CYCLIC DEFORMATION BEHAVIOR OF ZIRCALOY-4 AT DIFFERENT TEMPERATURES

ＣＹＣＬＩＣＤＥＦＯＲＭＡＴＩＯＮＢＥＨＡＶＩＯＲＯＦＺＩＲＣＡＬＯＹ－４ＡＴＤＩＦＦＥＲＥＮＴＴＥＭＰＥＲＡＴＵＲＥＳ￥ＸＩＡＯＬｉｎ;ＧＵＨａｉｃｈｅｎｇ;ＫＵＡＮＧＺｈｅｎｂａｎｇ（Ｘｉ＇ａｎＪｉａｏｔｏｎｇＵｎｉｖｅｒｓｉｔｙ,Ｃｈｉｎａ）Ｍ...     

应变速率对高氮奥氏体钢拉伸变形的影响

采用金相及透射电子显微镜对高氮奥氏体Fe-20Mn-19Cr-0. 6N钢在应变速率范围为3×10^-6～1 s^-1条件下的拉伸变形行为进行了研究。研究结果表明:N元素的固溶强化作用和促使位错平面滑移阻碍位错运动机制是高氮奥氏体钢的重要应变硬化机制,同时,随着应变速率的提升,这种强化机制不断提升,而应变诱导孪生机制不断削弱。随着应变速率的提升,高氮奥氏体钢的抗拉强度和屈服强度均呈逐步上升的趋势,断后伸长率则逐步下降。屈服强度提升超过60%,而抗拉强度提升仅10%。随着应变速率的提升,基体变形程度逐步下降,材料的位错密度和滑移带密度逐步下降。     

Neutron Diffraction Study of Low-Cycle Fatigue Behavior in an Austenitic–Ferritic Stainless Steel

By performing in situ neutron diffraction experiments on an austenitic–ferritic stainless steel subjected to lowcycle fatigue loading, the deformation heterogeneity of the material at microscopic level has been revealed. Based on the in situ neutron diffraction data collected from a single specimen together with the mechanical properties learned from the ex situ micro-hardness, a correlation has been found. The performance versus diffraction-profile correlation agrees with the cyclic-deformation-induced dislocation evolution characterized by ex situ TEM observation. Moreover, based on the refined neutron diffraction-profile data, evident strain anisotropy is found in the austenite. The high anisotropy in this phase is induced by the increase in dislocation density and hence contributes to the hardening of the steel at the first 10 cycles.Beyond 10 fatigue cycles, the annihilation and the rearrangement of the dislocations in both austenitic and ferritic phases softens the plastically deformed specimen. The study suggests that the evolution of strain anisotropy among the differently oriented grains and micro-strain induced by lattice distortion in the respective phases mostly affect the cyclic-deformationinduced mechanical behavior of the steel at different stages of fatigue cycles. The stress discrepancy between phases is not the dominant mechanism for the deformation of the steel.     

Cr原子在Fe_3Al金属间化合物亚点阵中占位测定SCIEI

利用中子衍射方法确定了Ｃｒ原子在Ｄ０_３有序结构的Ｆｅ－２８Ａｌ一５Ｃｒ合金亚点阵中的占位．结果表明：Ｃｒ原子仅替代Ｆｅ原子的位置，并占位于与Ａｌ原子次近邻的亚点阵，由于次近邻反相畴界能的降低，该合金四分位错组态中次近邻反相畴界的宽度变大，从而使交滑移变得容易，材料的变形能力得到改善，这一分析已分别为本工作对合金拉伸试样的表面滑移线金相观察和位错组态的ＴＥＭ研究所证实．     

High-temperature Tensile Behavior in Coarse-grained and Fine-grained Nb-containing 25Cr-20Ni Austenitic Stainless Steel

In this study, tensile behavior of Nb-containing 25Cr-20Ni austenitic stainless steels composed of coarse or fine grains has been investigated at temperatures ranging from room temperature to 900 °C. Results show that the tensile strength of fine-grained specimens decreases faster than that of coarse-grained specimens, as the test temperature increases from 600 °C to 800 °C. The rapidly decreasing tensile strength is attributed to the enhanced dynamic recovery and recrystallization, because additional slip systems are activated, and cross-slipping is accelerated during deformation in fine-grained specimens. After tensile testing at 700-900 °C, sigma phases are formed concurrently with dynamic recrystallization in fine-grained specimens. The precipitation of sigma phases is induced by simultaneous recrystallization as the diffusion of alloying elements is accelerated during the recrystallization process. Additionally, the minimum ductility is observed in coarse-grained specimens at 800 °C, which is caused by the formation of M₂₃C₆ precipitates at the grain boundaries.     

AZ91D镁合金屑的固相再生(英文)

利用固相再生技术回收利用AZ91D镁合金屑,具体工艺为先冷压再热挤。结果表明:制备的AZ91D镁合金具有较好的力学性能且晶粒明显细化。在热挤出过程中发生了动态再结晶,且动态再结晶组织受到热挤温度和应变速率的影响,在300-350 °C下基面滑移和孪晶协调变形导致动态再结晶晶粒产生,形成"项链"组织;在 350-400 °C下位错的交滑移控制动态再结晶形核;高于400 °C时位错攀移控制了整个动态再结晶过程,形成均匀的再结晶组织。随着应变速率增加AZ91D镁合金力学性能增大,改善了材料的力学性能,但应变速率过大,制备试样表面出现裂纹,影响材料的力学性能。     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

Effects of ausforming temperature on bainite transformation,microstructure and variant selection in nanobainite steel

The bainite transformation behavior after plastic deformation of austenite, i.e., ausforming was studied by in situ neutron diffraction and ex situ experiments, and the effects of ausforming temperature was made clear. Ausforming, at a low temperature (573 K) was found to accelerate bainite transformation and produce a characteristic microstructure, whereas at a high temperature (873 K), ausforming had little influence. The reason for the different results stems from the dislocation structure introduced in austenite; planar dislocations remaining on the active slip planes are believed to assist bainite transformation, accompanied by strong variant selection. The variant selection rule that focuses on Shockley partial dislocation was verified from electron backscatter diffraction results.     

Multiple plastic deformation mechanisms of NiTi shape memory alloy based on local canning compression at various temperatures

A Ni-rich NiTi shape memory alloy (SMA), which was in its austenitic state at ambient temperature, was subjected to plastic deformation by means of local canning compression at various temperatures ranging from room temperature to 800 °C. Depending on temperatures, NiTi SMA exhibited multiple plastic deformation mechanisms, such as dislocation slip, deformation twinning, grain boundary slide, grain rotation, dislocation climb and grain boundary migration. Amorphization, dynamic recovery and dynamic recrystallization of NiTi SMA were also observed at various temperatures. Mechanism of localized amorphization, in particular, was investigated based on dislocation slip and deformation twinning. Statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) were found to play an important role in the amorphization of the current NiTi SMA. There appeared a critical dislocation density below which NiTi SMA was unable to amorphize. Accordingly, at a fixed deformation strain, there should be a critical temperature above which amorphous phase would not occur in the NiTi SMA matrix. Furthermore, when NiTi SMA experienced plastic deformation at the critical temperature, amorphization and crystallization would occur simultaneously and compete with each other.     

Strain Hardening During Hot Compression Through Planar Dislocation and Twin-Like Structure in a Low-Density High-Mn Steel

In this work, we study the flow hardening behavior and microstructural evolution in a low-density Fe-21Mn-0.1C-2.0Al-2.5Si (wt.%) transformation-twinning-induced plasticity steel during hot compression. The substructures were examined by transmission electron microscopy and electron backscatter diffraction methods. The alloy exhibits a combination of strain hardening (at low and medium true compression strains, <0.2) followed by a strain softening at 800 °C. We explain the flow hardening behavior in terms of substructure refinement due to planar dislocation, as well as twin-like structure. The transmission electron microscopy results suggest that short-range ordering triggers slip planarity after straining to 0.05 at 800 °C. Twin-like structure with misorientation angle higher than 20° is formed during further plastic deformation.     

动态压缩条件下高纯钛微观组织特征研究

利用光学显微镜(OM)、背散射电子衍射(EBSD)技术及透射电子显微镜(TEM)对高纯钛低-中应变动态压缩变形的微观组织特征进行了研究。结果表明：随着应变量(ε)的增加,晶粒内部通过孪晶与孪晶,孪晶与位错以及位错与位错之间的交互作用逐步细化原始晶粒;变形初期,形变孪生以{11-22}孪晶为主,当ε达到0.2后,{10-12}孪晶转变为主要形变孪生类型,孪生改变了原始晶粒的取向,进一步促进晶粒内部的位错滑移。高纯钛动态压缩变形经历了由位错滑移到形变孪生,再到位错滑移主导的过程,但位错滑移和孪生始终共同作用协调动态压缩变形。     

SPD技术对锆及锆合金力学行为影响研究现状

综述了锆及锆合金剧烈塑性变形(SPD)后性能变化的研究进展,系统阐述了锆及锆合金经剧烈塑性变形后显微硬度、拉伸/压缩性能、高低周疲劳性能,重点介绍了SPD技术在纯锆、Zr-Nb系合金中的应用。经过剧烈塑性变形后,锆及锆合金的抗拉强度及屈服强度均显著提升,但依据剧烈塑性成形轨迹、合金成分、第二相分布、热处理制度不同,其提升程度存在一定的差别。位错滑移是锆及锆合金高周疲劳的主要损伤机制,位错运动(包括位错滑移及位错攀移)是锆及锆合金低周疲劳的主要损伤机制。文章最后指出现阶段锆及锆合金SPD技术的发展趋势及应用前景。     

Grain Size-Dependent Mechanical Properties of a High-Manganese Austenitic Steel

The effect of grain size on the mechanical properties of a high-manganese(Mn) austenitic steel was investigated via electron-backscattered diffraction, transmission electron microscope, X-ray diffraction, and tensile and impact tests at 25 °C and-196 °C. The Hall–Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm 0.5 at 25 °C, which increased to 14 MPa mm 0.5 at -196 °C. The effect that the grain boundary strengthening had on improving the yield strength at-196 °C was better than that at 25 °C. The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25 °C, while they remained nearly unchanged at -196 °C. The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at -196 °C, which promoted the cleavage fracture in the steels with large-sized grains. Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.     

The evolved microstructure ahead of an arrested fatigue crack in Haynes 230

The dislocation microstructure beneath surface slip traces produced by fatigue loading of Haynes 230 was revealed to be a function of distance from a crack tip. The microstructure beneath these traces evolves from planar slip bands with increasing dislocation density and decreasing interband spacing as the crack tip approaches one of refined subgrains and lamellar bands at and in the vicinity of the crack tip. Similarly, beneath fatigue striations the microstructure evolves from nanosized subgrains to a banded structure with increasing distance from the fracture surface. These structures are significantly different to those predicted to develop under fatigue loading of a planar slip material. The evolved structures are considered in terms of the microstructure generated by severe plastic deformation.     

激光熔化沉积Ti-6Al-2Zr-Mo-V合金

研究激光熔化沉积Ti-6Al-2Zr-Mo-V合金在高周疲劳失效中的变形行为,并提出相应的变形机制。结果表明,激光沉积钛合金疲劳裂纹萌生和扩展中的变形行为都与其位错运动和滑移行为密切相关。疲劳裂纹扩展中的滑移主要沿两个滑移面进行;裂纹扩展亚表面存在大量二次裂纹。解释两种不同类型二次裂纹的形成机制和作用。分析粗大柱状晶晶界变形特征。     

On the origin of deformation microstructures in austenitic stainless steel: part I—microstructures

A comprehensive characterization of room temperature deformation microstructures was carried out by transmission electron microscopy for ion irradiated and deformed AISI 316LN austenitic stainless steel. Deformation microstructures were produced by a recently developed disk-bend test method and also by a uniaxial tensile test. Cross-slip was dramatically suppressed by the radiation-induced defects and slip occurred predominantly by planar glide of Shockley partial dislocations. Deformed microstructures consisted of piled-up dislocations, nanotwin layers, stacking faults, and defect-reduced dislocation channel bands. Analyses revealed that all these features were different manifestations of the same type of deformation band, namely a composite of overlapping faulted layers produced by Shockley partial dislocations.     

Effects of Crystal Orientations on the Low-Cycle Fatigue of a Single-Crystal Nickel-Based Superalloy at 980 °C

The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.     

激光熔化沉积Ti-6Al-2Zr-Mo-V合金高周疲劳变形行为

研究激光熔化沉积Ti-6Al-2Zr-Mo-V合金在高周疲劳失效中的变形行为,并提出相应的变形机制.结果表明,激光沉积钛合金疲劳裂纹萌生和扩展中的变形行为都与其位错运动和滑移行为密切相关.疲劳裂纹扩展中的滑移主要沿两个滑移面进行;裂纹扩展亚表面存在大量二次裂纹.解释两种不同类型二次裂纹的形成机制和作用.分析粗大柱状晶晶界变形特征.     

大晶粒单相Ni-50Al金属间化合物高温塑性

研究了原始晶粒尺寸为220 (m的正化学计量比单相Ni-50Al金属间化合物的高温变形行为及组织演变规律.结果表明,该合金在温度1000～1100 ℃,应变速率7.5×10~(-4)～1×10~(-3) s~(-1)范围内具有良好的高温塑性变形能力;在1075 ℃,应变速率为8.75×10~(-4) s~(-1)时,最大延伸率可达139%.金相显微分析表明,原始大晶粒组织经高温塑性变形后显著细化;EBSD与 TEM分析表明,变形过程中小角度晶界持续产生,较小角度晶界向较大角度晶界不断演变,最终导致晶粒显著细化.显微结构综合分析表明,大晶粒Ni-50Al合金的高温塑性变形是由位错的交滑移与攀移等交互作用产生的连续动态回复和再结晶导致的.     

Compressive deformation behavior of CrMnFeCoNi high-entropy alloy

The compressive deformation behavior of a single phase CrMnFeCoNi high-entropy alloy (HEA) is investigated using experimental and theoretical approaches. The equiaxed microstructures are observed using optical microscope, electron backscattered diffraction, and synchrotron X-ray diffraction (XRD) techniques. Compressive results reveal that the CrMnFeCoNi HEA has a high strain-hardening exponent in spite of its large grain size due to increased dislocation density and severe lattice distortion. The compressive texture of the HEA resembles those of typical FCC metals. The phenomenological dislocation-based constitutive model well describes the compressive deformation behavior. The predicted dislocation density is in good quantitative agreement with the experimental value measured using whole-profile fitting of synchrotron XRD peaks. It can be confirmed from the experimental and theoretical findings that the deformation mechanism of the CrMnFeCoNi HEA is the conventional dislocation glide and mechanical twinning is negligible contrary to general belief.