Study On The Preparation, Casting Defects And Properties Of A New Kind Of Nickel Based Single Crystal Alloy

Nickel-based superalloys have been extensively used for thermal structural components of industrial gas turbines and aircraft engines for its excellent high temperature performance. However, it will be appreciated that the severe operating conditions required in this area necessitates nickel-based superalloy to withstand increasingly high temperatures. Thus, it plays a very important significance on researching preparation, casting defects and the improvement of heat treatment to the microstructure and property of a new kind of nickel-based superalloy.In this paper, the research object is a new kind of nickel-based superalloy, which was cast by HRS directional solidification technology. At first, study on the effects of temperature gradient, drawing speed on this nickel-based single crystal superalloy. Then, analyze casting alloy common defects and its formation mechanism that occur in preparation. Finally, based on this nickel-based superalloy, research the influences of heat treatment to the microstructure and mechanical properties at different temperatures. Here are some main conclusions in this paper:1 A new kind of nickel-based single crystal superalloy, which was cast by HRS directional solidification technology. With the different position to the chilling plate on the mould, temperature gradient between solid and liquid phase is different. The farther the distance to the chilling plate, the smaller the temperature gradient is. Drawing speed play an important significance on the microstructure of this superalloy, and the reasonable drawing rate is from 2.5mm/min to 3.5mm/min.2 At different withdrawal rate, the degree of segregation are influenced by the interaction of diffusion time and diffusion distance. When the withdrawal rate variation between 2.5mm/min to 4.5mm/min, segregation of Re, W is much serious with the upgrading drawing speed, and the other elements in the alloy goes in contrast. At most time, it is easily to form stray grain at the abrupt cross section, even the form position of the stray grain is uncertain. However, we can reduce the tendency of stray grain formed by thermal conductor technology.3 W, Re, etc. are high density elements. We found freckles on the sample surface which prepared in this paper, for this new kind of nickel-base superalloy added Re(3.5%) and W(8.5%).4 Heat treatment can reduce the segregation of the casting alloy, and uniformity of the distribution of compositions. At the same time heat treatment significantly improve the shape, size and distribution of the γ′ phase. It also eliminated most eutectic structures.5 Necking is not obvious in the macroscopic fracture of the alloy, the fracture form before and after heat treatment are respectively belongs to Positive break and negative break. With the increasing temperature, the tensile strength and yield strength of the alloy rose to the peak first and then decreased. Under any temperature conditions, samples after heat treatment had better tensile strength and yield strength than cast samples. Fracture morphology under 750 condition mainly were river pattern and cleavage step in cast samples, but dimple appeared in samples after heat treatment. The cleavage feature was not apparent at fracture when temperature reached 850. Under this temperature, tearing feature can be seen in cast samples. More dimple structure can be seen in heat treatment samples.