深过冷Cu—30Ni全金单向凝固组织的力学性能

研究了过冷０－２１０Ｋ的Ｃｕ－３０Ｎｉ合金的组织演化规律。在１０５－１５５Ｋ的过冷范围内实现了自由生长枝晶的单向凝固，获得了单向凝固的单昌组织，深过冷熔体的微观净化和单向快速凝固，有效地去除了合金名的微细夹杂物，减少了宏观偏析和枝晶偏析，显著改善了材料的均匀性，在拉应国作用下材料从沿晶断裂转变为穿晶断裂。     

冷却速度对无铅焊料Sn-3．5Ag合金微观组织和显微硬度的影响

在～10^2K／s、～10^3K／s和～10^4K／s的冷速条件下研究了凝固速度对无铅焊料Sn-3．SAg合金微观组织和显微硬度的影响。结果表明：由于非平衡凝固条件下动力学过冷的影响，导致了该共晶合金实际凝固过程开始于平衡共晶凝固点以下，合金凝固组织中包含初生β-Sn枝晶，且该初生β-Sn枝晶组织随合金凝固速度的提高而发生细化。另外，维氏硬度测试结果表明，无铅焊料Sn-3．5ag合金在不同冷速条件下的凝固组织与显微硬度的关系符合经典Hall-Petch关系式，即初生β-Sn枝晶细化能显著提高焊料合金的显微硬度。     

深过冷Cu-Pb偏晶合金的快速凝固行为和凝固组织

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-25%Pb合金,Cu-37.4%Pb偏晶合金和过偏晶Cu-40%Pb(质量分数)合金过冷熔体凝固行为和凝固组织的演化规律,以及Cu-37.4%Pb偏晶合金的过冷度对磨损率的影响.研究表明:在过冷亚偏晶Cu 25%Pb合金熔体凝固过程中先形成α(Cu)初生相,随着过冷度的增大,凝固组织经历粗大枝晶重熔形成的细化枝晶向准球状晶粒演化的过程;在过冷Cu-37.4%Pb偏晶合金熔体凝固过程中初生相为L2相,当过冷度在20～150 K区间时,得到第二相S(Pb)弥散在α(Cu)枝晶间的凝固组织,并且在该过冷区间内随着过冷度的增加,材料的磨损率也逐渐降低;在过冷过偏晶Cu-40%Pb合金熔体凝固过程中初生相为L2相,在过冷度区间42～80 K时,得到以偏晶胞形式分布的凝固组织.     

过冷过偏晶Cu-40wt%Pb合金凝固组织的演化

用熔融玻璃净化与循环过热相结合的方法,研究了Cu-40wt%Pb过偏晶合金过冷熔体凝固组织的演化规律.在过冷度40～296K的范围内,其凝固组织的形态有3次变化过程:第1次是在40～75K过冷度范围,经过液-液分离和偏晶反应形成了偏晶胞组织;第2次发生在75～196K过冷度范围,因枝晶熟化被抑制,由粗大的枝晶重熔形成的粒状晶转变为高度细化的细枝晶;第3次发生在196～296K过冷度区间,组织因细枝晶再结晶转变为均匀的准球状晶粒.     

过冷Cu-Ni-Fe合金凝固组织的演化

用熔融玻璃净化与循环过热相结合的方法，研究了Cu－390Ni－60Fe（mg·g－1）合金过冷熔体凝固组织的演化规律在过冷度25～304K的范围内，其凝固组织的形态有两次突变：第一次是在过冷度110K时，因枝晶熟化被抑制，由枝晶重熔形成的粒状晶转变成高度细化的细枝晶；第二次发生在过冷度180K时，组织因细枝晶再结晶转变成均匀的准球状晶粒     

深过冷Ni80.3B19.7合金的再辉和非规则共晶的形成

采用熔融玻璃净化结合气体保护的方法,使Ni80 3B19 7过共晶合金获得了407 K的大过冷度,研究了其在不同过冷度下快速凝固过程中的再辉行为.结果表明,Ni80 3B19.7过共晶合金在0～112 K过冷度范围内无明显再辉,在112～323 K过冷度范围内,其再辉曲线表现为两个再辉峰,而在323～407 K过冷度范围内,其再辉曲线为一个再辉峰.初生固相含量的随着过冷度的增大而增大,导致一次再辉度随着过冷度的增大而增大.深过冷Ni80 3B19.7合金凝固组织中非规则共晶的形成,归因于共晶两相在快速凝固阶段以自由枝晶的形式进行的非耦合生长和再辉后的慢速凝固阶段两相枝晶所发生的形态上的转变.     

深过冷DD3高温合金的两次细化机制

用复合熔盐净化与循环过热相结合的方法,获得了最大210K过冷度,研究了DD3高温合金过冷熔体凝固组织的演化规律,在所获得的过冷度范围内,凝固组织的形态发生两次晶粒细化,发生第一次细化的过冷度为30－70K,因枝晶熟化,重熔,高度发达的树枝晶转变为第一类粒状晶;发生第二次细化的过冷度超过153K,凝固组织因枝晶碎断和再结晶而志变为第二类粒状晶。     

冷却速度对无铅焊料Sn-3.5Ag合金微观组织和显微硬度的影响

在～102K/s、～103K/s和～104K/s的冷速条件下研究了凝固速度对无铅焊料Sn-3.5Ag合金微观组织和显微硬度的影响.结果表明:由于非平衡凝固条件下动力学过冷的影响,导致了该共晶合金实际凝固过程开始于平衡共晶凝固点以下,合金凝固组织中包含初生β-Sn枝晶,且该初生β-Sn枝晶组织随合金凝固速度的提高而发生细化.另外,维氏硬度测试结果表明,无铅焊料Sn-3.5Ag合金在不同冷速条件下的凝固组织与显微硬度的关系符合经典Hall-Petch关系式,即初生β-Sn枝晶细化能显著提高焊料合金的显微硬度.     

综论偏晶合金的制备技术:外场下凝固、快速凝固及激光技术

当偏晶合金液过冷至液相分离温度(Tsep)以下时,进入亚稳态难混溶区间,由单一液相分离成两个液相:L1(主体液相,质量分数大于50%)与L2(次生液相,收缩成液滴)。微观组织演化呈现三个阶段:(1)相分离自发进行阶段;(2)主体相合金熔体进入结晶过程;(3)残余的次生相合金熔体进入凝固阶段。尤其是当次生相凝固后弥散分布于主体相基体内时,偏晶合金具有高强、高导以及高耐磨性能,其在航空航天和汽车等工业领域具有重要的应用前景,长期以来受到了研究者的广泛关注。偏晶合金组织结构特征有两种,即第二相弥散型和核/壳结构型。然而,常规凝固条件下,制备的偏晶合金极易形成严重偏析或分层组织,导致制备大块匀质偏晶合金变得困难。为了深入研究偏晶合金液相分离行为,以及微结构特征对偏晶合金性能的影响,研究者提出了许多制备偏晶合金的方法。早在1958年,液相分离现象就在Cu-Fe偏晶合金中被发现,当即引起学者们的广泛关注。近年来,为了制备组织均匀和性能优异的偏晶合金,开发了许多外场作用下的偏晶合金制备方法,旨在消除常规重力场下熔体对流造成的凝固组织偏析、位错、空洞等缺陷。例如,在微重力场条件下,对流作用减弱,可制备接近无偏析的凝固组织;在电磁场条件下,实现了对材料工艺过程的控制和材料组织与性能的改善;在直流磁场和电场交互作用下,熔体流动得到抑制,实现了电磁搅拌控制凝固;在交流磁场和电场交互作用下,实现了电磁搅拌和电磁悬浮,达到减小偏析和改善组织结构特征的目的;在超声场作用下,实现了材料无容器凝固。此外,快速凝固是一个典型的非平衡相变过程,可以消除合金的溶质偏析,获得常规凝固条件下无法获得的成分、相结构和显微组织,显著提高合金的强度、塑性、韧性、延展性和磁性等。为深入了解各类偏晶合金的制备方法,本文主要从外场下凝固、快速凝固、激光技术角度综述了偏晶合金的各种凝固制备工艺和研究方法。     

快速凝固条件下Ni–Pb偏晶合金的组织演变机理研究

目的 研究过冷Ni–0.5%Pb(原子数分数)合金过冷组织的演化行为,阐明其组织演化和晶粒细化的基本机制。方法 采用熔融玻璃净化和循环过热方法制备出过冷度为0~255 K的试样,并结合枝晶生长的动力学–热力学模型,研究其深过冷快速凝固行为机制。结果 在0~255 K过冷度范围内,随着过冷度的增大,Ni–Pb偏晶合金的微观组织发生了2类晶粒细化现象,组织形态由粗大树枝晶向粒状等轴晶转变。结论 第1类粒状晶的形成是由于枝晶熟化和再辉重熔导致发达枝晶破碎,第2类粒状晶的形成是由于在应力和应变能的作用下,枝晶碎变和再结晶引起了晶粒细化。     

Structure and mechanical properties of unidirectionally solidified low alloy steels

Abstract

Two low alloy steels have been unidirectionally solidified in a liquid metal cooling Bridgman crystal grower. The dendrite morphology and dendrite arm spacing have been determined as a function of distance along the bars for solidification at various rates and under different temperature gradients. Microsegregation in the as solidified material was studied by electron probe microanalysis. The tensile properties of heat treated unidirectionally solidified material were determined and their values of elongation to failure found to be considerably greater than for the conventionally cast material. Similarly, the impact energy values of heat treated unidirectionally solidified material are higher than those of the conventionally cast alloy. The tensile and impact properties are discussed in terms of the strain incompatibility present during deformation both at the dendritic grain boundary and at the individual dendrite. Incompatibility of strain leads to a propensity for secondary cracking at these boundaries, the amount of which depends upon the detailed morphology of the dendrite which is determined by the solidification parameters.

MST/880     

铸型涂层对熔体过冷度和凝固组织的影响

利用溶胶－凝胶方法,在铸型内表面玻璃涂层上制备ＳｉＯ２晶态和非晶态薄膜涂层,将深过冷的Ｃｕ７０Ｎｉ３０合金熔体浇入两种涂层铸型中,分别获得９０Ｋ和１９８Ｋ的过冷度。提出了用重合密度和重合原子中心偏离度分析涂层结构对合金熔体形核惰性影响的模型。用ＢＣＴ模型分析过冷熔体凝固过程中枝晶生长与过冷度的关系,结果表明,８０Ｋ为临界冷度,△Ｔ〈８０Ｋ时,枝晶生长受成分过冷控制;△Ｔ〉８０Ｋ时,受热过冷控制。在深过冷范围内,凝固组织为细密挺直枝晶。     

Mechanical properties of undercooled Cu70Ni30 alloy

A series of Cu₇₀Ni₃₀ alloy ingots, each weighing 650 g, were solidified with various undercoolings prior to nucleation. The material was mechanically tested in the as-solidified condition. Although the decrease of dendrite segregation with increasing undercooling is favorable for the improvement of mechanical properties, the sophisticated evolution of structural morphology leads to a non-monotonic change of both elongation and strength. The maximum tensile strength is obtained in the quasi-spherical structure which forms above the critical undercooling 200 K, while the maximum elongation occurs in the longitudinal tension of the columnar dendrite structure solidified at undercooling 180 K.     

Microstructures and Mechanical Properties of Rapidly Solidified Mg-Al-Zn-MM Alloys

Mg-Al-Zn-M M （misch metal） alloy powders were manufactured by inert gas atomization and the characteristics of alloy powders were investigated.In spite of the low fluidity and easy oxidation of the magnesium melt,the spherical powder was made successfully with the improved three piece nozzle systems of gas atomization unit. It was found that most of the solidified powders with particles size of less than 50μm in diameter were single crystal and the solidification structure of rapidly solidified powders showed a typical dendritic morphology because of supercooling prior to nucleation.The spacing of secondary denrite arms was deceasing as the size of powders was decreasing.The rapidly solidified powders were consolidated by vacuum hot extrusion and the effects of misch metal addition to AZ91 on mechanical properties of extruded bars were also examined.During extrusion of the rapidly solidified powders,their dendritic structure was broken into fragments and remained as grains of about 3μm in size.The Mg-Al-Ce intermetallic compounds formed in the interdendritic regions of powders were finely broken,too.The tensile strength and ductility obtained in as-extruded Mg-9 wt pct Al-1 wt pct Zn-3 wt pct MM alloy wereσ-（T.S.） =383 MPa andε=10.6%,respectively.All of these improvements on mechanical properties were resulted from the refined microstructure and second-phase dispersions.     

Hydrogen-induced gas porosity formation in Al–4.5 wt% Cu–1.4 wt% Mg alloy

The effects of low (0.067 cm³/100 g) and relatively high (0.19 and 0.27 cm³/100 g) initial melt hydrogen concentration, solidification processing conditions, and grain refining on the formation of hydrogen-induced gas porosity in Al–4.5 wt% Cu–1.4 wt% Mg alloy have been quantitatively investigated. The study was conducted with unidirectionally cooled laboratory-size ingots solidified at 0.2–37 K/s. An optical microscope-based image analyzer and precision density measurement based on the Archimedes’ principle were used to quantify the characteristics of the hydrogen-induced porosity in the ingots. Predictably, increase in melt hydrogen concentration and decrease in solidification rate increased the amount of porosity and average pore size. However, the effect of solidification rate was greater at the very low melt hydrogen concentration (0.067 cm³/100 g). These results are consistent with reported effects of solidification rate and melt hydrogen content on porosity formation in other aluminum alloys. Addition of grain refiner slightly increased the amount of porosity and the average pore size, especially at solidification rates above 1 K/s.     

Effect of magnetic field on metallurgical structure of magnetostrictive alloys solidified unidirectionally in microgravity

TbFe₂ alloy solidification experiments were conducted in a static magnetic field in microgravity using a 10 m drop tower. When TbFe₂ melt was solidified in a magnetic field from 0 to 0.12T in microgravity, a [111] crystallographic alignment dominated with an increased magnetic field, but the planar macrostructure was random. The magnetostrictive constant of TbFe₂ solidified in magnetic field of 0.12T in microgravity was 2000 ppm at the external 1. 6T magnetic field. When TbFe₂ melt was solidified unidirectionally in a 0. 1 T magnetic field in microgravity, a [111] crystallographic alignment dominated, and the planar structure grew and oriented along the solidification direction. The magnetostrictive constant of TbFe₂ solidified unidirectionally in a 0. 1 T magnetic field in microgravity was 4500 ppm at the external 1. 6T static magnetic field. For all solidification in normal gravity, the maximum magnetostrictive constant remained at 2000 ppm at the external 1. 6T static magnetic field. TbFe₂ crystals grew predominantly along the same direction as the magnetic field, and the planar structure oriented along the solidification direction in microgravity.     

Study of grain refinement and eutectic transformation of undercooled IN718 superalloy

Abstract

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ～250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.