不同伸长率多晶Be室温拉伸断口分析

通过扫描电镜系统分析了伸长率5%和零伸长率多晶Be室温拉伸断口形貌。发现多晶Be无论伸长率高低,拉伸断口均平整、无颈缩;断口宏观形貌分纤维区和放射区,微观形貌既有裂纹沿一定结晶学表面扩展形成的解理断裂特征,又有一定塑性变形产生的撕裂棱,属准解理断口。但是,伸长率5%的多晶Be断口纤维区和放射区界限不清晰,放射花样细小且走向多变,断口没有明显的主裂纹源,断裂是由多个裂纹源汇合所致。而零伸长率多晶Be断口纤维区和放射区界限清晰,放射花样粗大且走向单一,并且纤维区占整个断口比例极小,放射花样则几乎遍布断口通区,断口上可见明显的主裂纹源,主裂纹源中心往往存在某种组织缺陷,断裂主要是单一裂纹扩展所致。这表明多晶Be的伸长率主要来自于裂纹形核阶段,微观组织缺陷造成裂纹过早地达到临界裂纹扩展尺寸,是导致多晶Be材室温伸长率降低的主要原因。     

TC4-DT钛合金电子束焊接接头的疲劳断口研究

采用单边缺口试样,用扫描电镜观察了TC4-DT钛合金母材及电子束焊接接头焊缝区与热影响区的疲劳断口,分别分析了疲劳裂纹起裂源、扩展区及瞬断区的断口形貌特征.结果表明,疲劳裂纹均起裂于试样缺口根部,并由试样的一侧扩展到另一侧,裂纹扩展初期均沿一定的结晶面扩展,属于脆性穿晶断裂;母材和热影响区的瞬断部位均有明显的韧窝出现,属于塑性沿晶断裂,而焊缝区断口上韧窝不明显;热影响区疲劳断口有疲劳条带出现,但在母材和焊缝区断口上很难看见.     

热暴露对Ti60合金性能及断裂行为的影响

采用光学显微镜和扫描电镜观察分析了Ti60合金热暴露前后拉伸试样的显微组织、断裂方式及其断口形貌.结果表明:Ti60合金塑性对试样表面富氧层较为敏感,带有富氧层的试样塑性较低;热暴露前后试样断裂方式发生变化,未经热暴露的试样,断裂起源于试样中心部位,断面凹凸不平,为典型的韧窝型断裂;毛坯热暴露断口上呈现出大量的解理小平面,也可以观察到韧窝形貌,为混合型断口;试样热暴露后,裂纹起源于试样表面,在微观断口上除了断裂小平面外,还有大量的撕裂棱,表现为断裂沿着α片层界面扩展的特征;在高温长时间暴露过程中,氧除了污染试样表面,还会溶解在基体中形成脆性富氧层,这是影响Ti60合金热稳定性能的重要原因.     

时效温度对ZL210A铸造铝合金力学性能和微观组织的影响

研究了不同时效温度对ZL210A铝合金砂型试样力学性能、微观组织和断口形貌的影响.结果表明:当时效温度为170℃时,试样的Rm、Rp0.2和硬度HB值达最大值.随着时效温度升高,试样的伸长率迅速下降,其后趋于稳定;合金析出相尺寸增大,边界清晰;试样断口韧窝变大,呈现韧性断裂.     

氢对X70管线钢预裂纹试样断裂性能的影响

将X70管线钢单边预裂纹试件在0.5 mol/L H2SO4 溶液中充氢至饱和,然后在空气中拉伸至断裂,获得裂纹体的条件断裂韧性.当固溶氢含量<0.978×10-4mass%时,氢使裂纹体断裂韧性提高11.8%;固溶氢含量>0.978×10-4 mass%时.裂纹体断裂韧性随氢含量线性降低;断裂特征为典型的韧窝断口,但韧窝直径随氢含量的增加逐渐变小.在动态充氢条件下进行慢应变速率拉伸实验,氢使裂纹体断裂韧性显著降低,裂纹体断裂韧性随充氢电流密度I 的幂函数而线性降低.断裂特征为典型的准解理断口.     

Ti-600合金的热稳定性

研究了Ti-600合金镦制饼材600 ℃热暴露前后室温拉伸性能的变化,并观察合金的拉伸断口、分析其断裂机制。结果表明,600 ℃热暴露100 h后,毛坯热暴露试样的强度较热暴露前固溶时效试样(STA)的提高了3%左右,延伸率降低20%左右;热暴露试样的强度稍有降低,延伸率则降低了45%左右。STA试样室温拉伸断裂起源于试样中心位置,断口形貌呈现韧窝型断裂特征;毛坯热暴露试样断口上可见解理小平面与韧窝并存的特征,为混合型断口;试样热暴露以后,裂纹萌生于试样表面,断口上观察到解理小平面,合金的断裂沿片层α相界面扩展。热暴露试样表层有脆性富氧层存在,富氧层内易诱发细微裂纹并向基体扩展。表面渗氧是Ti-600合金热暴露后塑性下降的重要原因之一     

Q&P钢韧性断裂机制的实验研究

采用扫描电镜（SEM）原位拉伸观察了Q＆P钢退火组织断裂过程中裂纹萌生和扩展的动态变化过程．并对其微观断裂特征进行了研究。结果表明：加载初期,裂纹在与最大主应力平面成45&#176;的位置萌生;加载中期,新的裂纹在主裂纹前方某处萌生,裂纹连接时伴随塑性变形;加载末期,裂纹失稳扩张,试样断裂。断口表面伴有韧窝的出现,呈延性断裂特征。     

铸态Mn18Cr18N钢轧制热压缩实验分析

针对铸态Mn18Cr18N电渣重熔钢进行了单道次直接轧制,其压下率分别为10%,30%,40%和50%,并借助电子背散射仪(EBSD)观察了微观组织,结果表明,单道次轧制时,随着压下率的增大,组织逐渐细化。在晶界周围发生再结晶,变形孪晶增多,促进再结晶形核发生。组织细化主要由动态再结晶机制和孪生机制起主导作用。分析了轧后板的拉伸力学性能,随着压下率的增大,屈服强度由690.16 MPa增加至743.24 MPa,抗拉强度由885.65 MPa增加至940.31 MPa,断后伸长率由39.73%增加至53.46%,断口微观形貌特征逐渐由韧性和脆性的混合型断裂向韧窝状韧性断裂转变。断口边部的第二相含量较少,而试样中心微孔洞内含有大量方形的第二相,导致形成了微裂纹,断裂先从中心部开始。     

球铁QT900-5和QT900-2微观组织与断口形貌观察

采用扫描电镜分析了QT900-5(1#试样)和QT900-2(2#试样)的微观组织及断口形貌.结果表明,1#试样石墨球细小,且形状较为圆整;2#试样石墨球数量较少,比较粗大.对断口的扫描结果发现,1#试样的微观断裂特征为石墨球与基体形成圆整的韧窝;而2#试样石墨球表面粗糙,且极不规整,断口处出现部分团絮状的硅酸盐.对两组试样的湿化学分析结果表明,1#和2#试样残留Mg含量分别为0.056%和0.035%;而P含量分别为0.036%和0.051%.2#试样较低的Mg残留量和较高的P含量是其韧性较低的原因之一.     

6063铝合金韧性断裂机制的研究

设计可实现不同应力状态的原位拉伸试样,在SEM下进行原位拉伸试验,对断裂过程做了详细的研究和分析.试验表明,不同应力状态下的试样表面在拉伸过程中都产生了大量的滑移带,但其韧性断裂机制不同.随着三轴应力度的降低,断裂从韧窝剪切机制向纯剪切断裂机制过渡,试件断口也由韧窝断裂模式向剪切断裂模式演变;6063铝合金的晶界最薄弱,微裂纹形核于晶界,随载荷增大,微裂纹之间通过扩展或剪切连接导致试样断裂;试样最小截面上的三轴应力度越小,试样断口的两个面上韧窝的取向越明显,而且断口越光滑.     

火焰调修工艺对转向架用钢SMA490BW性能的影响

采用火焰调修工艺对高速动车组转向架用钢SMA490BW材料进行了变形调修,并对不同加热温度后进行喷水冷却的材料组织和性能进行研究。结果表明:经火焰调修后,屈服强度、抗拉强度和延伸率都满足SMA490BW规定的要求,呈延性断裂特征,未改变材料断裂的特征属性。随着火焰加热温度的升高,材料的屈服强度、抗拉强度变化不大,断后伸长率有所降低。当火焰加热温度为750℃～850℃时,处于Ac1与Ac3的双相区,冷却后的组织为细化的铁素体、珠光体和少量粒状贝氏体以及少量的原始块状铁素体。当温度达到900℃时,加热温度超过Ac3,由于奥氏体快速冷却形成的贝氏体含量增加,导致材料冲击韧性降低。退火工艺对金相组织结构没有影响,但可以消除内应力,降低硬度,从而改善塑性和韧性。     

低碳球墨铸铁断口的观察和分析

利用扫描电镜(SEM)对珠光体、铁素体和准铸态贝氏体金属基体的低碳球铁拉伸断口形貌进行了分析和研究.结果表明,珠光体低碳球铁的断口应属于脆性断裂的范畴;准铸态贝氏体低碳球铁断口中韧窝明显增多,解理花样大大减少,属于韧-脆性混合型断口;铁素体低碳球铁的断口则完全呈现出韧性断裂的特点.研究还表明,ADI在固态相变时,高温奥氏体快速冷却到贝氏体转变温度的过程中,不可能再形成发达的奥氏体枝晶,也不可能再形成枝晶间的缩松区域.因此,ADI可在获得较高强度的前提下,使材料的韧塑性得到大大提高.     

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined. The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.     

Effects of forced cooling on mechanical properties and fracture behavior of heavy section ductile iron

To develop materials suitable for spent-nuclear-fuel containers, the effect of forced cooling on mechanical properties and fracture toughness of heavy section ductile iron was investigated. Two cubic castings with different cooling processes were prepared: casting A was prepared in a totally sand mold, and casting B was prepared in a sand mold with two chilling blocks placed on the left and right sides of the mold. Three positions in each casting with different solidification cooling rates were chosen. In-situ SEM tensile experiment was used to observe the dynamic tensile process. Fracture analysis was conducted to study the influence of vermicular and slightly irregular spheroidal graphite on the fracture behavior of heavy section ductile iron. Results show that the tensile strength, elongation, impact toughness and fracture toughness at different positions of the two castings all decrease with decreasing cooling rate. With the increase of solidification time, the fracture mechanism of conventional casting A changes from ductile fracture to brittle fracture, and that of casting B with forced cooling changes from ductile fracture to a mixture of ductile-brittle fracture.     

Fracture behavior of monofilament-reinforced multicrystalline copper composites

Studies on monotonic tensile fracture and fatigue fracture have been performed on tungsten monofilament-reinforced multicrystalline copper composites and the failure modes are reported. Monotonic tensile fracture is not very sensitive to the initial fiber breaks and grain boundaries in tungsten monofilament-reinforced multicrystalline copper composites. Multiple breaks of the fiber and multiple necking of the matrix near the fiber segment breaks, commonly observed in tensile tests of composites, are reported here. However, the fatigue fracture is very sensitive to a fiber break, and to microstructural features such as grain boundaries and dislocation structures. Once the fiber breaks into two segments, the composite will quickly fail. The fatigue fracture mechanism of the composites depends on applied plastic strain amplitudes. Most cracks initiate at grain boundaries in the matrix at low and intermediate plastic strain amplitudes and the fiber greatly improves the fatigue behavior. At a high plastic strain amplitude, fatigue cracks initiate at the fiber, and the fiber seems ineffective for improving the fatigue life.     

高速列车转向架轴箱铸件的生产试验——-40℃超低温高韧性球墨铸铁件的生产技术

介绍了高速列车球铁转向架轴箱铸件的技术要求,其中包括要求该铸件在-40℃、甚至-50℃超低温下必须有足够高的冲击韧度;对生产该铸件的技术难点进行了分析,并详细描述了试块试验和铸件生产试验的方法、控制要点和结果.检测铸件本体:基体组织100％为铁素体,球化率1～2级,球墨大小6级;抗拉强度超过400 MPa最高达420 MPa伸长率超过18％最高达26％;在-40 ℃的条件下冲击韧度最高可达16J.     

Effect of Cu content on microstructures and mechanical properties of ADI treated by twostep austempering process

The effect of Cu content on the microstructures and mechanical properties(yield strength,ultimate tensile strength,impact energy,fracture toughness) of austempering ductile iron(ADI) treated by two-step austempering process were investigated. High Cu content in nodular cast irons leads to a significant volume fraction of retained austenite in the iron after austempering treatment,but the carbon content of austenite decreases with the increasing of Cu content. Moreover,austenitic stability reaches its maximum when the Cu content is 1.4% and then drops rapidly with further increase of Cu. The ultimate tensile strength and yield strength of the ADI firstly increases and then decreases with increasing the Cu content. The elongation keeps constant at 6.5% as the Cu content increases from 0.2% to 1.4%,and then increases rapidly to 10.0% with further increase Cu content to 2.0%. Impact toughness is enhanced with Cu increasing at first,and reaches a maximum 122.9 J at 1.4% Cu,then decreases with the further increase of Cu. The fracture toughness of ADI shows a constant increase with the increase of Cu content. The influencing mechanism of Cu on austempered ductile iron(ADI) can be classified into two aspects. On the one hand,Cu dissolves into the matrix and functions as solid solution strengthening. On the other hand,Cu reduces solubility of C in austenite and contributes more stable retained austenite.     

Fracture Loci in Sheet Metal Forming: A Review

Fracture in sheet metal forming usually occurs as ductile fracture, rarely as brittle fracture, at the operating temperatures and rates of loading that are typical of real processes in two different modes:(1) tensile and(2) in-plane shear(respectively, the same as modes I and II of fracture mechanics). The circumstances under which each mode will occur are identified in terms of plastic flow and ductile damage by means of an analytical approach to characterize fracture loci under plane stress conditions that takes anisotropy into consideration. Fracture loci was characterized by means of the fracture forming limit line and by the shear fracture forming limit line in the fracture forming limit diagram. Experiments were performed with single point incremental forming and double-notched test specimens loaded in tension, torsion and in-plane shear give support to the presentation and allow determining the fracture loci of AA1050-H111 aluminium sheets with1 mm thickness. The relation between fracture toughness and the fracture forming limits was also investigated by comparing experimental values of the strains at fracture obtained from a truncated conical part produced by single point incremental forming and from double-notched test specimens loaded in tension.     

强韧化热处理对TA15钛合金组织和性能的影响

采用光学显微镜,扫描电镜和电子拉伸机等研究了TA15合金经两阶段强韧化退火热处理后的显微组织和性能。结果表明:采取两阶段的热处理工艺后,TA15合金的组织由约20%的初生等轴α,55%的片状α和β转变基体的组织组成;合金具有良好的塑性及较好的室温和高温强度,在975℃×1 h,WQ+850℃×2 h,AC的制度下,TA15合金的室温抗拉强度为1005 MPa,屈服强度为914 MPa,伸长率、冲击韧性分别为13%和72.2 J/cm^2。合金的冲击韧性I与次生片层α厚度t具有较好的线性关系I=26.504t+44.915,冲击断口形貌可以观察到大量的韧窝,表明合金的断裂机制以韧性断裂为主。随着第二重退火温度的升高,次生片层α厚度增加,韧窝逐渐变大,韧性增加。     

DEFORMATION AND FRACTURE BEHAVIOUR OF LOW CARBON MARTENSITE-FERRITE DUAL-PHASE STEEL

利用扫描电镜及其动态拉伸装置,对具有板条马氏体和铁素体双相钢组织的断裂过程进行了研究.结果表明,在拉伸应力作用下,裂纹在马氏体与铁素体界面及不同马氏体取向界面处成核,或者由于马氏体板条的断裂而产生.断裂系主裂纹钝化和微裂纹长大,最后沿强烈剪切形变带迅速连接所致.其微观形态为塑坑断口,属延性断裂机制.