研究了不同热处理条件下σ相的回溶规律及其对合金持久性能的影响. 研究发现, 在800~900 ℃范围内经过最长1×104 h时效后, 合金中产生的σ相率先在枝晶干的M23C6碳化物附近形成, 之后扩展到枝晶间; 随着时效温度的升高, σ相形成速度加快, σ相形核的孕育时间缩短. 激活能计算结果对比表明, σ相形成初期与Co, Cr的扩散相关, 稳态阶段与Mo的扩散相关; 长期时效后合金在1000~1170 ℃固溶时, σ相都可以回溶到基体, 且固溶温度越高, σ相回溶越快. σ相的回溶动力学研究表明, σ相的回溶速度受Co的扩散过程控制. 对比持久实验结果表明, 合金中的σ相并不能使合金变脆; 经过恢复热处理, 长期时效过程中析出的σ相回溶, 持久寿命提高.
The experimental alloy is designed and employed in high-performance industrial gas turbines as low-pressure turbine blades, working in temperature range of 750~900 ℃. The alloy contains high levels of refractory elements in order to increase the high-temperature mechanical properties. However, this can make the alloy prone to the formation of σ phase during service, which could deteriorate the properties further if the fraction of σ phase exceeds the safety allowances. In this study, the formation of σ phase during long-term thermal exposure, dissolution of the σ phase during rejuvenation process and their influence on stress-rupture properties of a hot-corrosion resistant nickel base superalloy have been investigated. During long-term thermal exposure at 800~900 ℃ for up to 1×104 h, the σ phase formation is mainly in dendrite cores with a few at interdendritic regions. As the aging temperature increases, the precipitation rate of σ phase increases and the incubation time for nucleation of σ phase decreases. From the kinetic analysis, the σ phase form firstly in the vicinity or on the M23C6 in dendrite cores with the strong segregation of W, Cr and Co. The calculated activation energies of σ formation show that the early stage is related to Co and Cr diffusions and the steady stage is related to Mo diffusion. During solid solution process at 1000~1170 ℃, the σ phase precipitated during long-term thermal exposure dissolves to γ matrix. As the solid solution temperature is higher, the dissolution of σ phase becomes faster. Moreover, the σ phase does not embrittle the alloy. The reheat treatment of the alloy leads to the dissolution of precipitated σ phase and further prolongs the stress-rupture life efficiently.
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