Fabrication And Mechanical Properties Of ZrO₂ /NiAl Matrix Composites

Currently, it is one of the hot hottest topics to improve the room-temperature toughness and high-temperature strength of NiAl through the preparation of its composites. Because of the similar thermal expansion coefficient with NiAl and the tendency of phase transformation under pressure, ZrO₂ has advantages over other kinds of reinforcements and attracted much attention in recent years. However, the influence of particle size and phase structure of ZrO₂ on the composites mechanical properties has not been fully understood. Therefore, research in this field has great theoretical value as well as practical significance. In this paper, ZrO₂/NiAl composite materials were prepared by mechanical alloying(MA) and spark plasma sintering(SPS). The goal is to produce composites with smaller grain size and enhance their mechanical properties by the transformation toughening mechanism and dispersion strengthening mechanism of ZrO₂ to improve the mechanical properties of NiAl. In this paper, the formation of NiAl during mechanical alloying and the microstructure and mechanical properties of the composites were investigated.The size of the Ni-Al powder increased at the beginning of the ball milling(BA) process and decreased by prolonging the time, and finally reached the dynamic equilibrium. NiAl nano crystalline could be obtained when the time of BA reached 30h. In addition, the degree of ordering of NiAl grains was decreased because of the BA process. Using SEM and TEM, atom diffusion zones could be observed within the powder particles after various alloying time, and phases of NiAl₃ and Ni3Al could be found in the powder when the time of BA reached 3h. Besides, relatively large NiAl grains were found by TEM after ball milling for 7h, these grains were formed under thermal explosion condition during the milling process. This result indicated that during mechanical alloying, there were both thermal explosion and diffusion mechanisms in the formation of NiAl compound.Grain growth could be avoided to the maximum and the grains with low degree of ordering could be preserved by SPS technique. It's noteworthy that there were two new phases, Al₃Zr4 and Al₂O₃, existing in the composite besides ZrO₂, which was not reported before. And their existence was confirmed by TEM analysis. After analyzing XRD and TEM results, it was conjectured that the new phase was formed by the reaction of NiAl with ZrO₂ during the sintering process.In this paper, the mechanical properties of the composites fabricated in the present work have been largely improved. Firstly, the fracture toughness of NiAl raises up to 29Mpam1/2, and the fracture toughness of the composites has the same value. Moreover, because of the transformation toughening mechanism of ZrO₂, fracture toughness of NiAl/TZ-3Y (2μm) composite is higher compared with NiAl. Secondly, because of the grain boundary and second phase strengthening, both the bending strength of the composite at room temperature and its compressing strength at elevated temperature were better than NiAl. Specifically, the compressing strength at 1000 and 1100℃were 244 and 144MPa, respectively, which is larger than that of any NiAl based composites which has ever been developed.