Mechanism of a-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

IN718is the worldwide most used nickel-basesuperalloy because of its unique mechanical propertiesand excellent technological behavior.Alloy IN718ismainly strengthened by phases y"-NijNb with orderedBCT DO22structure and Y-Ni3Al with FCC L12structure.Of these two phases,y"contributes majoreffect in strengthening because of its large lattice misfitwith matrix.But the instability of main strengtheningphase y"at high temperature has limited the highestservice temperature of IN718below650°…     

镍基合金晶界贫铬区演变的动力学模拟和试验验证

利用Thermo-Calc、DICTRA软件及其数据库,建立碳化物长大-Cr原子扩散模型,进行镍基690合金晶界贫Cr区演变的动力学模拟研究,并与扫描透射电子显微分析和能谱分析技术得到的合金晶界Cr含量实测值相比较,验证模型的合理性.结果表明:600～800℃敏化温度范围内进行等温处理时,敏化初期M23C6颗粒长大速度快,尺寸迅速增大;一定时间后长大速度急剧下降,尺寸增速减缓直至M23C6颗粒达到稳定的尺寸.等温条件下晶界Cr含量变化与敏化时间密切相关,敏化初期碳化物快速长大是影响晶界附近最低Cr含量的主要原因;而随着敏化时间延长,晶内Cr原子向贫Cr区的扩散愈加充分,贫Cr区最低Cr含量逐渐提高.该模型计算得到的晶界附近最低Cr含量值接近实测值,该模型可准确预测不同敏化过程中的晶界附近最低Cr含量.     

一种新型高温合金凝固组织和元素偏析行为研究

采用热力学平衡模拟计算、差热分析(DTA)和等温凝固淬火试验(ISQ)相结合的方法对一种新型镍基高温合金K4750合金的凝固组织和主要元素的偏析行为进行研究。结果表明,实际K4750合金的固相线温度为1270℃,液相线温度为1350℃,MC型碳化物析出温度为1320℃;凝固顺序为L→γ(1350℃),L→γ+MC(1320℃)。K4750合金在凝固初期凝固速度较快,液相体积分数下降明显,凝固后期速度开始变慢,在1310～1290℃温度区间内枝晶间连通残余液相开始断开,同时会有大块状的MC相析出,进一步阻碍液相的连通,从而极易导致合金凝固疏松的形成。合金中W、Fe、Cr的溶质分配系数大于1,富集于枝晶干,为负偏析元素;Ti、Nb、C、Mo的溶质分配系数小于1,富集于枝晶间的液相中,为正偏析元素。     

Mechanism of α-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

It is commonly accepted that the drop in mechanical properties of INCONE1718(IN718) after long time exposure at high temperature is a result of coalescence ofγ", γ and the transformation ofγ′ to δ-Ni3Nb phase. Besides the γ′, γ and δ,the precipitation of B.C.C. α-Cr is another factor that affects the mechanical properties of IN718. In this paper, the Thermal-Calc software is used to reveal the thermodynamic condition of α-Cr formation. The calculated sections of the pseudo ternary diagrams of Cr-Nb-IN718 and Cr-Fe-IN718 explain why α-Cr particles do not precipitate dispersedly in γmatrix but in the vicinity of δ phase. The selected area diffraction (SAD) of TEM confirms the crystallography relationship of α-Cr with δ is (010) δ// (-110) α-Cr, [100] δ//[111] α-Cr° In combination of TTT diagram with the Thermal-Calc and TEM results, the mechanism of α-Cr formation in IN718 is suggested.