Ti5Si3 whisker in-situ reinforced TiAl alloys

The effects of Fe, Cr, V, and Nb on the microstructures, tensile properties at 20 °C and 900 °C, and creep resistance at 800 °C of Ti₅Si₃ whisker-reinforced Ti₅₂Al₄₈-3Si2M alloys were investigated. The addition of Fe, Cr, Nb, and V modifies not only the morphologies but also the distribution of Ti₅Si₃ whiskers. A microstructure with a uniform and homogeneous distribution of Ti₅Si₃ whiskers was obtained in a Ti₅₂Al₄₈-3Si2Cr2V alloy by conventional ingot metallurgy. The Ti₅₂Al₄₈-3Si2Cr2V alloy has the best room-temperature tensile strength, relatively good ductility, an attractive tensile property at 900 °C, and good creep resistance at 800 °C. The improvement of properties results from not only the homogeneous distribution of Ti₅Si₃ whiskers but also from the higher fracture strength of the Ti₅Si₃ whisker and the interface property. The solubility of V in the Ti₅Si₃ phase is higher than that of Fe, Cr, and Nb. The element V is very effective in strengthening the Ti₅Si₃ whiskers. Different failure modes were found in the Ti₅Si₃ whisker-reinforced TiAl alloys at room temperature. Cleavage fracture dominates the failure of Ti₅Si₃ whiskers and γ phase in V-free alloys, whereas crack deflection and branching at the Ti₅Si₃-whisker/γ-matrix interface, subsequently followed by interface debonding and whisker bridging, were observed in Ti₅₂Al₄₈-3Si2V and Ti₅₂Al₄₈-3Si2Cr2V alloys. In addition, twinning and dislocation slip in Ti₅Si₃ whisker-reinforced TiAl alloys were investigated.     

Unidirectionally solidified Ti−TiB and Ti−Ti5Si3 eutectic composites

Induction melting and electron beam melting techniques were employed in the production of unidirectionally solidified eutectic composites of Ti-1.7 wt pct B and Ti-8.5 wt pct Si. The grown eutectics were reinforced by 7.7 volume pct of TiB fibers and 31 volume pct of Ti₅Si₃ fibers respectively. Controlled dendritic solidification of a hypereutectic composition of Ti-12 wt pct Si was also accomplished. Tensile, compressive, creep, and stress rupture specimens were cut from the eutectic composites and tested with reinforcing fibers parallel to the load axis. Ti?TiB eutectic was found to have less than the critical volume fraction of fibers necessary for reinforcement, while Ti?Ti₅Si₃ composite attained a compressive yield strength of 275,000 psi and a compressive Young's modulus of 30×10⁸ psi after heat treatment. The 500 and 4000 hr stress rupture properties of Ti?Si eutectic were superior to commercial titanium alloys at 1000° and 1200°F. The minimum creep rate of Ti?Ti₅Si₃ eutectic composite was lower than all other titanium alloys at 1000°F. Tensile, compressive, and creep properties of the Ti-8.5 wt pct Si eutectic are discussed in terms of the current theories of composite behavior.     

Electrosynthesis of Ti5Si3, Ti5Si3/TiC,and Ti5Si3/Ti3SiC2 from Ti-Bearing Blast Furnace Slag in Molten CaCl2

Metallurgical and Materials Transactions B - Ti5Si3, Ti5Si3/TiC, and Ti5Si3/Ti3SiC2 have been electrochemically synthesized from the Ti-bearing blast furnace slag/TiO2 and/or C mixture precursors...     

Ductile-phase toughening in V-V3Si in situ composites

This article describes the room-temperature fracture behavior of ductile-phase-toughened V-V₃Si in situ composites that were produced by arc melting (AM), cold-crucible induction melting (IM), and cold-crucible directional solidification (DS). Composites were produced containing a wide range of microstructures, interstitial impurity contents, and volume fractions of the ductile V-Si solid solution phase, denoted (V). The fracture toughness of these composites generally increases as the volume fraction of (V) increases, but is strongly influenced by the microstructure, the mechanical properties of the component phases, and the crystallographic orientation of the (V) phase with respect to the maximum principal stress direction. For eutectic composites that have a (V) volume fraction of about 50 pct, the fracture toughness increases with decreasing “effective” interstitial impurity concentration, [I]=[N]+1.33 [O]+9 [H]. As [I] decreases from 1400 ppm (AM) to 400 ppm (IM), the fracture toughness of the eutectic composites increases from 10 to 20 MPa √m. Further, the fracture toughness of the DS eutectic composites is greater when the crack propagation direction is perpendicular, rather than parallel, to the composite growth direction. These results are discussed in light of conventional ductile-phase bridging theories, which alone cannot fully explain the fracture toughness of V-Si in situ composites.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.     

Standard enthalpy of formation of Sc5Si3

The standard enthalpy of formation of Sc₅Si₃ has been determined by solute-solvent drop calorimetry at (1473±2) K. The following value is reported: ΔH _f ^o (mean)=−(719.1±34.0) kJ mol⁻¹. This result is compared with corresponding published values for the enthalpies of formation of Me₅Si₃, with Me=Mn, Cr, V, Ti. This comparison shows regularly increasing negative enthalpies of formation from Mn₅Si₃ to Sc₅Si₃. LETTTIA TOPOR, formerly Senior Research Associate, The James Franck Institute, The University of Chicago,     

C/C-SiC-Cu5Si复合材料的显微形貌及形成过程

以全网胎针刺整体毡为预制体,采用化学气相渗透法增密制备C/C多孔坯体。采用反应熔体渗透法,将Si粉,Ti粉和Cu粉均匀混合,在1 400~1 700℃下制备C/C-SiC-Cu5Si复合材料。利用X射线衍射分析该复合材料的物相组成,用扫描电镜分析其显微形貌。结果表明:C/C-SiC-Cu5Si复合材料由炭纤维,热解炭,β-SiC,TiC和Cu5Si相组成;生成的TiC相主要分布在热解炭周围,Cu5Si分布在TiC周围,而β-SiC弥散分布在Cu5Si基体中;TiC晶粒的形成是Ti原子向C原子扩散的结果,其长大受Ti原子扩散控制。     

热压反应合成Al_2O_3-Ho_2O_3/TiAl复合材料

利用Al-Ti-TiO2-Ho2O3体系原位反应合成了Ho掺杂Al2O3/TiAl复合材料。采用DTA结合XRD分析对体系反应过程进行了探讨。借助XRD、EDS和SEM等手段,对放热体系的物相组成及晶粒微观形貌进行了分析表征。结果表明:Al-Ti-TiO2-Ho2O3系原位合成的Al2O3/TiAl复合材料由TiAl、Ti3Al、Al2O3以及HoAl3相组成;Ho2O3的引入对基体相生成比例(TiAl:Ti3Al)有一定的调控作用,并使得基体晶粒和Al2O3晶粒均有所细化且逐渐分布均匀。力学性能测试表明:当Ho2O3的引入量为6%时,材料的抗弯强度达到最大值,约为593.5MPa;断裂韧度达到最大值,为8.74MPa.m1/2,具有可接受的力学性能。     

Temperature Dependence of Electrical Resistivity of the TiN/TiAl3/Ti2AlN Composite Material

Russian Journal of Non-Ferrous Metals - The TiN/TiAl3/Ti2AlN composite material is fabricated in the filtration combustion mode in nitrogen of porous samples made of the TiAl intermetallic powder...     

时效转变与Ti/(Ti,Zr)2Si复合材料的制备

通过真空自耗电极炉熔炼得到硅锆比为1，2的Ti—Zr—Si合金材料，对合金材料进行了1200℃，3．5h的固溶处理和850℃不同时间的时效处理，通过时效析出反应制备Ti／(Ti，Zr）2Si复合材料。利用光学显微镜、扫描电镜、X射线衍射、EDX分析了Ti—Zr—Si合金的铸造、固溶和不同时效阶段的显微组织及其相组成。分析结果表明，硅锆比为1／2(重量百分比)的Ti—Zr—Si合金，铸造组织为块状β—Ti、晶界由块状β—Ti和条状的5—3型硅化物组成；Ti—Zr—Si合金经过850℃，480min的时效处理，5—3型硅化物转变为2—1型硅化物；尺寸细小、分布均匀的2—1型硅化物使制备高性能的Ti／(Ti，Zr)2Si复合材料成为可能。     

原位烧结合成(Ti,V)C颗粒增强铁基复合材料的微观结构研究

将粉末冶金技术和原位反应技术相结合,制备了不同V/Ti原子比的 (Ti,V)C/Fe复合材料(V/Ti=0,0.4,0.6,0.8).用扫描电镜(SEM)、X射线衍射等测试方法对制备的复合材料进行了组织结构及相组成分析,用洛氏硬度计测试试样的硬度.研究结果表明:所制备的复合材料致密化好,硬度达60～65HRC,(Ti,V)C/Fe增强相颗粒尺寸大部分小于2μm,且随着V/Ti比的提高,(Ti,V)C的面间距和晶格常数相对TiC减小,(Ti,V)C增强相颗粒形态更规则,趋于球形.V/Ti为0.4时(Ti,V)C/Fe复合材料具备更好的微观结构.     

In situ formation of three-dimensional TiC reinforcements in Ti-TiC composites

Three-dimensional, single-crystal reinforcements of TiC were producedin situ during manufacture of Ti-TiC composites. The composites, containing 40 to 50 vol pct TiC, were produced using standard casting procedures. The presence of aluminum in Ti-TiC composites showed enhanced strength without loss of ductility at room and elevated temperatures. Aluminum additions were found to solid solution strengthen the Ti matrix and increase the strength of the TiC phase. The morphology of the TiC, which was controlled by processing parameters, influenced the properties of the Ti-TiC composites investigated. Refinement of the secondary dendrite arm spacing of the three-dimensional (3-D) TiC particles was found to dramatically improve the ultimate tensile strength (UTS) and ductility of the Ti-TiC composites.     

Si对(Ti5Si3+TiBw)/TC11组织与性能影响

采用粉末冶金法制备原位自生网状(Ti5Si3+TiBw)/TC11复合材料,对其金相组织、微观形貌、力学性能与断口进行研究分析.结果表明:TiBw、部分Ti5Si3分布于基体颗粒周围,呈准连续网状分布;其余Ti5Si3分布于基体周围、β-Ti内、α-Ti与β-Ti相界处.随着Si元素的增加,原位生成Ti5Si3增多,α...     

Oxidation of Ti3SiC2 composites in air

The oxidation of Ti₃SiC₂ composites (75 at. pct Ti₃SiC₂ and 25 at. pct TiC _x ), prepared via self-propagating high-temperature synthesis (SHS) and subsequent shock consolidation, has been studied in the range of 1073 to 1573 K in this research. The oxidation kinetics are parabolic with an activation energy of approximately 240±20 kJ/mol. As shown by transmission electron microscopy (TEM) during the very early stages of oxidation, the oxide layer formed contains amorphous SiO₂ and crystalline rutile (TiO₂). As oxidation proceeds, a two-layered oxide scale is observed with the outer oxide layer consisting of columnar TiO₂ with trace amounts of SiO₂ and the inner oxide layer being comprised of a mixture of amorphous SiO₂ and fine crystalline TiO₂. The grain size of the outermost oxide (TiO₂) increases with increasing oxidation temperature. The oxidation resistance of the Ti₃SiC₂ composites prepared by SHS and subsequent shock consolidation is similar to the oxidation of Ti₃SiC₂ prepared by other means with comparable parabolic constants.     

Rapid synthesis of ternary carbide Ti3SiC2 through pulse-discharge sintering technique from Ti/Si/TiC powders

Ti/Si/TiC powders with molar ratios of 1:1:2 (M1) and 2:2:3 (M2) were prepared for the synthesis of a ternary carbide Ti₃SiC₂ by using the mixture method for 24 hours in an Ar atmosphere. The synthesis process was conducted at 1200 °C to 1400 °C under a pressure of 50 MPa, using the pulse-discharge sintering (PDS) technique. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction (XRD) technique and observed by optical microscopy and scanning electron microscopy. The results showed that the phases in all the samples consisted of Ti₃SiC₂ and small amounts of TiC, and the optimum sintering temperature was found to be in the relatively low range of 1250 °C to 1300 °C. By the standard additive method, the relative content of Ti₃SiC₂ was calculated. For the M1 samples, the lowest TiC content can be only decreased to about 3 to 4 wt pct, whereas the content of Ti₃SiC₂ in the M2 samples is always lower than that in the M1 samples. When the M2 powder was sintered at 1300 °C for 8 to 240 minutes, the TiC peaks were found to show a very low intensity, and the corresponding content of Ti₃SiC₂ was calculated to be higher than 99 wt pct. The grain size of Ti₃SiC₂ increased from 5 to 10 μm to 80 to 100 μm in the entire applied sintering temperature range. The relative density of the M2 samples was measured to be higher than 99 pct at sintering temperatures above 1275 °C. It indicates that the PDS technique can rapidly synthesize high-content Ti₃SiC₂ from the Ti/Si/TiC powders in a relatively low temperature range.     

Refinement of Eutectic Si Phase in Al-5Si Alloys with Yb Additions

A series of Al-5 wt pct Si alloys with Yb additions (up to 6100 ppm) have been investigated using thermal analysis and multiscale microstructure characterization techniques. The addition of Yb was found to cause no modification effect to a fibrous morphology involving Si twinning; however, a refined plate-like eutectic structure was observed. The Al₂Si₂Yb phase was observed with Yb addition level of more than 1000 ppm. Within the eutectic Al and Si phases, the Al₂Si₂Yb phase was also found as a precipitation from the remained liquid. No Yb was detected in the α-Al matrix or plate-like Si particle, even with Yb addition up to 6100 ppm. The absence of Yb inside the eutectic Si particle may partly explain why no significant Si twinning was observed along {111}_Si planes in the eutectic Si particle. In addition, the formation of the thermodynamic stable YbP phases is also proposed to deteriorate the potency of AlP phase in Al alloys. This investigation highlights to distinguish the modification associated with the ever present P in Al alloys. We define modification as a transition from faceted to fibrous morphology, while a reduction of the Si size is termed refinement.