High-temperature and cyclic corrosion crack resistance of alloys of the Ti-Si-Al-Zr system
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HIGH-TEMPERATURE AND CYCLIC CORROSION CRACK RESISTANCE OF ALLOYS OF THE Ti – Si – Al – Zr SYSTEM O. P. Ostash,1 A. D. Ivasyshyn,1 B. D. Vasyliv,1 and I. Yu. Okun’ 2
UDC 539.43:669.295
We study new alloys of the Ti – Si – Al– Zr system. It is shown that the high-temperature and cyclic corrosion crack-growth resistances of cast materials remain practically constant up to temperatures of 700–800°C and in a 3% NaCl solution (pH = 7), respectively. Annealing and quenching from the ( α + β )- and β-phase regions of Ti– 4Si – 5Al– 5Zr cast alloy lead mainly to changes in the structure of the matrix and do not improve its strength and cyclic crack-growth resistance in the investigated working media, except the increase in the threshold of corrosion fatigue after treatment from the β-region and formation of a lamellar α-matrix. The increase in the strength, plasticity, and cyclic crack-growth resistance of these alloys at room and high temperatures is attained as a result of the formation of a globular structure of the matrix and silicide phase after thermomechanical treatment with deformation of at least 90%.
The development of contemporary machine building is connected with the necessity of creation new structural materials with high physicochemical and mechanical characteristics stable under the action of elevated temperatures and corrosive media. The indicated requirements are especially well satisfied by alloys based on titanium and extensively used, due to their high specific strength and corrosion resistance, in the aircraft, ship-building, chemical, and power-generating industries, in medicine (as biocompatible materials), etc. [1–3]. However, their working temperature is 400–600°C, which is insufficient in the contemporary stage of development of the equipment [4]. The restricted sphere of applicability of titanium alloys is also explained by their susceptibility to hydrogen embrittlement and corrosion cracking [5–7]. At present, extensive research works are carried out in the field of creation of new materials with improved characteristics of high-temperature strength. In this connection, we can especially mention cast alloys of the Ti – Si – Al – Zr system created at the Institute for Problems in Materials Science of the Ukrainian National Academy of Sciences and the Ukrainian National Metallurgical Academy [8–11]. Similar materials are also proposed by scientists from the other countries [12–15]. Due to the presence of frame-type eutectics ( α-Ti + Ti5 Si3 ), they are regarded as in-situ composites. These materials are characterized by high strength in temperature ranges broader than conventional titanium alloys, since the melting temperature of the reinforcing silicide phase (Ti5 Si3 ) is equal to 2130°C. However, in view of their low plasticity, these materials are sensitive to the influence of stress concentrators and brittle fracture. Their mechanical characteristics can be improved by thermomechanical treatment [16]. However, in this case, the heterogeneity of the structure increases, which can negativel
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