The mechanical properties of the NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites at room temperature are summarized in Table 1. Clearly, with the addition of Hf and Ho, the yield strength, compressive strength and compressive strain all increases obviously. Compared with the increase of strength, the improvement of the compressive ductility is much greater, which increases more than one time. The microhardness tests on the NiAl/Cr eutectic show the Hf and Ho addition increase the hardness of the eutectic structure. Such a change may be attributed to the increase of the interface dislocation, which impedes the movement of the mobile dislocations.
Table 1 Mechanical properties of the NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites
Based on the former researches, the optimizing microstructure is very important to the mechanical properties of the NiAl based eutectic composite. The fine microstructure is beneficial to the RT strength of alloys but detrimental to high temperature strength. However, if lamella is so coarse, the RT mechanical properties will decrease, especially the ductility. Hence it is easy to understood why the addition of Hf and Ho addition improves the NiAl/Cr composite obviously, because the NiAl/Cr(Hf,Ho) composite owns finest NiAl/Cr lamella inside of eutectic cell and the appropriate coarse phases in intercellular region. Moreover, the uniformly distributed Ni2AlHf and Ni2Al3Ho phases also contribute to improve the compressive properties, which could impede the movement of the dislocations. However, it also can find that the 0.1% Ho addition already results in many Ho based phases and promote the precipitation of Hf based phases along eutectic cell boundary. If more Hf and Ho are added, it will be detrimental to the compressive properties, due to their segregation along eutectic cell boundary. According to the recent research, the compressive ductility of eutectic alloy is sensitive to intercellular region. Because this is the last solidified region, the impurities and strengthen particles prefer to gather here, which increase its cracking sensitivity. Appropriate existence of Ho element can react with the impurities, such as S, O, etc, which decreases the crack sources and contributes to the RT mechanical properties. Furthermore, the previous investigations found that the rare earth elements tended to segregate to the grain boundary and phase boundary, reducing the harmful effect of impurities on the cohesive strength, by purifying the grain boundary. All these factors contribute to the improvement of compressive properties.
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