Effect of grain boundary precipitates on the stress rupture properties of K4750 alloy after long-term aging at 750°C for 8000 h
Effect of grain boundary precipitates on the stress rupture properties of K4750 alloy after long-term aging at 750°C for 8000 h
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DOI：
10.1016/j.jmst.2021.03.022
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发表时间：
2021-04
影响因子：
10.9
通讯作者：
Meiqiong Ou;Yingche Ma;Kunlei Hou;Min Wang;Guangcai Ma;Kui Liu
中科院分区：
文献类型：
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作者：
Meiqiong Ou;Yingche Ma;Kunlei Hou;Min Wang;Guangcai Ma;Kui Liu
K4750合金经750 °C × 8000 h长期时效后，晶界析出相的演变包括块状MC碳化物的退化和粒状/针状η相的析出。MC分解过程中，MC碳化物中析出的Ti和C分别与γ基体中的Ni和Cr结合，形成η-Ni 3 Ti相和富Cr的M23 C6碳化物。晶界处大量颗粒状η相析出，针状η相从晶界向晶粒内部逐渐长大。由于η相通过吸收γ '相而生长，在η相周围形成γ'贫化区。MC碳化物和η相的演变是导致持久寿命降低和伸长率提高的主要原因。在750 °C和360 MPa下测试LA样品之后，通过电子背散射衍射（EBSD）研究残余应变分布。结果表明，残余应变主要分布在晶界处，尤其是MC降解区和η相边缘，与相界面的出现密切相关。微孔洞/裂纹容易在相界面处萌生，然后沿γ′贫化区沿着扩展，因此LA试样的持久寿命明显低于标准处理试样。特别是750 °C /8000 h LA后，由于大量细小的MC、M23 C6和η相颗粒在晶界处分布，韧窝的形成阻碍了微孔/裂纹的形成，导致伸长率的增加。
In K4750 alloy, the evolution of grain boundary (GB) precipitates, including the degradation of blocky MC carbide particles and the precipitation of granular/needle-like η phase particles, were observed after long-term aging (LA) at 750 °C for 8000 h. During MC degradation, the Ti and C released from the MC carbide combined with Ni and Cr, respectively, in the γ matrix to form η-Ni3Ti phase and Cr-rich M23C6carbide. Large amounts of granular η phase precipitated at GBs and the needle-like η phase grew gradually from GBs toward the grain interior. Because of the growth of the η phase through absorbing γ′ phase, γ′-depleted zones were formed around the η phase. The evolution of the MC carbide and η phase was primarily responsible for the decrease of the stress rupture life and the increase of elongation. After an LA sample was tested at 750 °C and 360 MPa, the residual strain distribution was investigated by electron backscatter diffraction (EBSD). The results showed that the residual strain mainly distributed at GBs, especially in the region of MC degradation and at the edges of η phases, which was closely related to the appearance of phase interfaces. Microvoids/cracks easily initiated at phase interfaces, then easily extended along the γ′-depleted zones, thus the stress rupture life of LA samples was substantially shorter than that of samples subjected to the standard treatment. In particular, because of large amounts of fine degraded MC, granular M23C6and granular η phase particles distributed at GBs after 750 °C /8000 h LA and microvoid/crack formation could be hindered by the formation of dimples, which led to an increase of elongation.