THE PURIFICATION AND HYPERCOOLING OF Fe-B EUTECTIC ALLOY

通过真空熔炼和气体保护, 采用熔融玻璃与循环过热相结合的深过冷快速凝固技术,研究了影响富Fe端Fe-B共晶合金熔体净化效果的主要因素, 确定了该合金熔体获得超过冷的净化方法, 并使Fe83B17共晶合金熔体稳定获得了300-460 K的超过冷度, 使Fe80B20过共晶合 金熔体的过冷度达到了485 K, 从而使Fe-B共晶系合金熔体开始形核前的初始过冷度达到了0.3m-0.4m的水平. 通过对冷却曲线的分析, 讨论了表征超过冷快速凝固的热力学特征.     

热分析在Fe-B合金深过冷凝固过程中的应用

采用熔融玻璃净化与循环过热相结合的深过冷技术,研究了Fe-B共晶合金熔体的深过冷及超过冷凝固组织与冷却曲线的关系.凝固过程的热分析表明,合金熔体冷却曲线的变化体现了熔体的过冷程度,即通过对冷却曲线特征参量的分析能够直接确定深过冷/超过冷凝固组织的获得,而且过冷度与初生相的形核、分布、晶粒大小及共晶组织形貌等凝固特征的对应关系也能够通过冷却曲线反映出来.     

深过冷Fe-Co合金的凝固规律

采用熔融玻璃净化法研究了深过冷Fe-Co合金的凝固组织演化规律.结果表明,随着熔体过冷度的增加, Fe100-x-Cox(x=10-40)中除x=10的合金外,合金凝固组织都经历了两次晶粒细化过程;而当过冷度大于某一临界过冷度时,凝固组织不再是单相,而是出现了亚稳相,除了小过冷度下的组织转变以外,凝固组织转变过冷度以及亚稳相形成的临界过冷度随着Co含量的增加而增大.运用B-C-T模型和瞬态形核理论对凝固组织的演化规律进行了理论分析.     

深过冷Ni-P共晶合金凝固组织的形成和转变

采用玻璃熔覆法使Ni—P共晶合金实现了深过冷。随着过冷度的增加，其凝固组织发生了一系列的变化：晶粒逐渐细化，凝固组织从规则棒状共晶向异常粒状共晶组织转化，最后得到显微结构细小的胞状共晶团组织。利用单相枝晶状共晶的熔断模型，解释了过冷熔体形成异常粒状共晶团的形成原因。熔体在深过冷条件下形成的胞状共晶组织则可以固／液界面稳定性理论来描述。     

深过冷Al72Ni12Co16合金熔体中的相选择

采用惰性形核涂层型壳和氩气保护下循环过热的方法，使Al72Ni12Co16合金获得200K的大过冷度，在此基础上从实验和理论两方面研究了深过冷Al72Ni12Co16合金熔体中的相选择规律。实验和理论计算表明：熔体过冷度是决定Al72Ni12Co16合金中准晶相与晶体相竞争形核的主要因素，且存在一临界过冷度，其值约为60K，当Al72Ni12Co16合金的初始过冷度大于临界过冷度时，十面体准晶相将作为初生相从熔体中析出；反之，当Al72Ni12Co16合金的初始过冷度小于临界过冷度时，熔体初生相为β晶体相。     

深过冷Ni-31.44%Pb偏晶合金快速凝固行为

采用熔融玻璃净化和循环过热相结合的方法研究了Ni-31．44％Pb偏晶合金宽过冷区间的凝固组织演化规律；从形核热力学和动力学两方面分析过冷熔体中稳定相(α)和亚稳相(L2)两相的竞争形核规律。研究结果表明：过冷Ni-31．44％Pb偏晶合金在快速凝固阶段本质上是以枝晶方式生长，首先形成α枝晶骨架，再辉重熔后分布于枝晶间的残余掖相按照正常凝固模式进行分相／偏晶等后续反应。     

溶质截留对过冷共晶生长过程的影响

以过冷凝固条件下已有的共晶生长理论模型为基础,考虑了快速非平衡凝固条件下会出现溶质截留现象,探讨溶质截留对生长过程的影响.发现溶质截留的引入,增大了共品生长速度,减小了层片间距和枝晶尖端半径.初始平衡溶质分配系数越小,溶质截留对过冷共晶生长过程的影响越显著.     

Ag-Cu/Co-Sn共晶合金深过冷快速凝固反常共晶的形成研究

利用熔融玻璃净化循环过热的方法进行了Ag-Cu/Co-Sn共晶合金的深过冷快速凝固实验,并系统研究了熔体过冷度和微量Nb添加对反常共晶形成的影响。结果表明,快速凝固过程中共晶枝晶内部首先重熔形成反常共晶,随着过冷度的增大,共晶相的形貌从蠕虫状转变为球形颗粒,被重熔形成的固相颗粒将作为剩余液相形核生长的基底,Ag-Cu共晶中共晶两相形核具有非互惠性。Co-Sn共晶合金中添加Nb元素后样品内部反常共晶形成的临界过冷度由23 K降低至15 K,而表面组织中过冷度从45 K降低至30 K。由于样品表面与坩埚壁接触有利于结晶潜热消散,反常共晶形成的临界过冷度较高。Ag-Cu共晶合金温度再辉曲线上慢速凝固阶段持续的时间较Co-Sn共晶合金要长。     

纳米晶SmCo_(8.9)Si_(0.9)永磁合金相变特征及其磁性能变化研究

首次制备出具有高稳定性的Sm Co_(8.9)Si_(0.9)纳米晶合金,进而系统研究了亚稳相Sm Co_(8.9)Si_(0.9)的相变特征及相应的磁性能变化规律。发现添加元素Si可以有效提高过饱和固溶体亚稳相SmCo_(9.8)的稳定性,随着热处理温度的升高,SmCo_(8.9)Si_(0.9)纳米晶合金由SmCo_(9.8)(H)结构的单相转变为Sm_2Co_(17)(H)和Co(fcc)相,且伴随相变,矫顽力提高。其机理源于析出的细小Co相造成钉扎机制增强。进一步升高热处理温度,Sm_2Co_(17)(H)相转变为Sm_2Co_(17)(R)相,同时晶粒长大明显且晶粒尺寸分布不均匀,导致磁性能下降。     

Rapidly directional solidification of highly undercooled Ni-Fe-Ga shape memory alloy melts

In the present work, high undercooling rapid solidification technique is applied to the directional crystal growth of ternary Ni-Fe-Ga shape memory alloys (SMAs). Experimental results show that the phase selection of undercooled melts strongly depends on the triggering mode and nucleation undercooling. With the planar triggering nucleation at the undercooling of 160 K from the bottom of alloy melt, a directionally solidified sample with a preferred (2 0 0) orientation of γ phase is obtained. When the point triggering mode is employed at 180 K, γ phase is suppressed completely and the directionally solidified sample demonstrates almost identical phase transformation temperatures.     

Non-equilibrium solidification of undercooled Ni-31.44%Pb monotectic alloy melts

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The oxidation of a directionally solidified cobalt-tungsten eutectic alloy

A study of the oxidation of directionally solidified Co-W alloys were performed at 750, 900, and 1050°C. The study involved a comparison of the oxidation behavior of directionally solidified and as-cast alloys along with the oxidation behavior of the pure components. The study incorporated both thermogravimetric kinetic measurements and morphological studies of the oxides as a function of both temperature and time. Differences were noticed in both the oxidation rate and the oxide morphology and were attributed to differences in alloy microstructure. The directionally solidified Co-W alloy was found to be more spall resistant but also oxidized more rapidly than the conventionally as-cast alloy. Both alloys were more oxidation resistant than the pure components.     

Pb-Bi包晶合金定向凝固相和组织选择

针对Pb-Bi包晶合金中高速及低速定向凝固,利用最高界面生长温度判据以及充分形核和成分过冷准则,对Pb-Bi包晶合金定向凝固中初生α和包晶β两相的相互竞争规律进行预测。计算结果表明:在中高速凝固段,与Pb-26%Bi、Pb-28%Bi、Pb-30%Bi和Pb-34%Bi(质量分数)合金对应的α→β转变的临界生长速度分别为20、14、8.5和2 mm/s;在低速凝固段,合金的相选择图分为8个生长区,其中包括两相分别以单相生长区及两相混合生长区。利用该相选择图,结合温度梯度与凝固速度比值(G/V)及合金原始成分(C0)可预测相选择规律及组织形态。     

亚共晶Al-Si合金共晶团的形核与生长

采用光学显微镜、扫描电镜及电子背散射衍射技术研究亚共晶Al-Si合金共晶团的形核与生长。通过揭示共晶团和结晶位向的分析发现：在同一个共晶团中Si相和Al相都不是单晶体,而是由不同位向的小＂晶粒＂构成的。提出不能依据共晶团中Al相与周围初生枝晶Al相的位向关系来确定共晶的形核模式的建议。然而,初生枝晶铝相的演化显著影响随后的共晶形核与生长。涉及的杂质元素使共晶Si的形貌由粗大的片状转变成细小的纤维状的变质行为,可能与共晶的形核无关。     

中熵共晶CrCoNiNb0.4合金层状组织的热稳定性

对CrCoNiNb_0.4中熵共晶合金层片状组织的热稳定性进行了研究。结果表明:CrCoNiNb_0.4合金共晶层状组织在1023 K及以下温度可以保持较长时间的热稳定性;随热处理时间从24 h增加至168 h,CrCoNiNb_0.4合金共晶片层厚度从约0.19 μm增加至约0.39 μm,且片层间隙析出相逐渐增多,同时伴随着边界断裂,边缘球化等层状失效形式的发生,在析出强化相以及层状组织失效的共同作用下,CrCoNiNb_0.4合金共晶组织硬度值随热处理时间呈先增大后减小的趋势。     

The effect of thermal cycling on the subsequent oxidation behavior of an aligned Co-based TaC eutectic alloy

In situ aligned composites are envisioned for applications in environments involving simultaneous exposure to the effects of thermal cycling and oxidizing atmospheres. Therefore, to examine the extent of any synergism between these two effects, the oxidation behavior at 800°C and 1000°C of the aligned eutectic alloy TaC-Co50B-3W was examined in both the as-solidified state and after exposure to thermal cycling effects. The results presented here indicate that thermal cycling strongly influences the oxidation behavior of this alloy.     

Microstructures and evolution mechanism of highly undercooled Ni-Pb hypermonotectic alloy

The microstructures and evolution mechanism of the undercooled Ni-20%Pb(molar fraction) alloy were investigated systematically by high undercooling solidification technique. The experiment results indicate that the morphology of α-Ni phase and the distribution of Pb element in undercooled Ni-20% Pb alloys change with the in-crease of undercooling. The main evolution mechanisms of α-Ni are dendrite remelting and recrystallization. Pb phase in the microstructure of Ni-20% Pb hypermonotectic alloy originates from L2 phase separated from the parent melt during the cooling process through immiscible gap and L2 phase formed at the temperature of monotectic trans-formation. The solubility of Ph element in α-Ni phase under high undercooling condition is up to 5.83% which is ob-viously higher than that under equilibrium solidification condition. The real reason that causes the solubility difference is distinct solute trapping.     

Phase and microstructure selection in directionally solidified peritectic alloys with convection

A boundary layer model was used to investigate the convection effects on phase and microstructure selection in directionally solidified peritectic alloy. Due to the convection effects, the steady-state compositions of one phase at interface corresponding to an initial composition reduce, which causes its steady-state point moves upward along its solidus line and the compositional range is not consistent with the band cycle in banding. A criterion of critical interface temperature was put forward to determine whether a phase entered steady-state growth or not. Furtherly by equivalent transformation, the equivalent solidus lines and subsequent equivalent phase diagram were derived for peritectic solidification with convection. Using this equivalent phase diagram, a phase and microstructure selection map is built for a peritectic alloy with convection effect, which shows that the compositional range for banding reduces, and moves to the hyperperitectic region, and also the coupled growth region of both solids comparing with purely-diffusive limit. The predicted map for directionally solidified Pb-Bi alloy agrees well with its experimental observations.