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系统研究了高硼DZ444定向凝固合金的初熔行为及其对力学性能的影响.结果表明,在铸态合金中,枝晶间包含大量γ/γ'共晶、MC碳化物和由硼化物、Ni5Hf及η相组成的“团聚相”.在固溶处理期间,团聚相周围受硼化物显著影响的γ基体首先发生初熔.硼化物不是初熔的形核点,但是对初熔的形成具有关键作用.较高的B含量,使得合金具有较低的初熔温度,介于1160～1170℃之间,明显低于正常合金.提升温度或延长保温时间,初熔现象变得更加严重.采用水淬方式,初熔倾向于凝固为典型的γ枝晶和大量细小的沉淀相颗粒;而采用空冷方式时,初熔依次凝固为团聚相、γ基体和γ/γ'共晶相,团聚相形貌与铸态时没有明显差异.完整热处理时,固溶温度由1210℃提升到1230℃,初熔略微增加,而当温度达1250℃时,初熔区尺寸和面积分数剧烈增大,对合金造成严重破坏.由于温度较低,合金的高、低温时效对初熔组织影响不是很大.随着初熔区尺寸和面积分数增加,初熔区消耗了大量的固溶强化元素,同时初熔区内部易萌生大量微裂纹,从而使合金的拉伸性能稍有下降,持久性能显著降低.

A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications,which has high strength and excellent hot corrosion resistance at high temperatures.The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 γ'phase.To achieve a uniform distribution of precipitated γ'particles for optimized mechanical properties,the suitable heat treatments are used.However,the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting.Incipient melting microstructrue evolution during heat treatment has been hardly reported.Therefore,the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work.The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ'/γ eutectic,MC carbides and multi-phase eutectic-like constituent which are composed of boride,Ni5Hf and ηphases.During solution treatments,incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly,but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides.Compared to DZ444 alloy with the normal boron content,incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃.Incipient melting can occur significantly with the increase of the solid solution temperature or time.Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment.However,incipient melting forms multi-phase eutectic-like constituent,γ matrix and γ'/γ eutectic successively during air cooling (AC) following solution treatment,and the morphology of multiphase eutectic-like constituent is similar to that of as-cast alloy.Firstly,a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent;with the increase of the solid solution temperature or time,IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively.Finally,a incipiently melted pool forms gradually.Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly.However,incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃.Especially,IMR swallows up plenty of γ matrix,and many matrix islands,regions unmelted,exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly.The incipient melting has a minor effect on the tensile properties,nevertheless,decreases the creep-rupture properties remarkably.The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.