摘要
深过冷熔体的快速凝固是金属凝固领域的前沿研究课题。研究深过冷熔体中的枝晶生长动力学和凝固组织演化规律对于人们认识和利用液态金属的快速凝固过程具有重要的理论和实际意义。
本文首先采用纯金属Cu及Cu-10%Ge、Cu-14.8%Ge合金在熔融玻璃净化工艺条件下的过冷行为,就加热方式、保护气氛、冷却速率以及合金成分的影响进行了研究。在此基础上,本文采用红外测温技术和高速摄影技术研究了Cu-10%Ge和Cu-14.8%Ge合金深过冷熔体的快速凝固过程,对深过冷熔体中的枝晶生长速率进行了实验测定,并对其凝固组织中出现的晶粒细化现象进行了研究。
深过冷实验结果表明,无论是纯金属Cu还是Cu-10%Ge和Cu-14.8%Ge合金,其在感应加热条件下获得的最大过冷度值都明显高于电阻炉加热下的最大过冷度。分析认为,这一差别可能与感应加热条件下样品内存在的强迫对流有关。通过对感应加热下熔融玻璃净化工艺的实验发现,除加热方式外,保护气氛的种类和合金成分是影响熔融玻璃净化条件下液态金属过冷度的关键因素。
实验测量结果表明,Cu-10%Ge合金深过冷熔体中初生相α-(Cu)的枝晶生长速率随过冷度的增大而持续增大。在实验所能达到的过冷度范围内,枝晶生长速率与过冷度之间符合幂函数关系:V=0.00009×△T~(2.34)(m/s)。同样,Cu-14.8%Ge合金深过冷熔体中初生相α-(Cu)的枝晶生长速率也随过冷度的增大而持续增大。在实验所能达到的过冷度范围内,枝晶生长速率与过冷度之间符合幂函数关系:V=0.00001×△T~(2.57)(m/s)。在相同的过冷度条件下,Cu-14.8%Ge合金中初生相α-(Cu)的枝晶生长速率明显低于Cu-10%Ge合金中的数值。这是由于在中低过冷度条件下,枝晶生长速率主要受溶质扩散控制。合金中的溶质含量越高,枝晶生长速率越缓慢。
微观组织分析表明,Cu-10%Ge合金的凝固组织为单相组织,并在特定的过冷度范围内出现明显的晶粒细化现象。在电阻炉加热条件下,合金凝固组织出现晶粒细化的过冷度为45K~77K;而在感应加热下,合金凝固组织出现晶粒细化的最低临界过冷度为83K。Cu-14.8%Ge合金的凝固组织由初生相α-(Cu)和包晶相ζ-Cu_5Ge相组成。在深过冷条件下,其凝固组织中初生相α-(Cu)的晶粒也发生明显的细化现象。在电阻炉加热条件下,合金凝固组织出现晶粒细化的过冷度为74K~85K;而在感应加热下,合金凝固组织出现晶粒细化的最低临界过冷度为83K~103K。分析认为,Cu-Ge合金深过冷快速凝固组织中的晶粒细化现象不仅与过冷度有关,而且还和合金成分、冷却速率密切相关。
此外,对不同成分的Cu-Ge合金快速凝固过程的原位观察表明,在特定的过冷度内,样品表面的宏观固液界面发生了由锯齿状向平滑状的转变。其中,Cu-10%Ge合金宏观固液界面特征发生转变的过冷度范围为74K~83K,而Cu-14.8%Ge合金中宏观固液界面特征发生转变的过冷度范围则为83K~103K。分析认为,这种差异很可能与快速凝固过程中的溶质截留现象有关。
Rapid solidification of undercooled melts has been one of frontier research topics in the field of solidification of metals.Studies on dendritic growth kinetics and microstructure evolution in undercooled melts are of great importance not only to an understanding,but also to applications,of rapid solidification of liquid metals.
In the present thesis,the undercooling behavior of glass fluxed melted of pure Cu and Cu-10%Ge and Cu-14.8%Ge alloys was investigated with respect to heating mode, atmosphere,cooling rate and alloy chemistry.On this ground,the rapid solidification process of the undercooled melts of the two alloy compositions was in-situ monitored using a pyrometer and a high-speed camera.The dendritic growth velocities were determined,and grain-refinement of the solidification microstructure was examined.
The results of undercooling experiments showed that both for pure Cu and for Cu-10%Ge and Cu-14.8%Ge alloys,the maximum undercoolings of the induction-melted samples were always larger than those of the samples melted in an electrical resistance furnace.The larger maximum undercooling of the induction-melted samples was related to forced convection induced by electromagnetic stirring.A set of undercooling experiments under the induction melting condition showed that in addition to heating mode,nature of the atmosphere and alloy chemistry were key factors that influenced the undercooling of the glass-fluxed melt samples.
Measured data on Cu-10%Ge alloys showed that the dendritic growth velocity of primaryα-(Cu) phase increased with increasing undercooling,and followed a power law V=0.00009×△T~(2.34)(m/s) for undercoolings attained.The measured growth velocity data on Cu-14.8%Ge alloys showed the same tendency with increasing undercooling,and followed a power law V=0.00001×△T~(2.57)(m/s) for undercoolings attained.For an identical undercooling, the measured dendritic growth velocity on the Cu-14.8%Ge alloys was lower than that on the Cu-10%Ge alloys.The reason was that the dendritic growth velocity in low and medium undercooling regimes was controlled mainly by solutal diffusion ahead of the growth front. The higher the solutal content in the liquid phase,the lower the dendritic growth velocity was.
The solidification microstructure of the Cu-10%Ge alloys consisted of a singleα-Cu phase regardless of undercooling,but showed grain refinement for certain undercoolings. Under the resistance furnace-heating conditions,grain refinement phenomenon was observed for undercoolings between 45 K and 77 K.However,under the induction melting conditions, it was observed for undercoolings greater than 83 K.The solidification microstructure of the Cu-14.8%Ge alloys consisted of primary a-(Cu) phase and peritecticζ-Cu_5Ge phase.A grain refinement phenomenon similar to that of Cu-10%Ge alloys was observed.The corresponding undercooling regime varied from 74 K to 85 K for the induction-melted samples,whereas it ranged between 83 K and 103 K for the resistance furnace-heated samples.It was suggested that grain refinement in undercooled melts of Cu-Ge alloys depended not only on undercooling,but also on alloy chemistry and cooling rate.
In-situ monitoring of the rapid solidification process of the samples showed that the advancing macroscopic liquid/solid interface had a zigzag feature for low undercoolings,but transited to a smooth one at increased undercoolings.The undercooling regime for the transition of the interfacial feature ranged from 74 K to 83 K for the Cu-10%Ge alloys,but from 83 K to 103 K for the Cu-14.8%Ge alloys.It was supposed that the difference in the undercooling regime between the two alloy compositions might be related to solute trapping during rapid solidification.
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