The true stress-true strain compressive curves at RT of the as-cast and RS alloys with and without HIP treatment and their mechanical tests data are shown inFig. 1 andT able 1, respectively. Obviously, the true stress-true strain curves of the alloys with different states almost exhibit the similar trend with continuous hardening after yield. But the slopes of the compressive curve during elastic deformation of the RS and HIP treated RS alloys are higher than that of the as-cast and HIP treated as-cast alloys, which implies that the RS alloy has a relative higher modulus of elasticity. However the HIP treatment almost has no effect on the modulus ofelasticity, the RS alloy has the same slope with the HIP treated RS alloy. Such changes should be ascribed to the microstructure evolution caused by the rapid solidification, because the rapid solidification refines the NiAl and Cr2Nb phase, which can influence the modulus of elasticity.
Fig.1.True stress-true strain compression curves of the as cast and RS alloys with and without HIP treatment at room temperature (Inset picture showing the transverse section of the compressive sample).
Table 1
Results of mechanical properties of the alloys at RT with an initial rate
The mechanical properties data show that the RS alloy has the highest yield strength that is three times that of the as-cast alloy. Compared with the as-cast alloy, the compressive strength and compressive ductility of the RS alloy increase 1 13% and 60%, respectively. The HIP treatment increases the mechanical properties of the as-cast and RS alloy further. Especially the compressive ductility, the HIP treatment increases the as-cast and RS alloy four times and six times, respectively. The RS alloy with HIP treatment has the highest compressive strength and compressive ductility. What is interesting is that the HIP treatment decreases the yield strength of the RS alloy.According to the previous researches, the last solidified region always has the worst mechanical properties, due to the segregation of detrimental elements. In addition, the difference of lattice parameters between NiAl phase and Cr2Nb phase is great, so great interface stress would generate along the phase interface.
These factors make the NiAl/Cr2Nb phase interface the weakest place in the alloy. So it is easy to understand that the cracks prefer to propagate along the NiAl/Cr2Nb phase interface, as shown in the inset image inFig. 1. Moreover it alsofinds that the Cr2Nb phases along the NiAl phase boundary are broken into small ones, but no crack is found in the NiAl, which indicates that the Cr2Nb phases act as the protective shell and bear the main forces during compression test. In the present study, the yield strength of the as-cast alloy is almost the same as that of the NiAl. Based on the recent research, it can be concluded that the cracks generate in the NiAl and propagate rapidly in the as cast alloy. So the strengthening effect of Cr2Nb phase is almost no use. The rapid solidification refines the NiAl and Cr2Nb phases significantly, which contributes much to the compressive strength and yield strength but little to the compressive ductility. Because the rapid solidification just refines the phases, but does not change the morphology of the Cr2Nb precipitates and bulk phase. The great stress along NiAl/Cr2Nb phase interface still influences the mechanical properties greatly. The HIP treatment blunts the tip of the Cr2Nb precipitates and spheres the precipitates, which decreases the stress concentration and handicaps the generation of crack. Additionally the HIP treatment leads to more Cr and Nb elements solid soluted into NiAl matrix and small precipitates, which improves the strength of the alloy. Moreover the HIP treatment also results in many dislocations along Cr2Nb precipitates, as shown inFig. 2(a) and (b). This implies that the HIP treatment may results in some predeformation in the alloy. According to the recent research, these dislocations are movable and beneficial to the ductility of the materials. The decrease of yield strength in the HIP treated RS alloy confirms the conclusion, because the movable dislocations promote the deformation and then reduce the yield strength.
Fig. 2.(a) Bright-field TEM image showing the dislocations piling up near the rod-like Cr2Nb phase; (b) Bright-field TEM image of dislocations around the polyhedral Cr2Nb phase
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