Mechanical Properties of the TaSi2 Fibers by Nanoindentation

The Si-TaSi₂ eutectic in situ composite, which has highly-aligned and uniformly-distributed TaSi₂ fibers in the Si matrix, can be obtained when the solidification rate changes from 0.3 to 9.0 mm/min. It is very interesting that one or two TaSi₂ fibers are curved when the solidification rate reaches 6.0 mm/min, although it is very brittle in general. The formation mechanism of the curved fiber is discussed and mechanical properties of the TaSi₂ fibers are examined by nanoindentation. It is found that the hardness and the elastic modulus of the bended TaSi₂ fiber are much higher than that of the straight TaSi₂ fiber. Moreover, the reasons why the mechanical properties of the straight TaSi₂ fiber are different from that of the curved TaSi₂ fiber are discussed. This can be ascribed to internal stress which results from mismatch of the thermal expansion coefficients of the two phases and di®erent crystallographic orientations.     

凝固速率对Si-Ta合金固液界面稳定性的影响

以Si-Ta合金为研究对象,采用零功率法研究凝固速率对定向凝固固液界面稳定性的影响。结果表明:当凝固速率在0.3～9.0mm/min范围内,随着凝固速率的增大,固液界面经历了平界面→浅胞状界面→锯齿状界面→浅胞状界面→平界面的演化规律。在较低的凝固速率范围之内,实验结果与理论计算基本吻合。然而,在较高凝固速率时,实验结果与理论计算出现偏差,这是由于热力学和动力学均发生变化,使得在凝固速率V=5.0mm/min时固液界面就已经达到了稳定的平界面。     

Solidification Simulation of Single Crystal Investment Castings

The three dimensional solidification simulation of the single crystal investment castings at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min was performed with the finite element thermal analysis method.The calculated result were in accordance with the experimental ones.The results showed that with the increase of with-drawal rate the concave curvature of the liquidus isotherm was bigger and bigger and the temperature gradient of the castings decreased.No effects of withdrawal rate on the distribution of the temperature gradient of the starter and helical grain selector of the castings were observed at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min.The relatively high temperature gradient between 500℃/cm and 100℃/cm in the starter and helical grain selector was obtained at three withdrawal rates.The study indicates the three dimensional solidification simulation by finite element method is a powerful tool for understanding solidification and predicting defects in single crystal investment castings.     

Solidification Simulation of Investment Castings of Single Crystal Hollow Turbine Blade

The three-dimensional solidification simulation of the investment castings of single crystal hollow turbine blade at the withdrawal rates of 2 mm/min, 4.5 mm/min and 7 mm/min has been performed with the finite element thermal analysis. The calculated results are in accordance with the experimental ones. The results show that with the increase of withdrawal rate the concave curvature of the liquidus isotherm is larger and larger, and the temperature gradients of the blades increase. No effects of withdrawal rate on the distribution of the temperature gradients of the starter and helical grain selector of the blades are observed at withdrawal rates of 2 mm/min, 4.5 mm/min and 7 mm/min. The relatively high temperature gradient between 500℃/cm and 1000℃/cm in the starter and helical grain selector is obtained at three withdrawal rates.     

Effect of Solidification Rate on Microstructure and Solid/Liquid Interface Morphology of Nie11.5 wt% Si Eutectic Alloy

In this study NieN i3 Si eutectic in situ composites are obtained by Bridgman directional solidification technique when the solidification rate varies from 6.0 mm/s to 40.0 mm/s. At the low solidification rates the lamellar spacing is decreased with increasing the solidification rate. When the solidification rate is higher than 25 mm/s, the lamellar spacing tends to be increased, because the higher undercooling makes the mass transport less effective. The adjustments of lamellar spacing are also observed during the directional solidification process, which is consistent with the minimum undercooling criterion. Moreover, the transitions from planar interface to cellular, then to dendritic interface, and finally to cellular interface morphologies with increasing velocity are observed by sudden quenching when the crystal growth tends to be stable.     

Globular-to-needle Zn-rich phase transition during transient solidification of a eutectic Sn-9%Zn solder alloy

The aim of this article was to investigate the microstructural evolution of a eutectic Sn-9%Zn solder alloy as a function of growth rate during transient unidirectional solidification. It was found that globular-like and needle-like Zn-rich phases prevail at growth rates ranging from 0.5 to 2 mm/s and 0.3 to 0.1 mm/s, respectively, with a transition region occurring between these growth rate ranges. The microstructure control in soldering processes can be accomplished by manipulating solidification processing variables such as the cooling rate and the growth rate, since the resulting morphological microstructure depends on heat transfer conditions imposed by solidification, and as a direct consequence will affect the final properties.     

激光双面区熔Al2O3/Y3Al5O12共晶自生复合陶瓷的制备与表征

采用激光区熔凝固技术制备大尺寸Al2O3/Y3Al5O12(YAG)共晶自生复合陶瓷,考察双面区熔条件下大尺寸氧化物共晶陶瓷的熔化及凝固成形规律,采用扫描电镜、能谱和X射线衍射对其凝固组织特征进行了表征和分析.研究结果表明:在优化的凝固工艺下,激光双面区熔增加了熔凝层的厚度,获得了熔凝层厚度8.2 mm,长度65.0 mm,致密度达98.5%±1%的Al2O3/YAG共晶陶瓷;共晶熔凝层厚度随激光扫描速率的减小而增加,随激光功率的增大而增大,并且致密度随着激光功率的增大呈现先增大后减小的趋势;双面区熔后的Al2O3/YAG共晶陶瓷微观组织由均一分布、相互交织的Al2O3和YAG共晶相组成,共晶层片间距较小(1.0~3.5μm),且与凝固速度满足Jackson-Hunt公式;共晶间距随扫描速率的增大逐渐减小;双面区熔界面处共晶组织生长具有连续性,界面结合良好;共晶陶瓷的Vickers硬度为(18.6±1.0)GPa.     

Directional Solidification and Superconductive Transition of Bi_2Sr_2 CaCu_2O_(8+δ)

By laser heated pedestal growth method,Bi_2Sr_2CaCu_2O_(8+δ) as-grown fibers with differentmorphologies were solidified in non-equilibriumcondition at the rates of 0.1～14mm/min.At lowrates,a(Sr_(1-x)Ca_x)CuO_2 phase with long andstraight strips in shape and a white Cu-rich phasehaving a morphology of globular shape were found.With the increase of solidification rates,the phase(Sr_(1-x)Ca_x)CuO_2 becomes thinner and the Cu-richphase becomes smaller and dispersive.When thesolidification rates were last.another unstablephase(Sr_(1-x)Ca_x)Cu_2O_3 occurred.In fact,the tran-sition of the semiconductive as-grown fiber to thesuperconductor is a peritectoid reaction influencedby the annealing temperature and time.     

高铬 Nb Si金属间化合物基复合材料的定向凝固组织

采用液态金属冷却定向凝固技术制备了高铬 Nb-Si金属间化合物基复合材料, 分析了不同熔化温度和凝固速度下复合材料的相组成及组织特征。结果表明: Nb-22Ti-17Cr-16Si-2Al-2Hf(原子分数)合金相组成为Nb_ss、 α-Nb₅Si₃、 Laves相NbCr₂和少量的(Nb_ss+ Nb₅Si₃)共晶, 定向凝固没有改变复合材料的相组成。熔化温度为1550 ℃时, Nb_ss相呈颗粒状, Nb₅Si₃相呈随机分布的块状或短板条状; 随着熔化温度的提高, Nb_ss转变为枝晶状且沿轴向定向生长, Nb₅Si₃相转变为有一定定向效果的长板条状。随着凝固速度的增大, 组织明显细化且定向效果增强, 凝固速度为18 mm/min时, Nb₅Si₃呈定向效果良好的细长条状。      

浸渍熔渗复合法制备C_f/HfC复合材料及其性能

采用液相浸渍结合反应熔渗法制备了Cf/HfC复合材料。研究了浸渍效果、抗氧化烧蚀微粒分布形貌、熔渗效果与熔渗组织,并考核了C_f/HfC复合材料的抗氧化烧蚀性能和力学性能。结果表明:经5次浸渍-碳化循环和1次高温处理工艺制备了含13wt%抗氧化烧蚀微粒的ZrB_2+HfO_2+TaSi_2改性C/C预制体,其密度和开孔率分别为1.41g/cm~3和24.84%,微粒主要分布在纤维束之间的基体碳中,且分布均匀。改性C/C预制体经过Hf35Zr10Si5Ta合金反应熔渗制备的C_f/HfC复合材料密度和开孔率分别为2.98g/cm~3和12.95%,其线烧蚀率为0.017 1mm/s,弯曲强度为173.76 MPa,断裂方式为假塑性断裂。     

Rapid Directional Solidification with Ultra-High Temperature Gradient and Cellular Spacing Selection of Cu-Mn Alloy

The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 10^6K/m and 24.1mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum,λmax, minimum, λmin, and average primary spacing,-↑λ, as functions of growth rate, Vb, can be given by, λmax=12.54Vb^-0.61,λmin=4.47Vb^-0.52, -↑λ=9.09Vb^-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.     

Direct observation of field emission in a single TaSi2 nanowire

The electric potential change in a single TaSi2 nanowire during field emission was visualized by means of electron holography. During the field emission, the interference fringes of the electron hologram were blurred locally between the TaSi2 nanowire and anode. This phenomenon was interpreted as being due to a change in the electric potential of approximately 1 V in the TaSi2 nanowire after each ballistic emission. The experiments on the single TaSi2 nanowire field emission behavior provide the useful information for understanding the field emission in the nano-field-emitting device.     

Dendrite coarsening during solidification of hypo- and hyper-eutectic Al-Cu alloys

Dendrite coarsening during cooling at a constant rate was compared at various stages of solidification with that during isothermal holding for Al-Cu alloys of hypo- and hypereutectic compositions. For each specimen, the undercooling for the initial dendrite formation and the time elapsed after it were measured directly. The dendrite arm spacing was shown to be determined solely by the latter, and the dendrite structure was therefore coarsening-controlled from the early stage of solidification. The rate of coarsening in terms of the dendrite arm spacing during solidification at a constant cooling rate was same as that during isothermal holding in all the alloys tested. Numerical values of the fractional rate of solidification were evaluated for the hypo-eutectic compositions and the results show that the rate of dendrite coarsening does not depend on the fractional rate of solidification. Aluminium dendrites show structural coarsening with progressive solidification in the same way as during isothermal holding. CuAl₂ dendrites show curved boundaries after isothermal holding whereas those cooled at a constant rate are faceted.     

TaSi2 nanowires: A potential field emitter and interconnect

TaSi2 nanowires have been synthesized on a Si substrate by annealing NiSi2 films at 950 degrees C in an ambient containing Ta vapor. The nanowires could be grown up to 13 microm in length. Field-emission measurements show that the turn-on field is low at 4-4.5 V/microm and the threshold field is down to 6 V/microm with the field enhancement factor as high as 1800. The metallic TaSi2 nanowires exhibit excellent electrical properties with a remarkable high failure current density of 3 x 10(8) A cm(-2). In addition, effects of annealing temperatures and capability of metal silicide mediation layer on the growth of nanowires are addressed. This simple approach promises future applications in nanoelectronics and nano-optoelectronics.     

Fe-C-Cr合金定向凝固组织

对经定向凝固的共晶 Fe—2.94％C—29.1％Cr 合金及过共晶 Fe—3.12％C—35.9％Cr、Fe—2.80％C—33.1％Cr 合金的凝固组织作了研究。上述三种合金中的初晶及共晶碳化物均为(Fe,Cr)_7C_3它们都以小面状析出,其中初晶在与凝固方向垂直的截面上为六角形块。初晶碳化物在定向凝固时以六角形状进行包抄式的凝固,然后再从六角形壳向内凝固,最终生成完整六角形。随着凝固速度变小,初晶(Fe,Cr)_7C_3的截面尺寸及间距增大,且从实心的六角形棒变成空心的六角形棒。过共晶合金的液固两相区长度约为1.5mm。共晶合金凝固时,碳化物的形貌为不规则多边形棒,且在凝固速度大时,组织细小。上述三种合金,当凝固速度 R>2m/sec 时,液固界面将变得凹凸不平。     

Twin crystal structured Al-10 wt.％ Mg alloy over broad velocity conditions achieved by high thermal gradient directional solidification

Twin crystal structured Al-10 wt.％ Mg alloys that were grown over a broad solidification velocity range were prepared and studied for the first time.The high thermal gradient (G) and growth velocity (V) of directional solidification resulted in the dominant solidification of twins: the twinned dendrite trunks at constant high Vs curved in the G direction with large angles in 7 mm diameter crucibles and invaded regular columnar grains because of a distinct kinetics growth advantage.Transitive deceleration experiments were designed to produce twin crystals that evolved with lower values of V (100,10,and 0.5 μm/s) and had a structural coarsening trend.Twin cell growth in the absence of arms occurred at a growth velocity of 10 μm/s.A coherency loss was observed at a growth velocity of 0.5 μm/s with straight coherent twin boundaries turning into curved incoherent boundaries.Linear theoretical analyses were performed to understand the structural evolution of the twins.These results demonstrate the possibility of producing dense and controlled twin crystals in the Al-Mg system under most industrial production conditions;thus,this approach can be a new structural choice for designing Al-Mg-based alloys that have widespread commercial applications.     

Fabrication of continuous slim aluminum fibers using a multi-tooth tool

In this paper, we describe the development of a multi-tooth tool to fabricate continuous, slim aluminum fibers of an equivalent diameter of under 100?µm. Following an analysis of the process of the fabrication of aluminum fibers, we designed a geometric cutting model of the multi-tooth tool with a large inclination as well as the mechanism to form continuous, slim aluminum fibers. We conducted experiments to determine the influence of process parameters on the surface topography and the equivalent diameter of the aluminum fiber. We obtained the continuous, slim aluminum fibers with a micro-fin structure when the cutting speed, cutting depth, and feed rate were in the ranges of 0.08–0.18?mm, 5–15?m/min, and 0.1–0.2?mm/r, respectively. We found that the equivalent diameter of the aluminum fiber gradually increased with decreasing cutting speed, and increasing feed rate and cutting depth. Moreover, the result of a composition analysis indicated that the generated cutting heat had a minimal effect on the oxidation of the aluminum fiber.     

Numerical model for nucleation of peritectic alloy during unidirectional solidification

Based on transient nucleation theory, a numerical model has been constructed to describe the nucleation process of a new phase in front of the liquid–solid interface of a prior steady-growth phase in peritectic alloy with the combination of the concentration field calculated by a self-consistent numerical model for cellular/dendritic growth. The results show that the nucleation incubation time of a new phase varies with the solidification rate during unidirectional solidification. During unidirectional solidification of the Zn–4.0 wt.% Cu alloy, the incubation time changes very slightly when the solidification rate increases from 50 to 500 μm/s, but it increases significantly when the solidification rate exceeds 500 μm/s. The calculated results show a reasonable agreement with the experimental ones. This model reveals that nucleation of a new phase is time-dependent and reasonably explains the effect of the solidification velocity on the behaviors of nucleation and growth of ɛ dendrites in the matrix of the η phase in unidirectional solidification of Zn rich Zn–Cu alloys.     

Effect of growth rate on microstructure and tensile properties of Ti–45Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

Ti–45Al–2Cr–2Nb alloy was directionally solidified at different growth rates varying from 0.2 to 1.2 mm/min by applying an electromagnetic cold crucible directional solidification technique. It was determined that well-aligned α₂ (Ti₃Al)/γ(TiAl) lamellar structures, B₂ phase and blocky γ phase were generated in columnar grains. The interlamellar spacing (λ) decreases with the increasing growth rate (V) according to the relationship λ ∝ V^− 0.48, but the volume fraction of B₂ is increased as growth rate is increasing. Results of uniaxial tensile tests show that the B₂ phase and the blocky γ phase have significant influence on tensile failure when they are presented in the matrix of α₂/γ lamellar structures, because they are usually employed to act as cracking sources during the tension process.     

Effect of cooling rate on solidification of electron beam melted silicon ingots

Solidification rate has a significant influence on purification of silicon due to segregation of impurities at a liquid–solid interface of a solidifying silicon ingot. A mathematical model is developed to evaluate time-dependent position of the liquid–solid interface and solidification rate of electron beam melted ingots. A series of solidification experiments with different cooling rates are conducted to measure position of a line which separates directionally grown columnar crystals visible in cross-sections of the solidified ingots. Results show that not the whole ingot solidifies directionally when the reduction rate of the beam current is larger than 1.67 mA/s. The position of the dividing line depends on cooling rate and the experimental trend is consistent with that resulted from theoretical simulations. Modeling shows that the solidification rate changes fast when the beam current reduces linearly that is detrimental for segregation of impurities. It also predicts that an exponential reduction of the beam current leads to a uniform solidification rate which is beneficial to segregation of impurities, though not all exponential current reductions lead to this kind of solidification behavior.