铍中Be2C及硅基夹杂对铍材伸长率的影响

铍材伸长率是目前限制铍材使用的关键因素.对高伸长率和低伸长率铍材的拉伸断口进行对比分析,发现在铍材的断裂源处往往含有非金属夹杂,夹杂造成的微裂纹是铍材解理断裂的裂纹核心.通常情况下,这些夹杂是以硅为基的杂质,一般不会造成铍材伸长率的明显降低.而当杂质中存在碳元素时,形成脆、硬稳定的Be2C,因Be2C难以变形,其断裂强度较铍材更低,因此在Be2C处容易位错集束而加重应力集中,并沿Be2C内部生成主裂纹,该主裂纹与原有的微裂纹合并后,裂纹长度加长,导致铍材解理速度加快,伸长率显著降低.     

使用横梁位移测定铍材的断后伸长率

GB/T 228.1-2010规定了使用直接法和移位法测定金属材料的断后伸长率。但铍材拉伸试样的横截面积和断后伸长率较小,若使用直接法测定,其断后伸长率随断口所在位置及试样断裂部分拼接状态的变化而变化,不具备唯一性;若使用移位法测定,虽弥补了直接法的不足,但操作较为麻烦。笔者介绍了利用横梁位移计算铍材拉伸试样断后伸长率的方法,可弥补上述两种方法的不足。通过试验及数据统计分析,证明此种方法可以准确快速地测定铍材试样的断后伸长率。     

含铍聚碳硅烷的合成机理研究

含铍SiC陶瓷纤维是一种新型的高性能SiC纤维,而优质含铍聚碳硅烷先驱体(PBCS)的合成是得到该种纤维的重要保证。以乙酰丙酮铍(Be(acac)_2)和聚碳硅烷(PCS)为原料制备得到PBCS,并对其合成机理进行了深入研究,采用PCS和石蜡分别与Be(acac)2进行对比反应,用GC-MS对反应小分子产物进行分析,确定了Be(acac)_2与PCS的成键反应实际是Be与Si—H键反应,C—H键没有参与含铍键的形成反应;根据小分子反应产物的生成途径,合理分析了PBCS的合成反应原理。     

粉末冶金多晶态铍室温断裂行为的研究进展

介绍了粉末冶金法生产的多晶态铍的性能、生产工艺。综述了多晶态铍室温断裂行为的研究状况,包括铍的室温断裂特征,以及诸如铍粉粒度和形态、粉末压制过程、表面残余应力,机械加工损伤、多种杂质(BeO,Fe,Al,Cr,Ag,Si,C等)对铍室温断裂性能的影响。预测了未来关于铍在工程领域可能开展的基础性研究。     

镍基粉末高温合金中夹杂物导致裂纹萌生和扩展行为的研究

采用扫描电镜原位拉伸和原位疲劳的方法,跟踪观察了人工植入Al2O3夹杂物的镍基粉末高温合金P/M Rene95中夹杂物导致裂纹萌生、扩展乃至断裂的过程,结果表明,无论是在单轴拉伸还是低周疲劳实验中,裂纹均首先萌生于脆性非金属夹杂物Al2O3处,大于一定尺寸的夹杂物,还会使该裂纹扩展成为导致合金断裂的主裂纹,从而大大降低合金的屈服强度、断裂强度及低周疲劳寿命.     

金属铍材室温平面应变断裂韧性测试方法

介绍了金属铍材的室温断裂韧性测试方法,并对不同方法进行了讨论,认为采用载荷比（Pmin/Pmax)为－3的拉－压疲劳循环加载方式,可以制备出脆性金属铍材KIC测试所需要的疲劳裂纹,所测试的铍材室温断裂韧性KIC均值为9．91MPa.m^1/2,标准偏差为0．73MPa.m^1/2,符合国家标准要求。     

夹杂物和晶粒尺寸对洁净车轮钢室温冲击韧度的影响

对洁净车轮钢在不同温度下进行正火处理,得到不同晶粒尺寸的显微组织,然后对车轮钢进行室温冲击试验,利用扫描电镜对冲击试样断口形貌进行观察,研究了夹杂物和晶粒尺寸对洁净车轮钢室温冲击韧度的影响。结果表明：部分车轮钢冲击试样以夹杂物起裂,夹杂物类型为Ti（C,N）,另一部分冲击试样断口起裂源处则未发现有夹杂物,但尺寸在10μm以下的Ti（C,N）夹杂物对车轮钢的冲击韧度没有明显的影响,而晶粒尺寸对车轮钢的冲击韧度有明显影响;其主要原因是室温下车轮钢冲击断裂的临界事件是微裂纹穿过晶界扩展引发解理断裂,因此晶粒尺寸是决定洁净车轮钢冲击韧度的主要因素。     

P92钢等离子弧焊接接头原位拉伸的SEM观察研究

针对6 mm厚P92钢等离子弧焊接接头,利用带动态拉伸台的高分辨扫描电镜对接头的不同区域进行原位拉伸实验,通过试样动态断裂过程观察并结合断口分析研究了P92钢等离子弧焊接接头各区的微观断裂机理.结果表明:接头焊缝区在拉伸过程中经历了前期的钝化伸张区之后,裂纹起裂于中间相M23 C6处,而焊缝区的裂纹扩展路径沿着中间相M23 C6前进,并在断面上形成了撕裂脊,属于穿层断裂;热影响区是P92焊接接头的最薄弱环节,裂纹起裂于中间相M23 C6处,其断口起裂的孕育阶段基本没有前期的钝化,即前期的塑性裂纹的扩展长度Xf=0,裂纹以沿层断裂模式进行扩展,在断口处有分层现象,最终断面呈现出裂纹以解理断裂模式进行扩展;母材区的拉伸试样在经历了钝化伸张区之后,前期的裂纹以塑性断裂模式进行扩展,裂纹在扩展过程中遇到较多的中间相M23 C6时形成应力集中,同时在其附近出现空洞,此时裂纹以解理断裂模式进行扩展,属于沿层断裂.     

铍钛系复合材料

铍的价格、脆性以及毒性限制了其广泛的应用。每磅真空熔炼的铍锭价格为70美元,而每磅经加工的铍金属的价格竟达300美元。铍的低塑性问题尚没得到解决,但是预料Ti—6Al—4V合金基的铍纤维复合材料将可以作为一种塑性好、经济的材料。Be—Ti系复合材料可用共喷法或粉末冶金法制得。这种试制的材料的价格为25—30美元/磅。     

发动机用灰铸铁的低周疲劳行为

研究了发动机缸体用灰铸铁材料在室温、150℃和250℃下的低周疲劳行为。根据对拉伸应力-应变、循环应力-应变和应变-疲劳寿命数据的分析,给出了疲劳参数。结果表明:在高温下灰铸铁的弹性模量、强度降低,延伸率增大;循环应力响应表明,在较小的应变幅下经历初期循环硬化、循环软化、断裂,而在高温和较大应变幅下几乎没有硬化阶段,循环软化至断裂;其室温疲劳寿命最长,150℃最短,250℃居中。微观分析结果表明:疲劳裂纹萌生于片状石墨尖端、夹杂物及孔洞处,沿石墨扩展,夹杂物导致分支裂纹及裂纹偏转,延缓裂纹的扩展;灰铸铁的疲劳断裂方式为沿晶和准解理断裂的复合机制,存在扇形解理面和二次裂纹,解理台阶上观察到疲劳条带和韧窝。     

连杆精锻模具开裂分析

借助于光学显微镜、扫描电镜,对开裂的连杆精锻模具进行了断口分析、钢中非金属夹杂物检验、硬度测试、横截面低倍检验、裂纹源处以及距裂纹源处横截面上1/4,1/2(心部)处显微组织的检验和分析。结果表明:该模具材料中存在严重的微观组织偏析现象及明显的加工刀痕造成的应力集中,是使该模具发生早期脆性断裂的主要原因,并提出了改进措施。     

Quantitative acoustic emission source characterization of microcrackings in steel

Acoustic emission (AE) source wave analysis is a new NDE technique for the investigation of dynamic fracture process. We applied this technique to the quantitative characterization of crack sources in ductile fracture. Using two samples of ASTM A533B steel with different sulfur content, acoustic emissions during fracture toughness tests were detected, located, and analyzed. The detected AE signals were classified into two types according to the analyzed source waveforms. One was a signal due to microcracking at the MnS inclusion, and the other was a signal due to coalescence of the voids. The results of the source wave analysis showed that microcracking at the inclusions was due to Mode I type tension crack with sizes of 10–30 µm, and the coalescence of the voids was due to tension shear mixed cracks with sizes of 60–100 µm. It was confirmed that this technique is very effective for the quantitative evaluation of microcrackings and for the detection of the nucleation and growth of cracks.     

基体裂纹-异型夹杂相互作用焦散线实验研究

该文通过透射式静态焦散线方法利用三点弯曲梁断裂实验对异型夹杂与基体裂纹的相互作用进行研究。首先得到不同夹杂情况下I型裂纹尖端的焦散斑图,引入焦散斑纵横轴长之比β反映焦散斑在夹杂作用下的畸变特性;其次,提取相应的焦散斑特征尺寸,并得到I型裂纹的应力强度因子K_I;最后,基于不同夹杂情况下裂尖焦散斑、裂尖应力强度因子与裂尖和夹杂之间距离的关系,揭示不同夹杂对裂纹尖端应力场奇异性影响规律。实验研究结果为含异型夹杂结构的强度设计和断裂性能评估提供实验依据。     

2219铝合金TIG和FSW接头力学及疲劳性能

目的 研究钨极氩弧焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金(母材)力学及疲劳性能的影响。方法 通过拉伸试验,得到了母材、TIG和FSW接头的抗拉强度和伸长率;通过疲劳性能试验测试了母材、TIG和FSW接头在不同应力下相应的疲劳寿命,根据疲劳试验结果绘制了其试样的S-N曲线;使用扫描电子显微镜观察并分析了疲劳断口的形貌特征。结果 未焊接的铝合金母材抗拉强度和伸长率最高,分别为506 MPa和15.92%;TIG接头抗拉强度和伸长率分别为330 MPa和7.65%,FSW接头抗拉强度和伸长率分别为310 MPa和8.74%。母材、TIG和FSW接头等3种疲劳试样在2×10⁶次循环下的疲劳强度分别为129、108、115 MPa,其疲劳断口均可分为裂纹源区、裂纹扩展区和瞬间断裂区,疲劳裂纹分别起始于试样表面的局部变形区、第二相夹杂物和“吻接”缺陷。疲劳裂纹扩展区的主要形貌为疲劳辉纹和二次裂纹,瞬间断裂区以脆性断裂为主。结论 TIG和FSW等2种焊接工艺均导致了2219铝合金的强度、塑韧性和疲劳性能降低,其接头表面的第二相夹杂物和“吻接”缺陷促进了疲劳裂纹的萌生。     

Detrimental effect of nitride and aluminium oxide inclusions on fatigue life in rotating bending of bearing steels

Abstract

Rotating bending fatigue tests were performed on hardened AISI type 52100 bearing steel. Fracture surfaces after testing at a stress amplitude of 950 MPa showed that the Ti(C,N) inclusions which caused fatigue failure were significantly smaller than the corresponding alumina inclusions. The smallest crack initiating Ti(C,N) inclusion had a size of 3 μm and the smallest alumina inclusion was 17 μm. It was also shown that fatigue life was significantly shorter for a steel which showed cracked alumina inclusions on the fracture surfaces than for a steel which had non-cracked inclusions. Finite element calculations were performed to determine the driving forces of short cracks at Ti(C,N) and alumina inclusions. Two configurations were studied in each case, based on both non-cracked and cracked inclusions. The calculations incorporated heat treatment simulation and cyclic loading with successive growth of cracks. It was found that the Ti(C,N) configurations gave the highest driving forces for crack growth. The alumina configuration with a non-cracked inclusion gave the lowest driving force. It was concluded based both on experimental evidence and theoretical considerations that Ti(C,N) inclusions are more detrimental to fatigue life than alumina inclusions of the same size. It is their shape and thermal properties which make Ti(C,N) inclusions more detrimental than alumina inclusions. Internal cracking of alumina inclusions leads to reduced fatigue life.     

Der Mischbruch im Zugversuch

Mixed fracture in the tension test If a tensile test specimen does not break before by cleavage, voids are nucleated at second-phase particles and inclusions during plastic deformation. At the center of the necked region these voids coalesce by internal necking or shearing of the material between them forming a fibrous crack which expands radially. In a temperature range which is dependent on the material cleavage fracture is initiated by the fibrous crack, resulting in a mixed fracture. If no cleavage fracture is initiated a completely fibrous fracture is formed. Mixed fracture surfaces consist of a cleavage fracture zone surrounding the central fibrous fracture zone and the tensile specimen behaves like a fracture mechanics specimen. Fracture toughness can be calculated by equations for tensile specimens with a central penny shaped crack. A comparison of fracture toughness values obtained by the use of unnotched tensile specimens and of fracture mechanics specimens show good agreement inbetween the temperature range of valid K_lc values according to standards of linear-elastic fracture mechanics.     

分动箱齿轮断裂原因

某分动箱的20CrMnTi钢齿轮在工作过程中发生断裂.采用宏观分析、微观分析、化学成分分析、硬度测试、硬化层深度测量、非金属夹杂物分析、金相检验等方法对齿轮的断裂原因进行了分析.结果表明:齿面上残留的加工刀痕导致应力集中,在周期载荷的作用下,疲劳裂纹源首先在残留的加工刀痕较深处形成,随后裂纹逐渐扩展,最终齿轮发生疲劳断...     

Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing

Abstract

By observation of the fracture surfaces and of appropriate metallographic sections of C–Mn steel multipass MMA (manual metal arc) weld metals and simulated weld specimens which were fractured at ?60°C in Charpy V tests, it was found that the impact toughness of the specimen could be correlated with the length of the fibrous crack which was limited by unstable propagation of the cleavage crack; the latter could be initiated at a type of second phase particle transformed from carbon rich regions or non-metallic inclusions. The weakest zone in which the cleavage crack initiated was characterised by coarse grains of ferrite and the critical event which gave rise to unstable propagation of a cleavage crack was a crack in the ferrite grain larger than 30 μm cutting through the boundary and extending over the specimen. On the basis of these results, a model of the fracture mechanism is proposed and the effect of Mn content on increasing toughness is explained using the model.

MST/647     

A model of brittle fracture propagation part I—continuum aspects

The micromechanism of crack propagation in steel is described and analyzed in continuum terms and related to the macroscopic fracture behavior. It is proposed that propagation of cleavage microcracks through favorably oriented grains ahead of the main crack tip is the principal weakening mode in brittle fracture. This easy cleavage process proceeds in the Griffith manner and follows a continuous, multiply connected, nearly planar path with a very irregular front which spreads both forward and laterally and leaves behind disconnected links which span the prospective fracture surface. A discrete crack zone which extends over many grains thus exists at the tip of a running brittle crack. Final separation of the links is preceeded by plastic straining within the crack zone and occurs gradually with the increasing crack opening displacement. It is suggested that in low stress fracture, straining of the links is the only deformation mode. However, it is recognized that under certain conditions plastic enclaves may adjoin the crack zone. This deformation mode is associated with high stress fracture, energy transition and eventually with crack arrest.

Energy dissipation resulting from the two deformation mechanisms is related to crack velocity, applied load and temperature and the crack velocity in a given material is expressed as a function of the external conditions. Fracture initiation and crack arrest are then discussed in terms of the conditions which are necessary to maintain the propagation process. Finally, the dimensions of a small scale crack tip zone for a steady state, plane strain crack are evaluated as functions of material properties and the elastic stress intensity factor.

The microstructural aspects of brittle fracture will be discussed in a separate Part 2 [1].     

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.