Interface-controlled fatigue cracking of SCS-6/Ti-22Al-23Nb “orthorhombic” titanium aluminide composite

Abstract

The effect of aging at elevated temperature on interfacial stability and fatigue behavior of a SCS-6/Ti-22Al-23Nb ''orthorhombic'' (O) titanium aluminide composite is investigated. The composite was heat treated in vacuum at 900 ЊC for up to 250 hours to change the microstructural characteristics. The stability of the matrix alloy and interfacial reaction zone after extended thermal exposure was analyzed. The effect of interface on fatigue behavior, including stiffness degradation, evolution of fatigue damage, and crack growth rates, was characterized. Finally, a modified shear-lag model was used to predict the saturated matrix crack spacing in the composite under fatigue loading. The results demonstrate that aging at elevated temperature affects the stability of the interfacial reaction zone, which, in turn, degrades the fatigue properties of the composite. However, fatigue crack will not develop from the ruptured interfacial reaction layer until the thickness of the reaction zone or the maximum applied stress exceeds a critical value.

Growth behavior of short fatigue cracks in a SCS-6/Ti composite

“…In this section, an interface-controlled fatigue cracking model proposed by Her et al [15] will be used to predict several critical parameters which govern the initiation and growth behavior of short fatigue cracks. In this model, the fatigue cracks are assumed to initiate from the micro-notches in the interfacial reaction layer.…”

“…A detailed description of the analysis is given in Ref. [15]. The predicted saturated matrix crack spacing as a function of maximum applied stresses for the SCS-6/Ti -15 -3 composite is presented in Fig.…”

“…The presence of short fatigue cracks and their growth and coalescence during fatigue to form macroscopic defects in composites are of fundamental importance to the development of more a fatigue-resistant composite material. In an earlier study, Her et al [15] have reported that the micro-notches in the interfacial reaction layer serve as the primary fatigue crack initiation sites in an unnotched SCS-6/Ti-22Al-23Nb composite with a thick interfacial reaction layer. The threshold stress intensity factor range (DK th ) for short fatigue cracks was also found to be much lower than that for long fatigue cracks in the Ti-22Al-23Nb matrix alloy.…”