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.

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.

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.

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.

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.

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℃.

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。

临界淬火态热轧中锰钢单轴拉伸过程中的塑性失稳行为

结合数字图像相关法(digital image correlation,DIC)、红外热成像(infrared thermal imaging,IRT)、电子背散射衍射(electron backscatter diffraction,EBSD)和显微硬度分布,重点分析临界淬火态中锰钢在单轴拉伸过程中的塑性失稳行为。结果表明:实验钢在单轴拉伸过程中应变诱导马氏体转变主要发生在拉伸变形前期,存在明显的PLC(Portevin-Le Chatelier)带形成与传播,伴随着热量波动,经历了从A型到B型带再到A型PLC带的转变;PLC带的形成原因是增殖位错与间隙溶质原子的相互作用,即动态应变时效(dynamic strain aging,DSA),前期位错会优先向低密度位错区域运动即沿轴向单向传播(A型PLC带),中期位错的增殖和开始交互作用的影响致使形成跳跃性(B型)PLC带,后期位错密度的增大和交互作用的增强,再次致使A型PLC带的形成。

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。

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.

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.

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.

热轧V/Ti合金化TWIP钢的拉伸变形行为

以Fe-18Mn-1.0C-0.5V-0.1Ti TWIP钢为研究对象,利用单向拉伸、OM、XRD、EBSD和TEM等检测手段对比研究了锻造及热轧处理对V/Ti合金化TWIP钢拉伸变形行为的影响。结果表明,与锻造态相比,热轧态试验钢中晶粒尺寸显著减小,晶粒沿轧制方向拉长,大颗粒状碳化物消失;热轧试样的屈服强度、抗拉强度以及伸长率同时升高,加工硬化能力增强;两种状态试验钢拉伸变形过程中均产生动态应变时效现象,且热轧态试样的锯齿临界应变值较小,锯齿的应力幅和局部应变集中更大;热轧试验钢衍射峰半高宽增大,小角度晶界数量增加,位错密度增大。热轧态试验钢中较小的晶粒尺寸、较大的初始位错密度促使其强度升高,并产生更加强烈的动态应变时效强化,对试验钢强塑性提升有益。

Portevin-Le Chatelier Effect in Additively Manufactured and As-Cast Inconel 939 Nickel-Based Superalloy

Nickel-based superalloys, well-established in aeronautics, have recently gained significant traction in additive manufacturing. Inconel 939 is one of the alloys increasingly playing a vital role in this field. This paper examines the development of the Portevin-Le Chatelier (PLC) effect in additively manufactured Inconel 939 in comparison with cast Inconel 939. A detailed analysis of tensile test characteristics was conducted, complemented by a high-resolution scanning electron microscopy (HR-SEM) investigation. The PLC region exhibited several properties during tensile testing, such as stress-strain behavior, cycle scale, and overall stress increase. The HR-SEM analysis of Gamma prime (γ') precipitates revealed distinct morphologies, which are suggested to be linked to the features of the PLC region. Samples with a high amount of γ' precipitates showed a less pronounced PLC region, while those with fewer γ' precipitates displayed a more distinct PLC effect. A mechanism for the cyclic drop-and-rise stress behavior, based on the work of Varvenne and La-Rose, was proposed, possibly induced by the varying morphologies of γ' precipitates in the IN939 alloy. Further study is needed to deepen the understanding of the relationship between the γ' micro-(nano) structure and the PLC phenomenon.

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.