Anelasticity in austenitic stainless steel

Time-dependent anelastic deformation mechanisms arise in austenitic stainless steel when load is removed during a high-temperature creep test. This phenomenon is investigated by conducting creep tests, with intermittent load removal, on AISI Type 316H austenitic stainless steel at 550 and 650 °C, supported by in situ measurement of creep-induced intergranular residual strains by neutron diffraction. All the cyclic tests exhibit anelastic behaviour on unloading and develop substantially lower load-on creep strain rates, reduced ductility and longer rupture times than baseline steady-load creep tests at similar conditions. The mechanisms underlying the observed anelastic behaviour and changes in macroscopic creep properties are discussed with reference to the development of intergranular strains and dislocation behaviour.     

金属玻璃蠕变及回复行为研究综述

由于非晶合金独特的无序结构,其结构动力学特征涉及较大时间与尺寸跨度的粒子重排。作为深入研究金属玻璃体系弛豫行为与老化动力学的基础,对非晶合金结构动力学的表征和理解至关重要。大量研究表明,以镧基和铈基为代表的稀土基非晶合金的弛豫谱呈现明显次级弛豫过程,该体系亦成为探究非晶合金结构动力学与力学性能关联的理想载体。本文主要就金属玻璃的滞弹性变形作了详细评述。作为蠕变实验中变形的主要成分,这类变形在卸载后可完全回复,对其合理描述是深入理解非晶合金结构动力学的关键。此外,总结了蠕变和蠕变回复过程中滞弹性变形的主要特征,并介绍了几种可用于定量或定性分析的理论模型。     

不同时效态7050铝合金时效成形中析出相对应力松弛行为的影响

铝合金时效成形方法结合了合金的蠕变松弛和析出强化作用,作为一种先进的整体壁板制造技术倍受航空制造业青睐。7xxx系铝合金在时效成形过程中的应力松弛行为受到合金内析出相与位错蠕变交互作用的影响从而制约着成形后零件质量与性能。本文采用设计的应力松弛试验研究了不同时效态(固溶态,欠时效态和峰时效态)7050铝合金内析出相对时效成形过程中应力松弛行为的影响,并通过位错热激活动力学参数计算和显微组织表征分析析出相与位错运动的交互作用。结果表明时效成形过程中析出相对位错热激活运动有明显地阻碍作用,因此含有不同尺度析出相铝合金的应力松弛行为表现不同,随着析出相尺度的增加合金应力松弛速率减缓,应力松弛极限增大。不同时效态7050铝合金位错激活体积计算和显微组织表征结果都证明了应力松弛过程中析出相增大对位错运动的阻碍作用也越显著。峰时效态7050铝合金的位错激活体积最大,时效成形后塑性应变的转化率最低。此外,时效成形过程中,7050铝合金内析出相对位错热激活的阻碍作用引起了槛应力现象,且随着析出相的增大槛应力也逐渐增大。     

Directionality of yield point in strain-aged steels: The role of polar dislocations

The directionality of the sharp yield point in strain-aged steels has been investigated by modeling tension/compression and forward/reverse torsion tests separated by accelerated aging. The occurrence of a Bauschinger effect and the absence of a yield point after a forward straining–aging–reverse straining sequence are interpreted within the framework of a field dislocation theory coupling the evolution of statistical and polar dislocation densities with that of point defects due to strain aging. The polar dislocation density reflects lattice incompatibility and long-range internal stresses. By assisting yielding in reverse straining, the associated back-stress is seen as the origin of the Bauschinger effect. By also promoting dislocation unlocking, the back-stress is found to be responsible for the absence of a yield point in reverse straining. Polarized dislocation structures formed in forward straining in association with back-stress build up may annihilate and inverse polarization occur in reverse straining. This microstructure evolution translates into an inflexion of strain hardening after strain path reversal.     

铝合金7075蠕变时效成形回弹规律

蠕变时效成形技术是利用金属的蠕变特性,将成形与时效热处理同步进行的一种成形方法。文章以可时效强化型铝合金7075为研究对象开展蠕变时效成形试验,考察厚度、弹性预变形量、时效时间和温度的综合效应对成形曲率半径的影响规律,并通过正交多项式回归分析,建立了回弹率与4个试验因素之间的回归方程,进行的工艺试验验证结果表明,运用该回归方程可以对蠕变时效成形后零件的回弹率进行预测。     

Load relaxation and creep behavior of a spray cast hypereutectic Al-25Si-2Cu-1Mg alloy

We studied the high temperature deformation behavior of a spray cast Al-25Si-2Cu-1Mg alloy within the framework of an internal variable theory for inelastic deformation. We then carried out a series of load relaxation tests at elevated temperatures to obtain flow curves between flow stress and strain rate. The results were analyzed based on constitutive relations prescribed by an internal variable theory. The hypereutectic Al-25Si-2Cu-1Mg alloy exhibited two distinctly different deformation modes, viz. plastic deformation induced by dislocation glides and creep deformation by dislocation climbs at high temperatures. These load relaxation test results were then compared with those obtained from a series of creep tests performed at elevated temperatures under various stress levels. They were found to agree well with each other, providing a means to replace the time-consuming creep test by a simple load relaxation test in order to obtain high temperature creep data.     

Al-Zn-Mg-Cu铝合金变温双级蠕变时效模型

针对Al-Zn-Mg-Cu合金变温双级蠕变时效过程,建立了一种考虑蠕变应变与屈服强度的本构框架,通过实验数据的简单拟合方法获得了模型参数。模型不仅以简单的形式具备了处理蠕变时效过程中的应力松弛、强化响应和温度变化的能力,而且能够应用到有限元软件中模拟构件的蠕变量、屈服强度和回弹。模型结果不仅能够适应不同外加应力下实测的蠕变应变曲线,且有限元模拟结果与实测结果能够很好地吻合。     

Effect of temperature,strain rate and grain size on the mechanical response of Ti3SiC2 in tension

Tensile, relaxation and cycling loading–unloading tests indicate that the mechanical response of Ti₃SiC₂ has a strong dependence on temperature and strain rate, but a weak dependence on grain size. Loading at low temperatures, and/or high strain rates, results in elastic and anelastic deformation, followed by brittle fracture. Anelastic deformation in this regime can be attributed to the easy glide of dislocation into pileups during loading, and their run back during unloading. At high temperatures (≈1100–1200°C), and/or low (<10⁻⁵ s⁻¹) strain rates, the response is plastic. The resulting strain is elastic, anelastic and plastic. Even at 1200°C, intense stress-relaxation processes are observed, and a sizable fraction (≈13%) of the strain is anelastic. At intermediate temperatures and strain rates (transition regime) the mechanical response is controlled by simultaneous damage formation (microcracking) and localized plastic deformation. Combining the results obtained in this work with previous results, viz. tensile creep and strain transient dip tests, a deformation map that takes into account temperature, grain size and strain rate is defined.     

Microstructure Evolution of Extruded Mg-6Gd Alloy Under 175 °C and 150 MPa

The tensile creep behavior of extruded Mg-6Gd alloy, having the tensile yield strength of~110 MPa at 175 °C, has been investigated under 175 °C and 150 MPa. In this study, the extruded Mg-6Gd sample exhibits the total tensile strain of~10.5% after the creep time of 1100 h, and the fast plastic strain of~4.6% at the beginning of the creep test. The microstructure result suggests that the dislocation deformation is the main deformation mode during creep, and the grains with orientation close to〈0001〉|| ED disappear after creep. The creep process containing a low creep strain has no effective promotion for the precipitation compared with the aging process without strain. The origination of creep crack is related to the formation of precipitate-free zone during creep. The work offers an important implication to research the microstructure evolution under an applied stress in a weak aging response Mg alloy.     

铝合金2324蠕变时效成形试验研究

蠕变时效成形技术是利用金属的蠕变特性,将成形与时效热处理同步进行的一种成形方法。文章以可时效强化型铝合金2324为研究对象开展蠕变时效成形试验,考察了厚度、弹性预变形量、时效时间和温度的综合效应对成形曲率半径和材料性能的影响规律,并通过正交多项式回归分析,建立了回弹率与4个试验因素之间的回归方程,为实际工艺参数的优化以及回弹预测提供参考。     

A STUDY ON MICRO-PLASTIC DEFORMATION BEHAVIOR OF 2024 A1 ALLOY

1.~hon2024Alalloyusedincomponentsofaprecisiondeviceishighlyrearredfordimensionalstability.althoughdimensionalstabilityhadbeenstudiedinseverallit.rat~.[l--3),therelationshipsbetweenndcrostructureanddimensionalstabilitypropertiesandAnsotmpofmicro--plasticdeformationresistanceforhotextruded2024Alalloybarwerenotinvestigatedindetails.SOInanyengineeringproblemsPertinentto2024hialloyremaininsuspense.Additionallytheconventionalevaluationfordimensionalstabilityinunloadingconditionisobtainedbyend--len…     

Stress relaxation behavior of Ti-6Al-4V alloy

1　INTRODUCTIONTi 6Al 4VisoneofthemostimportantTial loys[1,2 ] .Butthisalloyhasbadformabilityforitshighelasticresilience .Therefore ,hotsizingisimpor tant[36 ] .Asthebaseofhotsizing ,thestudyofstressrelaxationhasimportanttheoreticalvalueandpracticalsignificance .Ontheotherhand ,Ti 6Al 4Visusedasfastenermaterialssometimes .Whenthefastenersworkatthetemperaturehigherthanroomtemperature ,stressrelaxationmayresultsinacci dents .Sohowtopreventthestressrelaxationisveryimportant[7] .Uptonow ,…     

Aging effects on the creep behavior of the near-alpha titanium alloy Ti-1100

During aging of the silicon-bearing, near-alpha titanium alloy Ti-1100 at the service temperature of 593 °C, the precipitation of Ti₃AI and silicide particles has been observed. The objective of this article is to determine the influence of these precipitates on creep behavior. Stress relaxation tests, with the advantage of needing only a short time to produce a complete creep curve, were used to determine the creep behavior of Ti-1100. These accelerated creep tests reduce the chance of metallurgical changes that could occur in the long time required to perform conventional creep tests. The creep results obtained from the stress relaxation tests were verified with those obtained from conventional monotonic creep tests. Aging of the material for 500 h at 593 °C, where full precipitation of Ti₃AI is assumed to be incomplete, resulted in a small decrease in creep resistance compared to the unaged condition. Specimens aged for 1000 h, which corresponds to the averaged condition in which the precipitation of Ti₃AI is expected to be completed, were also examined. It was observed that these specimens yielded creep resistance similar to that of the unaged condition. Material aging for 1000 h followed by a special heat treatment, which results in the dissolution of Ti₃AI particles, although not affecting the silicide precipitates, resulted in the poorest creep resistance of the aging conditions investigated. The major conclusion of this study is that the presence of Ti₃AI has a beneficial influence on the creep resistance of Ti-1100 alloy.     

AZ31镁合金蠕变初期的变形特征

通过蠕变曲线的测定和TEM的形貌观察,研究了AZ31镁合金在蠕变初期的变形特征及组织演化规律．结果表明,蠕变初期的变形特征是：大量形变产生的(α)位错在合金的基面和非基面滑移,(α c)位错在锥面滑移．其中(α)位错通过位错分解反应可由一非基面交滑移至另一非基面．随蠕变进行,高密度的形变位错发生动态回复,可进一步束集形成位错胞和位错墙．蠕变初始阶段,在应力的作用下,适当取向的晶体发生孪生,并作为一种附加的变形机制而改善合金的韧性．     

纳米压痕形变过程的分子动力学模拟

根据EAM多体势，利用分子动力学方法模拟了Ni压头压入Al基体的纳米压痕全过程．包括压头接近和离开基体时的原子组态；压入和上升时的载荷一位移曲线以及位错的发射和形变带的产生和变化；同时模拟了纳米尺度的应力弛豫行为．结果表明，当压头尚未接触基体时就能吸引基体原子，通过缩颈而互相连接．当压入应力Ts为1．9MPa时，基体Al开始发射位错；当分切应力Td=6．4MPa时，出现形变带．压头上升过程出现反向的拉应力，使基体反向屈服，在卸载过程中基体残留位错的组态不断改变．当压头上升离开基体后能拉着基体通过缩颈而相连，当压头和基体分离后仍粘有基体原子．在纳米尺度也存在应力弛豫现象，其原因是热激活引起的位错发射和运动．     

In situ TEM observations of reverse dislocation motion upon unloading in tensile-deformed UFG aluminium

Loading–unloading cycles have been performed on ultrafine-grained (UFG) aluminium inside a transmission electron microscope (TEM). The interaction of dislocations with grain boundaries, which is supposed to be at the origin of the inelastic behaviour of this class of materials, differs according to the main character of the dislocation segments involved in pile-ups. Pile-ups are formed by spiral sources and lead to the incorporation of dislocations into grain boundaries (GBs) during loading. Upon unloading, partial re-emission of dislocations from GBs can be observed. Stress and strain measurements performed during these in situ TEM loading–unloading experiments are in agreement with the rather large inelastic reverse strains observed during unloading in loading–unloading tests on bulk macroscopic UFG aluminium specimens.     

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.     

Modeling viscoplastic behavior and heterogeneous intracrystalline deformation of columnar ice polycrystals

A full-field formulation based on fast Fourier transforms (FFT) has been adapted and used to predict the micromechanical fields that develop in two-dimensional columnar Ih ice polycrystals deforming in compression by dislocation creep. The predicted intragranular mechanical fields are in qualitative good agreement with experimental observations, in particular those involving the formation of shear and kink bands. These localized bands are associated with the large internal stresses that develop during creep in such anisotropic material, and their location, intensity, morphology and extension are found to depend strongly on the crystallographic orientation of the grains and on their interaction with neighboring crystals. The predictions of the model are also discussed in relation to the deformation of columnar sea and lake ice, as well as with the mechanical behavior of granular ice of glaciers and polar ice sheets.     

Al2O3/Al合金复合材料的蠕变与回蠕变行为

研究了挤压铸造Al2O3短纤维(体积分数为15%)增强Al合金Al-Si12-Cu1-Mg1-Ni1复合材料的变载荷蠕变和回蠕变行为.结果表明,复合材料在主应力作用下蠕变当载荷突然降低后,表现为瞬态大应变量收缩以及随后的回蠕变研究表明初始瞬态收缩完全是复合材料的弹性应变,有别于前人采用复合材料或纯金属、合金所得到的数倍于弹性应变结果.实验结果经数值回归得出回蠕变速率εbc(t)=Atn,回蠕变应变εbc(t)=A 1n t+C.指数n在任一主应力应变阶段均接近于-1,系数A取决于降载前复合材料积累的主应力蠕变状态,并随降载前主应力蠕变程度的增加而增大,表明复合材料回蠕变行为受到降载前主应力蠕变状态的制约.     

变载条件下(Al2O3)f增强铝合金复合材料的瞬时变形和回蠕变

研究了由挤压铸造制备的体积分数为15％的Al2O3短纤维（Al2O3)f)增强ZL 109（Al-Si12-Cu1-Mg1-Nil)复合材料在降载前后的蠕变行为。结果表明：降载后的蠕变可分为瞬时弹性和回蠕变两阶段。瞬时应变与通过测量弹性模量计算的弹性应变基本一致，而随松弛时间的延长，反方向的应变增大，回蠕变的速率减小；回蠕变的出现与纤维在复合材料中引起的偏差应力在降载后变为负值有关，用偏差应力的方法以及应力松弛可以较好地解释回蠕变规律。