13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- material was more than 0.6%. The coating can be even super-plastic at high enough temperature [22]. The DBTT of NiAl is 693 °C measured by [23]), which however should also be dependent upon the coating thickness according to the results of [24]. In this type of sample through-coating cracks were commonly found with the oxidation around (Fig. 6(a) and (b)). The formation of γꞌ in OZ may weaken the strength grain boundary and induce cracks (Fig. 6(c)). When a through-coating crack was meeting or is located near a grain boundary in the substrate, it could continue to penetrate into the substrate, being identified by the “L”-shape crack path near the coating part (Fig. 6(b)). The cracking of the coating makes it a non-load carrying member and leaves the substrate in a somewhat similar creep situation as the uncoated sample. Figure 6. Fractured NiAl-NiAl sample for 850 °C: (a) morphology containing fracture surface, (b) “L”-shape path of though-coating cracks and (c) γꞌ precipitates in OZ. (The coating part is marked in Fig. 6-8). 3.2.3. PtAl-PtAl Figure 7. Fractured PtAl-PtAl sample: (a) morphology containing fracture surface (850 °C), (b) crack beneath PtAl (850 °C) and (c) morphology containing fracture surface (950 °C). The addition of Pt into aluminide diffusion coating was initially intended to slow down the inward diffusion of Al. However the “diffusion barrier” effect by Pt has been demonstrated to be not so effective [25], which can be further supported by the results in Fig. 4 in this study. Although the PtAl coating has a higher DBTT (795 °C [26]) than NiAl (693 °C [24]), no significant through-coating cracks but some ductile deformation shapes were observed in PtAl at 850 °C (Fig. 7(a) and (b)). Addition of Pt prevents the conversion of β to γꞌ according to the analysing results in section 3.1, which is further demonstrated by the absence of γꞌ in OZ (Fig. 7(b)). This is probably the reason for the ductile behavior of the OZ in PtAl at 850 °C, different from the brittle behavior of
RkJQdWJsaXNoZXIy MjM0NDE=