Microstructure And Mechanical Properties Of NiAl-Cr(Mo) Eutectic Alloys Processed By Various New Routes

NiAl has received much attention as a promising candidate in the search for a low-density, high-strength structural alloy for high-performance turbine engine applications owing to its attractive advantages over the currently used nickel-based superalloys, including low density, high melting point, excellent thermal conductivity and exceptional oxidation resistance. Unfortunately, the room temperature ductility and toughness as well as the high temperature strength of NiAl based alloys have not been successfully overcome which hold up the practical implementation of them. Research on NiAl has indicated that a number of eutectic alloys consisting of NiAl possess both improved toughness and creep strength. Additionally, incorporation of a refractory metallic phase within the brittle NiAl matrix is one useful technique which has been investigated as a method to obtain better property of NiAl alloy. In particular, the equiaxed and columar NiAl-Cr(Mo)-Hf alloys developed by Guo and his group exhibits much better elevated temperature strength and creep resistance with only 6% weight increase and inappreciable loss of oxidation resistance. But the Ni₂AlHf Heusler phase, existing at the NiAl/Cr(Mo) interface, makes the NiAl/Cr(Mo) interface serrated and leads to the lower ductility and toughness. In this paper, the microalloying and various fabrication methods are tried to improve the mechanical properties of the NiAl-Cr(Mo) eutectic alloys by microstructure refinement.The button billets containing different amount of Dy or B used for microalloying study are arc-melted under an argon atmosphere using a non-consumable tungsten electrode. The button billets are homogenized at 1523K for 24h in air followed by furnace cooling to RT. The maser alloys for rapid solidification, powder metallurgy and spray and deposition investigations are prepared in vacuum induction furnace. Both injection casting and suction casting are conduction for rapid solidification investigation. The hot pressing is used to consolidate atomized powders in powder metallurgy route. The molten master alloy is atomized by argon, and then at 1573K the deposited billet is hot isostatic pressed under 150MPa for 3 hours.The minor Dy and B additions lead to microstructural refinement and eutectic cell boundary strengthening through segregation at the cellular boundary in cast NiAl-28Cr-5.8Mo-0.2Hf alloy. The B is partitioned both into Cr(Mo) phase and at the phase interface in 0.1wt% B containing alloy, the extra B results in the dendritic NiAl and the dendriticmicrostructure dominates in 0.1 wt% B containing alloy. The Dy is distributed at the cellular boundary and no dendrite occurs in Dy containing alloys in the investigated Dy range of 0-0.2 wt%. The RT compressive ductility is proportional to the B content in the B containing alloys without improvement in high temperature compressive strength and the balance of properties is attained at the amount of 0.1wt% Dy for Dy containing alloy. The improvement in compressive is attributed to the boundary strengthening and microstructural refinement. The effecct of trace B and Dy is superior to that of small amount of Hf in the conventionally cast NiAl-Cr(Mo) eutectic alloy in terms of compressive behavior.The lamellar refinement and composition change in constituent phase takes place during the rapid solidification. The rapid solidification castability is weak for the NiAl-Cr(Mo) eutectic alloy owing to the narrow liquid/solid zone and the suction casting can improve the castability by accelerating the flow of the molt. The primary NiAl dendrites are introduced by the rapid solidification because the pseudo-eutectic region is moved to the high-molting-point phase in pseudo binary phase diagram. The improvement of high temperature compressive strength has been achieved for the injection cast NiAl-28Cr-6Mo and suction cast NiAl-28Cr-5.7Mo-0.3Hf alloys and dissolution of Hf in NiAl phase is beneficial to theThe atomization process produces the powders which are uniformly refined size. The powders are contaminated in the atomization process and the porosity was the main defect in the powder metallurgy alloy and the microstructure of the hot pressed prealloyed powders is also refined and uniform. In creep, the microstructure of the hot pressed NiAl-28Cr-6Mo is invarious and the cracks originate fron the porosity. The hot pressed NiAl-28Cr-6Mo alloy exhibited a high-level combination of fracture toughness and tensile strength, whereas the tensile creep resistance of the PM alloy was degraded. The addition of 0.5wt% Zr further enhances the mechanical properties of the PM alloy.Many irregular pores are unevenly distributed through thickness of the deposited NiAl-28Cr-6Mo alloy and the hot-isostatic-pressing can decrease the porosity greatly. The microstructure of the alloy produce by spray deposition technique is refined and the alloying element concentration of the constituent phases is extended. No new phase is introduced in the spray deposition process. The pores of the sprayed billet are decreased and the microstructure is hardly changed by HIPing. The compressive properties are improved by atomization and spray technique compared to convention! casting and the compressive creep mechanism is attributed to the dislocation climbing by self-diffusion.At the strain rate of 2×10^-3s^-1, the suction cast alloy containing Hf has the highest high temperature compressive strength, the spray deposited alloy exhibits the best RT compressive ductility and the hot pressed NiAl-28Cr-6Mo alloy containing Zr presents the optimal combination of RT and high temperature compressive properties in the all studed alloys.