STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY II. Microstructure Evolution in the Steady--State Growth Region

分析了定向凝固Ti--43Al--3Si(原子分数, %) 合金在3---90 μm/s 的生长速度下的稳态生长区组织. 在定向凝固过程中经历下列反应: L→Ti₅Si₃, L→α+Ti₅Si₃, α→α₂(Ti₃Al)+γ(TiAl), α₂→γ+Ti₅Si₃, 其中, α 与Ti₅Si₃ 共晶是合金最显著的凝固行为. 当生长速度大于20 μm/s 时, 还出现L→γ+Ti₅Si₃. 随着生长速度增大, 稳态组织逐渐由粗胞晶向细胞晶、胞状枝晶及枝晶转变, 起稳定α相作用的Ti₅Si₃ 相由低速时分布于α相中逐渐向高速时分布于凝固γ 相中转变, 不利于该合金的引晶. 选择10 μm/s 的初始生长速度, 既能减少到达稳态生长的距离, 又能保证引晶效果.     

RAPID SOLIDIFICATION WELDING OF ULTRA--THIN Zr55Cu50Al12Ni3 AMORPHOUS FOILS

应用微型储能焊机实现了厚度为25---35 μm的Zr₅₅Cu₃₀Al₁₂Ni₃非晶箔材的快速凝固连接. XRD测试表明, 接头仍为非晶结构. 计算的接头冷却速率高达10⁶ K/s, 远大于形成锆基非晶合金的临界冷却速率, 有效地抑制了接头区的晶化. 接头尺寸微小, 直径为60---90 μm, 未产生气孔、夹杂等焊接缺陷. 接头剪切强度高达1141 MPa. 高的电阻率特性使非晶合金的焊接能量明显低于晶态合金.     

Phase-field simulation of structure evolution at high growth velocities during directional solidification of Ti55Al45 alloy

A phase-field model whose free energy of the solidification system is derived from Calphad thermodynamic modeling of phase diagram is used to simulate structure evolution of Ti₅₅Al₄₅ alloy during directional solidification at growth velocities sufficiently higher than the critical velocity of transition from cells to dendrites, but lower than the absolute stability. The liquid–solid phase transition of L→L+β(Ti) is chosen. Firstly, the dynamics of the breakdown of initially planar interfaces into cellular structures then cellular dendrites are shown. Then the transition from cellular dendrites to fine cellular structures are shown at higher growth velocities. The solute segregation patterns are investigated at different growth velocities. The appearance of solute trapping is also investigated by determining the solute partition coefficients as a function of growth velocities. Agreement is reached with the theory of rapid directional solidification.     

凝固速度对Ni47Ti44Nb9铸态合金组织和织构的影响

对比研究了Ni₄₇Ti₄₄Nb₉合金快冷铸锭和慢冷铸锭的宏、微观组织和织构特点,分析了凝固速度对组织和织构的影响机理。采用光学显微镜(OM)和扫描电子显微镜(SEM)观察了微观组织,采用X射线衍射(XRD)分析了织构。结果表明,铸态Ni₄₇Ti₄₄Nb₉合金具有<113>方向的择优生长取向;凝固速度对Ni₄₇Ti₄₄Nb₉合金铸态组织和织构的形成具有重要影响,快速冷却条件下获得均匀的树枝晶组织,铸锭的横、纵截面均形成{113}织构,慢速冷却条件下形成具有特定方向的柱状树枝晶,纵截面织构以{113}<361>、{113}<332>和{113}<141>为主;凝固过程中热量方向性传导造成的树枝晶定向生长是影响合金铸态组织和织构形成的主要原因。     

Designing a toxic-element-free Ti-based amorphous alloy with remarkable supercooled liquid region for biomedical application

A series of toxic-element-free Ti–Zr–Ta–Si amorphous alloy ribbons have been successfully prepared by melt-spinning. The differential scanning carlorimetry (DSC), X-ray diffraction analysis, bending test and microhardness test are conducted for studying the thermal and mechanical properties. The results show that the Ti₄₂Zr₄₀Ta₃Si₁₅ metallic glass ribbon present excellent ductile behavior by the bending testing, without any fracture cracking after bending over 180 degree. In addition, this amorphous alloy possesses a very high glass transition and crystallization temperature of 799 and 898 K, respectively, as well as a very wide supercooled liquid region of 99 K. This amorphous alloy exhibits promising thermal stability during isothermal annealing at the middle temperature of its supercooled region, with more than 3000 s incubation time for isothermal annealing at 823 K (550 °C). This amorphous alloy also shows much lower value of corrosion current density (2.27 × 10⁻⁹ A/m²) than the 304 stainless steel in the 0.3 mass% sodium cloride solutions. This Ti₄₂Zr₄₀Ta₃Si₁₅ alloy is believed to be a promising based alloy for fabricating the bulk metallic glass foam by the spacer technique in the application of biomedical implants.     

Microstructure,high-temperature deformability and oxidation resistance of a Ti5Si3-containing multiphase MoSiBTiC alloy

A Ti₅Si₃-containing multiphase MoSiBTiC alloy with a composition of 38Mo–30Ti–17Si–10C–5B (at.%) was designed and produced by arc-melting. The alloy was composed of five phases—Mo solid solution (Mo_ss), Mo₃Si, Mo₅SiB₂ (T₂), Ti₅Si₃ and TiC—and consistently has good thermal stability at least up to 1700 °C. The density of the alloy was ∼7.0 g/cm³, which is considerably smaller than that of Ni-base superalloys. Microstructure was carefully examined and microstructural anisotropy was confirmed. The anisotropy was considered to be generated by thermal gradient during the solidification process. Microcracking was remarkable across the primary Ti₅Si₃ phase, which was caused by thermal expansion anisotropy of the Ti₅Si₃ phase. High-temperature deformability was examined by high-temperature compression tests at 1500 °C. Two kinds of loading axes were chosen for the compression tests with respect to the microstructural anisotropy. The alloy exhibited a peak stress of 450–550 MPa, followed by good deformability at the testing temperature. Microstructure refinement and reduction in microcrack density were observed after hot working. Oxidation tests were conducted on the alloy at 1100 °C and 1300 °C for 24 h. The oxidation curves demonstrated that rapid mass loss finished within several minutes. After that, the mass loss began to slow down and then the specimens' mass decreased almost linearly with increasing testing time. Cross-section observation indicated that oxygen propagated through Mo_ss, whereas T₂ and Ti₅Si₃ phases acted as barriers against oxygen attack during the tests. In addition, it was found that the alloy gained better oxidation resistance after high-temperature deformation, suggesting a positive effect of phase refinement on its high-temperature oxidation resistance.     

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.     

Low-Temperature Interface Reaction Between Titanium and the Eutectic Silver-Copper Brazing Alloy

Reaction zones formed at 790 °C between solid titanium and liquid Ag-Cu eutectic alloys (pure and Ti-saturated) have been characterized. When pure Ag-Cu eutectic alloy with 40 at.% Cu is used, the interface reaction layer sequence is: αTi/Ti₂Cu/TiCu/Ti₃Cu₄/TiCu₄/L. Because of the fast dissolution rate of Ti in the alloy, the reaction zone remains very thin (3-6 μm) whatever the reaction time. When the Ag-Cu eutectic alloy is saturated in titanium, dissolution no longer proceeds and a thicker reaction zone with a more complex layer sequence grows as the reaction time increases. Four elementary chemical interaction processes have been identified in addition to Ti dissolution in the liquid alloy. These are growth of reaction layers on Ti by solid state diffusion, nucleation and growth from the liquid of TiCu₄, isothermal solidification of silver and, finally, chemical conversion of the Cu-Ti compounds by reaction-diffusion in the solid state. A mechanism combining these processes is proposed to account for the constitution of Ti/Ag-Cu/Ti joints brazed at 780-800 °C.     

Microstructure transition from stable to metastable eutectic growth in Ni–25%Al alloy

The microstructural evolution during directional solidification of the Ni–25%Al (mole fraction) alloy was investigated in the range of growth velocity from 10 to 100 μm/s under a given thermal gradient of 10 K/mm. The solidification microstructures reveal a transition from γ‘–β equilibrium eutectic to γ–β metastable eutectic plus β dendrites. A mixed microstructure of γ‘–β and γ–β eutectics produced at a growth velocity of 25 μm/s illustrates that the transition occurs during the competitive growth between γ and γ' phases. The growth temperature for each phase was considered to understand the microstructure selection during solidification. The experimental results show that a phase or a microstructure solidifying with the highest temperature under a given growth condition is preferentially selected upon solidification. In addition, both stable eutectic and metastable eutectic are shown to coexist and simultaneously grow in the velocity range between 25 and 60 μm/s due to their similar growth temperatures.     

Cellular/dendritic transition,dendritic growth and microhardness in directionally solidified monophasic Sn-2%Sb alloy

Horizontal directional solidification experiments were carried out with a monophasic Sn-2%Sb (mass fraction) alloy to analyze the influence of solidification thermal parameters on the morphology and length scale of the microstructure. Continuous temperature measurements were made during solidification at different positions along the length of the casting and these temperature data were used to determine solidification thermal parameters, including the growth rate (V_L) and the cooling rate (T_R). High cooling rate cells and dendrites are shown to characterize the microstructure in different regions of the casting, with a reverse dendrite-to-cell transition occurring for T_R>5.0 K/s. Cellular (l_c) and primary dendrite arm spacings (l₁) are determined along the length of the directionally-solidified casting. Experimental growth laws relating l_c and l₁ to V_L and T_R are proposed, and a comparative analysis with results from a vertical upward directional solidification experiment is carried out. The influence of morphology and length scale of the microstructure on microhardness is also analyzed.     

P-Cr-Ti复合变质Al-25%Si合金时Cr-Ti作用机理

采用P-Cr-Ti复合变质处理Al-25%Si(质量分数)合金,重点研究了凝固组织的变化以及Cr、Ti元素的作用机理。结果表明:与单一的P变质相比,经P-Cr-Ti复合变质后,Al-25%Si合金凝固组织中初生Si的尺寸减小了12.2%~51.7%,并且初生Si分布的均匀程度增加。Al-25%Si合金中Cr、Ti主要以TiAl_3、Ti_7Al_5Si_(12)、Al_7Cr、Al_(13)Cr_4Si_4化合物的形式存在,同时有少量的P分布在含Ti化合物中。含Ti化合物呈长条状、短杆状;含Cr化合物呈菊花状、网状,分布在初生Si之间。含Cr、Ti化合物的数量随冷却速度的增加而增加。变质处理时Al-6.5%Ti合金带入的TiAl_3相,具有使初生Si持续析出和细化初生Si的作用,但细化能力有限。凝固过程中初生Si周围形成的含Cr化合物和αCr阻止初生Si的长大与聚集,促使增加初生Si分布均匀程度。     

Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti₅Si₃ has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm³) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti₅Si₃ alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti₅Si₃ in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti₅Si₃ is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti₅Si₃ is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N₂. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N₂, and the reaction rate increased as the concentration of N₂ increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

Rapid solidification and phase stability evaluation of Ti-Si-B alloys

Ti-rich Ti-Si-B alloys can be considered for structural applications at high temperatures (max. 700 °C), however, phase equilibria data is reported only for T = 1250 °C. Thus, in this work the phase stability of this system has been evaluated at 700 °C. In order to attain equilibrium conditions in shorter time, rapid solidified samples have been prepared and carefully characterized. The microstructural characterization of the produced materials were based on X-ray diffraction (XRD), scanning electron microscopy (SEM-BSE), high resolution transmission electron microscopy (HRTEM), High Temperature X-ray diffraction with Synchrotron radiation (XRDSR) and Differential Scanning Calorimetry (DSC). Amorphous and amorphous with embedded nanocrystals have been observed after rapid solidification from specific alloy compositions. The values of the crystallization temperature (Tx) of the alloys were in the 509-647 °C temperature range. After Differential Scanning Calorimetry and High Temperature X-ray Diffraction with Synchrotron radiation, the alloys showed crystalline and basically formed by two or three of the following phases: αTi, Ti₆Si₂B; Ti₅Si₃; Ti₃Si and TiB. It has been shown the stability of the Ti₃Si and Ti₆Si₂B phases at 700 °C and the proposition of an isothermal section at this temperature.     

Formation and properties of Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses with different (Ti + Zr)/Cu ratios for biomedical application

Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses (BMGs) free from highly toxic elements Ni and Be were developed as promising biomaterials. The influence of (Ti + Zr)/Cu ratio on glass-formation, thermal stability, mechanical properties, bio-corrosion resistance, surface wettability and biocompatibility were investigated. In the present Ti-based BMG system, the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy exhibited the highest glass forming ability (GFA) corresponding to the largest supercooled liquid region, and a glassy rod with a critical diameter of 3 mm was prepared by copper-mold casting. The Ti-based BMGs possess high compressive strength of 2014–2185 MPa and microhardness of 606–613 Hv. Young's modulus of the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy was about 100 GPa, which is slightly lower than that of Ti–6Al–4V alloy. The Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy with high GFA exhibited high bio-corrosion resistance, and good surface hydrophilia and cytocompatibility. The mechanisms for glass formation as well as the effect of (Ti + Zr)/Cu ratio on bio-corrosion behavior and biocompatibility are discussed.     

Effect of solidification process parameters on dry sliding wear behavior of AlNiBi alloy

As-cast samples of the Al–3wt.%Ni–1wt.%Bi alloy resulting from the horizontal directional solidification process were subjected to the micro-abrasive wear test. The effects of the solidification thermal and microstructural parameters, such as the growth and cooling rates and the cellular and primary dendritic spacings (V_L and T_R; λ₁ and λ_c; respectively), were evaluated in the wear resistance of the investigated alloy. The tribological parameters analyzed were the wear volume and rate (V_w and R_w). The solidification experiments and the wear tests were carried out by means of a water-cooled horizontal directional solidification device and a rotary-fixed ball wear machine, respectively. The results show lower V_w and R_w values correspond to finer microstructures and the V_w dependence on λ₁ is characterized by an experimental mathematical equation. A better distribution of Bi soft droplets and Al₃Ni hard intermetallic particles is observed within the finer interdendritic region and, in consequence, the better wear resistance is achieved in as-cast samples with dendritic morphology rather than cellular morphology. A transition of wear mechanism from adhesive to abrasive is observed.     

Selective Laser Melting of in-situ TiC/Ti5Si3 composites with novel reinforcement architecture and elevated performance

A novel Selective Laser Melting (SLM) process was applied to prepare bulk-form TiC/Ti₅Si₃ in-situ composites starting from Ti/SiC powder system. The influence of the applied laser energy density on densification, microstructure, and mechanical performance of SLM-processed composite parts was studied. It showed that the uniformly dispersed TiC reinforcing phase having a unique network distribution and a submicron-scale dendritic morphology was formed as a laser energy density of 0.4 kJ/m was properly settled. The 96.9% dense SLM-processed TiC/Ti₅Si₃ composites had a high microhardness of 980.3HV_0.2, showing more than a 3-fold increase upon that of the unreinforced Ti part. The dry sliding wear tests revealed that the TiC/Ti₅Si₃ composites possessed a considerably low friction coefficient of 0.2 and a reduced wear rate of 1.42 × 10^− 4 mm³/Nm. The scanning electron microscope (SEM) characterization of the worn surface morphology indicated that the high wear resistance was due to the formation of adherent and strain-hardened tribolayer. The densification rate, microhardness, and wear performance generally decreased at a higher laser energy density of 0.8 kJ/m, due to the formation of thermal cracks and the significant coarsening of TiC dendritic reinforcing phase.     

Formation of Brittle Phases During Pulsed Current Gas Tungsten Arc Welding of Titanium to Aluminum Alloys

Welding of titanium alloy TA15 to aluminum alloy Al 2024 was conducted by pulsed current gas tungsten arc welding using AlSi12 filler metal. Formation process of phases near the Ti/Al interface was discussed. Titanium and aluminum were partially fusion welded in the upper part while brazed together in the middle and bottom parts of the joint. In the upper part of the joint, intermetallics Ti₃Al + Ti₅Si₃, TiAl + Ti₅Si_3, and TiAl₃ were formed as three layers orderly from the titanium side to the weld metal. In the middle and bottom parts of the joint, intermetallics Ti₅Si₃ and TiAl₃ were formed as two layers near the Ti/Al interface.     

Joining of Cf/SiC Composite and TC4 Using Ag-Al-Ti Active Brazing Alloy

Carbon fiber reinforced SiC (C_f/SiC) composite was successfully joined to TC4 with Ag-Al-Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope, energy dispersive spectrometer, and x-ray diffraction. The mechanical properties of the brazed joints were measured by mechanical testing machine. The results showed that the brazed joint mainly consists of TiC, Ti₃SiC₂, Ti₅Si₃, Ag, TiAl, and Ti₃Al reaction products. TiC + Ti₃SiC₂/Ti₅Si₃ + TiAl reaction layers are formed near C_f/SiC composite while TiAl/Ti₃Al/Ti + Ti₃Al reaction layers are formed near TC4. The thickness of reaction layers of the brazed joint increases with the increased brazing temperature or holding time. The maximum room temperature and 500 °C shear strengths of the joints brazed at brazing temperature 930 °C for holding time 20 min are 84 and 40 MPa, respectively.     

Microstructure and microsegregation in directionally solidified Mg–4Al alloy

Directional solidification (DS) of a binary Mg–4Al (wt.%) alloy was carried out to investigate the microstructures and microsegregation under controlled solidification conditions. In directional solidification the microstructure depends on the growth rate V because the cooling rate, which governs the solidification microstructure, is the product of the growth rate and the temperature gradient. The ability to produce simple and uniform microstructures in directional solidification enables us to correlate the formation of the microstructure and its characteristic length scales quantitatively with processing parameters. The morphology of the solid–liquid interface and the microstructure of both the mushy zone and the steady-state region were characterized at different levels of growth rates. With the help of an electron microprobe, microsegregation was determined in a specimen directionally solidified with cooling rates ranging from 0.06 to 0.8 K/s. The calculated microsegregation results based on the Scheil model deviated significantly from the experimental data, which is anticipated since back diffusion was not included due to the lack of diffusivity data.