Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (˜200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.     

The effects of stoichiometry on the dry sliding wear of FeAl

The effects of alloy stoichiometry on the dry sliding wear behavior of B2-structured FeAl were investigated using four different FeAl alloys containing 40, 43, 48, and 50 at.% Al. Room temperature pin-on-disk tribotests were performed against an yttria-stabilized zirconia counterface in four different environments: air, oxygen, 4% hydrogen in nitrogen, and argon. It was found that the alloys had lower wear rates in both oxygen-free and water vapor-free environments, where the wear loss was roughly inversely related to the hardness. Interestingly, the wear rates were little affected by the presence of molecular hydrogen. In oxygen- or water vapor-containing environments, the effects of the environment predominated over any effects from the mechanical properties of the alloys. The tips of the worn pins were examined using both scanning electron microscopy and transmission electron microscopy, the latter using specimens produced by focused ion beam milling. Zirconia particles were found to be embedded in a tribolayer on the worn tips of the pins. The results indicate that both two-body and three-body abrasive wear, as well as plastic deformation and delamination were the main wear mechanisms. The abrasive particles largely consisted of the counterface material.     

Parabolic growth of solid-solution scales: Effect of deviations from stoichiometry

The effect of deviations from the stoichiometric composition of solid-solution oxides formed during the oxidation of a binary alloy is considered by taking into account its influence on the activities of the oxide components using an approximate expression. Calculations carried out for two systems, the Co-Ni and Co-Fe alloys, forming CoO-NiO and CoO-FeO solid-solution scales, show that the effect of this factor on the form of the calculated profile of oxide composition through the scale is negligible in the former case but significant in the latter. Inclusion of this effect in the theoretical treatment for the Co-Fe system yields results that are in better agreement with the experimental data than are those calculated by means of the classic treatment, which neglects this aspect. This finding results from the larger magnitude of the deviation from Stoichiometry in the CoO-FeO system as compared with the CoO-NiO system. In neither case does the nonstoichiometry affect significantly the magnitude of the calculated rate constants.     

不同形变温度下Mg-Al-Ca-Mn合金的变形机制

熔炼配制了Mg-Al-Ca-Mn合金,采用单轴拉伸实验测试了合金在25～475℃温度范围内的应力-应变曲线,通过数据处理分析了其加工硬化率曲线,并采用显微组织观察研究了不同形变温度下合金的形变机制。研究表明,合金拉伸的应力-应变曲线在不同温度下存在明显区别,且取决于其形变机制的不同。室温变形的形变机制以孪生为主,应力随应变增加而快速增加至最大值后断裂,塑性较差。425℃高温变形时主要形变机制为动态再结晶,应力在变形初期的极小应变范围内迅速达到最大值,由于动态再结晶的软化作用,应力值缓慢减小,表现出良好的变形能力。而当形变温度处于中温区(225℃)时,加工硬化阶段以孪生变形为主,随着应变的不断增加,部分区域发生动态再结晶,应力增加至最大值后缓慢减小,直到断裂。     

Mechanical properties of T-111 at low to intermediate temperatures

The overall objective of this study was to determine allowable design stresses for the solid-solution-strengthened alloy T-111 (Ta–8W–2Hf) to be used in a space power application at 300–650 °C. Tensile tests were conducted from 25 to 1100 °C and creep tests were conducted from 500 to 1171 °C. The allowable long-term stress intensity for T-111 was found to be about 180 MPa up to 850 °C; at higher temperatures the allowable stress intensity must be reduced because of creep. However, a single creep mechanism cannot predict creep rate over the entire temperature range of this study.     

Effect of microstructure on plastic deformation of Cu at low homologous temperatures

The mechanical properties of polycrystalline Cu (purity 99.95%) prepared by severe plastic deformation were studied at low homologous temperatures from 0.5 K to room temperature. Material with three different microstructures was prepared by annealing of ultrafine-grained Cu. At cryogenic temperatures (0.5 and 4.2 K) the material exhibited an inverse temperature dependence of the yield stress and unstable plastic deformation accompanied by serrations on the stress–strain curves. These low-temperature anomalies were accentuated with grain refinement. At cryogenic temperatures, enhanced ductility was observed and the Hall–Petch relation was found to hold. Microhardness and yield stress were much more temperature dependent in fine-grained than in coarse-grained material, and there is a correlation between the flow stress at a fixed strain and the microhardness. This study has demonstrated that, apart from enhanced discontinuous plastic flow, severe plastic deformation improves the strength of copper at cryogenic temperatures without sacrificing ductility.     

Mechanical and corrosion behaviour of a Ti‐Al‐Nb alloy after deformation at elevated temperatures

The mechanical properties of Ti6Al7Nb alloy deformed and heat treated at elevated temperatures were correlated with its microstructure and corrosion behaviour in Ringer (of different pH values: 2.49, 6.9 and 8.9) and Ringer–Brown solutions. Microstructural analysis revealed a Widmanstatten structure for the alloys deformed at 1100 °C (β field) and structure with α grains at 930 °C (α + β field). The thermo‐mechanical processing improved the electrochemical behaviour of Ti6Al7Nb alloys, especially their passive state. Open circuit potential variations in time reflected more compact, stable, resistant passive films on the surface of the treated alloys. Open circuit potential gradients simulating the non‐uniformities of pH along the implant surface have very low values that cannot generate galvanic corrosion. Corrosion rates and ion release are very much reduced. Impedance spectra were fitted with a two time‐constants equivalent circuit for some alloys and with three time‐constants equivalent circuit for other alloys.     

Deformation behaviour of iron-rich iron-aluminium alloys at high temperatures

The deformation behaviour of binary monocrystalline and polycrystalline Fe-Al alloys with Al contents up to 18 at.% and only low unavoidable impurity contents—in particular less than 100 wt.ppm C—has been studied above room temperature up to 800 °C. The effects of quenching and annealing treatments on the behaviour of as-cast materials were investigated in order to clarify the dependence of strength and ductility on Al content and possible short-range ordering. Besides solid-solution hardening by Al, serrated yielding and deformation twinning were observed at high temperatures. Yielding is affected by quenched-in excess vacancies.     

Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

Oxidation behaviour of PACVD TiBN coating at elevated temperatures

The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO₂. SEM revealed that the TiO₂ layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO₂ layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies.     

工业纯钛中低温蠕变的等时应力应变曲线

根据工业纯钛TA2在服役温度范围内(室温、353 K、423 K)的拉伸实验和蠕变实验获得了工业纯钛的拉伸应力应变曲线和蠕变曲线。由中低温蠕变实验数据发现工业纯钛在服役温度范围内会产生显著的蠕变现象,蠕变特征会随温度发生变化。随后,分别建立了工业纯钛在各温度下的弹塑性拉伸本构方程和蠕变本构方程。综合拉伸和蠕变本构方程建立了工业纯钛在服役温度范围内的等时应力应变曲线。等时应力应变曲线显著低于拉伸曲线,并且存在寿命相关性,随着寿命要求的提高等时应力应变曲线不断下降。值得注意的是在不同的温度下等时应力应变曲线随寿命要求的变化规律存在显著差别。根据等时应力应变曲线、结构允许的极限应变和结构设计寿命,本研究提出了工业纯钛与时间相关的许用应力曲线。     

AerMet100超高强度钢热变形行为

在热模拟机上对AerMet100超高强度钢进行了恒温和恒应变速率的热压缩实验,温度范围是900℃～1100℃,应变速率范围是0.01s-1～10s-1。实测了高温下应力-应变关系曲线,观察了变形后的显微组织,计算了材料的激活能,并建立了峰值应力与变形温度和应变速率的关系。结果表明,材料的流动应力随着变形温度的升高而降低,随应变速率的增大而增大;材料在不同变形条件下其软化机制分别受动态回复和动态再结晶控制;在实验条件范围内,AerMet100超高强度钢的再结晶温度在1000℃～1050℃之间,材料的热变形激活能为261.2kJ/mol。     

The high temperature oxidation behaviour of an ODS FeAl alloy

The oxidation behaviour of an oxide dispersion-strengthened (ODS) FeAl intermetallic, microalloyed with Zr and B and strengthened by a fine dispersion of Y₂O₃, is investigated at 1100°C for exposures of up to 200 h. The results show that a pure alumina scale is formed irrespective of the exposure time. The oxidation rate is far inferior to that found on PM 2000, a commercial alumina forming ODS ferritic superalloy. Limited scale spallation is observed in the intermetallic alloy from the early stages of oxidation. Scale failure, which is shown to occur during the cooling stage after oxidation and not at the high temperature of oxidation itself, results from the high compressive residual stresses in the scale induced by the misfit in the thermal expansion coefficients of the scale and the substrate. Failure of the scale may be supressed by using a very low cooling rate after oxidation.     

AZ71E镁合金的高温摩擦学行为(英文)

用销-盘摩擦磨损试验机考察 Z71E压铸镁合金在载荷为10~50 N时的高温摩擦学行为,利用光学显微镜(OM)和扫描电镜(SEM)对磨损表面和亚表面进行分析,通过光学显微镜(OM)、X射线衍射仪(XRD)、差示热扫描(DSC)等对AZ71E合金的高温微观结构、热稳定性和力学性能进行研究。结果表明:随着载荷和滑动距离的增加磨损率增大,而摩擦系数则随着载荷的增加而减少。在低载荷时,AZ71E镁合金的磨损机制主要是磨粒磨损;在150℃和高载荷下,粘着磨损和轻微的剥层磨损是主要的磨损机制;而在200℃及高载荷下,镁合金的主要磨损机制是严重的剥层磨损和熔融磨损。AZ71E镁合金的高温摩擦学性能提高的内在机制是AZ71E镁合金中第二相Al11Ce3使镁合金的高温拉伸和延展性能显著提高。     

Effect of the loading rate on the plastic deformation of corrosion-resistant martensitic-austenitic steels at cryogenic temperatures

Some modern products operate under conditions of rapid loading at cryogenic temperatures. To evaluate the serviceability of structural materials under such conditions, it is important to know how their mechanical properties and the nature of fracture change. In the present work, we investigated the effect of the loading rate in the range 10^–4–10^–1 sec^–1 on the mechanical properties of the martensitic-austenitic steels 03Kh12N10MT (VNS-25), 07Kh16N6 (SN-2A), and 08Kh14N8M (VNL-6) at 4.2–293 K.Deceased.Voronezh Polytechnic Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 33–36, February, 1994.