Microstructure and thermal expansion of Ti3SiC2

Abstract

A dense, bulk ceramic of Ti₃SiC₂ containing impurities of TiC and Ti₅Si₃ was fabricated by hot pressing elemental powders of Ti, Si, and C. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used to determine the crystalline phases and observe the microstructure of sintered compacts, respectively. Ti₃SiC₂ exhibits anisotropic grain growth and the size of exaggerated grain is ~25 μm in length. The average coefficient of thermal expansion of Ti₃SiC₂ was measured to be 9.29 × 10^-6 K^-1 in the temperature range 25-1400°C.     

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

Abstract

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni₅Si₂ phase and finely precipitated Ni₃Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni₃(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni₅Si₂ phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni₃Si and Ni₃(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni₃Si and Ni₃(Si,Ti) alloys.     

Mechanical properties of the Ni3(Si,Ti) alloys doped with carbon and beryllium

The Ni₃(Si, Ti) alloys doped with small amounts of carbon and beryllium were tensile tested in two environments, vacuum and air, over a wide range of test temperatures. The yield stresses of the carbon-doped alloys were almost identical to the undoped alloys while those of the beryllium-doped alloys were slightly higher than the undoped Ni₃(Si, Ti) alloys. The doping with carbon enhanced the elongation and ultimate tensile strength (UTS) whereas doping with beryllium reduced the elongation over the entire temperature range tested. The fracture patterns were primarily associated with the ductility behaviour. As the elongation (or UTS) increased, the fracture pattern changed from the intergranular to the transgranular fracture patterns. No environmental embrittlement of the ductility of the carbon-doped alloys was found at ambient temperatures but it was evident at elevated temperatures. Ductilities were reduced at high temperatures when the carbon-doped alloys were tensile tested in air. At high temperatures the environmental embrittlement observed is suggested to be due to the penetration of (free) oxygen into the grain boundaries causing the ductility loss in the carbondoped alloys.     

Corrosion resistance of Ti-6246 for high power aeroengine compressor discs and blades

Abstract

The corrosion resistance between 723 and 1123 K of Ti-6246 has been investigated. For temperatures between 723 and 823 K, the maximum weight gain in air was below the measurable range. For the temperature range 873–1123 K, there was substantial weight gain with reaction time with a maximum weight gain of 180×10^-4 g cm^-2 being obtained after 150 h at 1123 K. Weight gain for the vacuum tests remained constant over all temperatures and reaction times with a maximum observed increase of 3×10^-4 g cm^-2. There was evidence in favour of parabolic rather than linear increases in weight gain with time. The parabolic and linear rate constants increased with temperature and the activation energy associated with parabolic oxidation was estimated as 216 kJ mol^-1. The life of Ti-6246 in argon at 773 K was found at certain stresses to be almost twice that obtained in air. This difference could not be explained by the loss of load bearing cross-sectional area following oxidation.     

Microstructure and compressive flow stress of directionally solidified ternary Ni3(Al,Nb) and quaternary Ni3(Al,Nb, Ti) alloys with duplex phase

The microstructure and compressive flow stress of directionally solidified ternary Ni₃(Al, Nb) and quaternary Ni₃(Al, Nb, Ti) alloys were examined. Three compositions of Ni–16.0 at.%Nb–9.0 at.%Al (Alloy 1), Ni–13.3 at.%Nb–7.5 at.%Al–4.2 at.%Ti (Alloy 2) and Ni–10.7 at.%Nb–6.0 at.%Al–8.3 at.%Ti (Alloy 3) were selected for investigation. Alloy 1 was composed of the L1₂ and the D0_a phases while the constituent phases varied for the D0₂₄ and the D0_a phases for Alloys 2 and 3 with Ti content. The definite crystallographic relationship was observed between the D0₂₄ and the D0_a phases to be (0001)_D024//(010)_D0a and 〈112̄0〉_D024//〈100〉_D0a in Alloy 3. Compression tests were conducted along the growth direction in the temperature ranging from room temperature to 1000 °C. Alloy 1 exhibited high yield stress at low temperatures, but it rapidly decreased above 700 °C. Similar temperature dependence of yield stress was observed in Alloy 2, although the onset temperature of a rapid decrease in yield stress was somewhat lower. Alloy 3 with the highest Ti content showed the lowest compressive strength among the three alloys, while relatively good low-temperature ductility was obtained in Alloy 3. Yield stress of Alloy 3 exhibited anomalous strengthening behaviour accompanied by the basal slip in both D0₂₄ and D0_a phases. Transition in operative slip systems from the basal slip to the prism slip occurred at the peak temperature of yield stress anomaly (600 °C), resulting in a gradual decrease in yield stress. Slip transfer behaviour between the D0₂₄ and the D0_a phases was briefly discussed.     

Mechanical properties of Ni3(Si,Ti) polycrystals alloyed with substitutional additions

The mechanical properties of the L1₂-type Ni₃(Si, Ti) polycrystals, which were alloyed with 1–2 at% of various transition metals and also doped with boron, were investigated over a wide range of temperatures. The addition of Hf enhanced the levels of yield stress whereas the addition of Cr, Mn and Fe reduced the levels of the yield stress over a wide range of temperatures. Ni₃(Si, Ti) alloyed with Cr, Mn and Fe showed a shallow minimum in the yield stress-temperature curves. This result was correlated with the modification of the micro-cross-slip process by the additives. At low temperatures, the addition of Hf and Nb slightly reduced the elongation, while the addition of Cr, Mn and Fe improved elongation. This elongation behaviour was interpreted as the alloying effect on the intergranular cohesive strength of L1₂ ordered alloys. At high temperatures, the elongation of Ni₃(Si, Ti) alloyed with Hf showed a particularly high value. This elongation behaviour is discussed based on the alloying effect on the competition between dynamic recrystallization and cavitation at grain boundaries. The fracture surfaces exhibited a variety of fracture patterns, depending on temperature and the alloy, and were primarily well correlated with the elongation behaviour. The ductilities of most of the alloys at high temperatures were reduced by the tests in air.     

Mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

The mechanical properties of the Ni₃(Si, Ti) alloys undoped and doped with 50 p.p.m. boron, both of which were polycrystalline specimens prepared by recrystallization, were investigated by tensile testing. The yield stress was found to increase with increasing test temperature to a maximum at 800 K, followed by a decrease. The tensile elongation was highest at room temperature and tended to decrease with increasing temperature for both alloys, but was consistently higher in the boron-doped Ni₃(Si, Ti) alloys than in the undoped ones over all the test temperatures. The change in the ultimate tensile stress (UTS) with temperature was similar to that of tensile elongation. The transgranular fracture became dominant as the elongation increased, regardless of the alloys and the testing temperature. Thus, this work again verified that the alloying method proposed by the present authors is useful for improving the grain-boundary cohesion of L1₂-type ordered alloys.     

Production of MgAl2O4 – Ti(C,N) composite ceramics by aluminothermic reduction in reducing atmosphere

Abstract

In the presence of N₂ and CO gases, MgAl₂O₄ – Ti(C,N) composite has been synthesised by aluminothermic reduction. The phase characterisation and microstructure of this novel composite were investigated by powder X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the nitridation reaction begins at 1100°C. With an increase of temperature, TiC starts to appear and forms Ti(C,N) solid solution with TiN; the grain size of Ti(C,N) grows with increasing temperature and the final product is MgAl₂O₄ – Ti(C,N).     

Suppression of moisture-induced embrittlement of Ni3(Si, Ti) alloys by a surface nickel alloying layer

The effect of a surface nickel alloying layer on moisture-induced embrittlement of Ni3(Si, Ti) alloys has been investigated by tensile tests at room temperature, using nickel-deposited materials. Undeposited Ni3(Si, Ti) alloy became remarkably embrittled in air. However, nickel-deposited Ni3(Si, Ti) alloy showed a high elongation value, indicating the suppression of embrittlement caused by hydrogen decomposed from moisture in the air. When the surface nickel alloying layer consists of f c c(γ) solid solution with a high nickel concentration and good adhesion to the substrate, improvement of tensile elongation is the greatest. The results have been discussed from the chemical and structural viewpoints of the surface nickel alloying layer. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of P on hot ductility of high C,Al, and Nb containing steels

Abstract

Hot ductility curves for high carbon Nb and Nb free steels have been determined immediately after casting at two P levels, ~0.01% and ~0.045%. High strain rates of 0.1-0.55 s^-1 were generally used but some limited low strain rate testing at 7 × 10^-3 s ^-1 was carried out on Nb containing steels. Nb containing steels showed, as expected, worse ductility than the Nb free steel but high P level was detrimental to ductility for both steels and ductility in general was very poor. Failure was intergranular with the presence of films of a P rich phase at the boundaries in the case of the Nb free steels and in addition to this, in Nb containing steel there was a Nb rich phase. The films were thicker and more continuous in the higher P steels. It is suggested that the P rich films are probably the low melting point phase Fe₃P or Fe₃(Mn)P, which can remain liquid down to temperatures as low as 950°C. Some back diffusion of P into the grain interior is possible if the strain rate is reduced and/or at high testing temperatures during the 5 min hold prior to testing. This allowed some improvement in ductility to occur in the lower P containing steels by reducing the amount of the low melting point phase at the boundaries.     

The effect of Nb addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The alloying behavior, the microstructure and the high-temperature tensile properties of the Nb-added Ni₃(Si,Ti) alloys were investigated. The solubility limit of Nb element in the L1₂ Ni₃(Si,Ti) phase at 1273 K was shown to be approximately 2.7 at%, and thermodynamically discussed. The second-phases (dispersions) formed beyond the solubility limits were identified as D0_a-type Ni₃Nb and Ni₁₂Si₄TiNb₂ compounds, and contained some lattice defects with incoherency with the L1₂ Ni₃(Si,Ti) matrix. The Nb-containing second-phase dispersions in the L1₂ Ni₃(Si,Ti) phase matrix resulted in strengthening over a wide range of temperature, and also an improvement of the high-temperature tensile elongation.     

Environmental effect on mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

The environmental effect on the mechanical properties of boron-doped and undoped Ni₃(Si, Ti) polycrystals was investigated by tensile testing in air from room temperature to 1073 K, and the results were compared with those obtained previously by tensile testing in vacuum. The environmental effect for the Ni₃(Si, Ti) alloys was significant at ambient temperatures whereas that for the boron-doped Ni₃(Si, Ti) alloys was considerable at elevated temperatures. When these samples at associated temperatures were tensile tested in air and also at low strain rate, intergranular fracture was dominant. It was suggested that the environmental embrittlements at low and high temperatures were due to hydrogen and oxygen absorbed from the air, respectively, and were caused by the weakening of the grain-boundary cohesion. It was proposed that boron competing with hydrogen, for site occupation or for its effectiveness at grain boundaries, has the effect of suppressing hydrogen embrittlement, whereas it was suggested that the low-melting phases, consisting of boron and oxygen (and/or constituent atoms), may be responsible for the ductility loss in the boron-doped Ni₃(Si, Ti) alloys.     

Microstructure characterization and tensile properties of squeeze-cast AlSiMg alloys

A research program was conducted to study the effects of squeeze pressure (70, 100 and 160 MPa) and heat treatment T6 on the structure, hardness and tensile properties of cast Al6Si0.3Mg alloys. The influence of squeeze pressure on macro- and microstructures of Al6Si0.3Mg alloys has been investigated. Some of castings were solution treated at 540 °C for various times and others were subjected to aging at 170 °C after solution treatment. The results indicated that precipitation occurred within about 30 min for both cast and squeeze cast alloys. The hardness began to increase and maximum values were observed after about 10 h for as-cast alloy. Increasing of squeeze pressure (70–160 MPa) accelerated strength of the alloys from 8 to 4 h, respectively. Squeeze pressures decreased the percentage of porosity and increased the density, also it decreased the grain size of α-Al and modified the Si eutectic. Hardness and tensile properties increased with both heat treatment and increasing of squeeze pressure.     

Microstructure and tensile properties of FeMnNiCuCoSnx high entropy alloys

High-entropy FeMnNiCuCoSn_x (x denotes the atomic fraction of Sn) alloys with good plasticity have been developed. The systematical investigation demonstrates that the concentration of Sn element plays a significant role in the microstructure and tensile properties. As 0.03 < x < 0.05, the alloys exhibit high tensile strength and plasticity, and the maximum elongation strain and strength are 16.9% and 476.9 MPa, respectively, because of their single FCC solutions. While the concentration of Sn is higher than 0.05, an intermetallic compound (Cu_5.6Sn) in the interdendritic regions forms, which degrades the ductility of alloys.     

Microstructural characterisation and mechanical properties of Ni–Ge alloys containing Ni3Ge

Abstract

Six Ni–Ge alloys were prepared and equilibrated at 1000°C. The microstructures, compositions of phases, solubilities in various phases, volume fractions of phases, lattice parameters of Ni₃ Ge, and microhardness values of phases were determined. Two alloys contained Ni₃Ge and α, one alloy was single phase Ni₃Ge, and three alloys contained Ni₃Ge and Ni₅Ge₃. The volume fractions of second phase were 6 and 33%α and 37 and 54%Ni₅Ge₃. The amount of second phase in the third Ni₃Ge–Ni₅Ge₃ alloy was too low to be determined accurately. The two phase alloys containing 6%α, 37%Ni₅Ge₃, or 54%Ni₅Ge₃ were tested in compression. A yield strength maximum was observed in the three alloys. The deformation behaviour of the alloy containing 6%α was similar to that of single phase Ni₃Ge: crack formation along Ni₃Ge grain boundaries and very low plastic strain were revealed. In alloys containing Ni₃Ge and an appreciable percentage of Ni₅Ge₃, crack formation was not observed along Ni₃Ge grain boundaries. Instead, at low temperatures, deformation and crack growth occurred within Ni₅Ge₃ grains and, at high temperatures, decohesion occurred along the Ni₃Ge/Ni₅Ge₃ interphase. The plastic strain in the alloy containing 54%Ni₅Ge₃ increased to about 40%.

MST/1724     

Microstructure,phase formation and properties of rapid solidified Al–Fe–Cr–Ti alloys

Improvements in the mechanical strength of Al–Fe–Cr–Ti alloys have been demonstrated when non-equilibrium microstructures are developed. This paper investigated the effect of cooling rate and composition on the phase formation, microstructure and properties of new Al_96.6Fe_1.5Cr_1.7Ti_0.2 and Al_91.6Fe_4.9Cr_2.2Ti_1.3 (at.-%) alloys. Wedge-shaped samples produced by suction casting were characterised by optical, scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, energy dispersive X-ray spectroscopy and microhardness. The results showed that the morphology and size of the phases precedent of the flower-like phases change from small, spherical particles to large flower-like phases with decreasing the cooling rate. The presence of intermetallic phases Al₁₃Fe₄, Al₁₃Cr₂ and Al₃Ti in the Al_91.6Fe_4.9Cr_2.2Ti_1.3 alloy, resulted in a hardness 1.6 times higher compared to the Al_96.6Fe_1.5Cr_1.7Ti_0.2 alloy.     

含钪Al-Cu-Li-Zr合金的组织与性能

为了研究微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金组织与性能的影响,采用铸锭冶金法,制备了4种不同Sc含量的Al-3.5Cu-1.5Li-0.12Zr合金.采用室温拉伸力学性能实验、金相显微镜和透射电镜研究了微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金微观组织和拉伸性能的影响.结果表明:添加0.1%Sc能消除铸态合金的枝晶组织,有效地抑制了再结晶的发生,具有一定的强化作用和明显的增塑效应;添加O.15%Sc和0.25%Sc能显著细化合金铸态的晶粒组织,但添加0.15%Sc不能抑制合金固溶过程中再结晶;添加0.25%Sc会促进合金固溶过程中的再结晶,从而降低合金的强度.合金中较适宜的Sc加入量为0.10%～0.15%,此时合金既具有较高的强度,又兼具较好的塑性.     

Microstructure and internal friction of Ni–Ti alloys absorbing hydrogen

The microstructure and internal friction of Ni–Ti alloys after hydrogen absorption have been investigated by means of optical microscopy, X-ray diffraction, differential scanning calorimetry and low frequency torsional internal friction apparatus. The results show that, after hydrogen absorption, the grains tend to be elongated, and the nucleation and growth of hydrides are mainly concentrated at the grain boundaries. In addition, a new phase ultimately identified as Ti₂NiH_0·5 phase forms after absorption of hydrogen. The hydrogen induced martensite promotes the emergence of a two-stage transformation. However, the growth of hydrides causes a reduction of the hydrogen induced martensite. The hydrides act as strong pinning points, resulting in a dramatic increase in the internal friction. In addition, the marked change of the internal stress, caused by the microscopic strain and the mismatching of the volumes, also improves the internal friction.