应变速率对Q&P钢拉伸变形行为的影响

以汽车用先进高强度Q&P钢为研究对象,分析了应变速率对Q&P钢拉伸性能及变形行为的影响。结果表明,随应变速率增加,Q&P钢的强度增加,断裂延伸率则呈先下降(10-4s-1～10s-1),后上升至峰值(8×10s-1),之后再下降(102s-1～103s-1)的趋势。变形过程中强度的增加可能同形变回复受限,位错运动受阻有关。而断裂延伸率的变化主要与不同应变速率下Q&P钢中残余奥氏体向马氏体转变(即TRIP效应)有关。     

Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel

In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and α′ martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower α′ martensite fraction observed at fracture for a strain rate of 10^?2 s^?1, as compared to 10^?3 s^?1. This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson–Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The α′ martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to α′ martensite during loading.     

变形速率对普碳钢中形变诱导铁素体相变的影响

针对普通碳素钢(Q235类型),研究在Ae3～Ar3温度区间内采用形变诱导铁素体机制获得超细晶铁素体的数量与变形速率的相互关系。实验在Gleeble 1500热模拟实验机上进行。实验方案为：1000℃保温2min,以10℃／s的速度冷却到变形温度[Ae3(840℃)至Ar3(780℃)],变形量为30％～50％,变形后立即水淬。结果表明,在840℃变形时,随着变形速率的增大,形变诱导铁素体量增多;在780℃变形时,随着变形速率的增大,形变诱导铁素体量减少;而在840-780℃之间变形时,变形速率存在最佳值,在该值下诱导生成的铁素体量最大。     

淬火-配分马氏体钢的组织特征

淬火-配分(Quenching and Partitioning,简称QP)工艺是针对马氏体钢提出的热处理新工艺。利用QP工艺处理40Si2Ni2钢,并通过扫描电镜观察其微观组织特征。结果表明,与传统淬火+回火工艺得到的组织不同,QP组织为低碳(回火态)和高碳马氏体(淬火态)共存,其中高碳马氏体呈现为有规则几何形状、边界清晰、无析出物析出的块状组织,淬火温度(QT)对高碳马氏体(淬火态)量有影响。     

具有马氏体组织的TC11合金两相区变形的组织机制及工艺优化

采用等温压缩实验研究了具有马氏体组织的TC11合金在两相区的变形行为及微观组织演变规律。等温压缩实验在Gleeble3500热模拟实验机上进行,其中变形温度为920～980℃,应变速率为0.1～10s-1,变形量为70%。基于动态材料模型(DMM),建立了具有马氏体组织的TC11合金两相区变形的热加工图。在低温(<940℃)区和高温(>960℃)高应变速率(>1s-1)区域存在失稳现象,主要表现为低温时的表面开裂和高应变速率区的绝热剪切带;在塑性加工安全区域,分别对应着片层扭折和片层球化的组织机制,其中变形量70%时应变速率敏感因子在980℃,0.1s-1时取得峰值为0.73,此时可得到完全球化的细晶组织(等轴α尺寸约为0.7μm)。具有马氏体组织的TC11合金两相区变形时,为避免缺陷并得到细的等轴组织,合适的加工工艺为温度:950～980℃,应变速率0.1～1s-1。     

Effect of Strain Path Reversal on Austenite Grain Subdivision and Deformation Induced Ferrite Transformation Studied by Hot Torsion

The effects of large strain and strain path reversal on the deformation microstructure evolution in austenite below the recrystallisation temperature were studied by hot torsion using a non-transforming Fe-30wt%Ni model austenitic alloy.Results show that the high angle boundaries (HABs) can be generated by both microstructure mechanism through dislocation accumulation and texture mechanism via subgrain rotation.However,multiple strain path reversals lead to less well-developed HABs in the original grains compared to single reversal deformed to the same amount of total accumulative strain.This effect is attributed to the subgrain rotation mechanism being less effective at small strains.In comparison,the same hot torsion tests were conducted using a microalloyed steel at a temperature between Ae 3 and Ar 3.After single strain path reversal,substantial deformation-induced austenite-to-ferrite phase transformation was observed.Meanwhile,a test with multiple strain path reversals but with the same total strain produces much lower levels of strain-induced ferrite formation.This difference is correlated to the observations made in the Fe-30wt%Ni model alloy.It is believed that the different amount of strain-induced ferrite originated from the different levels of strain-induced HABs within the austenite which act as ferrite nucleation sites.     

铁基合金马氏体相变宏观点阵变形的特征

利用原子力显微镜观察并定量分析了铁基合金4种典型惯习面马氏体相变的宏观点阵变形特征。结果表明：{3,10,15}f薄片马氏体和{2,5,9)f透镜马氏体的宏观点阵变形呈一次均匀切变特征,其宏观点阵变形完全符合“不变平面应变”特征;而{2,2,5}f片状马氏体和板条马氏体的宏观点阵变形特征不符合“不变平面应变”特征。     

Martensite lattice changes during tempering

An X-ray diffractometer study of martensite formed in an 18 wt pct nickel, 0.98 wt pct carbon austenite single crystal yields the shapes, positions, and integrated intensities of 200, 020, and 002 peaks. Martensite, which forms below ? 60 °C, was tempered at successively higher temperatures from ?45 to 450 °C. The results show that after subambient aging, during which C atoms in c-oriented octahedral sites have clustered, carbide precipitation starts and small regions (～30Å in the [001]) with negative tetragonality appear. Upon subsequent tempering these are augmented by larger regions which have small positive tetragonality. In this process the “c” lattice parameter changes markedly but the “a” and “b” lattice parameter increase very little. These results indicate the formation of carbon depleted martensite which is coherently strained by the carbide particles. At and just above 100 °C the 200, 020, and 002 peaks all become doublets as the martensite matrix discontinuously breaks free of coherency and becomes highly imperfect ferrite. This change also occurs during the so-called “first stage of tempering.” Further tempering decreases the defect content of this ferrite. The lattice of the martensite is extensively reoriented during tempering just above room temperature. These reorientations probably accommodate the lattice parameter changes described above and may be carried out by movement of twin boundaries.     

3104铝合金热变形流变应力模型

采用等温压缩试验，研究了3104铝合金在应变速率为0．001-1s^-1、变形温度为573-773K条件下的流变应力行为。结果表明，3104合金流变应力对应变速率和变形温度十分敏感，合金高温塑性变形时存在稳态流变特征，并建立了合金热变形流变应力模型。     

Effect of Deformation in Controlled Rolling on Ferrite Nucleation

Abstract

Nucleation of ferrite either at austenite grain boundaries or within them on deformation defects has been examined experimentally and analyzed on the basis of the theory of heterogeneous nucleation. Low-carbon low-alloy steels were deformed by controlled rolling schedules to a total reduction of 50 or 68% in finish rolling at 730-800 °C. Deformation accelerates the kinetics of the γ→∞ transformation and strongly activates ferrite nucleation at grain boundaries. Both experimental and theoretical estimations showed that the rate of intragranular nucleation is much less compared to nucleation at grain boundaries. Intragranular nuclei develop notably only in the final stages of transformation in deformed austenite and affect the formation of structure only in the small separated areas.

On a examiné expérimentalement et analysé la nucléation de la ferrite, soit aux joints de grain de l'faustenite, ou à l'finterieur de ceux-ci, aux défauts de déformation, en se basant sur la théorie de la nucléation hétérogéne. On a déformé des aciers peu alliés à faible carbone, en suivant des plans de laminage contrôlé, jusqu'fa une réduction totale de 50 ou 68% pour le laminage final a 800-730 °C. La déformation accélère la cinétique de transformation de γ→α et active fortement la nucleation de la ferrite aux joints de grains. Tant les évaluations expérimentales que theoriques ont montré que le taux de nucléation intragranulaire était beaucoup moins élevé en comparaison avec la nucleation aux joints de grain. Les noyaux intragranulaires se développent particuliérement seulement lors des étapes finales de transformation de l'faustenite déformée et affectent la formation de la structure seulement dans de petites zones séparées.     

Deformation and Strain Storage Mechanisms during High-Temperature Compression of a Powder Metallurgy Nickel-Base Superalloy

High-temperature compression testing combined with high-resolution electron backscatter diffraction (EBSD) analysis has been used to study microstructural-scale straining processes that occur during high-temperature deformation of a powder-consolidated nickel-base superalloy, René 88DT (GE Aviation, Evendale, OH). Orientation imaging has been employed to study grain-level straining and strain storage at temperatures between 1323 K (1050 °C) and 1241 K (968 °C) for strain rates between 0.1/s and 0.00032/s at nominal strain levels between 0.1 and 0.7. Two distinct deformation mechanisms were observed. At strain rates below 0.01/s, superplastic deformation dominates, while power-law creep occurs during high rate compression. Stored strain and evolution of the grain structure during deformation are dependent on strain rate during compression. At low strain rates in the superplastic regime, low levels of stored strain and some grain growth are observed. At high strain rates, dynamic recrystallization occurs along with higher levels of stored strain within selected grains, particularly those at the high end of the grain size distribution. A constitutive model for superplastic deformation was employed to predict the temperature and strain rate dependence of the transition from superplastic to power law deformation. The transition in rate sensitivity was consistent with the transition in stored strain measured by EBSD. Superplasticity-enhanced grain growth is observed and the implications for the transition in deformation mechanisms are discussed.     

Yield Strength Enhancement by Carbon Trapping in Ferrite of the Quenching and Partitioning Steel

The split quenching and partitioning (S-QP) process allows researchers to investigate microstructure and properties separately, i.e., before and after partitioning. After the partitioning process, the yield strength increases by approximately 300 MPa in the ferrite-bearing δ-quenching and partitioning (δ-QP) steel. We propose that carbon trapping in dislocations at the ferrite grain boundaries during partitioning process is responsible for the yield strength enhancement of ferrite. Combined transmission electron microscopy and 3D atom probe tomography observations demonstrate carbon atoms segregating in dislocations. The mechanisms for the high yield strength of ferrite presented QP steels are clarified for the first time in this research.

     

超细晶工业纯钛热变形流动应力特征研究

在室温下以等径弯曲通道变形(ECAP)技术制备超细晶工业纯钛,采用Gleeble-1500热模拟实验机对粗晶和超细晶工业纯钛在热变形条件下的流动应力特征进行了研究。结果表明:超细晶工业纯钛在热压缩过程中,热压缩条件不同流动应力变化规律会有所差异。在较低应变速率条件下,随变形程度的增加,流动应力增加到峰值后开始下降,呈现明显的动态再结晶特征;而在较高应变速率下,呈现稳态流变特征。此外,与粗晶工业纯钛相比,超细晶工业纯钛的屈服应力显著增强。     

新型Al-Zn-Mg-Cu合金热变形流变应力特征

采用Gleeble-1500热模拟机进行热压缩变形实验,研究了一种新型Al-7.5Zn-1.6Mg-1.4Cu-0.12Zr合金在变形温度为380-460℃、应变速率为0.001～0.1 s-1条件下的流变应力特征,并利用TEM分析了合金在不同变形条件下的组织形貌特征.结果表明,应变速率和变形温度对合金流变应力的大小有显著影响,流变应力随变形温度的升高而降低,随应变速率的提高而增大;合金平均亚晶尺寸随温度补偿应变速率Zener-Hollomon参数的升高而减小.可用Zener-Hollomon咖参数描述该Al-Zn-Mg-Cu合金热变形时的流变应力行为.     

Impact of Martensite Spatial Distribution on Quasi-Static and Dynamic Deformation Behavior of Dual-Phase Steel

The effects of microstructure parameters of dual-phase steels on tensile high strain dynamic deformation characteristic were examined in this study. Cold-rolled steel sheets were annealed using three different annealing process parameters to obtain three different dual-phase microstructures of varied ferrite and martensite phase fraction. The volume fraction of martensite obtained in two of the steels was near identical (~ 19 pct) with a subtle difference in its spatial distribution. In the first microstructure variant, martensite was mostly found to be situated at ferrite grain boundaries and in the second variant, in addition to at grain boundaries, in-grain martensite was also observed. The third microstructure was very different from the above two with respect to martensite volume fraction (~ 67 pct) and its morphology. In this case, martensite packets were surrounded by a three-dimensional ferrite network giving an appearance of core and shell type microstructure. All the three steels were tensile deformed at strain rates ranging from 2.7 × 10^?4 (quasi-static) to 650 s^?1 (dynamic range). Field-emission scanning electron microscope was used to characterize the starting as well as post-tensile deformed microstructures. Dual-phase steel consisting of small martensite volume fraction (~ 19 pct), irrespective of its spatial distribution, demonstrated high strain rate sensitivity and on the other hand, steel with large martensite volume fraction (~ 67 pct) displayed a very little strain rate sensitivity. Interestingly, total elongation was found to increase with increasing strain rate in the dynamic regime for steel with core–shell type of microstructure containing large martensite volume fraction. The observed enhancement in plasticity in dynamic regime was attributed to adiabatic heating of specimen. To understand the evolving damage mechanism, the fracture surface and the vicinity of fracture ends were studied in all the three dual-phase steels.     

Nb的析出对变形诱导铁素体相变的影响

通过Gleeble2000热模拟实验机，研究了X65管线钢中Nb在变形奥氏体中的析出状态对变形诱导铁素体相变（DIFT）的影响。试验结果表明，在奥氏体临界区变形时，第一道次变形后，随变形后等温时间延长，诱导铁素体量变化不大。等温时间达120S时，变形奥氏体仍未发生再结晶。在道次间随时间延长，Nb的析出量增加，第二道次变形后诱导的铁素体也显著增加。微合金元素Nb通过碳氮化物的析出作用促进变形诱导铁素体相变。     

Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

Metallurgical and Materials Transactions A - Quenching and partitioning (Q&amp;P) processing of third-generation advanced high strength steels generates multiphase microstructures containing...