The true stress-true strain curves at RT of the alloys with and without SMF treatment are show inFig. 1. It is clear that the SMF treatment improves compressive properties of the alloys at RT, especially compressive ductility. The difference between SMF treated alloys is that the slop of compressive curves at the elastic deformation stage increases with the increase of the treatment temperature. Except the measurement errors, this also indicates that the treatment may produce some influenceon thee lastic modulus. Simultaneously, the yield strength and compressive ductility Of the SMF treated alloys both increase,compared with the HT alloy.
Fig.1.True stress–true strain curves of the HT, MT1, MT2 and MT3 alloys at RT with an initial strain rate of 1.0×10−3s−1.
In order to explain the significant improvement of compressive properties of the SMF treated alloys, the micro-mechanism of Deformation and fracture are discussed as follow. The fracturemor-phologies of RT compressive test alloys with HT and SMF treatment are showninFig. 2. The fracture surface of the HT alloy exhibits typical debonding characteristics with some cleavage and furthermore there are obvious crack along the eutectic cell boundaries. While the fracture morphology of the SMF treated alloy trend to change from debonding to cleavage, as shown inFig.2(b). Except some cleavage of the NiAl, deformed Cr(Mo) phases are observed on the fracture surface, which is beneficial to the compressive ductility. So in the alloy with 10T 1073K SMF treatment, the alloy gets the best compressive ductility.With more high treatment temperature, the fracture surfaces of the SMF alloys present almost completely cleavage, as shown inFig. 2(c) and (d), which implies that the cohesion of grain boundaries get significant improvement, compared the HT alloy. The improvement of grain cohesion is beneficial to the strength, but not to the compressive ductility. So it is understand-able that the yield strengths of the MT2 and MT3 alloys improve, while the compressive ductilities decrease.
Fig. 2.Typical fracture surfaces of RT compressive samples of (a) the HT alloy; (b) the MT1 alloy; (c) the MT2 alloy; and (d) the MT3 alloy.
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