| TiAl alloys is a new promising material used as high temperature structures due to its advantages, such as, low density, high specific strength and excellent oxidation resistance at high temperature. It has been found that the casting defects, including shrinkage and gas porosities, often exists if the liquid pressure head is not enough during casting. In general these kind cast defects can be partly eliminated by the hot isostatic pressing (HIP) treatment, and some defects usually remained in the casting. The researches for the effect of these remained defects on fatigue property of TiAl alloys have hardly reported. For engineering application of TiAl alloy it is necessary to study the effect of the remained defects on fatigue property of TiAl alloy with remained full lamellar structure. Therefore the topic of "the effect of the remained casting defects on fatigue property of TiAl alloy with preferential full lamellar structure" has been selected in the present investigation.The composition of the alloy was Ti-47.5Al-2.5V-1.0Cr-0.2Zr (at.%). The casting samples were HIP treated under the condition of 1270℃/180MPa/1.5h, and annealed and aged at 950℃ for 12h in vacuum. The fatigue test specimens were cut and machined from the HIPed plate. The high frequency rotating bending fatigue tests were carried out on a fatigue testing machine at the temperature of 750℃. The microstructures of samples before and after HIP treatment as well as the fatigue fracture surface were observed by optical microscopy and SEM, and effect of remained defects on the fatigue life of TiAl alloy was studied accordingly.It was found that HIP treatment didn’t change the microstructure of TiAl alloy basically, only few equiaxedygrains precipitated at the grain boundaries. It was found that two kinds defect of the bulk micro-porosity and the bridged porosity interface existed in the alloy after the hot isostatic pressing.The fatigue test result indicated that the fatigue strength σ-1 of TiAl alloy was 360 MPa. According to fatigue life test results under the test condition of σ max=400MPa, the fatigue lives of the HIP treated specimens could be divided into two areas, one area was longer life group where the fatigue lives of the specimens were in the range between 1.08×106to 5.71×106 cycles, and another area was shorter life group where the fatigue lives of the specimens were in the range between 3.48×103 to 1.96×104 cycles.The fatigue fracture observation indicated that the fatigue cracks propagated across the lamellar structure by both twinning and dislocation slip at both the fatigue initiation site and fatigue propagation site. The type of remained defects in high and low fatigue life samples were the bulk micro-porosity defects and the bridged porosity interface defects, respectively. And the type of the defect affected the fatigue life of TiAl alloy significantly. The bulk micro-porosity defects reduce the fatigue life of the specimen up to three orders of magnitude. In addition, the size, the shape and the location of defects of the defects are all affecting factors for the fatigue life of the alloy. It was found that the fatigue lives decreased as the defect size increased; the samples with the round shape defects had longer fatigue life than the samples with sharp shape defects; and the fatigue life of the specimens with the defects in the area of subsurface was significantly longer than that of the specimens with the defects at the surface.The influence rule of the remained defects on the fatigue life of TiAl alloy with preferential direction lamellar structure has been systematically studied in the present investigation. It has been found that the bulk-porosity defects affected the fatigue life of the alloy significantly. Based on this study it can be proposed that the elimination of the bulk-porosity through optimizing HIP parameters is an effective way to decrease the instability of fatigue life of the TiAl alloy with preferential direction lamellar structure. Therefore this investigation is important for increasing the service reliability and promoting the engineering application of TiAl alloy. |
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