磷钙无铅黄铜及其切削变形机理

为取代铅,减轻对环境的污染,采用磷和钙共同替代铅,获得了综合性能优良的磷钙无铅黄铜铸锭.为研究磷钙无铅黄铜的切削变形机理,采用卧式车床进行了切削实验,通过扫描电镜及能谱仪对切屑组织进行研究,并结合Griffith脆性断裂理论和切削力学计算,对切屑变形过程进行了分析.结果表明:磷钙无铅黄铜切削性能优良,切屑的尺寸和形貌与HPb59-1相当;沿晶界弥散分布的金属间化合物,割断了基体组织的连续性,切削时在剪切力的作用下,金属间化合物颗粒经变形后发生断裂或脱落,甚至在切削热的作用下部分熔化,引发基体产生应力集中,萌生裂纹并致使扩展,导致刀具前面剪切区剪切带的周期形成,并向自由表面扩展成层片结构,造成切屑断裂,从而提高了磷钙无铅黄铜的切削性能;磷钙无铅黄铜的综合性能优良,其替代含铅黄铜,具有可行性.     

30CrNi3MoV低合金超高强钢中的马氏体相变

通过对马氏体的显微组织进行分析,并结合线膨胀试验得到的相变动力学信息研究了30CrNi3MoV低合金超高强钢中的马氏体相变特征.结果表明:淬火冷却30CrNi3MoV钢的相变产物包括低碳板条状和高碳针状两种马氏体形态,两者的形成在动力学曲线中截然分开.板条马氏体形成于Ms以下的较高温(310℃～260℃),相变过程中发生了碳的重新分配,造成富碳奥氏体微区的形成;高碳针状马氏体形成于Ms以下的较低温(260℃～170℃),由富碳奥氏体微区转变而成.板条马氏体形成速率远高于针状马氏体.     

精密硬态切削过程中已加工表面变质层生成研究

精密硬态切削过程中刀具与工件发生剧烈的热力耦合作用,使得一定条件下已加工表面出现变质层.为揭示变质层的生成机制,研究了PCBN刀具硬态切削淬硬模具钢Cr12MoV的工艺过程.通过实验手段揭示了表面变质层生成机理,得到了不同刀具磨损情况下变质层中白层厚度的变化规律;采用MATLAB数学分析软件构建了已加工表面温度场分布曲线,预测了特定切削条件下变质层中的白层厚度.实验验证结果表明:该模型预测精度较高,可以为精密硬态切削加工过程中工件加工质量的控制提供依据.     

工业纯钛TA2剪切带中微观组织的演变

剪切变形局域化是结构材料经受冲击时的一种重要失效机制,为研究密排六方晶体结构金属材料的绝热剪切带形成条件与扩展规律,采用HOPKINSON压杆装置对精加工后的工业纯钛帽形样品进行高速冲击,利用扫描电镜和高分辨透射电镜研究了剪切带形貌和剪切带微观组织的演化过程.结果表明,工业纯钛TA2经高速冲击后,在帽形样品的韧带部位形成了明显的剪切带,剪切带组织由细小的再结晶晶粒组成,剪切带内没有相变发生,剪切带内的动态再结晶过程通过渐进式亚晶位相差再结晶机制完成.     

纳米磨削过程中加工表面形成与材料去除机理的分子动力学仿真

目前，以线性断裂力学为基础的加工理论对解释微切削加工机理还存在不足．分子动力学分析的方法在研究纳米尺度或原子尺度下的固体变形方面具有独特的优势；辅以压痕挤压机理分析，解释纳米磨真削过程中加工表面形成和材料去除机理．研究表明：静水压力对非结晶变形程度影响很大；晶格重构原子与一部分非晶层原子堆积在磨粒的前上方。由于磨粒不断前移，最终形成磨屑而实现材料去除，处在磨粒前下方的非晶层原子在压应力的作用下与已加工表层断裂的原子键结合重构形成已加工表面变质层；变质层由内外两层组成，外层是非晶层，内层是晶格变形层。     

正交切削高强度钢绝热剪切带组织和硬度研究

为了研究切削速度和工件硬度对高强度钢锯齿形切屑内绝热剪切带显微组织和硬度的影响,利用光学显微分析、SEM和TEM以及硬度测量等方法观察和测量了不同切削速度下正交切削两种回火硬度的30CrNi3MoV钢形成的锯齿形切屑中绝热剪切带的微观组织和显微硬度的变化过程.结果表明:低速下形成以组织剧烈拉长为特征的形变带,高速下形成以组织严重细化为特征的转变带;工件硬度的提高有利于形成转变带;增加切削速度和工件硬度对转变带硬度影响很小,但会显著提高形变带硬度.     

干摩擦应变诱导板条马氏体固态非晶化的能量分析及其模型

使用定点离子束切割制样(FIB)并根据透射电镜(TEM)表征，分析了板条马氏体钢干摩擦层内部板条马氏体协调塑性变形、演变为纳米层片结构并发生非晶化的全过程。结果表明，高密度位错缠结和缺陷集中是纳米层片结构的典型特征，这种结构产生的界面在高应变驱动下发生非晶化。这些非晶产物，为进一步细化磨屑和形成表面自润滑层提供结构条件。基于上述实验结果并结合摩擦学和材料学理论建立了干摩擦过程中的非晶化形核模型，计算了发生非晶化的热力条件和能量壁垒。结果表明，根据经典形核理论和晶体向非晶转变的吉布斯自由能壁垒计算公式所建立的干摩擦非晶化形核能量模型，可用于计算发生非晶化必需的临界位错密度值。根据对应的计算结果，可控制摩擦条件用干摩擦应变诱导板条马氏体的固态非晶化。     

等原子比TaRu高温记忆合金的显微组织、压缩特性和形状记忆效应

利用光镜、XRD、DTA、压缩试验和TEM研究了等原子比TaRu超高温形状记忆合金的显微组织、相变、压缩特性和应变恢复特性.研究发现在高温到室温的降温阶段,合金发生了β(母相)→β′(中间马氏体相)→β"(马氏体)两步相变.室温显微组织为规则排布的马氏体板条,晶格结构为单斜结构.马氏体板条内部的孪晶关系经标定为(101)I型孪晶.室温压缩过程中变体内部的马氏体板条发生再取向及粗化.β′→β逆相变提供了主要的形状记忆效应,最大可恢复应变为2%.     

刀具磨损对于单点金刚石切削单晶硅的影响（英文）

单点金刚石切削(SPDT)是加工单晶硅最常用的方法,刀具磨损是影响加工表面或工件表面质量的重要因素,但是其中的磨损机制尚不清楚。为了研究刀具磨损对于切削机制的影响,本研究建立了单点金刚石切削单晶硅的分子动力学(MD)仿真模型。仿真结果表明随着刀具磨损程度的增加,切削力、表面损伤层厚度、位错分布面积、剪切变形和相变程度均增加。当使用已经磨损的刀具切削单晶硅时,挤压起主要作用,当使用未磨损刀具时,剪切变形起主要作用,工件表面损伤层主要是由硅的非晶相组成,使用磨损的刀具时产生的轴向力F_t约是未磨损刀具的四倍。模拟结果同时表明使用未磨损金刚石刀具时会导致工件发生塑性变形,当刀具发生磨损后切削过程中会伴随有脆性断裂。     

20号钢高速变形时热塑剪切局部化的结构特征

本文报导了20号钢在 Hopkinson 扭转杆上以1500 s~(-1)高速变形时产生的热塑剪切局部化的微观结构特征。结果指出,在试样标距截面的变形区内分布着一些宽度约为50μm,间距为100μm 彼此平行的微细剪切带。变形区的平均应变为0.83而剪切带内的应变高达1.95。剪切带内组织结构损伤严重,主要表现在大量的微裂纹生核和聚合,剪切带内的位错密度很高,位错胞在与剪切带成一定角度方向上沿晶粒拉长方向排列。由于带内的高度变形,剪切中的晶粒严重变形而拉长并产生一定程度的碎化但还未出现非晶现象。     

Microstructural study of adiabatic shear bands formed in serrated chips during high-speed machining of hardened steel

The characterization of the microstructure and phase transformation in adiabatic shear bands (ASB_s) within the serrated chips generated during high-speed machining of hardened 30CrNi₃MoV steel has been performed. The observations showed that the microstructure gradually changes from the center of the ASB to the matrix of the chip, the fine equiaxed grains appear with size of about 0.4–0.6 μm in the center of the ASB_s, the transitional region adjacent to the ASB is characterized by the broken and elongated martensite laths in shear direction. The analysis indicated that the serrated chip formation was likely due to adiabatic shear instability that occurred in the primary shear zones and the transformation to martensite within the ASB. Dynamic recovery and recrystallization are the dominant metallurgical processes during microstructural evolution of ASB.     

Microcosmic mechanism of material softening and fracture in primary shear zone during high speed machining of hardened steel

The metallurgical observations of microstructure and fracture characteristics of the adiabatic shear bands, within the primary shear zones of the serrated chips produced during high speed machining 30CrNi3MoV hardened high strength steel, have been performed using optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure observations showed that the material softening occurred in the primary shear zones during high speed machining. A microcosmic model of microstructure development and rotational dynamic recrystallisation in the primary shear zone during high speed machining high strength steel was suggested by analysis of material softening mechanism. The fracture observations showed that adiabatic shear fracture was the primary reason of serrated chip formation during high speed machining. A microcosmic fracture model during adiabatic shear was proposed.     

Dynamic shear response and evolution mechanisms of adiabatic shear band in an ultrafine-grained austenite–ferrite duplex steel

The dynamic properties of an intercritically annealed 0.2C5Mn steel with ultrafine-grained austenite–ferrite duplex structure were studied under dynamic shear loading. The formation and evolution mechanisms of adiabatic shear band in this steel were then investigated using interrupted experiments at five different shear displacements and the subsequent microstructure observations. The dynamic shear plastic deformation of the 0.2C5Mn steel was observed to have three stages: the strong linear hardening stage followed by the plateau stage, and then the strain softening stage associated with the evolution of adiabatic shear band. High impact shear toughness was found in this 0.2C5Mn steel, which is due to the following two aspects: the strong linear strain hardening by martensite transformation at the first stage, and the suppressing for the formation of shear band by the continuous deformation in different phases through the proper stress and strain partitioning at the plateau stage. The evolution of adiabatic shear band was found to be a two-stage process, namely an initiation stage followed by a thickening stage. The shear band consists of two regions at the thickening stage: a core region and two transition layers. When the adjoining matrix is localized into the transition layers, the grains are refined along with increasing fraction of austenite phase by inverse transformation. However, when the transition layers are transformed into the core region, the fraction of austenite phase is decreased and almost disappeared due to martensite transformation again. These interesting observations in the core region and the transition layers should be attributed to the competitions of the microstructure evolutions associated with the non-uniformly distributed shear deformation and the inhomogeneous adiabatic temperature rise in the different region of shear band. The 0.2C5Mn TRIP steel reported here can be considered as an excellent candidate for energy absorbers in the automotive industry.     

Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy

The adiabatic shear band (ASB) was obtained by split Hopkinson pressure bar (SHPB) technique in the hat-shaped specimen of a near beta-Ti alloy. The microstructure and the phase transformation within the ASB were investigated by means of TEM. The results show that the elongated subgrains with the width of 0.2-0.4 μm have been observed in the shear band boundary, while the microstructure inside the ASB consists of fine equiaxed subgrains that are three orders of magnitude smaller than the grains in the matrix. The β → ω_(althermal) phase transformation has been observed in the ASB, and further analysis indicates that the shear band offers thermodynamic and kinetic conditions for the ω_(althermal) phase formation and the high alloying of this alloy is another essential factor for this transformation to take place. The thermo-mechanical history during the shear localization is calculated. The rotational dynamic recrystallization (RDR) mechanism is used to explain the microstructure evolution mechanism in the shear band. Kinetic calculations indicate that the recrystallized fine subgrains are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Microstructural and mechanical behavior of Zr-based metallic glasses with the addition of Nb

The effect of Nb content on the microstructure and mechanical properties of Zr-based bulk metallic glass (BMG) were investigated. The addition of Nb led to the formation of the Zr-based metallic glass composites with a ductile dentritic phase by in situ precipitation. The presence of the in situ precipitated phase enhanced significantly the plasticity of the composite under uniaxial compressive test. The interactions between the precipitated phase and the shear band affect the deformation mechanism and fracture mode of the BMG by enhancing the affecting level of the normal stress on the shear surface, and the constant α in the Mohr–Coulomb criterion can reflect the extent of the interactions among particles and the amorphous matrix.     

Laser-induced graphite superhardness and transformation into amorphous carbon in a near-surface layer of cast iron

An analysis of the microstructure and Raman spectra shows that the formation of superhard structures in a near-surface layer of cast iron, observed upon a special laser treatment of the gray iron surface covered by a thin layer of inductor (copper), is related to the transformation of graphite (present in the cast iron) into amorphous carbon. A possible mechanism of this process is proposed, which includes the stages of graphite transition into a liquid phase, supercooling of the liquid carbon, and its transformation into a condensed amorphous phase.     

Über adiabatic shearbands und die Entstehung der „Steilen Weißen Bänder”︁ in Wälzlagern

About adiabatic shear bands and the generation of “high‐angle white bands” in roller bearings During the rolling motion of roller components structural changes are generated by over‐elastic distressing below the bearing face. Micro‐ and macro‐plastic distortions of the microstructure due to the three‐axial distressing accompany the process of rolling contact fatigue with the start from the first load cycle. They determine the life‐duration of the roller component. Further structural changes in ball bearings besides the plastic deformations are the so called “butterflies” and the so called low‐ and high‐angle “white bands”. The “white bands” you only can detect later, after an extended number of rolling actions. They are inclined to average circumferential angles at ≈ 30 ° respectively ≈ 80 °. Butterflies and white bands develop obviously due to a two‐axial material stressing. An influence on the life duration is not proved. The structure and the mechanism of their generation are under discussion. There exist phenomena of the microstructure which are similar to the white bands, the so called adiabatic shear bands. These generate after a local macro‐shear process of the microstructure, caused by a single, rapid shock‐shear stressing. Flash‐temperatures near the melting point generate in the shear zone of martensitic hardened materials the formations of new hardened zones; these microstructures cannot be etched too. The paper contributes to the question, if there exists a stress‐constellation which causes the adiabatic formation of butterflies and of white bands during the steady and long during process of plastic deformation. If one considers the latest knowledge about rolling contact fatigue and stressing by EHD (elasto‐hydro‐dynamic) flow, he can come to the conclusion, that at least the “high‐angle white bands” are adiabatic shear bands.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

10Ni5CrMoV钢奥氏体焊缝接头熔合区马氏体带的显微组织

本文研究了A507填充10Ni5CrMoV钢接头熔合区马氏体带的显微组织。"原位取样"TEM分析表明熔合区存在板条马氏体、孪晶马氏体、奥氏体、贝氏体和部分分解的板条马氏体混合组织;彩色金相显示熔合区存在马氏体和奥氏体混合组织;结合Schaeffler图分析结果,可以确定这种混合组织位于OM下熔合区后续熔化滞留层的马氏体带。     

Heat transfer in shape memory alloy thin films

The thermal response of periodic, shape memory alloy (SMA) island structures is addressed in this paper. Incorporated into the thermal modeling are several ingredients that are encountered at length scales typical to micron-sized thin films: (i) the microstructure may be heterogeneous, e.g., there may be a mix of an amorphous layer, a non-transforming crystalline layer and a crystalline layer that does undergo phase transformation; (ii) given the typical large surface-to-volume ratios and the attendant thermal losses to the environment, the strength of the heat source as well as its duration during a martensite to austenite transformation become very important. These issues, especially the latter, are investigated for a range of periodic structures with different spacings including the limiting structures of vanishing inter-island spacing on the one hand (infinitely extended thin film) and infinite inter-island spacing on the other (isolated islands). It is seen that a reduced inter-island spacing results in lower heat losses to the substrate, that a heterogeneous microstructure has a minimal effect on the thermal field, and that there appears to be a threshold in the strength of the heat source (represented by a current density, if the heat source is electrical in origin) above which the reduction in the heating time does not improve substantially. Interestingly, the evolution of martensite volume fraction with respect to time seems to be insensitive to the thickness of the film undergoing transformation to austenite.