Al-3.36wt-%Mg合金在中温循环蠕变时的应变突发行为

Al-3.36wt-%Mg 合金在中温下的循环蠕变实验结果表明,在一定的温度和应力范围内,循环蠕变的第一和第二阶段都有应变突发发生,且这类应变突发具有明显的周期性,是动态应变时效的结果。根据 McCormick 提出的位错与溶质原子交互作用模型,讨论了应变突发时的位错运动机制,从而说明了应变突发周期随应变速率单调变化的规律。     

纳米晶Al及Al-Mg合金的合成与性能研究进展

主要总结了纳米晶Al及Al-Mg合金的合成与性能研究进展,详尽介绍了用低温球磨法制备纳米晶Al及Al-Mg合金的过程,简要分析了影响低温球磨过程的因素,并探讨了球磨过程对显微组织的影响;介绍和评述了纳米晶Al的热稳定性,并在与粗晶Al及Al-Mg合金对比的基础上,介绍了纳米晶Al及Al-Mg合金的力学性能以及蠕变性能等方面的研究进展.     

不同Mo含量对Al-Mg合金轧板微观组织和短时高温力学性能的影响

基于高强度耐火Al-Mg合金开发需求,设计并制备了6种Mo含量(质量分数)的Al-Mg合金,经变形热处理获得H3xx态轧板,结合光学显微镜、X射线衍射仪、拉伸试验机、带有能谱仪(EDS)的蔡司扫描电镜(SEM)等表征设备对各合金轧板微观组织和短时高温力学性能进行检测分析,揭示了微量Mo对Al-Mg合金的强韧化机理。结果表明:Mo合金化H3xx态Al-Mg合金具有较高的力学性能,这主要归功于与铝基体呈半共格关系的Al₁₂Mo弥散相起到的弥散强化效果和抑制再结晶作用,但过量的Mo易使Al-Mg合金形成较高Mg固溶度的难熔Al₁₂Mo结晶相,不利于合金性能提升。Al₁₂Mo弥散相具有一定的高温稳定性,高温状态下显著阻碍再结晶晶粒长大,进而提高Al-Mg合金高温性能。Mo含量为0.08%时的高温性能最佳,高温强度最大提升22.5%。随着Mo含量的增加,Al-Mg合金常温力学性能和短时高温力学性能都有所提高。     

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。     

Influence of grain size on tensile properties of Al-Mg alloys

Abstract

Grain size refinement is an important strengthening mechanism in Al-Mg 5000 series alloys, which have a relatively large Hall-Petch slope compared with other Al alloys. In addition, the high work hardening rate exhibited by Al-Mg alloys provides excellent formability. This paper investigates the influence of grain size on the flow stress over a range of strains, and in several different Al-Mg alloys. It is found that the Hall-Petch slope decreases after yield, indicating that the large grain size effect is primarily associated with initiating plasticity in these alloys. Beyond yield the slope decreases to a value equivalent to other, non-Mg containing alloys, and shows no clear dependence on strain. The intercept stress from the Hall-Petch plots at different strains is non-linear with ? 1/2 for alloys containing up to 3 wt-%Mg, which indicates that the free slip distance is strain dependent in these alloys. In an Al-5 wt-%Mg alloy the intercept stress is linear with ? 1/2, indicating that solute atoms are controlling the free slip distance. If Mn is added to the Al-5 wt-%Mg alloy, as it is in commercial alloys, it has little influence on the grain size dependence, but it does increase the frictional stress at the highest Mn level of 0.7 wt-%.     

Zum Wechselverformungsverhalten der Magnesiumbasislegierung AZ31

The cyclic deformation behaviour of Mg – base alloy AZ31 The cyclic deformation behaviour of Mg – base alloy AZ31 was investigated in stress controlled tension-, compression-tests. Experiments with zero mean stress (R = -1) as well as with tensile or compression mean stress (R = 0, R = - ∞ resp.) were carried out. Cyclic strain hardening and a pronounced anisotropy of strength during the first loading cycles was observed with higher yield strength in tension compared with compression. Consequently, in tests with zero mean stress cyclic creep and compressive mean strains occured.     

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.     

热挤压AZ31镁合金的组织结构与蠕变行为

通过对热挤压态AZ31镁合金进行组织形貌观察、内摩擦应力测定及蠕变性能测试,研究了热挤压AZ31合金的组织结构和蠕变行为.结果表明:热挤压AZ31镁合金的组织具有带状结构特征,并沿轧制方向分布,且有β-Mg17Al12相在合金中弥散析出.蠕变期间,位错运动的内摩擦力有较强的温度敏感性,随温度增加,内应力值明显降低,致使合金具有较高的蠕变速率.合金在蠕变期间,大量位错的形成与运动是蠕变初期的变形机制;蠕变稳态阶段,高密度位错逐渐束集形成位错胞,进一步发生蠕变期间的动态再结晶.随裂纹在晶界处萌生使蠕变进入第三阶段,而裂纹沿晶界韧性撕裂扩展是合金的蠕变断裂机制.     

A predictive mechanism for dynamic strain ageing in aluminium-magnesium alloys

Dynamic strain ageing (DSA) is the phenomenon in which solute atoms diffuse around dislocations and retard dislocation motion, leading to negative strain-rate sensitivity (nSRS) and thus to material instabilities during processing, an important issue in commercial metal alloys. Here, we show the mechanism of DSA and nSRS on experimental strain-rate, temperature and stress scales for Al-Mg to be single-atomic-hop motion of solutes from the compression to the tension side of a dislocation core. We derive an analytic expression for the strengthening versus strain rate and temperature that justifies widely used phenomenological forms, provides specific dependences of the parameters on material properties and is supported by atomistic kinetic Monte Carlo simulations. Using literature material properties, the predicted strengthening quantitatively agrees with the experimentally derived behaviour of Al-2.5% Mg at 300 K, and qualitatively agrees with the strain rate and temperature ranges of DSA and nSRS in Al-Mg alloys. The analyses herein show a clear path for multiscale design, from quantum to continuum mechanics, of solute strengthening in face-centred-cubic metal alloys.     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

Strain dependence of creep cavity nucleation in low alloy and 12%Cr steels

The nucleation of creep cavities was analysed using data from the literature for creep resistant low alloy and 12%Cr steels. The number of cavities per unit area was in most cases found to be a linear function of the creep strain, the cavity nucleation rate increasing linearly with increasing minimum creep rate. As a result, the cavity nucleation rate showed the same type of stress and temperature dependence as the strain rate, i.e. a power law function of the stress, where the stress is raised to a power close to the exponent in the creep rate equation (Norton's exponent). Simple models relating the number of cavities to strain and time as well as relating the cavity nucleation rate to time were derived. The theoretically estimated cavity numbers and cavity nucleation rate in relation to strain and time showed satisfactory agreement with the observations for both low alloy and 12%Cr steels.

MST/3353     

Creep behavior of nanostructured Mg alloy at ambient and low temperature

Tensile creep test at temperature <0.35 T_m was carried out to investigate the creep behavior in nanostructured Mg alloy with an average grain size of 45 nm consolidated from mechanically alloyed powders using power creep law. The stress exponent is found to be larger than one and with a threshold stress. The activation energy for the creep is measured to be 76 kJ mol⁻¹ smaller than that for grain boundary diffusion in Mg. It is deduced that creep behavior is affected by the presence of impurities and nanovoids inherited from the processing history.     

Primary and Anelastic Creep of a Near α-Ti Alloy and Their Dependencies on Stress and Temperature

The primary creep behaviour of a high temperature near -Ti alloy Ti6242Si has been investigated in the temperature range from 500 to 625°C, and the stress range from 80 to 450 MPa. The results are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). Furthermore, full unloading experiments were conducted in order to gain additional information as to the nature of primary creep. It is shown that primary creep can be described by an athermal component, strain hardening, with a mean strain hardening coefficient of 0.37, and a thermally activated component, strain rate sensitivity, with a strain rate sensitivity coefficient suggesting a mechanism based on climb controlled recovery. This is confirmed by the activation energy of 259 kJ/mol determined at different stresses, which is similar to the activation energy of Ti self diffusion in -Ti. The anelastic strain obtained on full unloading was analysed in its fast stage in a similar way. The kinetics of anelastic creep and its activation energy are in many aspects very similar to those of primary creep. It is thought that, in the stress and temperature range investigated, primary creep is to a relatively high extent anelastic in nature, and is controlled by the climb controlled bow out of pinned dislocation segments, particularly dislocations pinned at lath boundaries.     

Effects of Sn segregation and precipitates on creep response of Mg‐Sn alloys

Mg‐Sn alloys are promising for the development of new cheap creep resistant magnesium alloys. In the present paper, the creep behaviours of Mg‐Sn and Mg‐Sn‐Ca alloys were examined at the constant temperature and different stresses. The measurements of stress exponents indicate that the dislocation climbing is the dominant mechanism during the creep of Mg‐3Sn or Mg‐3Sn‐2Ca alloys. The poor creep resistance of the binary Mg‐3Sn alloy is caused by the easy movement of dislocation and the segregation of Sn at the boundaries. Both T4 and T6 heat treatments improve the creep resistance of Mg‐3Sn alloy due to the alleviation of Sn segregation at grain boundaries and the precipitation of Mg₂Sn particles, respectively. Ca is an effective alloying element to increase the creep resistance of Mg‐Sn alloys. The Ca addition leads to the formation of thermal stable phases Mg₂Ca and CaMgSn in Mg‐3Sn‐Ca alloys. These two phases effectively hinder the movement of dislocations and the sliding of grain boundaries. On the other hand, the addition of Ca alleviates the segregation of Sn by the interaction of Ca with Mg and Sn to form the phase CaMgSn.     

钛合金高温短时蠕变与应力松弛的关系研究*

蠕变或应力松弛被认为是钛合金板材热成形降低回弹的主要机理。目前对热校形阶段中的蠕变与应力松弛的区别及联系尚缺乏深入研究。本文主要进行了钛合金高温短时蠕变及应力松弛实验, 利用TEM对实验后的显微组织进行了观察。分别研究了温度、应力及时间对蠕变和应力松弛行为的影响规律, 从蠕变率-时间和蠕变-时间角度建立了蠕变与应力松弛之间的联系。研究表明: 钛合金在低温低应力下蠕变以原子扩散为主, 高温高应力下以位错滑移和攀移为主, 而应力松弛在不同温度时均以位错攀移为主要变形机制, 基于蠕变数据预测的应力松弛行为与实验结果符合较好。     

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...     

Effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

Magnesium alloys are increasingly used in the automotive and aerospace industries for weight reduction and fuel savings. The ratcheting behavior of these alloys is therefore an important consideration. The objective of this investigation was to study the effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy. The experiments have shown that the extruded AZ31B Mg alloy presented the following characteristic behavior with increasing number of loading cycles: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. This generic behavior can be explained by the fact that the variation trend of the maximum strain with the number of cycles differs from that of the minimum strain. The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.     

Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing

Abstract

High precision stress relaxation tests (SRT) at four temperatures were conducted on T91 (9%Cr) steel after extended boiler service, and also after re-heat treatment. Relative differences in creep strength, measured over five decades in strain rate were dependent on test temperature. Using an established correlation between strain rate sensitivity and elongation at failure, intrinsic ductility values as a function of stress and test temperature were determined. The general trend of a minimum in ductility in terms of stress or strain rate was consistent with long term creep rupture data on T91, and with literature data on alloy steels. However, the precision and repeatability of the SRT analysis contrasted with the appreciable scatter and heat to heat variation in traditional testing. It is argued that the current creep strength evaluation based on the nearly constant state measurement from the SRT test is superior to the measurement of stress dependence of minimum creep rate in traditional creep rupture testing. The complexity of primary creep in laboratory testing, which may not be significant at operating stresses where loading strains may be fully recoverable (anelastic), does not apply to the SRT. Since very low strain rates are achieved in a one day test, the procedures for setting of design allowables and design analysis based on the SRT data should not be significantly different from current practice. This technique offers accelerated alloy development and optimisation for creep strength and also ductility, and hence resistance to notch sensitivity.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

A STUDY OF FATIGUE AND CREEP BEHAVIOUR OF FOUR HIGH TEMPERATURE SOLDERS

Creep and cyclic deformation behavior of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu, a high lead alloy 97.SPb-1.SAg-1.0Sn, and an Ag-modified eutectic alloy 62.SSn-36.1Pb-1.4Ag, were studied. Room temperature and high (100°C and 150°C) temperature fatigue tests (with cyclic strain amplitude up to 6.0%) for the four solders were conducted, with the fatigue lives ranging from a few cycles to more than 100,000 cycles. It is shown that among the alloys studied, 62.SSn-36.1Pb-1.4Ag (the modified Sn-Pb eutectic alloy) has the lowest fatigue resistance in term of low cycle fatigue life (strain controlled). The high lead alloy, 97.SPb-1.5Ag-1.0Sn, has the highest strain fatigue resistance in the large strain region (Δ > 2.0%). Temperature has a significant effect on alloys 95Sn-5Ag and 99Sn-1.0Cu, but has a negligible effect on the Ag modified Sn-Pb eutectic alloy 62.5Sn-36.1Pb-1.4Ag and 97.5Pb-1.5Ag-1.0Sn. Creep studies show that these alloys generally have a very significant primary creep regime (up to 20%); thus, any realistic constitutive relation has to take such a primary creep phase into consideration. Cyclic deformation of alloy 95Sn-SAg was simulated by using a constitutive relation built upon a 2-cell model, which covers both primary and secondary creep. This model provides a good estimate of the peak stresses (the minimum stress and the maximum stress in each cycle); it agrees with experimental results when the applied cyclic strain is small and/or the applied strain rate is very low.