Effect of Hot Rolling on the Microstructure and Mechanical Properties of Ti 3 Al Based Dual Phase Alloys
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INTRODUCTION The intended application of %2 -Ti3A1 based alloys in aerospace industry requires improved high temperature strength, oxidation resistance and creep properties while retaining their light weight. Development of a 2-Ti3A1 based dual phase alloys has shown some promising potentials in property improvement by introducing Ti5 Si3 sulicide phase in the matrix via Si alloying [1]. A Ti-AlSi-Nb alloy with the nominal composition of Ti23.5A16.5SilONb developed by Es-Souni et al [2] exhibits excellent creep and oxidation resistance behaviour. However, the presence of coarse network of (Ti,Nb) 5(Si,A1) 3 phase formed by eutectic reaction in the as-cast state also embrittles the alloy [3]. Both hot rolling and powder metallurgy are considered to be the possible ways to refine the Ti5Si3 phase in the matrix. Our previous study demonstrated that the morphology of Ti5 Si3 had a great influence on the toughness of the Ti-Al-Si-Nb dual phase alloys [4]. Recently, P/M process and hot rolling have been applied in (Ti,Nb)3(A1,Si)+ (Ti,Nb)5(Si,A1) 3 dual phase alloys in order to obtain finely distributed Ti5Si3 second phase in the matrix. Both the strength and ductility are improved by refining the brittle second phase [5]. Hot rolling is an effective way of refining the brittle second phase. In order to gain a further understanding of the effect of rolling parameters on the microstructure and mechanical properties of the dual phase alloys, two Ti-Al-Si-Nb alloys have been hot rolled to different amount of deformation in this paper. Microstructure characterisation and mechanical property evaluation are performed accordingly.
EXPERIMENTAL PROCEDURE Two alloys, 402 (Ti-24Al-1 1Nb-2Si) and 405 (Ti-24AI- lNb-5Si) (at. %) were prepared by arc melting in the laboratory furnace under an argon atmosphere. Each button was melted 4 times and annealed at 1150 0C for 30 hours to ensure chemical homogeneity. Hot rolling was carried out KK5.10.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
at adequately high temperature (1200TC). The ingots were covered by stainless steel sheet and rolled several times to the final deformation amounts of 65%, 75% and 90% respectively. The microstructures of the alloys were characterised using an optical microscope. Tensile tests were performed at temperatures from room temperature to 800'C and the fracture surfaces were examined by scanning electron microscopy (SEM) after testing. RESULTS Microstructure
The as-cast microstructures of the two alloys are essentially hypoeutectic (Fig. 1(a) and Fig. 2(b)) consisting of (Ti,Nb) 3(AI,Si) and (Ti,Nb)5(Si,A1) 3, as shown in our previous work [6]. The matrix regions of the two alloys are clusters of Widmannstitten laths within the grain which is typical in the Nb containing a 2 based alloys. Alloy 405 contains higher volume fraction of (Ti,Nb)5(Si,A1) 3 phase than alloy 402 does since its Si content is higher. In alloy 405, the coarse (Ti,Nb) 5(Si,A1) 3 phase forms network along the grain boundary; while in alloy 402, discontinuous
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