SiC纤维增强TiAl基复合材料残余应力的调控

Regulation of Residual Stress in SiC Fiber Reinforced TiAl Matrix Composites

武器装备的轻量化设计以及更苛刻的高温服役要求,对材料提出了更高的要求。连续SiC纤维增强钛基复合材料(SiCf/Ti复合材料)具有良好的比强度和综合力学性能,成为了备受关注的轻质高温结构材料。但受局限于SiCf/TiAl复合材料的工艺特点,在成型过程中不可避免的会引入热残余应力而引起材料内部缺陷,对材料的力学性能带来不利影响。研究表明,SiCf/TiAl基体中具有较高的残余张应力,是SiCf/TiAl复合材料基体缺陷产生的主要原因。当SiC纤维体积分数(V_(f))为40.54%时,TiAl基体中最大残余张应力为1083.7 MPa(高于TiAl室温断裂强度)。为了调控SiCf/TiAl复合材料内部残余应力,采用有限元分析法计算了不同位置引入不同厚度的钛合金涂层,获得涂层的位置、厚度以及钛合金种类对SiCf/TiAl复合材料热残余应力的影响,从而获得SiCf/TiAl复合材料的残余应力的优化方案。对于SiC纤维V_(f)为40.54%的SiCf/TiAl复合材料而言,采用引入5μm(内)/1μm(外)涂层时,相比原始SiCf/TiAl复合材料,TiAl基体中最大残余张应力可降低约10%(低于TiAl室温断裂强度),从而在一定程度上降低复合材料中基体开裂等缺陷产生的风险。

Continuous SiC fiber reinforced titanium matrix composites(SiC/Ti composites)were a kind of composites containing SiC fiber and titanium alloy,respectively as the reinforcement and the matrix.It could be widely used in the aerospace field due to its high specific strength,specific stifness and favorable fatigue resistance and creep resistance properties.The service temperature of Sic/Ti composites was closely related to the properties of the titanium alloy matrix.With the service temperature increasing,the titanium alloy matrix had been developed from medium-temperature titanium alloy gradually to high-temperature titanium alloy,and then to intermetallic compounds,such as Ti,AlNb and TiAl.The thermal residual stress would be induced by the mismatch of thermal expansion coefficient between titanium alloy and SiC fiber during the high temperature consolidation processing,which adversely affected the mechanical properties of the material.Therefore,it was important to control the residual stress of SiC/Ti composites.Although the measurement method and calculation of residual stress had been researched in the previous studies,definite scheme was still absent for the residual stress control.The finite element analysis(FEA)was applied to calculate the thermal residual stress of SiC/TiAl composites through establishing the hexagonal fiber arrangement model for exploring the stress optimization resource.Thus,the influence of coating position,thickness and type on the thermal residual stress of Sic/TiAl composites as well as the optimal scheme was obtained.Specifically,Workbench was used for finite element model building,in which the three-dimensional(3D)model including SiC fiber,C coating,TiAl matrix and the added inner and outer coatings.An 1/4 hexagonal model was used to calculate the residual stress.All of the displacements of the bottom node(x-z plane)in the y direction,the left side node(y-z plane)in the x direction and the top node(x-y plane)on the hexagon in the z direction were zero.In addition,the hypotenuse surface node,the right-side node and the lower bottom node of the hexagon were coupled to ensure the consistent displacement of the material during shrinkage in cooling processing(from 940℃ to room temperature).Meanwhile,SiC/TiAl composites were prepared by matrix coating method to compare and verify with the calculated results.based on the above calculation and research,the influence of different SiC fiber volume fraction(V)on the residual stress of SiC/TiAl composites was studied.The results showed that all of the tensile stress in TiAl matrix with different V_(f) was near C coating,and the maximum tensile stress located at the fiber distance which was the shortest.The maximum tensile stress gradually decreased with the increase of the distance from the fiber,which decreased from 1272.2 to 983.81 MPa with V_(f) from 53.86% to 31.60%.The maximum tensile stress of TiAl matrix was 1083.7 MPa at V_(f) of 40.54%,which was quite close to the ro0m temperature fracture strength of TiAl alloy(1120 MPa).Therefore,the crack in TiAl matrix was prone to produce in SiC/TiAl composites.Lots of crack were observed indeed in TiAl matrix actually located at the shortest distance along the line of two fibers,which was consistent with the calculation results.Furthermore,the effects of position,thickness and type of titanium alloy coating on the thermal residual stress of SiC/TiAl composites were studied.The results showed that the maximum tensile stress increased with the thickness of the outer coating when the thickness was≥2μm(except 1μm)was introduced.Therefore,it was suitable to introduce thin(1μm)ductile titanium alloy coating in the outer layer of TiAl matrix.While the ductile titanium alloy coating was introduced between C coating and TiAl matrix,the maximum tensile stress in TiAl matrix decreased compared to the original SiC/TiAl composites.Also,the maximum tensile stress in hoop direction decreased gradually with the thickness growth of the inner titanium alloy layer,which was due to the mismatch between the thermophysical properties of C coating and TiAl matrix.It was more beneficial to reduce the maximum residual tensile stress in TiAl matrix by introducing a coating with matched thermal expansion coefficient at this position.The results showed that the maximum residual stress in TiAl matrix gradually decreased,companied with the thickness of the inner titanium coating increased from 1 to 5μm when the outer titanium coating designed as 1μm.Based on the above research results,the titanium alloy introduction scheme was set as 5μm(inner)/1μm(outer),and also different coating types including of nearlyβtitanium Alloy A,intermetallic compound Alloy B andαtitanium Alloy C were applied in the calculation.The final results showed that the maximum residual stress of SiC/TiAl composites could be reduced to 977.69 MPa when inserting 5μm(Alloy B)/1μm(Alloy B)coating.The 10% reduction of maximum residual stress certainly reduce the risk of the cracking in TiAl matrix to a certain extent.This was because there was similar thermal expansion coefficient between Alloy B and TiAl alloy,contributing to easier coordinate deformation with TiAl matrix in the cooling process,further better stress reduction could be attained with the scheme of 5μm(Alloy B)/1μm(Alloy B).Compared with the original SiC/TiAl composites,the cracks in TiAl matrix were reduced to some degree and the cracking degree was also weakened obviously in SiC/TiAl composites prepared by precursor with the structure of 5μm(Alloy B)-TiAl-1μm(Alloy B)through microstructure observation,which was consistent with the calculated results.