| Referencing a lot of Ti-Al-Nb ternary phase diagrams and affection of alloy elements on binary Ti-Al phase diagram, a series of Ti-Al-Nb ternary alloy samples were produced by adjusting the contents of Nb and fixing the ratio of Ti and Al to 1. The main purpose of the composition configuration was to prepare the Ti-Al-Nb ternary alloys with different microstructures. Microstructures and phase configurations at different test temperatures were obtained by many different means such as differential thermal analysis, X-ray diffraction analysis, electron probe microanalysis, microscopy, transmission electron microscopy and selected area electron diffraction. All the samples were treated at 1473K for 5hrs before testing.Four Ti-Al-Nb ternary alloys with different composition and microstructures were selected. The nominal composition of the four alloys were Ti-47.5Al-5Nb, Ti-42.8Al-14.2Nb, Ti-40Al-20Nb and Ti-30Al-40Nb (at%), with microstructure γ-TiAl, γ-TiAl+a2-Ti3Al, γ-TiAl+a2-Ti3Al+Nb2Al and γ-TiAl+a2-Ti3Al+Nb2Al respectively. The interrupted isothermal oxidation experiment of the four alloys was carried out at 1273K in air for 100 hours. The results showed that the oxidation behavior of the four Ti-Al-Nb alloys was superior to that of the binary Ti-Al alloys. The mass gains of the alloys after interrupted oxidation test at 1273K for 100 hours were all less than 8mg/cm2. In particular, the y-TiAl+a2-Ti3Al two-phase alloy (Ti-42.85Al-14.3Nb(at%)) showed the best oxidation resistance with a mass gain of only about 2.5mg/cm2. The mass gain of a binary TiAl alloy used in this study was found to be 69mg/cm2 after treatment at 1273K for lOOhours. The mass gain of Ti-Al binary alloys oxidized at 1073K for 100hours was about l-3mg/cm2 , the same order as that of Ti-Al-Nb ternary alloys. Among the four Ti-Al-Nb ternary alloys investigated, better oxidation resistance showed by the two-phase y-TiAl+a2-Ti3Al alloy may be attributed to the higher amount of Nb presented in both y and a2 phases (13.3at% and 19.4at% respectively). On the other hand, aluminum content of main phase γ in γ+a2 two-phase alloy was relatively higher due to the presence of the a2 phase. The rich aluminum y phase promoted the formation and growth of Al2O3. In the single-phase γ alloy (Ti-47.5Al-5Nb (at%)), less amount of Nb solution in y phase could not inhibit the growth of TiO2 and at the same time accelerate the formation of A12O3, as a result, non-protective oxide of TiO2 dominated the outer oxide scale. The emergence of Nb2O5 in Ti-40Al-20Nb(at%) and Ti-35Al-30Nb(at%) alloys when the Nb content in the Ti-Al-Nb alloy is over 20at% is discussed, and the exfoliation of oxide scale was observed. The relationship between poor oxidation resistance of Ti-40Al-20Nb(at%) and Ti-35Al-30Nb(at%) alloys and the Nb-enriched phase of Nb2Al was investigated.On the basis of the above investigation on the oxidation behavior of Ti-Al-Nb ternary alloys, the three alloys with γ(Ti-47.5Al-5Nb(at%)), y+a2(Ti-42.85Al-14.3Nb(at%)), y-+a2+Nb2Al(Ti-40Al-20Nb(at%)) respectively were selected for compression testing at room temperature, 973K, 1173K and 1373K and tensile testing at room temperature and 1373K. The results showed that the yield strength of the y single phase Ti-Al-Nb ternary alloy was 450Mpa,which was the same as that of binary y-TiAl alloy at about 1200K, but at higher temperatures, the yield strength was higher than that of y-TiAl binary alloy, Inconel715 and Ti-48Al-2Cr-2Nb etc. alloys. This attributes to that Nb solutes in y phase. The yield strengths of the two-phase and three-phase alloys were higher than that of the single-phase alloy from room temperature to 1173K, which were 1150MPa and 950MPa respectively at room temperature. The yield strength decreased slightly when temperature increased.The results of tensile property experiments showed that plastic property of the two-phase alloy was better than that of single-phase alloy and three-phase alloy. The main reason was that the appearance of a% phase effectively decreased average grain size. Another reason was that the lamellar microstructure consisted of y and a.2 phases and y/a2 interface were clearly favorable for plastic deformation. The ductility of the two-phase alloy improved when temperature increased, tensile elongation was 40.4% at 1173K. However, the ductility of single-phase and three-phase alloys could not increase at 1173K. Characteristics of fracture of the two-phase alloy transformed from cleavage fracture at room temperature to i ductile fracture at 1173K, but that of single-phase and three-phase alloys were all cleavage fracture at room temperature and 1173K. Nt^Al-phase in ternary alloys decreased the continuity of lamellar microstructure consisted of y-phase and a2-phase. The Tab of Nb2Al was 1443K, hence brittle Nb2Al at 1173K made the RT ductility and high temperature ductility of the three-phase alloy lower.In a word, the results of oxidation resistance and mechanical properties showed that the two-phase alloy of Ti-42.85Al-14.3Nb(at%) had better oxidation resistance and room temperature and elevated temperature general mechanical property. It is potential to assume the application for elevated temperature components below 1173K.On the basis of research results of oxidation and mechanical property test of Ti-42.85Al-41.3Nb(at%), referring to benefit effect of W, Cr on Ti-Al alloy, four alloys were designed and prepared by adding W, Cr to Ti-Al-Nb ternary alloys. Their normal chemistry composition were Ti-41.85AM4.3Nb-2W, Ti-42.85Al-12.3Nb-2W, Ti-42.85Al-12.3Nb-2Cr and Ti-42.85Al-10.3Nb-4Cr(at%). The interrupted oxidation behaviors of the four alloys at 1273K in static air were studied.The microstructure of the four alloys was y+p, due to Nb, W, Cr stabilized (J phase. Addition of W or Cr to Ti-Al-Nb ternary alloys accelerated accumulation of Nb in p phase, this made Nb content of matrix y phase less. This explained why Nb could not improve the oxidation resistance. The addition of Cr made the adhesiveness between oxide and matrix weaker, the formed oxide is liable to breaking off due to thermal stress while cooling. Not only the oxidation resistance of the four alloys with W, Cr to Ti-Al-Nb ternary alloys could not be improved but also get lower than that of Ti-42.85Al-14.3Nb (at%) two-phase alloy.In order to further study the effect of P phase on oxidation resistance, the alloy such as Ti-42.85Al-12.3Nb-2Cr(at%) was heat-treated at 1553K for 2hours and cooled down to 1473K for 6hours and to room temperature in furnace(process2) or in water(process3). The microstructure of the two kinds of the treated alloys were y-TiAl+a2-Ti3Al,...
|
PDF Full Text Request