定向凝固制备内生晶体增塑的锆基非晶复合材料

通过Bridgman定向凝固成功制备了成分为Zr58.8Ti14.3Nb5.2Cu6.1Ni4.9Be1.0的内生枝晶增塑的非晶复合材料.内生枝晶的尺寸和体积分数可以经抽拉速度的改变得以控制,进而实现了对其力学性能的调节.研究表明,枝晶的大小与抽拉速度呈线性关系,体现出定向凝同在制备非晶复合材料方面可调控析出相的优势.通过对实验参数抽拉速度的优化得出,当抽拉速度为1.0 mm/s时,最高压缩强度达到了1930 MPa,断裂塑性达到11.3%.     

Bridgman法制备塑性钛基轻质非晶复合材料

通过Bridgman定向凝固法成功制备了内生枝晶增塑的轻质钛基非晶复合材料. 与传统的Cu模吸铸法相比, Bridgman法有效消除了铸态组织中的孔洞, 得到了更均匀的微观组织, 且能通过调节抽拉速度来控制枝晶相的尺寸和分布, 进而优化其力学性能. 当抽拉速度为1.4 mm/s时, 合金压缩屈服强度、断裂强度和断裂塑性分别达到1956 MPa, 2706 MPa和18.0%, 且有明显的加工硬化现象. 进一步讨论了枝晶跨越长度$L$和枝晶间距$S$与力学性能的关系, 发现L在约40 μm时对材料的塑性贡献最大.     

定向凝固法制备非晶基自生复合材料初步研究

定向凝固技术可使材料凝固组织按特定方向排列,获得定向及单晶组织结构,大大改善材料的力学和物理性能。在大温度梯度的宽调频电磁成形定向凝固设备上,制备出了不同拉拔速度的合金试样,对其微观组织及显微硬度进行了分析。研究表明：在温度梯度一定的情况下,随拉拔速度的增加,合金组织得到了细化,并由胞状晶向树枝晶再向颗粒弥散状分布组织演化,制备出了内生枝晶增强非晶基自生复合材料和微米晶相增强的非晶基自生复合材料。     

STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY I. Microstructure Evolution in the Transition Region

对Ti--43Al--3Si (原子分数, %) 合金在3---100 μm/s 的生长速度下进行了系统的定向凝固实验. 研究了生长速度对固/液界面形态及初始过渡区组织演化规律的影响. 合金在3---60 μm/s 的生长速度范围内均以胞晶形态生长, 胞晶间距随着生长速度的增大而减小; 当生长速度达到90 μm/s 时, 开始出现枝晶生长. 在定向凝固初始启动阶段, 存在清晰的热过渡区, 热过渡区内Ti₅Si₃ 相分布及过渡区组织与定向凝固区组织的关联性对于籽晶材料的引晶效果有重要影响. 生长速度在10 μm/s 以内时, 热过渡区内Ti₅Si₃ 相分布连续, 且热过渡区组织与定向凝固区组织的关联性好, 有利于该合金的引晶.     

一种镍基单晶高温合金的高温度梯度定向凝固组织及枝晶偏析

采用双区加热和液态金属冷却法 (LMC) 相结合, 对一种含4%Re (质量分数) 的镍基单晶高温合金进行了高温度梯度定向凝固. 结果表明: 与传统的“ 高速凝固法 (HRS) ” (温度梯度G=20-40 K/cm, 抽拉速率V=50-100 μm/s, 一次枝晶间距 λ₁=200-400 μm)相比, 该技术可以显著提高凝固界面前沿的温度梯度 (G=238 K/cm) 和抽拉速率 (V=500 μm/s). 随着抽拉速率的提高, 凝固界面形态呈现出平面、胞状、粗大枝晶和细枝晶形态, 一次枝晶间距不断减小, 通过固态相变析出的γ' 强化相也被显著细化, 当G=238 K/cm, V=500 μm/s时, λ₁和枝晶干γ' 相平均尺寸分别减小到61.3和0.04 μm. 电子探针测定表明, 随着抽拉速率的提高, 枝晶偏析呈现先增大后减小的趋势. 这是高温度梯度条件下, 固相反扩散作用强烈影响元素在枝晶中分布的结果.     

温度梯度对定向凝固Al-Zn-Mg-Cu合金微观组织和硬度的影响

采用定向凝固方法制备不同温度梯度下的高锌Al-Zn-Mg-Cu合金，表征了该合金的一次枝晶臂间距λ₁、二次枝晶臂间距λ₂以及其维氏硬度。在此基础上，采用线性回归和曲线拟合分析方法建立了温度梯度、枝晶间距和显微硬度之间的关系，结果与枝晶生长理论模型吻合，并获得了高锌Al-Zn-Mg-Cu合金的凝固特征参数，同时分析了温度梯度对显微硬度的影响机制。研究结果对高锌Al-Zn-Mg-Cu合金制备工艺优化有指导作用。     

定向凝固Al-4.5%Cu合金枝晶组织与抽拉速率的关系

用自制下拉式定向凝固设备,在一定的温度梯度下,在20-220μm/s的抽拉速率范围制备定向凝固Al-4.5%Cu合金,并对其微观组织、特别是一次枝晶间距随抽拉速率的变化规律进行研究。结果表明：定向凝固微观组织随抽拉速率的增大呈细化趋势,其一次枝晶间距减小;当抽拉速率小于100μm/s时,枝晶间距随抽拉速率而减小的幅度较大;当抽拉速率大于100μm/s时,枝晶间距减小幅度较为平缓。在综合分析抽拉速率、界面生长速率、温度梯度等影响因素的基础上,推导出界面局域平衡条件下预测定向凝固次枝晶间距的理论模型,该模型能够较为准确地反映定向凝固一次枝晶间距随抽拉速率在100-220μm/s范围的变化规律,为定向凝固工艺获得特定组织而预先选配合适的工艺参数提供理论参考。     

DESIGN AND PREPARATION OF IN SITU Pb-RICH PARTICLES/Al BASE METALLIC GLASS MATRIX COMPOSITE

在Al--Pb二元难混溶合金的基础上添加其它的合金元素Ni, Y和Co, 优化设计了新的 Al_82.87Pb_2.5Ni_4.88Y_7.8Co_1.95多元难混溶合金. 开展了该多元难混溶合金的快速凝固实验, 对制备的薄带样品进行了结构表征、热稳定性分析以及显微组织形成的研究. 结果表明, 合金熔体在快速冷却过程中发生了液-液相分离, 生成富Al和富Pb两液相; 随后, 富Al基体液相发生玻璃转变, 形成铝基非晶合金基体, 而富 Pb液相凝固结晶后以球形晶态粒子形式均匀分布于铝基非晶基体中. 研究表明, 利用难混溶合金液-液相变原理, 通过快速凝固技术可以设计和制备原位球晶粒子/非晶合金基复合材料.     

EFFECTS OF WITHDRAWAL RATE ON MICROSTRUCTURE AND STRESS RUPTURE PROPERTIES OF A Ni3Al–BASED SINGLE CRYSTAL SUPERALLOY IC6SX

采用螺旋选晶法制备Ni₃Al基单晶合金IC6SX试棒, 研究了不同拉晶速率对凝固组织和高温持久性能的影响. 结果表明, Ni₃Al基单晶合金IC6SX凝固组织为树枝状, 随着拉晶速率从1.5 mm/min增加到6 mm/min, 一次枝晶间距逐渐减小. 与普通镍基高温合金不同, 位于枝晶干处的次生γ'相尺寸比枝晶间处的大. 随着拉晶速率的增加, 枝晶干和枝晶间处的γ'相尺寸都逐渐减小, 由不规则形状逐渐立方化, 枝晶间处的初生γ'相的数量逐渐增多. 一次枝晶间距和γ'相尺寸对高温持久性能影响显著, 随着拉晶速率的增加, 组织细化, 铸态IC6SX单晶合金的高温持久寿命增加.     

液态金属冷却法制备重型燃机定向结晶空心叶片凝固过程的实验与模拟

利用高温度梯度定向凝固-液态金属冷却(LMC)技术制备了重型燃机定向结晶空心高压涡轮叶片,采用Pro CAST有限元模拟软件计算了LMC定向凝固工艺下,不同抽拉速率时空心定向结晶叶片凝固过程的温度场、晶粒组织以及一次枝晶间距(PDAS),预测了抽拉速率对杂晶、雀斑等缺陷的影响.结果表明,模拟结果与实验结果吻合良好.随着抽拉速率增加,叶片的凝固速率、冷却速率均增加,远高于高速凝固法(HRS)的凝固速率、冷却速率;叶片不同部位达到最大纵向温度梯度时的抽拉速率不同,纵向温度梯度是评价定向工艺的有效方法;LMC工艺制备的燃机叶片消除了雀斑缺陷,PDAS远小于HRS工艺.     

On the microstructure–tensile property correlations in bulk metallic glass matrix composites with crystalline dendrites

Bulk metallic glass (BMG) matrix composites with crystalline dendrites as reinforcements exhibit a wide variance in their microstructures (and thus mechanical properties), which in turn can be attributed to the processing route employed, which affects the size and distribution of the dendrites. A critical investigation on the microstructure and tensile properties of Zr/Ti-based BMG composites of the same composition, but produced by different routes, was conducted so as to identify “structure–property” connections in these materials. This was accomplished by employing four different processing methods—arc melting, suction casting, semi-solid forging and induction melting on a water-cooled copper boat—on composites with two different dendrite volume fractions, V_d. The change in processing parameters only affects microstructural length scales such as the interdendritic spacing, λ, and dendrite size, δ, whereas compositions of the matrix and dendrite are unaffected. Broadly, the composite’s properties are insensitive to the microstructural length scales when V_d is high (～75%), whereas they become process dependent for relatively lower V_d (～55%). Larger δ in arc-melted and forged specimens result in higher ductility (7–9%) and lower hardening rates, whereas smaller dendrites increase the hardening rate. A bimodal distribution of dendrites offers excellent ductility at a marginal cost of yield strength. Finer λ result in marked improvements in both ductility and yield strength, due to the confinement of shear band nucleation sites in smaller volumes of the glassy phase. Forging in the semi-solid state imparts such a microstructure.     

Deformation and fracture behaviors of Co-based metallic glass and its composite with dendrites

Microstructures and mechanical properties of Co-based metallic glass with nominal composition of Co₄₃Fe₂₀Ta_5.5B_31.5 (at.%) cast at the different cooling rates were investigated. When cooling rate is low enough, some dendritic crystalline phases were in situ precipitated from the glass matrix, forming the Co-based metallic glass composite with dendrites. Macroscopically, the fully amorphous samples often split apart or were broken into some particles, displaying a fragmentation failure mode. The size of particles became larger with the decrease in cooling rate. But, strength reduces slightly. Besides, the composites with dendrites show a small compressive plasticity, plus local melting behaviors with vein-like structure on the fracture surfaces. Furthermore, the effects of cooling rate on microstructures, deformation and fracture behaviors were discussed systemically. It is proposed that the fragmentation mechanism can be attributed to the inherent brittle character and high stress concentrations around the free volume. And the local melting behavior is due to the more receiving elastic energy and local shearing.     

定向凝固Sn-36%Ni包晶合金凝固组织特征长度的表征

通过对Sn?36%Ni包晶合金在恒定温度梯度(G=20 K/mm)下进行的一定速度范围内(v=2~200μm/s)的定向凝固实验，研究凝固组织特征尺度随生长速度v的变化；测量包括一次与高次枝晶间距及枝晶尖端半径在内的凝固组织特征尺度。通过实验结果与理论模型的对比，发现凝固组织特征尺度随生长速度的变化关系为：对于一次枝晶间距有λ1=335.882v?0.21,且与 Kurz?Fisher 模型吻合；对于二次枝晶间距有λ2=44.957v?0.277,且与 Bouchard?Kirkaldy模型吻合；对于三次枝晶间距有λ3=40.512v?0.274；对于枝晶尖端半径有R=22.7v?0.36。实验结果表明，λ1/λ2随着生长速度的增加而增加，λ1/λ3的变化明显较λ1/λ2的小，表明三次枝晶具有与一次枝晶类似的生长特征；而λ1/R的比值随着生长速度的增加而由2增加到2.3，变化很小。     

Composition dependence of the microstructure and mechanical behavior of Ti–Zr–Cu–Pd–Sn–Nb bulk metallic glass composites

Ti-based bulk metallic glass composite alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈, Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ and Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ were designed to obtain the microstructure composing of β-Ti dendrites and glassy matrix. The compressive and three-point bending properties were investigated. It was found that the actual microstructure of the Nb-added alloys consisted of primarily precipitated β-Ti dendrites, network-like glassy matrix, and extra island-like Ti₂Cu intermetallic phase with different volume fractions. Under compressive loading, all the Nb-added alloys presented higher yield strength combined with remarkably increased plasticity. Under bending condition, however, the alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈ and Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ with higher Ti₂Cu volume fractions became completely brittle. The alloy Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ could keep its plastic deformability due to the decreased Ti₂Cu volume fraction. Compressive deforming behavior of the Nb-added alloys was determined by the ductile β-Ti phase and glassy matrix, nevertheless, bending deforming behavior of the alloys was determined by the volume fraction and distribution of the brittle intermetallics.     

Optimized interface and mechanical properties of W fiber/Zr-based bulk metallic glass composites by minor Nb addition

W fiber/Zr-based bulk metallic glass composites were prepared by melt infiltration casting and the interface reaction in composites was studied in detail. It was found that minor Nb addition to the matrix can suppress the interface peritectic reaction and optimize the interface structure in W fiber/Zr-based bulk metallic glass composites. Taking the interface characteristics of composites and glass forming ability of the matrix into account, an optimized alloy Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ was selected as a new metallic glass matrix. The interface in the W fiber/Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ bulk metallic glass composite has excellent interface bonding, and the compressive strength of 70% W fiber/Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ metallic glass composite is 2.6 GPa, 58% higher than the unreinfored matrix. Multiple shear band formation in the matrix, which results from the interaction between W fiber and the matrix, can answer for the high ultimate fraction strength and excellent plastic deformation of the composite.     

A Successful Synthesis of the CoCrFeNiAl0.3 Single-Crystal, High-Entropy Alloy by Bridgman Solidification

For the first time, a face-centered-cubic, single-crystal CoCrFeNiAl_0.3 (designated as Al0.3), high-entropy alloy (HEA) was successfully synthesized by the Bridgman solidification (BS) method, at an extremely low withdrawal velocity through a constant temperature gradient, for which it underwent two BS steps. Specially, at the first BS step, the alloy sample underwent several morphological transitions accompanying the crystal growth from the melt. This microstructure evolves from as-cast dendrites, to equiaxed grains, and then to columnar crystals, and last to the single crystal. In particular, at the equiaxed-grain region, some visible annealing twins were observed, which indicates a low stacking fault energy of the Al0.3 alloy. Although a body-centered-cubic CoCrFeNiAl (Al1) HEA was also prepared under the same conditions, only a single columnar-crystal structure with instinctively preferential crystallographic orientations was obtained by the same procedure. A similar morphological transition from dendrites to equiaxed grains occurred at the equiaxed-grain region in Al1 alloy, but the annealing twins were not observed probably because a higher Al addition leads to a higher stacking fault energy for this alloy.     

分段钨丝/锆基非晶复合材料穿甲试验研究

为了研究不同结构和材料弹芯的侵彻能力,实验中采用80%体积分数、钨丝/锆基非晶复合材料(钨丝直径0.3mm)弹芯,在1450-2100m/s范围内,对不同方案的弹芯垂直侵彻均质靶板进行了实验研究。根据实验结果可以发现：(1)材料的性能和结构都可以影响弹丸的侵彻效率,突破了传统认识,为复合材料杆式穿甲弹设计提供了重要依据;(2)弹芯结构不同,其侵彻深度-着靶速度变化曲线也是不同,对于两段和三段结构弹芯其侵彻曲线变化是凸的,其侵彻深度峰值分别出现在1750m/s~1800m/s和1850m/s附近,最大侵彻穿深均为x=1.7L;对于未分段弹芯,其侵彻深度-速度变化曲线呈渐进线变化,在速度大于1850m/s时接近于流体动力学极限穿甲深度L*(r_p/r_t)_1/2,约为1.5L;(3)结合单向纤维复合材料的动态破坏特点、动态裂纹传播和弹芯高速撞击的侵蚀速度特性,给出了最优分段概念,分析了(2)中发现的问题的原因,为分段弹芯结构设计提供了参考。     

A thermoplastic forming map of a Zr-based bulk metallic glass

A thermoplastic forming (TPF) map of a Zr₃₅Ti₃₀Be_26.75Cu_8.25 bulk metallic glass was constructed through systematic hot-embossing experiments, spanning a wide range of strain rates and temperatures in the supercooled liquid region. By comparison with the corresponding deformation map, it is found that Newtonian flow, non-Newtonian flow and inhomogeneous flow regions correspond well to fully filled, partially filled and non-filled regions, respectively, in the hot-embossing TPF map. Furthermore, the spatio-temporally homogeneous flow facilitates the thermoplastic formability of the Zr-based bulk metallic glass, which is rationalized in terms of free volume theory as well as by finite element simulations. Finally, our results are corroborated by potential application tests.