750℃热暴露对定向层片组织铸造TiAl合金室温拉伸塑性的影响

EFFECT OF THERMAL EXPOSURE AT 750℃ ON ROOM TEMPERATURE TENSILE DUCTILITY OF CAST TiAl ALLOY WITH DIRECTIONAL LAMELLAR MICROSTRUCTURE

研究了大气中750℃,48~300 h热暴露对定向层片组织铸造TiAl合金室温拉伸塑性的影响,并采用拉伸中途卸载、染色渗透后再次加载直至断裂的方法,分析因表面脆性层诱发的微裂纹的形成和扩展行为,以揭示定向层片组织在热暴露后保持更好室温拉伸塑性的原因.结果表明,定向层片组织TiAl合金在750℃热暴露150 h后室温塑性仍大于2.0%,300 h热暴露后尚保持1.0%的水平,其热暴露致脆程度远小于双态组织和其它层片组织.在430 MPa应力下,微裂纹起源于脆性贫Al层,并在后续加载过程中扩展进入基体.此裂纹起到尖锐缺口的作用,约束了材料的塑性变形,导致TiAl合金室温拉伸塑性降低.对于定向层片组织,由于层片界面平行于基体表面,有利于抑制微裂纹在后续加载过程中向基体扩展,从而使合金在热暴露后保持较高的室温塑性.

The effect of thermal exposure on room temperature tensile ductility of cast TiAl alloy with directional lamellar microstructure was evaluated at 750 ℃ for 48~300 h in atmosphere.By preloading,unloading,dyepenetrating followed by reloading until fracture for exposed samples,initiation and propagation behavior of the microcrack triggered by surface brittle layer was mainly analyzed in order to explain that the directional lamellar structure retains a better ductility at room temperature after thermal exposure.The results show that room temperature tensile ductility is still retained above 2.0% and 1.0% after exposure for 150 and 300 h at 750 ℃,respectively.The embrittlement of the directional lamellar microstructure caused by thermal exposure is much less than that of duplex microstructure and the other lamellar microstructures.At a stress of 430 MPa,the microcrack forms at the Al-depleted brittle layer and propagates into the substrate during subsequent loading.Just as the sharp notch,the microcrack can constrain the plastic deformation,which is the main mechanism of the brittlement for TiAl alloy by thermal exposure.The directional lamellar microstructure with the lamellae interface parallel to the substrate surface is obtained,which is good for restraining the micro-crack propagation into the substrate and retaining higher ductility at room temperature after thermal exposure.