Microstructure And Properties Of TiAl Alloys With Hot Shot Peening

TiAl alloy, as high-temperature structural material, has light weight, highspecific strength, high specific stiffness, and excellent high-temperaturemechanical properties. TiAl alloy is one of excellent high-temperature structuralmaterials in the field of contemporary aerospace industry. In order to furtherenhance the fatigue life of titanium aluminium alloy, shot peening treatment ofalloy materials can effectively improve the microstructures and residual stress inthe surface layer and near surface layer of materials. And then the fatigueperformance of materials can be improved. Because of high ductile-brittletransition temperature and large room temperature brittleness of titaniumaluminium alloy, this research mainly explores the feasibility of shot peeningtreatment of Ti-48Al-2Cr-2Nb at the high temperature and study the effect ofprocess parameters on microstructure and properties of the alloy.The shot peening treatment of Ti-48Al-2Cr-2Nb alloy samples at differenttemperatures, with different kinds of projectiles for different shot peening timewere conducted. The surface morphology, microstructure and microhardness ofthe alloy after shot peening were investigated. The research shows that thesurface of materials is easy to appear microcracks after shot peening treatment tothe alloy at room temperature to400℃. But when the alloy is done shot peeningtreatment at600℃or800℃and reaches200%coverage rate of projectiles, itcan get better strengthening effect. And with the depth of strengthening layerincreases, the strengthening effect becomes weak gradually. Due to the certainplastic deformation in strengthening layer and smooth macroscopic surfaceachieved by ceramic projectiles, the ceramic projectiles are the best.The casting Ti-48Al-2Cr-2Nb alloy was heat treated by isothermal andcyclical heattreatment. The microstructures of original coarse lamellar structureare both refined after two kinds of heat treatment. The microstructures of the alloy after isothermal and cyclical heattreatment are duplex microstructure andnear lamellar microstructure, respectively. And the microhardness is alsoimproved. And the cyclical heattreatment gets better performance.Using ceramic projectiles to do high-temperature shot peening treatment toalloy samples after cyclical heattreatment, in a certain temperature range(600℃to675℃) the strengthening effect is improved with shot peening temperatureincreasing, and too high temperature(700℃and725℃) decreases alloyhardness.Hot shot peening with ceramic projectiles at different temperature wasapplied to the alloy after cyclical heattreatment. The microstructure andmicrohardness of the alloy after shot peening were investigated. The resusltsindicated that, in a certain temperature range (600℃to725℃), thestrengthening effect improved with temperature increasing from600℃to675℃,then decreased after675℃.After shot peening, the alloy was heattreated under different temperature fordifferent time. Then the hardness of the samples were tested to evaluated thethermostability of the shot peeninged alloy. The resuslts indicated that afterheatteated at high temperature for long time makes microhardness ofstrengthened layer decrease, but still retains the strengthening effect of shotpeening at a certain level.