SYNTHESIS OF PLASTIC Zr–BASED BULK METALLIC GLASS WITH CRYSTAL PHASE BY DIRECTIONAL SOLIDIFICATION

通过Bridgman定向凝固成功制备了成分为Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be_11.0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, 体现出定向凝固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。     

冷却速率和高径比对钛基非晶复合材料力学性能的影响

通过制备不同尺寸的Ti45.7Zr33Ni2.9Cu5.9Be12.5非晶复合材料样品,研究了冷却速率和高径比对内生枝晶相增强钛基非晶复合材料力学性能的影响.随着制备过程中冷却速率的降低,非晶复合材料中枝晶相的尺寸逐渐增大,同时枝晶相熟化的现象也趋于明显.在力学性能方面表现为非晶复合材料的强度降低而塑性增强.与以往非晶复合材料性能对高径比比较敏感所不同的是,本工作中的Ti45.7Zr33Ni2.9Cu5.9Be12.5非晶复合材料的力学性能对高径比的变化并不敏感,原因在于晶态相的存在以及其中的形变诱发马氏体相变行为的发生对非晶复合材料内部应力分布的调节.     

抽拉速度对高温合金DZ125定向凝固中缩松的影响

研究了不同抽拉速度下镍基高温合金DZ125定向凝固试样中显微缩松的分布、变化规律.结果表明,在一定抽拉速度范围内(≤100μm/s),随抽拉速度增加,定向凝固试样中缩松水平降低.v=50μm/s时试样中缩松水平约为v=5μm/s时缩松水平的10%,v=100μm/s时试样中的缩松水平约为v=50μm/s时缩松水平的70%～90%.随抽拉速度增大,一次枝晶间距和二次枝晶间距减小,枝晶发生细化,抽拉速度对缩松水平的影响主要由枝晶间距的变化引起.     

易切削钢9SMn28定向凝固过程中夹杂物的研究

应用B ridgm an区域熔炼定向凝固设备制备出不同抽拉速度下的定向凝固易切削钢9SMn28。利用光学显微镜、扫描电镜及电子探针显微分析仪对所得试样中的夹杂物及树枝晶进行了观察,并通过凝固理论对其进行了计算和分析。研究发现,9SMn28钢中的夹杂物主要以MnS的形式存在,普遍聚集在树枝晶间隙之中,其形态以球形为主,在二次枝晶臂周围也存在少量纺锤状和条状的夹杂物,其尺寸随抽拉速度的增大而减小。二次枝晶臂间距的计算结果与测量结果比较吻合,同样随抽拉速度的增大而减小。     

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。     

基于ProCast的镍基单晶高温合金数值模拟

采用ProCast软件和CAFE模型对不同工艺条件下的DD3镍基高温合金的定向凝固过程进行了模拟,分析了抽拉速度对糊状区的影响及加热区温度和抽拉速度对定向凝固温度场和枝晶二次臂间距的影响规律。结果表明,提高加热区温度和抽拉速度,可以获得更细化的微观组织,降低二次枝晶臂间距,但是抽拉速度过大,糊状区宽度变宽,糊状区位置下移,界面下凹,叶片平台处容易出现杂晶,因此抽拉速度需要控制在一个合理范围内。     

内生晶相对Cu46Zr46Al8非晶复合材料力学性能的影响

通过不同退火工艺分别制备出了含内生晶相的Cu46Zr46Al8非晶复合材料,并研究了其显微组织及室温压缩性能.结果表明,在淬火态非晶复合材料的晶化组织中出现了亚稳初生相与共晶共存的形态,这种组织形态是导致其力学能大幅度降低的主要原因.通过控制退火工艺在Cu46Zr46Al8非晶中内生出不同形态和尺寸的晶体相.枝晶臂细且长的树枝晶对材料的强度有较大的危害作用,而细小的胞状初生相晶粒则对材料的强度提高有一定的作用.     

纵向强静磁场对定向凝固DZ417G合金枝晶形态和数目的影响

纵向强静磁场可明显影响高温合金DZ417G的定向凝固组织.为控制定向组织树枝晶数目提供了一种新手段.在抽拉速率为5μm/s时,强磁场影响了该合金组织的定向凝固生长特性,影响程度随磁场强度的加大而增加.当抽拉速率达到40μm/s及其以上时,施加强磁场使得单位面积上的枝晶数目增加,枝晶数目随磁场强度的增大而增大,增大幅度最大达到一倍左右.当温度梯度增大时,强磁场在低抽拉速率(5μm/s)下影响该合金定向凝固的效应加剧,在抽拉速率40μm/s及其以上时增加枝晶数目的效应也加大.从磁抑制对流和热电磁效应方面分析了上述现象,提出了作用机理.     

Tensile stress-strain response of metallic glass matrix composites reinforced with crystalline dendrites: Role of dendrite morphology

Bulk metallic glass composites (BMGCs) consisting of soft crystalline phases (commonly referred to as dendrites) in a metallic glass matrix have shown enhanced tensile ductility compared to conventional bulk metallic glasses (BMGs). Experiments and atomistic simulations suggest that a large number of geometrical parameters such as aspect ratio, spacing and orientation of dendrites as well as their spatial distribution can affect the mechanical response of BMGCs. However, the precise mechanism by which these parameters influence shear band initiation and propagation is not well understood. Therefore, continuum simulations of tensile loading on BMGCs with different morphologies are performed in this work. The results show that aspect ratio of dendrites has weak effect on the mechanical response up to the peak stress stage. However, it influences ductility considerably, albeit in a different manner for BMGCs with high and low hardening dendrites. The present analysis suggests that a BMGC capable of displaying mildly strain hardening response with large strain to failure can be designed by using closely spaced dendrites of high aspect ratio, and aligning them parallel to the maximum tensile stress direction.     

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.     

Temperature,strain rate and reinforcement volume fraction dependence of plastic deformation in metallic glass matrix composites

A systematic study of mechanical properties is presented for Zr-based bulk metallic glass matrix composites, spanning a wide range of strain rates and temperatures, as well as various levels of reinforcement volume fraction. All of the experimental materials exhibit mechanical properties dominated by deformation of the amorphous matrix phase, including inhomogeneous flow and fracture at low temperatures, as well as homogeneous flow of both Newtonian and non-Newtonian character at high temperatures. In the homogeneous flow regime, the composites exhibit clear strengthening as the volume fraction of reinforcement increases. This strengthening effect is quantitatively explained in both the Newtonian and non-Newtonian regimes, and is found to arise from two contributions: (i) load transfer from the amorphous matrix to the reinforcements; and (ii) a shift in the glass structure and properties upon precipitation of the reinforcements. An additional source of apparent strengthening – in situ precipitation of reinforcement during deformation – is also discussed.     

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.     

In-situ dendrite reinforced Dy-based amorphous matrix composites

In-situ dendritic reinforced Dy-Fe-Al amorphous matrix composites with a diameter of 3 mm were designed and fabricated by conventional Cu-mold casting method. XRD and SEM analyses were conducted to investigate the microstructure, the mechanical properties and the deformation and fracture behaviors of the composites. The forming mechanism and the deformation and fracture mechanism of the composites were discussed. The results indicate that the microstructures of composites consist of metallic glass matrix and α-Dy dendritic phase. The composites exhibit good mechanical properties with compressive fracture strength of 1 063 MPa, which is attributed to the effective bearing-load ability of the α-Dy dendrites and the glassy matrix and the restriction to the expanding of shear bands and cracks of the α-Dy dendrites. The nature of in-situ crystalline phases embedded in the amorphous matrix for in-situ crystallite reinforced Dy-Fe-Al amorphous matrix composites has a more important influence on the mechanical properties, the deformation and fracture behavior of the composites.     

铍含量对钛基非晶复合材料摩擦行为的影响

通过真空电弧熔炼制备了一系列钛基非晶复合材料和钛基非晶合金,研究了合金中铍元素的含量对整体合金摩擦行为的影响。随着合金中铍元素的减少,非晶复合材料中的枝晶体积分数逐渐增加,整体合金的摩擦系数降低,但是合金的磨损率升高。所有材料的磨损表面都展现出了磨粒磨损的磨损机制,并且磨屑的尺寸随着枝晶体积分数的升高而逐渐降低。     

High strain rate compressive behavior of Ti-based metallic glass matrix composites

The compression behaviors of Ti-based metallic glass matrix composites with dendrites scale were tested at different loading rates. It was found that the composites exhibited not only high strength, but also large plasticity under quasi-static compression. Under the dynamic loading, however, the TZ1 alloy with fine dendrites demonstrated a catastrophic failure. Although both the strength and plasticity decreased for the TZ2 composite sample with coarse dendrite, the total strain is over 7%. Discussions on the strain rates and dendrite scale are provided by analyzing the effects of dendrite, which can present the possible deformation mechanism of the composites.     

Microstructure studies of Zr65Cu17.5Al7.5Ni10 and Zr65Cu15Al10Ni10 glass forming alloys: Phase morphologies and undercooled melt solidification

Zr₆₅Cu_17.5Al_7.5Ni₁₀ (at.%) and Zr₆₅Cu₁₅Al₁₀Ni₁₀ (at.%) glass forming alloy microstructures have been investigated by means of optical and electron microscopies. They are composed of a fine eutectic matrix with eutectic dendrites (EDs) that have peculiar morphologies. Al and Cu concentrations, in these alloys, favour primary dendrites and determine the ED morphologies and compositions. Their locations within the microstructures suggest a two-step solidification process of the two undercooled melts. The identified crystalline phases indicate the occurrence of solid state phase transformations in agreement with the structural defects observed in the grains. The crystalline phases can be classified into Zr-rich, Cu-rich, Ni-rich and Al-rich compounds resulting from competing diffusion between Cu, Ni, and Al in the melts.