Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

DSA(dynamic strain aging)phenomenon in SUS316 steel was investigated using isothermal and non-isothermal tensile tests of simulated HAZ(heat-affected zone)thermal cycles.Isothermal tensile tests were performed on SUS316 in the peak temperature range of 20-700°C,with strain rates varying from 4.2×10^(-3)to 4.2×10^(-5)s^(-1).Based on the appearance of discontinuous plastic flows,expressed as serrations,and the hardening phenomenon of the tensile samples,the conditions for the occurrence of DSA in the SUS316 steel were investigated.Furthermore,the extent of hardening due to DSA was evaluated by comparing the hardness values of the SUS316 and SUS316EHP steels after the tensile tests.To confirm the effect of DSA on hardness in the HAZ of the welded SUS316 steel,non-isothermal tensile tests of the simulated HAZ thermal cycles were performed using a Thermec Master.The relationship between the increase in Vickers hardness due to DSA and the strain in the HAZ was determined;the effect of DSA on hardness in the HAZ could be predicted.The DSA in SUS316 steel was found to be mainly attributed to the dynamic interaction of dislocations with C and N interstitial atoms during high-temperature deformation.

Improving Tribological Performance of Gray Cast Iron by Laser Peening in Dynamic Strain Aging Temperature Regime

A high and stable brake disc friction coefficient is needed for automobile safety, while the coefficient degrades due to elevated temperature during the braking process. There is no better solution except changes in material composition and shape design optimization. In the dynamic strain aging（DSA） temperature regime of gray cast iron, micro-dimples with different dimple depth over diameter and surface area density are fabricated on the material surface by laser peening（LP） which is an LST method. Friction behavior and wear mechanism are investigated to evaluate the effects of surface texturing on the tribological performance of specimens under dry conditions. Through LP impacts assisted by DSA, the friction coefficients of the LPed specimens increase noticeably both at room temperature and elevated temperature in comparison to untreated specimens. Moreover, the coefficient of specimen with dimple depth over diameter of 0.03 and surface area density of 30% is up to 0.351 at room temperature, which dramatically rises up to 1.33 times that of untextured specimen and the value is still up to 0.3305 at 400℃ with an increasing ratio of 35% compared to that of untreated specimen. The surface of textured specimen shows better wear resistance compared to untreated specimen. Wear mechanism includes adhesive wear, abrasive wear and oxidation wear. It is demonstrated that LP assisted by DSA can substantially improve wear resistance, raise the friction coefficient as well as its stability of gray cast iron under elevated temperatures. Heat fade and premature wear can be effectively relieved by this surface modification method.

EFFECT OF DYNAMIC STRAIN AGING ON LOW CYCLE FATIGUE BEHAVIOUR OF 18-8 AUSTENITIC STAINLESS STEEL

Studies were made of the symmetric tensile-compressive low cycle fatigue behaviour and the influence of dynamic strain aging(DSA)pre-treatment of 18-8 austenitic stainless steel. Within the testing amplitude range of strain.±0.5 % to±1.5 %,the three processes of cyclic hardening,cyclic saturation and cyclic softening were observed.In the same amplitude of strain,the peak stress of the samples pre-treated by DSA is higher than that of solid-solu- tion and cold working pre-treatment,but no remarkable differences of the fatigue lives of them were found.TEM observation shows that the uniform and stable dislocation networks with high density form after DSA pre-treatment,which increases the cyclic peak stress.The cyclic softening results from the low dislocation density and elongated cell structure with low energy.

A DISLOCATION-MECHANICS-BASED CONSTITUTIVE MODEL FOR DYNAMIC STRAIN AGING

Dynamic strain aging (DSA) is an important phenomenon in solutehardened metals and seri- ously affects the mechanical properties ofmetals. DSA is generally induced by the interaction between themoving dislocations and the mobile solute atoms. In this paper, onlythe interaction between moving disloca- tions and mobile solute atomsin a dislocation core area (core atmosphere) will be taken intoaccount. To es- tablish the constitutive model which can describe theDSA phenomenon, we improved the Zerilli-Armstrongdislocation-mechanics-based thermal viscoplastic constitutiverelation, and added the effect of the interaction between the movingdislocations and core atmosphere.

Stress-state dependence of dynamic strain aging:Thermal hardening and blue brittleness

This study aims to discover the stress-state dependence of the dynamic strain aging(DSA)effect on the deformation and fracture behavior of high-strength dual-phase(DP)steel at different deformation temperatures(25-400°C)and reveal the damage mechanisms under these various configurations.To achieve different stress states,predesigned specimens with different geometric features were used.Scanning electron microscopy was applied to analyze the fracture modes(e.g.,dimple or shear mode)and underlying damage mechanism of the investigated material.DSA is present in this DP steel,showing the Portevin-Le Chatelier(PLC)effect with serrated flow behavior,thermal hardening,and blue brittleness phenomena.Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness.Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness.Accordingly,the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect.

EFFECT OF DYNAMIC STRAIN AGING ON PURE BENDING FATIGUE STRENGTH OF AUSTENITIC STAINLESS STEEL

The effect of various dynamic strain aging(DSA)pre-treatment processes on pure bending fatigue strength of an 18-8 austenitic stainless steel was investigated.The results show that DSA pre-treatment processes increase remarkably the fatigue strength and the strengthening effects increase with increasing pre-strain temperature and pre-strain.The fatigue limit of specimens pre-treated by DSA is 87% higher than that by solution treatment,and 20% high- er than that by cold-working.TEM observations show that the uniform and stable dislocation networks with high density formed after DSA pre-treatment,which suppress effectively the initiation and propugation of fatigue cracks and increase the fatigue strength of materials.

DYNAMIC STRAIN AGING BEHAVIOR OF K40S ALLOY

The dynamic strain aging behavior during tensile tests of K40S alloy has been investigated in the temperature range of 25-1100℃ with the strain rate range from 10-4 to 10-3s-1. The results show that four different types of serration, identified as A, B, C and E type serration were observed in the temperature range of 300-600℃. The strain exponents for onset of the serrated flow were calculated as 1.21, 2.19 and 1.61, and the activation energies as 121, 40 and 67kJ/mol for E, B and C type serration respectively. The main mechanism for dynamic strain aging discussed in light of the strain exponent and the activation energy.

挤压态Mg-Al-Mn-(0.2Sr)镁合金的准静态压缩变形行为

采用准静态压缩实验研究了挤压态Mg-Al-Mn-(0.2Sr)镁合金在0.0001~0.33 s^(-1)应变速率范围内的变形行为。结果表明:挤压态Mg-Al-Mn-(0.2Sr)镁合金的流变应力响应行为具有一定的应变速率相关性,具体表现为流变应力随加载应变速率的增加呈现先降低,后增加,再降低的演变趋势。在压缩变形过程中,两种镁合金试样均产生了较为明显的动态应变时效(DSA)。产生DSA所需的临界应变随加载应变速率的增加而增加,且应力-应变曲线的锯齿类型具有强的应变速率相关性。添加Sr元素降低了镁合金沿径向压缩的流变应力,但对DSA行为具有一定的促进作用。相同应变速率下,挤压态Mg-Al-Mn-0.2Sr镁合金产生DSA的临界应变明显较小,且锯齿幅值略大。此外,Sr元素对孪生也具有一定的抑制作用,导致挤压态Mg-Al-Mn-0.2Sr镁合金的孪晶界出现不同程度的扭曲。

V对S30403奥氏体不锈钢微观组织和高温力学性能的影响

研究了3种不同V含量(0 mass%、0.097 mass%和0.19 mass%)的S30403奥氏体不锈钢在500~900℃高温下的拉伸行为。结果表明:在500~900℃范围内,3种实验钢的抗拉强度都随着拉伸温度的升高而降低。V含量为0.097 mass%的实验钢在600、700和800℃的极限抗拉强度是3种钢中最高的,分别为362.9、271.4和156.1 MPa,这是不同温度下动态应变时效、动态再结晶和析出强化的协同作用的结果。含V钢在500~900℃范围内拉伸后的断面收缩率高于不含V钢,当V含量为0.19 mass%时,实验钢的断面收缩率最高,这是由于基体中固溶的V元素促进了实验钢的动态再结晶。

温度对核电压力容器用SA508-Ⅲ钢拉伸性能的影响

对核电压力容器用SA508-Ⅲ钢进行了在不同温度(25,90,180,260,320℃)下的拉伸试验,研究了温度对其拉伸性能的影响及机理。结果表明:当温度为260,320℃时,试验钢拉伸应力-应变曲线塑性变形阶段均出现锯齿波,呈典型动态应变时效特征;随着温度升高,试验钢的屈服强度减小,抗拉强度先减小再增大,断后伸长率先增加后减小,当温度为180℃时,抗拉强度最小,断后伸长率最大;随着温度升高至180℃以上,动态应变时效发生,试验钢中位错胞尺寸减小,位错密度增大,沿亚晶界和晶界析出的碳化物数量增加、尺寸减小,这也是强度提高、塑性减小的主要原因。

变形条件对Mg-Gd-Y-Nd-Zr合金动态力学行为的影响

采用金相观察、扫描电子显微镜、能谱分析、显微硬度计及分离式霍普金森压杆等手段,研究了峰值时效挤压态Mg-8Gd-3Y-0.5Nd-0.5Zr合金在变形温度及应变速率下的动态冲击力学变形行为。结果表明:合金在220℃/14 h到达峰值时效,其硬度约为143.2 HV,提升了55%左右。在不同的变形温度下均表现出优异的抗冲击性能,在室温及应变速率为3000 s^(-1)条件下合金抗压强度可高达682 MPa;在100℃及应变速率为1500 s^(-1)条件下抗压强度为635 MPa;在400℃及应变速率为3000 s^(-1)条件下抗压强度为583 MPa。合金在不同温度下优异的抗冲击性能主要得益于时效强化相、稳定存在的块状富稀土粒子以及冲击过程中在晶界形成的动态析出相协同强化机制。随着应变速率和变形温度的增大,合金热软化效应增强,合金力学性能有所降低。

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel （AISI 316L） and one nickel-base alloy （Alloy 617） have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromech- anisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650℃.

铝合金中Portevin-Le Chatelier效应的研究进展

铝合金具有高的比强度与耐蚀性,在航空航天及轨道交通领域应用广泛。作为一种常见于铝合金中的塑性不稳定现象,Portevin-Le Chatelier(PLC)效应是一种典型的多尺度效应。该效应在宏观尺度上表现为,在一定温度和应变速率范围内,材料塑性变形呈现锯齿状波动的应力-应变曲线,并在试样表面伴随出现局部变形带;而在微观尺度上,这种现象则涉及位错与溶质原子间的动态交互作用。室温下铝合金形变过程中可能伴随的PLC效应会造成其塑性的降低和表面粗糙化,影响其可成型性及力学服役性能,一定程度上制约了其工程应用。因此,研究铝合金PLC效应的外在表现和内在机理具有重要的理论意义和工业应用价值。主要从类型特点、机理分析、影响因素和发展趋势4个方面对铝合金PLC效应的研究进展进行综述讨论:首先介绍了3种常见的PLC效应类型,重点阐述其分类标准、各自特点以及存在的类型转变;其次从动态应变时效理论的发展出发,描述了现阶段对PLC效应的主要理论解释;进而以温度、应变速率、晶粒尺寸、沉淀相颗粒等为代表,讨论了多种因素对铝合金PLC效应的影响;最后对铝合金PLC效应的研究发展趋势进行了展望。

国产核电主管道用316LN钢的低周疲劳性能评估

对CAP1400核电机组国产主管道用316LN钢开展室温(25℃)、服役温度(350℃)、500℃和600℃下的低周疲劳试验,得到该钢的循环应力响应特性和迟滞回线,基于RambergOsgood模型和Manson-Coffin模型拟合稳定循环应力幅-应变幅曲线和应变幅-失效反向数曲线,采用ASME锅炉及压力容器规范最佳拟合疲劳曲线和设计疲劳曲线对该钢的低周疲劳性能进行评估。结果表明:316LN钢的循环应力响应过程包含循环硬化、循环软化、稳定循环和最终失效4个阶段;500,600℃下316LN钢发生显著的动态应变时效,350℃下动态应变时效特征不明显,室温下不存在动态应变时效现象;Ramberg-Osgood模型和Manson-Coffin模型可以较好地描述316LN钢的稳定循环应力幅-应变幅和应变幅-失效反向数关系;在室温和350℃条件下316LN钢的疲劳试验数据均高于修订后的最佳拟合疲劳曲线,应变调整系数取2或疲劳寿命调整系数取12的试验数据均高于设计疲劳曲线,低周疲劳性能满足ASME规范要求。

A New Strategy for Restraining Dynamic Strain Aging in GH4169 Alloy During Tensile Deformation at High Temperature

The dynamic strain aging(DSA) behavior was investigated in GH4169 alloy during tensile deforming with electric-pulse current(EPC) at 750 ℃.The results show that DSA is restrained in the alloy when deformed with 40 Hz-EPC.The size ofγ " phase inner grains increases obviously and δ phase is facilitated to precipitate on grain boundary in the alloy applied with EPC,due to the promotion effect of EPC on the diffusion and segregation of atoms.Transmission electron microscopy(TEM)results indicate that dislocations can cut through small γ" precipitate with the size of less than 10 nm,while dislocations can only bypass dislocations when γ " precipitate grow up over 20 nm.The growth of precipitates consumes large amounts of atoms as well as the velocity of dislocation increase,which makes dislocations difficult to be pinned.Therefore,when γ" precipitates grow up to a large size more than the critical size of dislocation pinning,DSA is significantly restrained in the alloy after necking deformed with EPC.

Effect of strain rate and temperature on the serration behavior of SA508-Ⅲ RPV steel in the dynamic strain aging process

Dynamic strain aging （DSA） effect on SA508-III reactor pressure vessel （RPV） steel was investigated. The SA508-III RPV steel was subjected to tension tests at different strain rates （1.1× 10-5 s-1 and 6.6× 10-5 s-1） and different temperatures （500 and 550 ℃） to evaluate the influence of strain rate and temperature on the serrated flow behavior, which is the repetitive and discontinuous yielding phenomenon on the stress-strain curves. The higher temperature leads to the higher density of precipitates, M23C6 carbides and needle-like Mo2C carbides. It was found that the samples under tension test of 6.6 × 10-5 s-1 and 500 ℃ possess superior mechanical properties and mainly show A-type serrations on the tension test curves. Then, the local regress method was used to filter the DSA curves, thus to show the real trend of the curves. It has been found that the less time of interaction between dislocations and precipitates under higher strain rates leads to a higher strength of the sample. The more tiny-stress drops on the 550 ℃ serration curve can be attributed to the hardening phase, M23C6 carbides and needle-like Mo2C carbides. The higher percentage of the small stress drops on the serration curves represents the higher mechanical strength.

Strain Hardening Associated with Dislocation,Deformation Twinning,and Dynamic Strain Aging in Fe–20Mn–1.3C–(3Cu) TWIP Steels

The effects of Cu on stacking fault energy,dislocation slip,mechanical twinning,and strain hardening in Fe–20Mn–1.3C twinning-induced plasticity（TWIP） steels were systematically investigated.The stacking fault energy was raised with an average slope of 2 mJ/m2 per 1 wt% Cu.The Fe–20Mn–1.3C–3Cu steel exhibited superior tensile properties,with the ultimate tensile strength reached at 2.27 GPa and elongation up to 96.9% owing to the high strain hardening that occurred.To examine the mechanism of this high strain hardening,dislocation density determination by XRD was calculated.The dislocation density increased with the increasing strain,and the addition of Cu resulted in a decrease in the dislocation density.A comparison of the strain-hardening behavior of Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels was made in terms of modified Crussard–Jaoul（C–J） analysis and microstructural observations.Especially at low strains,the contributions of all the relevant deformation mechanisms—slip,twinning,and dynamic strain aging—were quantitatively evaluated.The analysis revealed that the dislocation storage was the leading factor to the increase of the strain hardening,while dynamic strain aging was a minor contributor to strain hardening.Twinning,which interacted with the matrix,acted as an effective barrier to dislocation motion.

Dynamic Strain Aging in a Newly Developed Ni-Co-Base Superalloy with Low Stacking Fault Energy

Characteristics of dynamic strain aging （DSA） in a Ni-Co-base superalloy were studied by tensile tests at temperatures ranging from 250 ℃ to 550 ℃ and strain rates ranging from 3 x 10-5 to 8 x 10-4 s-1. Serrated flow in the tensile stress-strain curves was observed in the temperature range from 300 ℃ to 500 ℃. Normal DSA behavior was found at temperatures ranging from 300 ℃ to 350 ℃, while inverse DSA behavior was observed at temperatures ranging from 400 ℃ to 500 ℃. The yield strength, ultimate tensile strength, elongation, work hardening index, and fracture features were not affected by temperature and strain rates in DSA regime. Negative strain-rate sensitivity of flow stress was observed in DSA regime. The analysis suggests that the ordering of the substitutional solutes around some defects like mobile dislocations and stacking faults due to the thermal activated process may cause the serrations on the tensile curves.

Verification of Interpretation of Dynamic Strain Aging for Intermediate Temperature Embrittlement in Ni-Bi Alloy

Intermediate temperature embritttement （ITE） is a general phenomenon in Ni alloys and recently was interpreted by dynamic strain aging （DSA）. The relationship between ITE and DSA was studied by a binary Ni-Bi alloy. The experimental alloy of well-controlled purity was produced by vacuum induction inching and then heat-treated properly. Tensile tests were performed at various tensile temperatures, and the elongation at fracture was used to indicate the ductility. In order to identify the mechanisms of fracture and ITE, fracture morphologies of the samples of low ductility were observed by scanning electron microscopy. According to the tensile ductility, Ni-Bi alloy shows an obvious embrittlement behavior in the intermediate temperature range （700--750℃ ）. However, the stress strain curves of Ni-Bi alloy and the fracture morphologies indicate that DSA does not exist over the whole temperature range. Based on the experimental results and literatures, the interpretation of DSA was then discussed and proved to be invalid for elucidating the general feature of ITE in Ni-Bi alloy and Ni-based superalloys.

金属材料中“塑性不稳定现象”的研究进展

对于金属合金材料而言,出现的"塑性不稳定"现象或Portevin-Le Chatelier(PLC)效应主要表现为应力-应变曲线上的锯齿状波动,并伴随着空间上可传播的应变局域化,致使材料经受力变形后表面会出现凹凸化,最终导致材料整体塑性的降低。近年来,关于PLC效应的研究及其相应的实验手段已有很多相关报道。根据出现PLC效应的时空特性进行分类,锯齿波可分为A、B和C 3种类型。目前,关于PLC效应的理论解释主要归因于动态应变时效和位错切割机制。将重点介绍传统金属材料中存在塑性不稳定现象的研究进展,叙述并归纳微观结构、加载速率、受力状态和微观变形机制等对PLC效应的影响和作用规律,指出目前研究中存在的问题,提出将来的研究重点和发展方向。