过冷二元合金凝固组织演化及晶粒细化机制研究

发布时间：2018-03-11 22:11

  本文选题：单相合金　切入点：过冷度　出处：《西北工业大学》2016年博士论文　论文类型：学位论文

【摘要】：采用熔融玻璃包覆与循环过热相结合的方法,在预抽真空及氩气保护条件下,通过高频感应熔炼使大体积Co-20at.%Pd合金和Ni-20at.%Cu合金获得了深过冷。讨论了影响合金净化效果的因素及获得稳定深过冷的控制条件。结合理论模型分析,系统研究了Co-20at.%Pd合金和Ni-20at.%Cu合金的组织演化和晶粒细化现象。探讨了深过冷合金熔体快速凝固组织中的应力累积规律。基于快淬装置设计,系统研究了大过冷度条件下实验合金系晶粒细化的再结晶机制,实现了大过冷大体积Ni-20at.%Cu合金快速凝固组织的再结晶。本文主要研究内容和结果如下:1.采用熔融玻璃净化与循环过热相结合的深过冷技术,在预抽真空和惰性气体保护条件下,使大体积Co-20at.%Pd合金获得了340K的超过冷度,并且在过冷度?T约265K时,合金熔体已达到实际的超过冷状态。在获得的过冷度范围0-340K,Co-20at.%Pd合金的凝固组织发生了两次晶粒细化。在50 K(27)(35)T(27)265K时,合金的晶粒细化归因于枝晶重熔。在265 K(27)(35)T(27)280K时,合金的晶粒细化机制由枝晶重熔机制向应力诱导再结晶机制过渡。利用透射电镜分析技术,发现超过冷快淬合金具有大量位错,层错及小角度亚晶界。这些亚结构说明超过冷快淬合金发生了部分再结晶,但再结晶不充分。在(35)T(29)280 K时,Co-20at.%Pd合金的晶粒细化完全由再结晶机制主导。2.随初始过冷度的提高,Ni-20at.%Cu单相合金凝固组织经历了“粗大树枝晶—粒状晶—定向细枝晶—粒状晶”的转变过程。第一类晶粒细化来源于枝晶重熔。通过再结晶过程中的形核、生长和应力累积理论计算,并结合深过冷Ni-20at.%Cu合金第二类细化组织中位错形貌的透射电镜观察结果,间接证明了导致第二类晶粒细化现象的应力诱导再结晶机制。3.基于两相模型和应力累积模型建立了快速凝固过程应力累积模型,该模型可以半定量地估计深过冷熔体快速凝固过程中初生组织内部的应力累积。当初始过冷度超过临界过冷度值时,凝固收缩和热应变诱导的枝晶间液相流动将导致初生枝晶骨架内产生应力累积,应力将促使初生枝晶骨架发生碎断和塑性变形,同时以应变能的形式存储于枝晶碎片中,并进一步作为后续回复和再结晶过程的热力学驱动力。4.利用Ga-In合金液对深过冷Ni-20at.%Cu合金熔体在再辉之前进行快淬,结果发现,快淬微观组织含有大量显著塑性变形亚结构,例如密集位错网络等。进一步对快淬组织进行等温退火,首次在深过冷快速凝固组织中发现了再结晶现象。而对自然冷却合金进行相同条件下的等温退火,微观组织几乎没有发生变化,这说明自然冷却合金内部储存的塑性应变能远远小于快淬合金组织内的塑性应变能。显然,这些实验研究结果揭示了过冷单相合金在大过冷度区间内发生晶粒细化现象的潜在物理机理之一,即再结晶机制。5.研究了不同过冷度快淬Ni-20at.%Cu合金的微观组织,发现合金试样具有沿其轴线方向的晶粒尺寸梯度,且随初始过冷度的增加,试样顶部近表面微观组织晶粒尺寸逐渐减小。在沿晶粒尺寸梯度方向上,随试样的深度增加,快淬Ni-20at.%Cu合金微观组织内的小角度晶界分数逐渐减小,而孪晶分数逐渐增大,这些是再结晶转变的典型特征,从而说明快淬合金在等温退火过程中发生了再结晶。
[Abstract]:By using molten glass coating method combined with thermal cycle, in vacuum and under argon, by induction melting of the massive Co-20at.%Pd alloy and Ni-20at.%Cu alloy were obtained. The influence factors of purification effect of Undercooled Alloy are discussed and obtain stable control conditions of deep cold. Combined with the analysis of the theoretical model, the microstructure evolution and grain refinement of Co-20at.%Pd alloy system and Ni-20at.%Cu alloy. The effect of the stress accumulation of microstructure of highly Undercooled Alloy Melts in rapid solidification. The design of quenching device based on recrystallization mechanism of grain refinement than the experimental condition of the alloy cold system, realize the recrystallization than the rapid solidification structure of cold mass Ni-20at.%Cu alloy. The main research contents and results are as follows: 1. using molten glass purification and superheating cycle, high undercooling technique In the vacuum, and under the protection of inert gas, the large volume of Co-20at.%Pd alloy was 340K more than cold, and in the degree of supercooling? T about 265K, more than the cold state alloy melt has reached the actual. In the undercooling range of 0-340K, occurred two times grain refinement of solidification structure of Co-20at.%Pd alloy. In 50 (27) K (35) T (27) 265K, grain refining alloy is attributed to the dendrite remelting. In the 265 K (27) (35) T (27) 280K, the grain refinement mechanism of the alloy is composed of dendrite remelting mechanism to stress induced recrystallization mechanism transition analysis. By using the technique of transmission electron microscope, found that more than cold quenched alloy has a large number of dislocations, stacking faults and small angle subgrain boundaries. These sub structure shows more than cold quenched alloy was recrystallized, but recrystallization is not sufficient. In (35) T (29) 280 K, fine crystal grain of Co-20at.%Pd alloy by the recrystallization mechanism leading.2. with early It had increase the cooling and solidification of Ni-20at.%Cu single-phase alloy experienced a transition from the process of coarse dendrite - granular crystal orientation - granular crystal crystal twigs ". The first kind of grain refinement from dendrite remelting. Through recrystallization in the process of nucleation, growth and stress accumulation and observe the results with the theoretical calculation, transmission electron microscope dislocation morphology of undercooled Ni-20at.%Cu alloy second kinds of refinement in the organization, leading to grain refinement phenomenon indirectly proved that the second types of stress induced recrystallization mechanism of.3. two-phase model and stress accumulation model is established for the rapid solidification process of stress accumulation based on the model, the model can semi quantitatively estimate the internal primary tissue during rapid solidification of undercooled melt in the stress accumulation. When the initial undercooling exceeds the critical undercooling, solidification shrinkage and thermal strain induced by the interdendritic liquid flow will cause the primary dendrite bone 鏋跺唴浜х敓搴斿姏绱�绉�,搴斿姏灏嗕績浣垮垵鐢熸灊鏅堕�ㄦ灦鍙戠敓纰庢柇鍜屽�戞，

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