A simple model for lamellar peritectic coupled growth with peritectic reaction

The lamellar peritectic coupled growth in Fe-Ni peritectic system was investigated using the equilibrium Boettinger-Jackson-Hunt model. It was found that the slope of the undercooling vs. lamellar spacing is very near zero around the minimum overheating, and the coupled growth can exist under this condition even if the slope of the undercooling vs. lamellar spacing curve is slightly smaller than zero. In addition, the peritectic reaction can never reach completion during the peritectic coupled growth. So the equilibrium peritectic coupled growth was modified by considering the incompletion of the peritectic reaction. It was shown that when the fractions of the peritectic reaction reach 60%–80% completion, the calculated undercooling vs. lamellar spacing curves agree well with the experimental observations in the directionally solidified Fe-Ni alloys. Supported by the National Natural Science Foundation of China (Grant Nos. 50395102 and 50271020) and the Program for New Century Excellent Talents in University (Grant No. 05-0530)     

强磁场对Mn-Sb包晶合金相变及凝固组织的影响

以Mn-56.5 wt%Sb包晶合金为研究对象, 进行了不同磁场、不同冷速条件下的凝固实验. 通过对液相线温度、包晶温度的考察, 发现强磁场可以提高Mn-56.5 wt%Sb合金的液相线温度, 且该上升值随磁感应强度的增加而增加, 当所施加的磁感应强度为11.5 T时, 液相线温度升高大约3 ℃, 但施加磁场后包晶反应温度没有明显改变. 对该合金的凝固组织进行定量金相分析发现, 施加磁场后MnSb相明显减少, 该结果与磁场对相变温度的影响相一致. 另外通过X射线衍射分析发现, 强磁场诱发包晶反应生成相MnSb的c轴垂直于磁场方向取向, 而Mn₂Sb相的(311)面平行于磁场方向取向. 对不同冷速凝固的Mn-56.5 wt%Sb合金组织进行定量金相分析结果显示, 强磁场对合金凝固过程的作用效果受到冷却速度的影响. 随着冷却速度的增加, 强磁场对该合金凝固组织中MnSb相的相对含量变化影响效果减弱. 关键词： 强磁场 包晶合金 凝固 相变温度     

Heat and mass transfer characteristics during rapid solidification of Fe-Cu peritectic alloys

The viscose flow and microstructure formation of Fe-Cu peritectic alloy melts are investigated by analyzing the velocity and temperature fields during rapid solidification, which is verified by rapid quenching experiments. It is found that a large temperature gradient exists along the vertical direction of melt puddle, whereas there is no obvious temperature variation in the tangent direction of roller surface. After being sprayed from a nozzle, the alloy melt changes the magnitude and direction of its flow and velocity rapidly at a height of about 180 μm. The horizontal flow velocity increases rapidly, but the vertical flow velocity decreases sharply. A thermal boundary layer with 160–300 μm in height and a momentum boundary layer with 160–240 μm in thickness are formed at the bottom of melt puddle, and the Reynolds number Re is in the range of 870 to 1070 in the boundary layer. With the increase of Re number, the cooling rate increases linearly and the thickness of thermal boundary layer increases monotonically. The thickness of momentum boundary layer decreases slowly at first, then rises slightly and decreases sharply. If Re < 1024, the liquid flow has remarkable effects on the microstructure formation due to dominant momentum transfer. The separated liquid phase is likely to form a fiber-like microstructure. If Re>1024, the heat transfer becomes dominating and the liquid phase flow is suppressed, which results in the formation of fine and uniform equiaxed microstructures. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

δ/γ transformation in non-equilibrium solidified peritectic Fe-Ni alloy

Through phase transformation kinetic analysis and experimental observation, the δ/γ transformation occurring in the non-equilibrium peritectic Fe-4.33at.%Ni alloys was systematically investigated. According to JMA solid-state transformation kinetic theory, the Time-Temperature-Transformation (TTT) curves of the δ/γ transformation in peritectic Fe-Ni alloy were calculated. On this basis, the physical correlation between the δ/γ transformation and the initial undercooling of melt (△T) was elucidated. The results indicate that the change of △T can alter not only the overall δ/γ transformation pathways but also the transformation fraction with respect to each transformation mechanism.     

Peritectic phase growth in directionally solidified Cu-70% Sn alloy

The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t ^1/2, which is consistent with the theoretical model. Supported by the National Science Foundation of China (Grant No. 50395102)     

Ti-45at.% Al合金定向凝固过程中显微组织演化的计算机模拟

采用基于溶质扩散控制模型CA方法对Ti-45at.% Al合金定向凝固过程中显微组织演化过程进行了数值模拟.模拟结果表明,在温度梯度较高时,初生晶核间距无论大小,均可通过分枝或竞争生长而使一次胞晶/枝晶间距得到调整.在抽拉速度一定情况下,随着温度梯度增加,胞晶/枝晶间距减小,在温度梯度一定情况下,随着抽拉速度增大,观察到胞晶/枝晶混合结构,混合结构区随温度梯增大而增大,模拟结果与实验观察相符合. 关键词： Ti-45at.% Al合金 定向凝固 组织演化 数值模拟     

快速凝固Co-Cu包晶合金的电学性能

研究了Co-Cu包晶合金快速凝固过程中的相选择和组织形成特征, 探索了冷却速率、组织结构和晶体位向与合金电阻率之间的相关规律.实验发现, 快速凝固可使Co在(Cu)中的固溶度扩展至20%.Cu含量大于80%时, L+αCo→(Cu)包晶转变被抑制, (Cu)可从过冷熔体中直接形核析出.Cu含量在40%—70%范围时, Co-Cu合金的液相分离受到抑制, 凝固组织沿条带厚度方向分为两个晶区.细晶区中αCo和(Cu)相竞争形核并生长, αCo枝晶形态细密，细小的（Cu）等轴晶均匀分布于αCo的基体之中.粗晶区αCo相为领先相, 富Cu相分布于αCo枝晶的晶界处.随着冷速的增大, 合金组织显著细化, 晶界增多,对自由电子的散射作用增强, 合金电阻率显著增大.当晶界散射系数r＝0996—0999时, 可采用M-S模型综合分析快速凝固Co-Cu合金的电阻率. 关键词： 电阻率 快速凝固 相结构 晶体生长     

定向凝固生长对Ni-Mn-Ga-RE (RE=TbSm)合金磁感生应变的影响

关键词：     

Phase separation and rapid solidification of liquid Cu<Subscript>60</Subscript>Fe<Subscript>30</Subscript>Co<Subscript>10</Subscript> ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu₆₀Fe₃₀Co₁₀ ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V _M) and Stokes motion velocity (V _S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V _M/V _S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V _M/V _S value for Fe(Cu,Co) droplets with the same size. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105) and the Scientific and Technological Creative Foundation of Youth in Northwestern Polytechnical University of China (Grant No. W016223)     

Mode selection in a caricature of eutectic solidification

We have studied the problem of mode selection, specifically, selection of the lamellar spacing, in eutectic solidification, by constructing a caricature of a thin-film eutectic solidifying at constant velocity in the presence of a finite temperature gradient and in a slightly noisy environment. Our model incorporates mechanisms which allow termination of substable lamellae and creation of new lamellae by the splitting of lamellae larger than a spacing _max. We have studied this model both by a simple analytic approximation and by computer simulation. We find that both the steady state spacing and the regularity of the pattern are sensitively dependent on the ratio _max/_min, where _min is the minimum spacing required for steady state stability. An additional result to emerge from our investigation is that when _max becomes less than a critical value, the system apparently undergoes a transition into a chaotic state.For a review of the eutectic problem see Ref. 2.     

烧结Nd-Fe-B磁体的微观结构和冲击韧性研究

对烧结Nd₁₅Fe_72-xCo_yNb_xB₈(y=0, 5;x=0, 05, 10, 15, 20, 25)永磁体的微观结构和冲击韧性及二者间的关系进行了研究.结果表明,添加Nb能够改善Nd-Fe-B磁体的微观结构,提高磁体的冲击韧性.当无Co磁体中Nb的原子百分含量为15%时,其冲击韧性达到最大;对于添加了Co的磁体,其冲击韧性的 关键词： Nd-Fe-B 冲击韧性 微观结构 晶界     

Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO₂ laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210~mm/s. An implicit expression of the critical solidification velocity for the cellular--planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M--S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS was successfully achieved in the present experiments.     

Effects on the evolution of microstructure and properties of low-silicon cast aluminum alloys in service

ABSTRACT

The microstructure evolution and property change of four kinds of low silicon cast aluminum alloy exposed to heat for 0–50?h at 200°C were studied by means of Brinell hardness test, tensile property test, friction and wear property test and XRD analysis. The results show that with increasing thermal exposure time, the tensile strength of each group of samples decreased and the amount of wear increased. The tensile strength of samples with more Si content decreased slowly. When the time increased to 50?h, the increase of wear loss was the largest. The hardness of samples after thermal exposure increases compared with that before thermal exposure. The residual stress of (311) diffraction crystal surface of AlSi3.5Mg0.66 under different thermal exposure time was measured. The type of residual stress changed from residual tensile stress to residual compressive stress after thermal exposure. There is an abnormal phenomenon that the hardness of the sample increased and the amount of wear increased, and it is evident that the distribution of residual stress was inhomogeneous after thermal exposure. It is found that with increasing thermal exposure time to 50?h, the average lattice distortion ε of the low-index crystal plane and the high-index crystal plane in the aluminum alloys gradually increased.     

Phase field investigation on the initial planarinstability with surface tension anisotropy during directional solidification of binary alloys

Phase field investigation reveals that the stability of the planar interface is related to the anisotropic intensity of surface tension and the misorientation of preferred crystallographic orientation with respect to the heat flow direction. The large anisotropic intensity may compete to determine the stability of the planar interface. The destabilizing effect or the stabilizing effect depends on the misorientation. Moreover, the interface morphology of initial instability is also affected by the surface tension anisotropy.     

聚乙烯醇水溶液二维定向凝固的微观组织演化

为了深入探究定向多孔聚合物材料的微观组织形成机理,利用定向凝固原位实时观察手段,研究不同浓度及不同分子量聚乙烯醇(PVA)水溶液在不同抽拉速度下的定向凝固形貌演化.PVA水溶液的定向凝固形态在低浓度(1 wt%,2.5 wt%)和小分子量(M_w=24000)情况下,一次枝晶间距随着抽拉速度的增加而减小.随着PVA浓度和分子量的增加,一次枝晶间距随抽拉速度变化不明显,枝晶主轴尺寸则随速度增加呈现减小的趋势.与传统凝固形态形成机理相比,PVA水溶液的凝固形态由PVA分子的扩散引起的凝固界面不稳定性机理和PVA高分子链交联引起的局部相分离机理竞争决定.     

添加Dy元素对钕铁硼速凝片微观组织和磁特性的影响

本文讨论了不同含量的Dy元素对速凝条带微观组织的影响,得出Dy的添加量为1%—2%(at%)时条带的微观结构得到最优化,柱状晶生长均匀一致,具有良好的沿c轴取向度.同时,急冷面的细小等轴晶生长区域减少,氢破后相应的磁粉抗氧化性能增强. 关键词： 速凝片 织构 微观组织 氢破     

抽拉速度对SCN-DC共晶生长形貌的影响

本文采用类金属透明模型合金丁二腈-23.6 wt%樟脑 (SCN-23.6 wt%DC) 合金, 研究了棒状共晶定向凝固组织的演化行为, 考察了抽拉速度对棒状共晶合金组织形貌演化的影响规律. 结果表明, 在共晶生长初期, 共晶组织首先起源于晶粒晶界或者试样盒型壁处, 随后沿液/固界面和平行于热流方向生长; 在较小的抽拉速度 (0.064–0.44 μm/s)下, 棒状共晶界面前沿呈现平界面形态, 内部两相棒状组织平行生长, 并且随着抽拉速度的增大,棒状共晶逐渐细化, 棒状间距减小; 而在较大的抽拉速度 (0.67–1.56 μm/s)下, 共晶界面前沿呈现胞状生长形貌, 胞内的棒状共晶呈放射状生长, 同样, 随着抽拉速度的增大, 胞内棒状共晶逐渐细化, 棒状间距减小. 关键词： 定向凝固 共晶形貌 抽拉速度 共晶间距     

各向异性作用下合金定向凝固界面稳定性的渐近分析

采用渐近分析方法对考虑了界面能各向异性的单相二元合金平界面定向凝固过程进行了线性稳定性分析，得到了特定条件下的零级、一级渐近解，并通过对长波段渐近解的讨论得出了适用于整个波段的色散关系.分析表明零级渐近解等效于成分过冷理论，而一级渐近解则与M-S稳定性理论一致.在稳定状态控制参数(抽拉速度和温度梯度)的选择图中，界面能各向异性增大了不稳定区域，且在高速和高温度梯度时的影响更强.     

各向异性作用下合金定向凝固界面稳定性的渐近分析

采用渐近分析方法对考虑了界面能各向异性的单相二元合金平界面定向凝固过程进行了线性稳定性分析,得到了特定条件下的零级、一级渐近解,并通过对长波段渐近解的讨论得出了适用于整个波段的色散关系.分析表明零级渐近解等效于成分过冷理论,而一级渐近解则与M-S稳定性理论一致.在稳定状态控制参数(抽拉速度和温度梯度)的选择图中,界面能各向异性增大了不稳定区域,且在高速和高温度梯度时的影响更强. 关键词： 定向凝固 界面稳定性 渐近分析 各向异性     

高压对大块（PrNd）xAl0．6Nb0．5Cu0．15B1．05Fe97．7-x合金微观结构和性能的影响冰

经配料、熔炼、制粉、成型和烧结后制备了（PrNd）xAl0．6Nb0．5Cu0．15B1．05Fe97．7-x（质量百分比）合金,将该合金分别采用1．5GPa和3．0GPa的压强进行压制,研究了此高压对其显微结构和磁性能的影响．分析发现,该块状合金承受的压强越高,其外观和微观结构破坏越严重,抗弯强度也会降低,但高温抗氧化性能却有一定程度的提高．与没有经过高压处理的磁体相比,经过1．5GPa和3．0GPa高压的样品最大磁能积分别提高了7．69kJ．m^-1和0．94kJ．m^-1,剩余磁通密度分别提高了0．02T和0．01L内禀矫顽力分别提高了20．06kA．m-1和30．33kA．m^-1．结果表明,高压对块状NdFeB烧结磁体的显微结构和力学性能及磁性能均有一定的影响．