Microstructure and oxidation resistance of porous NbAl3 intermetallic prepared by thermal explosion reaction

Porous Nb–Al intermetallic was prepared by thermal explosion (TE) of combustion synthesis (CS). The temperature profile, phase composition, open porosity and oxidation resistance of Nb–Al compact were investigated. The results showed that the significant exothermic reaction occurred, which means that an obvious TE appeared during heating. The volume expansion of 190% was observed, and the prepared sample exhibited interconnected pores with an open porosity of 65.7%. The porous NbAl₃ intermetallic showed ‘pest’ oxidation in the temperature range of 500–600°C, and followed the parabolic oxidation law at 400°C. Moreover, due to the highly porous structure, this material had great potential for application to separation and heat insulation at a temperature range of room temperature to 400°C.     

热处理对TiAl基合金相变和显微组织的影响

TiAl基合金净形成形是克服其难加工成形的有效方法，但该合金铸态组织较粗大，不利于提高其综合性能，如何细化该合金的微观组织成为当前研究热点之一。本文重点讨论了通过热处理改变TiAl基合金微观组织的技术途径，包括双相区热处理，双温热处理，循环热处理，快速加热循环热处理等，并分析了各种热处理条件下组织演变的微观机制，其中快速循环热处理对细化原始组织效果最为显著。     

Effect of heating conditions on the magnetic properties of micron-sized carboxyl modified-magnetite particles synthesized by a spray pyrolysis and heating process

Superparamagnetic carboxyl (COOH) modified-magnetite (Fe₃O₄) (COOH-Fe₃O₄) micrometer-sized porous particles were synthesized by the spray pyrolysis of a 0.1 M Fe(NO₃)₃·9H₂O and 0.2 M citric acid solution and a subsequent heating process in either an Ar + 1% H₂ (P_O2 = 10^?20 Pa) or N₂ (P_O2 = 1 Pa) gas atmosphere. Fe₃O₄ formed due to carbon and hydrogen gas generation of thermal decomposition of citric acid, even under reduction conditions with a P_O2 below 10^?23 Pa. The COOH-Fe₃O₄ particles heated in an Ar + 1% H₂ or N₂ atmosphere were porous (40% porosity) and about 1.2 to 1.3 μm in diameter. The particles consist of nano-sized COOH-Fe₃O₄ crystallite about 7 nm in diameter. The specific surface area increased from 116 to 127 m²/g by increasing the heating time in an Ar + 1% H₂ atmosphere from 10 to 20 h. The saturation magnetization of the COOH-Fe₃O₄ particles (38.7 A·m²/kg) heated in Ar + 1% H₂ for 10 h was much higher than that of commercial magnetic microbeads (17 A·m²/kg) and the coercivity was 0 kA/m. These superparamagnetic COOH-Fe₃O₄ particles dispersed in distilled water were attracted to a NdFeB magnet for 7–10 s under a 400 Mt external magnetic field, compared with 10 s for commercial magnetic microbeads.     

Effect of heating rate on the grain refinement of a TiAl alloy by cyclic heat treatment

This letter reports the effect of heating rate on the grain refinement of a Ti–46Al–2Cr–2Nb (at.%) alloy by cyclic heat treatment. Results show that microstructures with two different sizes were developed at two different heating rates. A coarse fully lamellar structure can be refined to a fine one with a size of 15 μm if the heating rate, together with the heating temperature, is appropriately controlled.     

Effect of heating rate on the hysteresis in the phase transition in silver telluride thin films

The electrical resistance of vacuum deposited silver telluride thin films was measured in the temperature range from 300 to 430 K at different heating rates. It is found that silver telluride films undergo a structural phase transition, with a hysteresis. The phase transition occurs over a wide temperature range of about 30 K and the transition temperature as well as the hysteresis width are found to be influenced by the heating rate. The effect of heating rate on the phase transition temperature and the hysteresis are discussed.     

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.     

粉末粒度对FeAl金属间化合物多孔材料孔结构的影响

研究了粉末粒度对FeAl金属间化合物多孔材料孔结构的影响.研究表明,当粉末粒径在60μm以上时,随着粉末粒径的增加,FeAl金属间化合物多孔材料的总孔隙度和开孔隙度变化不大,即粉末粒度不是决定FeAl多孔材料孔隙度的主要因素;粉末粒度是决定FeAl金属间化合物多孔材料最大孔径的主要因素,在18～125μm的粒度范围内,多孔体最大孔径与粉末粒径之间严格遵循dm=0.4·d.的直线变化规律.     

Al含量对热爆合成Al-Ti-C组织及细化性能的影响

为改善Al-Ti-C中间合金的晶粒细化性能,采用铝液中热爆合成法制备出用于铝及铝合金晶粒细化的Al-Ti-C中间合金.通过DTA、XRD和SEM等手段分析了Al含量对热爆反应过程及合成产物组织形态的影响,并比较了Al-Ti-C中间合金对工业纯铝的细化效果.结果表明,铝液中热爆合成的Al-Ti-C中间合金由Al、Al3Ti、TiC三相组成,Al含量对反应体系的合成温度、反应速度及合成产物组织产生重要影响.调整原料Al含量可有效控制中间合金中第二相粒子组织形态,获得良好的晶粒细化性能.     

微量磷(P)对等原子比NiAl的微观组织与力学性能的影响

研究了微量P对挤压态等原子比NiAl的微观组织与高温力学性能的影响.结果表明:微量P的添加对NiAl的晶格常数有一定的影响,P偏聚于NiAl晶界处;并对其高温延伸率有重要影响.P偏聚于晶界阻碍了合金变形过程中的动态回复和再结晶,加剧了晶界处孔洞的形成,造成了NiAl-P合金与二元NiAl合金高温力学性能的显著差异,主要表现在:应力-应变曲线经历了较长的加工硬化阶段;最大延伸率明显下降;变形激活能升高,应变速率敏感指数下降.NiAl-P合金的高温变形机制为变形过程中位错的滑移与攀移共同作用.     

Evolution of texture at the initial stages of continuous annealing of cold rolled dual-phase steel: Effect of heating rate

The performance of cold rolled dual-phase (DP) steels depends on their microstructure, which results from the thermomechanical processing conditions, involving hot rolling, cold rolling and continuous annealing. In the present work, the effects of intercritical annealing parameters i.e. heating rate, soaking temperature and time and the cooling rate on the texture and microstructure of a cold rolled DP steel (0.08%C–1.91%Mn) were investigated after simulating through Gleeble thermomechanical simulator. The soaking temperature was chosen in a way that all the ferrite has recrystallized before the temperature was reached. The three different heating rates allowed the samples to get recrystallized in three different ways: below Ac₁, just around Ac₁ and above Ac₁. {3 3 2} fiber texture along with {1 1 2} 1 1 1 texture component were observed after heating to the soaking temperature as well as after slow cooling. The overall intensity of the texture as well as textural component was observed to be nearly independent of the heating rate as well as cooling rate. The textural evolution was correlated with the volume fractions and morphology of carbides, which depend on the annealing processing parameters.     

Fabrication and characteristics of porous NiTi shape memory alloy synthesized by microwave sintering

Porous nickel titanium (NiTi) shape memory alloy (SMA) was successfully fabricated by microwave sintering method. This method allows formation of porous structures without using any pore-forming agents. Moreover, microwave sintering of NiTi SMA can be successfully performed at a relatively low sintering temperature of 850 °C and a short sintering time of 15 min. The pore characteristics, microstructure, phase transformation and stress-strain behavior of the porous NiTi SMA were investigated. The porous NiTi SMA exhibited porosity ratios from 27% to 48% and pore sizes range from 50 to 200 μm when using different sintering temperatures and holding times. The predominant B2 (NiTi) and B19′ (NiTi) phases were identified in the porous NiTi SMA. A multi-step phase transformation took place on heating and a two-step phase transformation took place on cooling of the porous NiTi SMA. The irrecoverable strains decreased with increasing sintering temperature, but the holding time had little effect on the stress-strain behavior at 60 °C.     

Na2O对MoSi2/oxide发热元件特性的影响

为了克服半导体热处理炉中MoSi2发热元件的低温氧化问题，通过添加高Na2O粘土获得了由MoSi2/oxide复合材料制成的发热元件，考查了脱Na工艺前后MoSi2/oxide发热元件使用特性的变化。研究结果表明，脱Na工艺较好地解决了半导体制品热处理装置中石英玻璃管的失透问题，使发热元件的弯曲强度和高温蠕变特性有了明显地改善，提高了发热元件的使用寿命。     

预设升温速度对电场作用下Fe-Cu-Ti-C体系燃烧合成的影响

采用Gleeble-3500D热模拟机,研究了电场作用下预设升温速度对Fe-Cu-Ti-C体系燃烧合成的影响.结合X射线衍射分析(XRD)、扫描电镜(SEM)及金相显微镜分析了合成产物的相组成及显微组织,同时基于能量守恒定律对体系中TiC的转化率进行理论计算.结果表明:随着预设升温速度的提高(50～100℃/s),体系的点火温度相应下降(752～629.78℃);试样的致密度则相应地提高;合成产物TiC颗粒逐渐变细.同时转化率计算结果显示:Fe-Cu-Ti-C体系中TiC的转化率随着预设升温速度的提高而有所增加.     

Synthesis of TiC-TiB2-Ni cermets by thermal explosion under pressure

TiC/TiB₂-based cermets were fabricated in situ by means of the thermal explosion under pressure technique starting from Ti-B₄C powders with the addition of varying contents of Ni metal binder to achieve near-net-shape bulks. The combustion reaction was ignited in a graphite die heated by current. Full conversion of the reactants was obtained by thermal explosion and the process yielded TiC-TiB₂-Ni materials characterised by a fine microstructure. Appreciable differences in terms of microstructure, hardness and fracture toughness by indentation were observed between core and external surface of the products due to fast cooling caused by heat transfer to the die walls. Cermets with a high content of Ni showing high hardness and fracture toughness were obtained, with values of HV₅ = 2182 and K_Ic = 8.8 MPa m^1/2 for 30 wt.% Ni and of HV₅ = 1684 and K_Ic = 12.7 MPa m^1/2 for 47 wt.% Ni.     

Effect of Cr on the high temperature oxidation of L12-type Al3Ti intermetallics

Three kinds of L1₂-type Al₃Ti–Cr alloys, Al₆₇Ti₂₅Cr₈, Al₆₆Ti₂₄Cr₁₀ and Al₅₉Ti₂₆Cr₁₅, were prepared by induction melting followed by thermomechanical treatment. The oxidation behavior was investigated at 1273, 1373 and 1473 K in air. The oxidation resistance of the prepared alloys was excellent, however, there were some differences for each alloy. The isothermal oxidation resistance increased in the order of Al₅₉Ti₂₆Cr₁₅, Al₆₆Ti₂₄Cr₁₀ and Al₆₇Ti₂₅Cr₈, while the order became reversed in terms of the cyclic oxidation resistance. As more Al₂O₃ formed owing to the increased Al content in the alloy, the isothermal oxidation resistance increased, whereas the cyclic oxidation resistance decreased. The oxide scale was primarily composed of Al₂O₃, contaminated with Cr₂O₃ and TiO₂ that were present mainly at the lower part the oxide scale.     

真空高温热退火对单晶磷化铟纳米孔阵列的形貌、微结构影响

利用真空热退火的方法，研究了单晶InP方形纳米孔阵列退火后的形貌和晶相等特征。发现随着退火温度的不断升高，单晶InP中P的挥发性也越来越剧烈，到650℃后P完全挥发，样品变成h单质，而单晶InP方形纳米孔阵列的形貌也逐渐变得不均一，当退火温度高于550℃，整个纳米孔阵列被完全破坏。     

Effect of high heating rate on thermal decomposition behaviour of titanium hydride (TiH2) powder in air

DTA and TGA curves of titanium hydride powder were determined in air at different heating rates. Also the thermal decomposition behaviour of the aforementioned powder at high heating rates was taken into consideration. A great breakthrough of the practical interest in the research was the depiction of the P _H2-time curves of TiH ₂ powder at various temperatures in air. In accordance with the results, an increase in heating rate to higher degrees does not change the process of releasing hydrogen from titanium hydride powder, while switching it from internal diffusion to chemical reaction. At temperatures lower than 600 °C, following the diffusion of hydrogen and oxygen atoms in titanium lattice, thin layers TiH _x phase and oxides form on the powder surface, controlling the process. On the contrary, from 700 °C later on, the process is controlled by oxidation of titanium hydride powder. In fact, the powder oxidation starts around 650 °C and may escalate following an increase in the heating rate too.