Deformation of Ni-Al at high temperature

Nickel-rich Ni-Al was deformed in the temperature range 720 K to 1200 K. The deformation mechanism was found to be independent of the composition. An anomalous behaviour of the activation volume is described in the stoichiometric compound and is attributed to the easy activation of [1 10] slip in the temperature range 850 K to 950 K.     

Influence of chemical and mineralogical composition of concrete aggregates on their behaviour at elevated temperature

The nature of aggregates has an important influence on the behaviour of concrete at high temperature. The aggregates used in concrete are classified into two categories: siliceous (S) and calcareous (C). Most publications and Eurocode 2 Part 1–2 underline concrete containing C aggregates have a better thermal resistance in comparison to concrete with S aggregate. Recent studies show that rocks of identical chemical nature can have different behaviour during a temperature rise. Therefore, the improvement for understanding the thermal damage process of aggregates is necessary. An experimental study performed on three different aggregates (limestone, flint and quartzite) underwent heating–cooling cycles at 150, 300, 450, 600 and 750 °C is discussed in this paper. For a same S nature, the flint showed a spalling phenomenon from 300 to 500 °C, while quartzite had a good thermal stability up to 750 °C. C aggregates presented instability due to decarbonation/hydration after the heating/cooling cycle at 750 °C. The physico-chemical, mineralogical and microstructural evolutions of these aggregates with temperature were analysed to better understand the instability process of concrete aggregates. The evolution of flint damage is especially described by a series of observations of cracking from macroscopic to nanoscopic scale through microscopic scale. Aggregates have a high temperature behaviour very different depending on their physicochemical properties. Distinguishing aggregates according to their mineralogical nature only may not completely be sufficient to anticipate their thermal stability. The distinction between S versus C prescribed by the Eurocode 2 Part 1–2 for calculating concrete compressive strength at elevated temperature is not sufficiently precise or relevant.     

Ultrasonic characterization of refractories at high temperature

A knowledge of the elastic constants of refractory materials is necessary for carrying out stress analysis of high temperature structural design. Together with internal friction such measurements can give useful NDT information. In developing new materials these parameters show well-defined discontinuities at their high temperature mechanical limits. Techniques for these measurements are also based mainly on disks but rods and rings are described. Observations on conventional materials such as silica, molybdenum and tungsten have been extended to graphite, graphite fabrics and the silicon nitrides. In general these materials have very stable thermophysical properties but show wide variations from batch to batch.     

Behaviour of Al-Mg alloys at high temperature

The behaviour of a few Al-Mg alloys (up to 7.1 wt% Mg) has been studied on heating in air at 500°C for 8 h or more. The precipitation of a spinel phase, MgAl₂O₄, is found to take place on the surface and along the transverse section (depth) of the specimen. The alloys seem to be resistant to internal oxidation below 4.5% Mg and thereafter prone to it. Beryllium modification does not suppress the progress of internal oxidation taking place in the Al-Mg alloys. The results have been interpreted on the basis of anodized photomicrographs and X-ray diffraction data.     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Crystallization of amorphous carbon at high static pressure and high temperature

Amorphous carbons prepared from furfuryl alcohol resin have been studied in a high-pressure apparatus of octahedral anvil type at pressures up to 18 GPa and at temperatures up to 2000° C. The amorphous carbons, when heated under pressure, crystallized first into graphite at 450 to 600°C and then into diamond at 1120 to 2000° C. The temperatures for the onset of these crystallizations,T ₉ andT _d, were determined by a simple technique. As the temperature for the preparation of the amorphous carbons was raised from 700 to 1000° C,T ₉ at 15 GPa increased slightly whereasT _d at the same pressure turned from a decrease into an increase beyond 750° C for the preparation temperature. For amorphous carbon prepared at 850° C,T _g increased a little whileT _d decreased markedly with increasing pressure.     

Microstructure of Y-PSZ after ageing at high temperature

Partially stabilized zirconia (PSZ) materials containing 2.5 and 5.0 mol % Y₂O₃ were prepared by pressureless sintering and aged at 1200° C for 1000 IS, and their microstructures were analysed by transmission electron microscopy and electron diffraction methods. Tetragonal zirconia polycrystal (TZP) containing 2.5 mol % Y₂O₃ before ageing showed nearly 100% tetragonal microstructure and 0.5 m grain size, but after ageing the microstructure changed greatly, exhibiting no simple grain structure over wide areas. Repeated twin structures within the grains were observed. Y-PSZ material containing 5.0M01% Y₂O₃ before ageing showed a tetragonal (I structure within a cubic (c) stabilized ZrO₂ matrix, After ageing, structures of fine strip crystals crossed each other orthogonally within the cubic matrix and typical diffuse scattering in the diffraction pattern was observed. Repeated twins were found on the plane of (100)_m, and the orientational relationship between tetragonal (t) and monoclinic (m) crystal was determined to be (100)_m [(100)_t, [010]_m \tT [001b]_t.     

Fe-Cr合金钢高温性能研究

本文试验研制了四种Fe-Cr合金材料,其在900 ℃时的热膨胀率在10～11×10-3范围内,其中FeCr-4合金的热膨胀率为10.27×10-3,与钇稳定氧化锆(YSZ)相比,热膨胀率逐渐接近;与传统合金材料相比,热膨胀率之差均有了明显下降,因而降低了界面热应力,有效改善层间的热失配现象.随着温度的升高,各合金试样的电阻率呈平缓增大的趋势.在850℃时,四种合金试样的电阻率在0.95～1.17×10-3 Ω·cm范围内,其中Fe-Cr-4试样的电阻率较低,为1.17×10-3 Ω·cm,满足用作SOFC连接体材料导电性能的要求.Fe-Cr -1、4两种合金试样的抗氧化性能较好,随着氧化时间的延长,试样表面形成稳定的3Cr2 O3 Fe2 O3氧化层,氧化增重逐渐趋于稳定.累计氧化455 h后,Fe-Cr-1、4两种试样对应的氧化增重在0.0002 g/g左右;而Fe-Cr-2、3两种试样仍在0.0004 g/g以上.     

Endless recrystallization of high-entropy alloys at high temperature

In traditional physical metallurgy, once recrystallization occurs, it will proceed to 100% along with time even at relatively low temperatures, resulting in the limited thermal stability of partially recrystallized alloys. Here, we proposed the strategy of achieving the endless recrystallization state at high temperature(～0.6Tm) in high entropy alloys for the first time. The partially recrystallized microstructures remained stable after annealing at 700 °C for 1440 h toward endless recrystalliza...     

Shock response of stainless steel at high temperature

Measurements of the dynamic tensile strength of HR-2 (Cr-Ni-Mn-N) stainless steel have been carried out over the initial temperature range of 300 K–1000 K at shock stress of 8 GPa, the corresponding spall strength _f and Hugoniot elastic limit _HEL are determined from the wave profiles. In the temperature range of 300 K–806 K, _f and _HEL decrease linearly with increasing temperature T, i.e., _f = 5.63-4.32 × 10^–3T, _HEL = 2.08-1.54 × 10^–3T, but when heated to 980 K, _HEL increases from 0.84 GPa at 806 K to 0.93 GPa at 980 K and _f keeps at an almost fixed value of 2.15 GPa. The TEM analysis on recovery samples identified the existence of intermatallic compound Ni₃Al and the carbide Cr₂₃C₆ in the sample of 806 K, another intermatallic compound Ni₃Ti was found in the sample of 980 K. All these products emerge along crystal boundary. While no such products were found in the samples of 300 K and 650 K.     

Capillary ion chromatography at high pressure and temperature

The application of high pressure and temperature in ion chromatography (IC) can significantly improve the efficiency and reduce the analysis time. In this work, the kinetic-performance limits of capillary IC columns with inner diameters of 400 μm packed with 4 and 7 μm macroporous anion-exchange particles were investigated employing a capillary ion-exchange instrument allowing column pressures up to 34 MPa and column temperatures up to 80 °C. Plate heights below 10 μm could be realized using capillary columns packed with 4 μm particles. Compared to conventional IC using 7 μm particles and pressures up to 21 MPa, a 40% improvement in plate number could be achieved when working at the kinetic performance limits at 34 MPa and using columns packed with 4 μm particles. Using coupled columns with a total length of 400 mm, a mixture of seven anions was separated within 7.5 min while yielding 20?000 plates. Increasing the temperature improved the performance limits when operating in the C-term region (for fast IC separation using columns <75 cm). Temperature also affected the retention properties and hence the selectivity. At higher temperature, retention for monovalent ions was mainly governed by ion diameter. An increase in retention with temperature was observed for small ions, and there was a decrease for ions having a larger diameter. The retention factor for divalent and trivalent anions increased with temperature.     

高温氧化对超高温陶瓷材料耦合传热的影响

超高温陶瓷材料暴露于极端高温飞行环境中会导致其发生氧化,表面生成的氧化物具有不同的热物性从而对传热过程造成影响。针对预氧化的ZrB₂和ZrB₂-SiC,基于氧化模型预测氧化层(ZrO₂、B₂O₃、SiO₂和SiC耗尽层)厚度,利用有限元建立圆柱形代表性体积单元,并与外部高超声速流场的CFD (Computational Fluid Dynamics)求解器相耦合,研究了高温氧化对超高温陶瓷材料的耦合传热的影响。计算中采用分区求解方法,通过耦合界面处非匹配网格间的插值完成实时数据交换,实现了基于Navier-Stokes方程的流动求解器与有限元求解器的多场耦合计算。ZrB₂、ZrB₂-SiC以及氧化生成物的热物性均为温度相关,通过理论计算给出了B₂O₃挥发及SiC耗尽导致的多孔结构的有效热导率和有效比热容。瞬态耦合传热分析的结果表明:ZrB₂在预氧化后其热阻能力略有提高, ZrB₂-SiC氧化前后的热阻变化很小,并且在相同流动环境条件下,氧化后ZrB₂的热阻能力高于氧化后ZrB₂-SiC的热阻能力。