nature relatively abundantly (the clarke in the crust is about 30). It has a lower density than the mentioned elements (r Y = 4.469 g/cm 3 , r La = 6.145 g/cm 3 , r Ce = 6.749 g/cm 3 , r Nd = 7.007 g/cm 3 ), a higher melting temperature [1795 K (Y), 1294 K (Nd), 1194 K (La), 1072 K (Ce)] [12] and affinity for oxygen (the enthalpy of formation of yttrium oxide is 1905 kJ/mole; that of neodymium oxide is 1808 kJ/mole; that of cerium oxide is 1800 kJ/mole; that of lanthanum oxide is 1794 kJ/mole). The enthalpy of formation of yttrium oxide is higher than that of titanium oxide (944 kJ/mole for TiO 2 ) and its alloying elements. Yttrium introduced as an alloying element promotes removal of oxygen from the matrix forming an oxide, the presence of which hinders substantially the development of recrystallization and provides formation of finer grains under heating to the b-range [13]. When present in the solid solution of titanium, yttrium enriches primarily the grain boundaries being a surface-active element [14], which stimulates formation of complexes of harmful impurities with yttrium [15]. This leads to refinement of the matrix and deceleration of grain growth during heating to the b-range [1]. In addition, yttrium additives in titanium alloys may help with control of the processes of decomposition during heat treatment [1,15] and provide elevation of the process and operating characteristics, such as weldability [16], superplasticity [17], high-temperature strength [9], structural strength [1,13], and thermal stability [11]. However, the effect of yttrium in titanium alloys on the occurrence of phase transformation is still arguable. For example, by the data of foreign researchers [18,19] yttrium additives in titanium lower the temperatures of polymorphic transformations (T pt ) from 882 to 870°C and promote subsequent eutectoid transformation, because yttrium behaves as a b-eutectoid-forming stabilizer. However, by the data of domestic scientists presented in monographs [20,21] alloying of titanium with yttrium raises T pt to 890°C, which is followed by occurrence of a peritectoid transformation, i.e., yttrium may be classified as a weak a-stabilizer.The aim ...