【作者】 童欣；

【导师】 徐锦锋；

【作者基本信息】 西安理工大学 ， 材料科学与工程， 2008， 硕士

【摘要】 本文采用单辊实验装置研究了Ni-28%Sn,Ni-30%Sn,Ni-32.5%Sn和Ni-33%Sn合金的快速凝固及组织特征,通过将金属熔体热传导方程与Navier-Stokes方程相耦合,理论计算了液态合金的冷却速率,并借助XRD和SEM分析技术,深入研究了急冷快速凝固条件下Ni-Sn共晶合金的相选择和组织演变规律,定量分析了快速凝固合金的电阻率、抗拉强度、伸长率和耐腐蚀性能,探索了冷却速率和组织形态与合金性能的相关性。研究结果表明:在快速凝固条件下,Ni-Sn合金不形成亚稳新相,凝固组织由a-Ni和β-Ni₃Sn相组成。Ni-32.5%Sn共晶合金的组织形态以不规则共晶为特征。随着冷却速率的增大,层片状规则共晶组织逐渐减少至完全消失,共晶组织明显细化,沿条带厚度方向的均匀性显著提高,晶体形态由粗大的柱状晶向细小的等轴晶转变。快速凝固合金条带存在明显的边缘效应,条带边缘凝固组织异于中部。Ni-28%Sn亚共晶合金的凝固组织沿垂直辊面方向大致分为三个晶区:急冷面细晶区、中部柱状晶区和自由面粗大等轴晶区。随着冷速的增大,柱状晶区厚度减小,晶体形态由柱状晶向等轴晶转变。Ni-30%Sn和Ni-33%Sn近共晶合金处在共生区内接近共生区边界的成分点上,在冷速较低时,熔体凝固时有先析相析出;但是,当冷却速率较高时,先析相析出被抑制,a-Ni和β-Ni₃Sn相竞相形核,交互生长,生长方式由枝晶生长向共晶生长转变,形成细小、均匀的不规则共晶组织。随着冷速的增大,一方面,合金组织细化、晶界增多,对自由电子的散射作用增强,有效电荷数降低,Ni-Sn共晶合金的电阻率显著增大,急冷面电阻率高于自由面电阻率。另一方面,细晶强化作用增强,合金的抗拉强度呈增大趋势,伸长率相应减小。另外,组织细化和均匀性提高使合金内部电位更趋均匀,快速凝固合金的抗腐蚀性能明显提高。更多还原

【Abstract】 In this paper,the rapid solidification and microstructural characteristic of Ni-28%Sn, Ni-30%Sn hypoeutectic,Ni-32.5%Sn eutectic and Ni-33%Sn hypereutectic alloys are investigated by using a single roller experimental apparatus,the cooling rate is calculated theoretically by coupling the melt heat conduct equation and Navier-Stokes equation,the phase selection and microstructural evolution of the alloys are studied by XRD and SEM techniques, meanwhile,the electrical resistivity,tensile strengthe,longation and corrosion resistance of alloys are measured experimentally,and the relationships between cooling rate and alloy properties are further analyzed theoretically.The results show that:under rapid solidification conditions,Ni-Sn alloy is formed with the a-Ni phase andβ-Ni₃Sn phase,which is same with under equilibrium condition.For Ni-32.5%Sn eutectic alloy,with the rise of cooling rate,the amount of regular lamellar eutectic decreases,the growth form of eutectic microstructure transforms from columnar to fine equiaxed morphology,and the directivity of crystal growth becomes weakening and eutectic microstructure is refined markedly;the rapid solidification ribbon presents edge effect,so the edge morphology is different from that of middle portion.The micrstructural morphology of Ni-28%Sn hypoeutectic alloy along the direction vertical to wheel surface can be roughly divided into three crystal zones:fine equiaxed zone near roller side,columnar zone in the middle part of ribbon and coarse equiaxed zone at free surface respectively.With the rise of cooling rate,the thickness of columnar zone decreases and the complete fine equiaxed grain is formed.For Ni-30%Sn and Ni-33%Sn near-eutectic alloys,the primary phase will precipitate from undercooled melt at low cooling rate.It will be surpressed at high cooling rate,the a-Ni andβ-Ni₃Sn phase will nucleate competitively and grow cooperatively,the microstructure will be markedly refined,the homogeneous irregular eutectic microstructure will be formed. On one hand,with the increase of cooling rate,the microstructure is refined and the amount of grain boundary increases,which intensifies the scattering of free electrons and cut down the amount of effective electric charge,resulting in the rise of alloy electrical resistivity. The alloy resistivity near roller side is usually larger than that at free surface because cooling rate near roller side is larger than that at free surface.On the other hand,with the increase of cooling rate,the tensile strength of alloy foils increases,while the elongation of alloys decreases.Due to the uniformity of internal potential in rapidly solidified alloys,the corrosive resistance of the alloys increases obviously with the rise of cooling rate.更多还原