冷却速度对快速凝固Ti-48Al-4Cr合金组织转变规律及力学性能研究

采用单辊旋淬快速凝固设备制备了不同辊速条件下的Ti-48Al-4Cr(at.%)薄带,研究冷却速度对快速凝固Ti-48Al-4Cr合金的组织及力学性能变化规律。结果表明,快速凝固Ti-48Al-4Cr合金凝固在辊速为10m/s和20m/s时,基体为等轴的γ相,基体中含有少量的B2相、α₂相颗粒和片层组织;辊速进一步增加至30m/s时,基体转变为α₂相,片层组织消失。快速凝固Ti-48Al-4Cr纳米硬度随着冷却速度的增加而增加,纳米硬度由常规凝固时的5.04±0.09GPa增加至辊速为30m/s时的10.48±0.13GPa。该结果为研究TiAl合金组织转变,减小TiAl合金偏析,提高其力学性能提供了基础。     

热处理对快速凝固Ti-48Al-4Cr合金组织影响规律研究

采用单辊旋淬快速凝固设备制备了Ti-48Al-4Cr(at.%)薄带,并在真空封装后进行热处理实验,研究热处理对快速凝固Ti-48Al-4Cr合金的组织演变规律。结果表明,快速凝固Ti-48Al-4Cr合金凝固基体为等轴的γ相,基体中含有少量的B2相、α2相颗粒和片层组织;经723℃保温1h空冷后,亚稳的α2相颗粒失稳,但片层组织仍然比较稳定;热处理温度升高至932℃时,片层组织中的杆状α2相开始按照瑞利分解失稳,分解成大量短杆状或者颗粒状的α2相;在995℃保温1h以后,基体中已较难发现片层组织存在,但存在着排列方向相同的棒状α2相,同时在等轴γ相晶界处发现了数量较多的颗粒状B2相。本论文的研究为进一步研究快速凝固条件下的TiAl金属间化合物的组织与相转变提供了基础,丰富了快速凝固理论。     

高压凝固Ti-48Al合金片层组织失稳机理研究

采用高压凝固设备制备了Ti-48Al(at.%)合金,并在真空封装后进行热处理实验,研究热处理对高压凝固Ti-48Al合金的片层组织失稳机理。结果表明,在低于共析转变温度进行热处理时,常压凝固Ti-48Al合金组织中较难发现α₂相颗粒,而高压凝固片层组织界面处已开始析出α₂相颗粒。加热至1280℃进一步确定,相比于常压凝固,高压凝固Ti-48Al合金更易分解,且片层组织中γ相优先分解,长杆状的残余α₂相经历球化长大,本文的研究为进一步研究高压下组织与相转变提供了基础,丰富了高压凝固理论。     

高压凝固对Ti-48Al合金片层组织及其力学性能影响

研究了高压下凝固Ti-48Al合金片层间距及其力学性能的变化。结果表明,压力下凝固时,随着凝固压力的增加扩散系数降低,从而引起片层增厚速度的降低,即片层间距的减小。在常压、2GPa、4GPa下凝固时,片层间距的平均值分别为495,345,227nm。片层组织的显微硬度随着凝固压力的增加而增加。该实验为细化片层间距提供了一种新的方法。     

快速凝固二元Al—Cr合金的退火组织及显微硬度

研究了快速凝固Ａｌ－Ｃｒ合金的微观组织及显微硬度，确定了α－Ａｌ过饱和固溶体在退火过程中沉淀析出相为θ－Ａｌ７Ｃｒ，析出开始温度取决于过饱和度和沉淀位置。     

快速凝固Ni—Al合金中的组成相

选取了Ai-Al系六种合金成分（Ni25Al75,Ni31.5Al68.5,Ni39Al61,Ni70Al30,Ni72.2Al27.8,Ni74Al26),采用熔体旋铸法分别制备出不同厚度（30－120μm）的条带试样。用XRD的K值法对其组成相作了定量测定。结果表明,快速凝固组成相与常规凝固组成相有较大差异,快速凝固析出的相中,具有较低液相线湿度的化合物相有较大幅度的增加;冷速不同,组成相中各相的含量也不相同。用当前快速凝固的形核理论和枝晶生长模型对实验结果作了理论分析与计算。结果表明, 具有较高液相线温度的化合物是先析出相,而具有较低液相线温度的化合物具有生长速度上的优势。分析认为,凝固的冷却速率和竞争相之间的生长速度的差异是最终相组成变化的主要原因。     

快速凝固二元Al－Cr合金的退火组织及显微硬度

研究了快速凝固Ａｌ－Ｃｒ合金的微观组织及显微硬度，确定了α－Ａｌ过饱和固溶体在退火过程中沉淀析出相为θ－Ａｌ７Ｃｒ，析出开始温度取决于过饱和度和沉淀位置．     

快速凝固TiAl合金冷却速度的计算

利用传热模型和物理模型计算了等离子旋转电极法（简称PREP）和锤砧法（简称HAP）制备的TiAl合金的冷却速度。计算结果显示：以高纯氩为冷却介质,PREP法制备的TiAl合金的冷却速度为10^2K/s-10^4K/s;锤砧法的冷速为10^5K/s-10^6K/s。     

Microstructure and properties of AZ31 magnesium alloy with rapid solidification

Rapidly solidified (RS) AZ31 magnesium alloy ribbons were made using melt spinning technique. The results show that its microhardness increases with the wheel speed, and after heat treatment, the microhardness of the ribbons produced at 1 600 r/min also increases. Rapid solidification leads to reduction of grain size. When the wheel speed reaches 1 600 r/min, no Mg17 Al12 phase precipitates, while heat treatment at 200℃ leads to precipitation of Mg17 Al12 phase. Al-Mn intermetallic compounds with size no larger than 10 nm appear in as-spun ribbons. The corrosion potential of the as-cast ingots is lower than that of the as-spun ribbons.     

过共晶Al-Si合金快速凝固组织演变

以群体动力学模型为基础,在充分考虑合金的热物性参数、过饱和度以及第二相形核率变化的基础上,提出描述该合金快速凝固组织演变的数学模型,并结合A390合金进行计算。结果表明:随着雾化合金液滴尺寸的减小,平均冷却速度增加,而初生Si相长大时间减少,所析出初生Si相的尺寸及体积分数也不断减小;当粉末尺寸减小到某一临界值时,初生Si相消失,从而使不同尺寸粉末的微观组织发生很大的变化。计算结果与实验结果吻合较好,说明此提出的模型可以较好地预测过共晶Al-Si合金快速凝固组织的形成,从而为进一步研究高硅合金材料的组织演变奠定基础。     

Microstructural evolution of Al-20Si-5Fe alloy during rapid solidification and hot consolidation

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400°C/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43−1.2 cm/s and 15–28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 10⁵ K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 10⁷ K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural refinement and modification. The size of the primary silicon decreased from approximately 1 μm to 30 nm while the formation of iron-containing intermetallic compounds was suppressed. Supersaturation of the aluminum matrix in an amount of ∼7 at.% Si was noticed from the XRD patterns. During the hot consolidation process, coarsening of the primary silicon particles and precipitation of β-Al₅FeSi phase were observed. Evaluation of the compressive strength and hardness of the alloy indicated an improvement in mechanical properties due to the microstructural modification.     

Microstructure of a two-phase titanium alloy by rapid solidification technique

Ribbons of the two-phase titanium alloy were fabricated by single-roller rapid solidification technique, and aged at high temperature. The microstructure of ribbon samples were characterized with X-ray diffractometer （XRD） and environmental scanning electron microscope （ESEM）. The microstructures of the alloy are composed of a phase and supersaturated β phase, and X-ray diffraction results show that all peaks of the a and β phases shift slightly to smaller angles, which can be explained by the disordering growth pattern caused by the rapid solidification process. After aging at 960℃in vacuum, the ribbon is composed of homogeneous a phase and β phase.     

W对Ti-42Al-5Mn凝固组织与元素分配行为的影响

以低成本、易变形的Ti-42Al-5Mn合金（原子分数,下同）为研究对象,系统研究了0.5~1.0 at.%W含量对合金凝固组织和元素在相中分配行为的影响。结果表明,采用CaO坩埚一次真空感应熔炼方法制备的合金铸锭不同位置难熔金属W分布均匀且O含量控制在556~660 ppm。W添加后,合金凝固组织得到明显细化,随着W含量的增加,凝固组织中片层组织尺寸降低、含量减少。通过对不同相合金元素分布行为特征分析发现,Al在合金不同相中的富集顺序为：γ＞α2＞βo,Ti、Mn、W的为：βo＞α2＞γ,W添加不能改变Ti、Al元素在相关相中的分配行为,但会在一定程度上改变Mn元素的分配行为。随着W含量的增加,W在βo和α2相中的分配系数线性增加,分别满足kWα2/γ=1.4026CW(at.%)+0.4313,kWβo/γ = 1.3290CW(at.%) +1.8745,且W在βo相中的富集倾向约为Mn的5.6倍,在α2相中的富集倾向约为Mn的7.5倍。     

Phase and its morphologies of Ti-45 % Al alloy directionally solidified at different growth rates

The microstructures of Ti-45%Al(molar fraction) alloy directionally solidified at different growth rates in alumina tube by electromagnetic heating zone melting were studied. The measured temperature gradient of the system is about 104K/m. The microstructures show that the primary solidified phase is β phase at different growth rates. The growth at low rates from 1.94 × 10-6 m/s to 4. 16 × 10-6 m/s results in a transient solid/liquid interface structure from planar to shallow cellular. This transient rate is larger than the theoretical value of vc = 6.94 × 10-7 m/s.Compared with vt = vc/k = 1.01 × 10-6 m/s, the cellular-dendritic transient rate of experiment is observed in the range of 1.67 × 10-5- 2.50 × 10-5 m/s. The primary arm spacing decreases with increasing growth rate.     

冷却速度对过共晶铝硅合金凝固组织和耐磨性能的影响

试验研究了在不同的冷却速度下凝固的Al-20%Si和Al-30%Si(质量分数,下同）合金的组织和耐磨性。实验结果表明,冷却速度对过共晶铝硅合金的凝固组织和耐磨性能有显著的影响。随着冷却速度的增加,Al-20%Si和Al-30%Si合金的凝固组织组成,初生硅的形貌和尺寸都发生明显的变化：冷却速度小于0．1K/s 的炉冷试样和冷却速度小于1K／s耐火砖型铸造试样的凝固组织由（α＋Si)共晶和初生Si相组成,初生Si相呈粗大的片状,共晶Si呈针状;冷却速度约10K/s的金属型铸造试样的凝固组织由（α＋Si)共晶,枝晶状α相和初生Si相组成,初生Si相为块状或长条状,共晶Si呈细小的针状,并且凝固组织中出现的枝晶状α相;凝固速度为（10^3-10^5)K/s的过喷粉末的凝固组织也是由（α＋Si)共晶,枝晶状α相和初生Si相组成,初生Si相为块状,而喷射沉积快速凝固Al-20%Si和Al-30%Si合金的沉积组织都是由Si相和α相组成,细小的Si相均匀分布在α基体中。随着冷却速度的增加,Al-20%Si和Al-30%Si合金的凝固组织中初生硅的尺寸明显减少,磨损机制发生变化,合金的耐磨性显著增加。     

Modeling of solidification grain structure for Ti-45% Al alloy ingot by cellular automaton

A cellular automaton model for simulating grain structure formation during solidification processes of Ti45% Al(mole fraction) alloy ingot was developed, based on finite differential method for macroscopic modeling of heat transfer and a cellular automaton technique for microscopic modeling of nucleation, growth, solute redistribution and solute diffusion. The relation between the growth velocity of a dendrite tip and the local undercooling,which consists of constitutional, thermal, curvature and attachment kinetics undercooling is calculated according to the Kurz-Giovanola-Trivedi model. The effect of solidification contraction is taken into consideration. The influence of process variables upon the resultant grain structures was investigated. Special moving allocation technique was designed to minimize the computation time and memory size associated with a large number of cells. The predicted grain structures are in good agreement with the experimental results.