Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep of a Fe₃Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873–1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms of the threshold stress concept. A rapid fall of the stress exponent and of the threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results of long-term creep tests in tension performed recently on the same alloy. __________ Translated from Problemy Prochnosti, No. 1, pp. 117–120, January–February, 2008.     

Evaluation of Oxidation of Ti-Al and Ti-Al-Cr Coatings Arc-ion Plated on Ti-60 High-temperature Titanium Alloy

High-temperature titanium alloy for aeroengine compressor applications suers from high-temperature oxidation and environmental corrosion,which prohibits long-term service of this kind alloy at temperatures above 600-C.In an attempt to tackle this problem,Ti-48Al(at.pct)and Ti-48Al-12Cr(at.pct)protective coatings were plated on the substrate of alloy Ti-60 by arc ion plating(AIP)method.Isothermal and cyclic oxidation tests were performed in static air at elevated temperatures.Phase composition,morphology of ...     

Ti-6Al-4V合金的室温蠕变行为

研究了Ti-6Al-4V钛合金板材的室温蠕变行为及其对合金后续使用性能的影响.结果 表明:合金的宏观织构、应力水平以及预塑性应变都显著影响其室温蠕变行为.在加载方向上合金的<0001>峰值极密度越高,则其加工硬化指数越大、蠕变指数越小、室温蠕变性能越好.足够大的应力,是合金发生室温蠕变的必要条件.只有在蠕变应力不小于0...     

Influence of the Heat Treatments on Creep Behaviour of Nickel Aluminide with Boron

The nickel aluminide with boron alloy is being considered for elevated-temperature structural application in particular for jet turbine engine components. The alloy is attractive due to its ease of production, the low cost of its components, and its property advantages relative to superalloys. Therefore, if alloys based on Ni₃Al are successfully developed, substantial increases in engine performance and efficiency may be realized.The creep characteristics of an intermetallic Ni₃Al alloy containing boron produced by hot isostatic pressing were investigated in the temperature range 800 to 900^°C. Various heat treatments were used to produce different initial grain sizes of this alloy.Parameters studied were steady state strain rate, time to fracture, ductility and Larson-Miller parameter. The stress exponent, activation energy for creep and grain size exponent were calculated.It was found that by increasing the temperature of the heat treatment, the grain size increased. The results showed that the creep behaviour for this alloy improved as grain size increased. Furthermore, a comparison of the resulting creep data with data obtained from references is discussed.     

Ti-6Al-3Nb-2Zr-1Mo合金室温压缩蠕变行为及位错类型研究

研究Ti-6Al-3Nb-2Zr-1Mo合金在不同外加应力下的室温压缩蠕变行为,拟合了蠕变曲线,计算出蠕变发生第二阶段的临界值,并对不同应力水平压缩后的合金显微组织进行TEM观察,研究其位错滑移类型.结果表明:室温条件下,Ti-6Al-3Nb-2Zr-1Mo合金压缩蠕变-时间曲线符合时间强化指数模型,该合金发生蠕变第二...     

Effect of chromium addition on microstructure, tensile properties and creep resistance of as-cast Ti-48Al alloy

The effect of Cr on the microstructure, tensile properties and creep resistance of as-cast Ti–48Al–xCr (x = 0, 2, 4 at.%) alloys were studied. The dependence of the tensile properties and creep resistance of as-cast TiAl on the solid solution strengthening and formation of β phase due to addition of Cr was investigated.     

Measurement of the compressive creep strain rates of the individual phases within a lamellar microstructure

A fuducial line technique has been developed to determine the creep properties of the constituent phases within a lamellar composite subject to compression creep deformation. The technique can yield information on the total strain, creep rate, and the stress exponent and activation energy for creep of the individual phases within a lamellar microstructure. The contribution of interphase interfacial sliding to the strain of lamellar composites can also be evaluated by using the fiducial line technique. Application of the fiducial line analysis to a two-phase TiAl/Ti₃Al lamellar alloy deformed in compression at 1080K and 380MPa yields good agreement between the creep strain determined using the fiducial line analysis and the value directly measured from the crept specimen. The fiducial line analysis reveals that the TiAl phase within the two-phase TiAl/Ti₃Al lamellar microstructure creeps 2.2 times faster than the Ti₃Al phase and that interfacial sliding does not contribute to creep deformation of this alloy, within the resolution limit of the fiducial line experiment.     

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Abstract

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n?=?6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Q_c) was estimated to be 500 kJ mol^?1. Based on the n and Q_c values, it is concluded that the rate controlling mechanism is dislocation creep.     

TiAl基合金环境脆性的研究

研究了不同环境气氛对成分为Ｔｉ－４６Ａｌ－２Ｃｒ－０．２Ｓｉ－０．１Ｎｄ（ａｔ％）的ＴｉＡｌ基合金室温塑性的影响。结果表明该合金在室温下对环境脆性是敏感的，在不同气氛中的拉伸塑性依次为Ｏ２〉Ａｉｒ（空气）〉Ｈ２〉Ａｒ＋Ｈ２Ｏ。水汽和氢气均引起环境脆性，但水汽比氢气更有害。     

Creep behaviour of cast TiAl based intermetallics

Abstract

Constant load tensile creep tests were carried out on the cast TiAl based intermetallics Ti–47 Al–2Mn, Ti–47 Al–2Zr, and Ti–48Al (at.-%), prepared by plasma arc melting. Two microstructural conditions dependent on heat treatment were evaluated as follows: a fully lamellar (FL) scheme consisting of a fully transformed coarse lamellar structure with α₂ laths plus γ laths within the grain interiors; and a duplex scheme consisting of fine equiaxed grains of γ with α₂/γ lamellae. The steady state creep behaviour of both microstructural conditions, for each composition, was studied under stresses of 70–300 MN m^?2 in the temperature range 700–900°C. The microstructure was found to have a pronounced influence on the creep resistance. The FL microstructure exhibited superior creep resistance to the duplex microstructure. At temperatures and stress levels at which direct comparisons can be made, the steady state creep rates of the FL structures are an order of magnitude lower than those of the duplex structure. The apparent creep activation energies and stress exponents were measured for both microstructural conditions for each composition. The temperature and stress dependence of the steady state creep rate of both microstructures can be described by the power law creep equation, suggesting dislocation motion as the operative deformation mechanism.

MST/1962     

渗碳处理提高TiAl基合金高温抗氧化性

研究了表面渗碳处理Ｔｉ－４８Ａｌ－２Ｃｒ－２Ｎｂ合金高温抗氧化性的影响，结果表明，渗碳处理显著地提高ＴｉＡｌ基合金高温抗氧化能力，归因于渗碳处理在试样表面层形成了具有良好的抗氧化性和高温热稳定性的多层结构的渗碳层。     

Steady State Creep Rate Equation of Inconel 718 Superalloy

The steady state creep rate equdtion ofa nickel base superalloy Inconel 718, strengthenedby coherent ordered disc-shaped bct γ~(11) phaseand coherent spherical fcc γ~1 phase precipitates,has been established in the stress and temperatureranges of 620-840 MN m~(-2) and 853-943K, respecti-vely. Constant stress tensile creep tests wereused to medsure the values of steady state creeprate, ε_s, and the consecutive stress reductionmethod was used to measure the back stress duringcreep deformation. The values of effective stressexponent, n_e, were detemined from the slopesof the lgε_s vs. lg(σ_a-σ_0)/G plots. The effectof grain size, d, on steady state creep rdtehas been also studied in this investigation,and the grain size sensitive exponents m weredetemined from the slopes of lgε_s vs. lg(b/d)plots. The creep rate equations of Inconel 718,in the above stress and temperature ranges,have been proposed to beε_s=1.6×10~(-5)(D_1Gb/KT) (b/d )~(0.19)[(σ_a-σ_0)/G]~(1.35)in diffusional creep region, andε_s =75(D_1Gb/KT) (b/d)~(-0.42)[(σ_a-σ_0)/G]~(5.5)in dislocation power law creep region.     

磷对IN718合金蠕变性能的影响

研究了磷对ＩＮ７１８合金蠕变性能的影响,结果表明,磷显著提高蠕湾抗力。磷对应力指数没有明显的影响,掺杂０．０２％磷使表观蠕变激活能由６７８．５ｋＪ／ｍｉｌ提高至７４６．１ｋＪ／ｍｏｌ。磷的有益作用可能是抑制晶界扩散实现的。     

Effect of stress-temperature regimes on creep mechanisms in Ti-24Al-11Nb

The steady state creep rate (SSCR) of various microstructures of Ti-24Al-11Nb (a/o) has been determined. SSCR vs. stress and vs. temperature curves were determined to find Q_a, apparent creep activation energy, and to investigate n, power law stress exponent. At low stresses, apparent creep activation energies determined for all microstructures were found to be between 106 kJ mol⁻¹ and 156 kJ mol⁻¹, which agrees fairly well with the energy for self-diffusion in alpha titanium, and the energy of interdiffusion on the Al-rich side of the ₂ phase in Ti₃Al, both having a value of 150 kJ mol⁻¹. In many SSCR vs. stress curves, a slope change was observed in the stress range investigated. As temperature increased, slopes decreased towards unity, suggesting that different creep mechanisms, i.e. dislocation creep and diffusional creep, may be rate-controlling in different stress-temperature regimes.     

Al_2O_3-SiO_(2(sf))/AZ91D镁基复合材料蠕变本构方程

利用常应力拉伸蠕变试验法对体积分数为25%的硅酸铝短纤维(Al2O3-Si O2(sf))增强AZ91D镁基复合材料及其基体合金AZ91D在温度为473 K和573 K、外加应力为30～100 MPa下进行蠕变测试。根据应变和应变速率曲线,计算出复合材料的真应力指数、真蠕变激活能、真门槛应力、载荷转移因子和蠕变本构方程。TEM分析结果表明,复合材料蠕变后的门槛应力来源于短纤维表面上的MgO颗粒和Mg17Al12析出相对可动位错的钉扎作用,短纤维具有承载和传递载荷作用,从而提高了复合材料的抗蠕变性能。     

金属蠕变律及蠕变行为研究

蠕变过程中 ,材料内部状态的不断演化 ,使得材料的蠕变行为发生改变。本文提出考虑损伤和硬化影响的蠕变律。利用该蠕变律讨论了 12 Cr1Mo V钢蠕变行为。分析结果表明 ,ε·c在蠕变过程中始终变化 ,第二阶段仅仅是ε·c相对稳定的阶段 ,其相对稳定程度和持续范围与载荷大小有关。在相同寿命分数下 ,不同应力水平引起的硬化状态也不相同     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Determination of creep behaviour of Ti60 alloy by small punch creep test

The small punch (SP) creep test has distinct advantages in the creep property assessment of materials at elevated temperatures. However, there are few creep properties of Ti alloys obtained by the SP creep test in the current literature. In this paper, the SP creep behaviour of Ti60 alloy has been evaluated under various loads in the range 550–800 N over a temperature range 873–973 K. The SP creep curves obviously indicated the primary, secondary and tertiary stages of creep. The test results have been compared with those of conventional creep tests. The European Code of Practice (CoP) for Small Punch Testing, Dorn equation and Monkman–Grant relationship have also been used to analyse the results of the SP creep tests. The ratio of load of the SP creep tests to equivalent stress of conventional creep tests, the load exponent value of steady deflection rate and activation energy for creep deformation were estimated from the SP creep tests. In conclusion, it was found that dislocation creep may be the main mechanism that dominates the SP creep deformation of Ti60 alloy in the range of load and temperature.     

An analysis of microstructure and impression creep response of squeeze-cast AZ91–xBi–ySr alloys

ABSTRACT

The present investigation deals with the microstructural modification following the Bi?+?Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi?+?Sr additions form the Al₄Sr and Sr₂Bi phases besides the α-Mg and β-Mg₁₇Al₁₂ phases, and improves creep resistance of the AZ91 alloy. The AZ91?+?1.0Bi?+?0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and the activation energy values of all the alloys are in the range of 4–7 and 100.2–112.7?kJ?mol^?1, respectively, depicting the pipe diffusion-controlled dislocation creep is the governing creep mechanism. The post-creep microstructural study confirms several dislocations pile-ups around the Al₄Sr and Sr₂Bi phases resulting in improved creep resistance of the modified AZ91 alloys.