In situ synthesis of TiC particulate-reinforced aluminum matrix composites

Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere. The onset of formation of TiC particles began at about 1150 K by reaction of TiAl₃ with Al4C₃. Subsequent formation of TiC particles at higher temperatures to approx-imately 1265 K occurred by direct reaction of carbon with TiAl₃. Above this temperature, the TiC particles coarsened with increasing temperature from an initial size of about 0.15μm. TiC particles were also produced in preforms that were not infiltrated; however, the presence of liquid aluminum in infiltrated specimens inhibited particle agglomeration and sintering. Infil-trated preforms could, therefore, serve as excellent "master alloys" for subsequent dilution in an aluminum melt and processing of metal-matrix composites (MMCs) reinforced with sub-micron TiC particulates. Formerly Research Scientist, Massachusetts Institute of Technology, Cambridge, MA 02139     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

In situ formation of metal-ceramic microstructures,including metal-ceramic composites,using reduction reactions

Partial reduction reactions were used to form a metallic phase either around or inside oxide grains in polycrystals in the FeMnO system. By suitable choice of oxide composition, partial pressure of oxygen, annealing time and temperature, it is possible to control the nucleation and growth of the metallic phase to produce a wide range of metal-ceramic microstructures. These include ceramic grains with a thin layer of metallic phase at their boundaries; ceramic grains with a thick layer of metallic phase at their boundaries —essentially, a metal-ceramic composite; and ceramic grains containing a fine distribution of metal particles—essentially, a ductile phase toughened ceramic. The presence of the metallic phase increases the fracture toughness of all the metal-ceramic microstructures with respect to that of the pure ceramic, with the largest increase observed for the metal-ceramic composite. It is believed that the principles established by studying the FeMnO system can be used on more practical mixed oxide systems to produce metal-ceramic microstructures, which in some cases are unique, and in particular, if the starting oxide material is in the form of plate- or rod-shaped crystals, to produce metal-ceramic composites in situ.     

Production of Ni3Ti-TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi-TiC (stoichiometric) or Ni₃Ti-TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3 ≤ x ≤ 0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi; i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti-TiC _x composites are produced. Ignition, combustion, and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented.     

Microstructure and mechanical properties of in situ (TiC + SiC)/FeCrCoNi high entropy alloy matrix composites

In situ (TiC + SiC) particles (5 vol.％ and 10 vol.％, respectively)-reinforced FeCrCoNi high entropy alloy matrix composites were fabricated via vacuum inductive...     

TiC颗粒增强铜基复合材料的研究

以电解铜粉和TiC粉为原料, 采用粉末冶金法制备了增强体质量分数为5%、10%、15%、20%的TiC颗粒增强铜基复合材料。通过对显微组织的观察和对相对密度、硬度、电导率、磨损率、摩擦系数的测试, 研究了增强相质量分数、烧结温度对复合材料组织性能的影响。研究结果表明, TiC颗粒除少量团聚外均匀分布在基体上, 并与基体结合良好; 随烧结温度升高, 铜基复合材料的密度和硬度均有所增加; 随增强相质量分数的增加, 硬度增加, 相对密度和电导率均有所下降; 磨损率则表现为先降低后有所增加的趋势, 磨损率在TiC质量分数为15%时最低; 铜基复合材料的摩擦系数明显低于纯铜, 其磨损机制主要以磨粒磨损为主。     

In situ observations of the formation of martensite in stainless steel

Specimens of stainless steel have been deformed at room temperature, or cooled to below M_s in an HVEM and the formation of martensite observed. The M_s temperature, orientation relationships and habit planes of the martensites formed in specimens thicker than 0.5 μm were found to be identical to those of the bulk material. It has been shown that the ϵ-martensite occurs in regions where appropriately, but usually irregularly, spaced stacking faults are formed, while α-martensite nucleation is associated with dislocation pile-ups on the active slip plane.     

In situ combustion synthesis of dense ceramic and ceramic-metal interpenetrating phase composites

A model exothermic reaction is used to demonstrate the application of simultaneous combustion synthesis, conducted under a consolidating pressure, as a one-stepin situ synthesis technique for the production of dense ceramic and ceramic-metal interpenetrating phase composites (IPC). The addition of an excess amount of metal,e.g., Al, and/or a diluent,e.g., Al₂O₃, lowers the combustion temperature and aids in the refinement of the microstructure, facilitating an increase in compressive strength and elastic modulus. The effects of the important process parameters,e.g., reaction stoichiometry and diluents, green density, pressure, and temperature, on microstructure and properties of these high-performance composites are discussed.     

Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr₂Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr₂Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr₂Nb particles.     

Phase transitions in reactive formation of Ti5Si3/TiAl in situ composites

A new method is developed for preparing Ti₅Si₃/TiAl in situ composites by incorporating metastable phases (called metastable precursors) into TiAl (a mixture of elemental Ti and Al) matrix powders. Metastable precursors with a starting composition of Ti-14Al-21Si are prepared by mechanical alloying (MA). They have been proven through X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observations to be mainly consisting of mixtures of nanostructured solid solutions and milling-formed TiAl compound. Particularly, phase reactions and transitions in the precursors and the composites during heating have been investigated in detail by using diffraction thermal analysis (DTA) in conjunction with XRD. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, TiSi₂ → Ti₅Si₄ → Ti₅Si₃. When the composite powder (precursor, Ti and Al) is heated, a combustion reaction first occurs in the matrix, which results in the formation of TiAl₃ and/or TiAl followed by the completion of the previously mentioned silicide transitions in a very short time. Scanning electron microscope (SEM) observations indicated the locations of reinforcements in the reaction-formed composite, and TEM observation provided some details of the structures for the reinforcements and their neighborhood. This method is intriguing because a designed phase hierarchy is possible.     

原位生成TiC颗粒对铁基合金组织及性能的影响

通过向Fe—Al-B合金中添加C和Ti元素的方法，在合金中原位生成了TiC颗粒，利用X射线衍射(XRD)、扫描电子显微镜(SEM)和能谱仪分析了添加TiC前后材料中相的变化、显微组织的变化及材料中各相的化学成分。结果表明，在合金中原位生成TiC颗粒后，合金组织明显得到细化，其耐熔锌腐蚀性能及强度也有大幅度提高。     

热压原位合成（Cu-Mo）／Mo5Si3复合材料

为改善Mo5Si3的室温脆性,以Mo、Cu、Si粉体为原料,通过热压反应烧结原位合成制备了(Cu-Mo)/Mo5Si3复合材料。其微观组织由Mo5Si3和少量Mo形成的相和Cu基固溶体相两相组成,且各相分布均匀、组织致密。随着Cu含量(质量分数)的增加,Mo5Si3的体积分数减少,材料的硬度下降,而相对密度、抗弯强度和断裂韧度提高。Cu和Mo的协同增韧,加之Mo5Si3的高强度和高硬度使(Cu-Mo)/Mo5Si3复合材料具有良好的强韧性配合。     

Role of Al2O3 particulate reinforcements on precipitation in 2014 Al-matrix composites

Precipitation in commercial aluminum alloy 2014, without and with alumina particulate reinforcements, was studied using microhardness, electrical resistivity, differential scanning calorimetry (DSC), and transmission electron microscopy. The precipitation sequence in 2014 Al was confirmed to be α_ss→α + GPZ →α + λ’→a + λ’ + gH→α + λ (AlCuMgSi) + θ (CuAl₂). Reinforcement addition decreased the time to peak hardness, but also reduced the peak matrix microhardness. This was traced to a decrease in the amount of λ’ formed in the composites. Further, it was observed that while Guinier-Preston (GP) zone and θ’ formations are accelerated in the composites, λ’ precipitation is decelerated. The acceleration is attributable primarily to enhanced nucleation resulting from an increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements, whereas the deceleration is associated with a decrease of low-temperature solute diffusivity due to absorption of vacancies at dislocations and interfaces. It was also observed that the degree of overall acceleration in hardening and the reduction in peak matrix microhardness with reinforcement addition decreased with decreasing aging temperatures. The causal relationships of these observations with the associated mechanisms are discussed. Formerly Graduate Student, Department of Mechanical Engineering, Naval Postgraduate School.     

反应热压制备FeAl/TiC复合材料

采用混合粉热压工艺制备了FcAl/TiC复合材料。研究了TiC含量、粘结相成分以及反应热压工艺对致密化过程和力学性能的影响。研究结果表明:复合材料的密度随TiC含量增加而减小;硬度和抗弯强度随TiC体积分数增加而出现峰值,增加Al含量有利于致密化,但因引入过多的氧化夹杂和热空位会导致力学性能降低;热压温度和压力等工艺参数也对材料的性能有影响。