Study On The Preparation And Thermostability For Mechanical Properties Microstructure Of Low Density MA/ ODS Nickel-Based Superalloy

In this thesis, the development and application prospects of Nickel-based superalloys were discussed. Based on the previous work, low density MA/ODS Nickel-based superalloys were prepared by powder metallurgy procedures. By analyzing the mechanical properties, microstructure composition and fracture morphology, we studied the effects of different metallurgical techniques, including the nano-Y₂O₃ content, mechanical alloying, compressing molding, sintering, HIP and heat treatment process on the mechanical properties and microstructure as well as detailed causes of the effect. In order to explore thermal stability, high temperature oxidation resistance of the alloy was also studied with oxidation tests.The properties and sintering performance of powders produced by different milling processes were compared. All alloy powders were milled at different rotational speed for 48h with ball-to-powder weight ratio of 20:1. Two methods were adopted to add process control agents. One way was adding 2wt% ethanol as process control agents in the whole milling process and the other way was that milling without process control agents for the first 43h then adding 2wt% ethanol for the following 5h. The first method with milling speed of 300rpm got flake powders, which increased the sintering performance. The second approach could obtain smaller granules dispersion and better mixed powders, which easily absorbed impurities in the air due to the improvement of their activity. This reduced the sintering performance of powder pressing characterize.One kind of ODS nickel-based superalloys with different content of nano Y₂O₃ (0～20wt%) as dispersion phases were prepared by super-solidus liquid phase sintering method at different sintering temperatures. The experimental results indicated that samples with 1.5wt% Y₂O₃ sintered at 1265℃for 2h obtained optimal comprehensive properties of which relative density and tensile strength were 98.13% and 770MPa respectively. The relative densities of the samples were increased with the hot isostatic pressing at the following stage. And the tensile strengths were improved to 838MPa. Along with the increase of the content of nano Y₂O₃, higher sintering temperature leaded to specimens with higher relative densities. For example, alloy specimens with 5wt%Y₂O₃ got optimal relative density 96.79% at 1285℃. Therefore, the best sintering temperature for powders is near their melting point (+ 10℃ 15℃).For the alloy specimens containing Al and Ti and nanoY₂O₃, the tensile properties at room temperature got great promotion because of the precipitation of second phaseγ′. However their relative densities appeared a certain degree of reduction. The heat treatment was taken as follow: 1200℃,4h,AC+900℃,16h,AC.High temperature oxidation tests at 1000℃for 100h were conducted on specimens that had been sintered at different temperature with optimal mechanical properties. The oxidation kinetic curve, surface morphology and chemical composition of oxidation film were analyzed by OM, SEM and XRD. The relationship between Y₂O₃ content and thickness, densification and tightness of the matrix of the oxidation film were compared. At the same time, various theories were used to explain the mechanism of improving high temperature oxidation resistance by dispersive oxide particles. The best adding amount was determined based on these analyses.The surface modification, which includes coarsening, sensitization and activation treatment and electroless nickel plating, were used on nano Y₂O₃ powders. The dispersion properties, compatibility and bonding of oxide particles and matrix were improved and the mechanical performance rose remarkably. ?The relative density of alloy specimens contain 5wt% Y₂O₃ raised from 96.87% without nickel plating to 98.13% after treatment. With improvement of the relative density, the microhardness and tensile strength at room temperature were also given the large enhancement, achieving to 404.6HV and 764MPa respectively.