7020铝合金型材的疲劳行为及微观机制

采用光学显微镜、扫描电镜、扫描透射电镜和电子背散射衍射等分析方法研究了 7020铝合金型材疲劳行为及微观机制.结果表明在应力比R为0、疲劳极限寿命取107周次时,合金疲劳强度为232.9 MPa.疲劳裂纹尖端应力强度因子△K=8 MPa·m1/2时,裂纹扩展速率约为6.44×10-5 mm/cycle.合金中尺寸在3～...     

Ti5Al2.5Sn ELI钛合金铸造组织与力学性能研究

Ti5Al2.5Sn ELI钛合金是航空航天等领域的重要结构材料。本文采用TEM、XRD等分析方法对该合金的铸造组织和力学性能进行了研究。结果表明,Ti5Al2.5Sn ELI钛合金铸态显微组织由α相组成,并呈片状分布,经HIP处理后显微组织未有明显变化,缺陷弥合部位有再结晶现象;该合金在常温下具有中等强度和良好塑、韧性,疲劳裂纹扩展速率较低,断裂韧性采用25 mm试样无法获得准确K_Ic值,只能得到条件值K_q,ELI级合金的抗断裂和抗裂纹扩展能力高于非ELI级合金。     

Study of fatigue crack growth rate for austenitic Fe-Al-Mn alloys

A study was made of the crack growth rate (da/dN) versus stress-intensity variation ΔK behavior of Fe-Al-Mn alloys with different percentages of carbon, aluminum, and manganese at ambient temperature. The experimental results are described with respect to a Paris equation,da/dN = C(†K)ⁿ, where the exponent n, index for crack growth resistance of materials, was strongly influenced by alloy composition. It was found that higher manganese content provided better crack growth resistance, and that carbon and aluminum had an opposite effect. Scanning electron microscopy, x-ray diffraction, and mechanical properties evaluation were performed and correlated to the change of n values.     

钛合金TC21车削过程的三维数值建模与试验研究

针对一种新型高强度钛合金材料TC21钛合金难加工的特点,建立了车削过程的三维有限元模型。采用建立的三维车削有限元模型对钛合金材料TC21的切屑成形过程进行了数值模拟,并获得了切削过程的切削力变化曲线及应力值等物理量。同时,通过钛合金TC21的车削试验研究了刀具前角、进给速度及主轴转速对切削过程的影响规律。在相同切削条件下对试验结果与数值模拟进行了对比,结果验证了数值模拟的可靠性。实验结果表明,钛合金材料TC21的切屑也是锯齿状,且随着刀具前角的减小,TC21材料切屑锯齿状形态越明显。     

Fracture and R-curve behavior of an intermetallic β-stabilized TiAl alloy with different nearly lamellar microstructures

Four different microstructures of an engineering multi-phase Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%) were fracture mechanically tested under monotonic loading conditions from room temperature up to 700 °C. Monotonic loading crack resistance curves were measured using the potential drop technique and the calculated crack lengths were compared to optically measured crack lengths during in-situ experiments under the light-optical microscope. By examining the fracture surfaces and the polished side faces of the samples in the scanning electron microscope the fracture mechanisms were evaluated. It was found that the fracture toughness mainly depends on structure and phase arrangement at the boundaries and can be enhanced up to 30% by having thick boundary areas with a few micrometers in width, consisting of unconnected β_O and γ grains. In contrast, a connected β_O-phase at the colony boundaries as well as a texture which provide lamellae parallel to the subsequent fracture surface reduce the fracture toughness.     

An estimation of stress intensity factor in a clamped SE（T） specimen through numerical simulation and experimental verification： case of FCGR of AISI Hll tool steel

A finite element analysis of stress intensity factors(K I) in clamped SE(T)C specimens(dog bone profile) is presented.A J-integral approach is used to calculate the values of stress intensity factors valid for 0.125≤a/W ≤0.625.A detailed comparison is made with the work of other researchers on rectangular specimens.Different boundary conditions are explored to best describe the real conditions in the laboratory.A sensitivity study is also presented to explore the effects of variation in specimen position in the grips of the testing machine.Finally the numerically calculated SIF is used to determine an FCGR curve for AISI H11 tool steel on SE(T)C specimens and compared with C(T) specimen of the same material.     

Three-dimensional characterization of fatigue cracks in Ti-6246 using X-ray tomography and electron backscatter diffraction

In the present study, fatigue crack propagation was imaged non-destructively in three dimensions during in situ fatigue loading of Ti-6246 using X-ray microtomography on beam line ID19 at the European Synchrotron Radiation Facility. Phase contrast enabled the visualization of the two-phase microstructure and in combination with absorption contrast the crack chronology was recorded in situ during the fatigue experiment. In order to obtain the crystallographic orientation of individual grains along the crack path, a 3D electron backscatter diffraction volume was subsequently recorded. By combining both techniques it was possible to identify the role of prior β grain boundaries and the crystallographic orientation of the α lamellae on the crack path. It is shown that a near-surface crack that cuts across a prior β grain boundary is often diverted and crack bifurcation takes place. This results in local retardation of crack propagation and a first-order undulation of the crack front. In addition, the lamellar grain orientation and morphology causes a second-order crack front undulation with the tendency of large misorientations between α lamellae preferably orientated for prismatic a slip or pyramidal c + a slip deflecting the crack path.     

Effect of welding parameters on morphology and mechanical properties of Ti-6Al-4V laser beam welded butt joints

The influence of welding speed and laser power on weld quality of 1.6 mm thick Ti − 6Al − 4V sheets autogenously laser beam welded in butt configuration using a Nd-YAG laser was studied. The joint quality was characterized in terms of weld morphology, microstructure and mechanical properties. An underfill defect, controlling the whole weld geometry, was observed both at the weld face and root surface. In dependence of the specific heat input, this defect showed a maximum, which separates two different welding regimes: keyhole welding, at low heat input, and a welding regime where heat conduction around the keyhole is predominant, at high heat input. Influence of the underfill radius on the weld fatigue life was also assessed.     

Fatigue mechanics of WC–Co cemented carbides

The fracture and fatigue behavior of a fine-grained WC–10 wt% Co hardmetal is investigated. Mechanical characterization included flexural strength and fracture toughness as well as fatigue limit and fatigue crack growth (FCG) behavior under monotonic and cyclic loads, respectively. Considering that fatigue lifetime of cemented carbides is given by subcritical crack growth of preexisting defects, a linear elastic fracture mechanics (LEFM) approach is attempted to assess fatigue life–FCG relationships for these materials. Following the experimental finding of an extremely high dependence of FCG rates on the applied stress intensity for the hardmetal studied, the LEFM analysis is concentrated, from a practical design viewpoint, on addressing the fatigue limit–FCG threshold correlation under infinite fatigue life conditions. Thus, fatigue limit associated with natural flaws is estimated from FCG threshold experimentally determined for large cracks under the assumptions that (1) similitude on the FCG behavior of small and large cracks applies for cemented carbides, and (2) critical flaws are the same, in terms of nature, geometry and size, under monotonic and cyclic loading. The reliability of this fatigue mechanics approach is sustained through the excellent agreement observed between estimated and experimentally determined values for the fatigue limit under the different load ratios investigated.     

Phase assemblage effects on the fracture and fatigue characteristics of magnesia-partially stabilized zirconia

A detailed investigation on the relationships between phase assemblage and fracture and fatigue characteristics of Mg-PSZ has been conducted. In doing so, three completely different microstructural conditions were first attained through different thermal treatments and then their flexural strength, fracture toughness and crack growth resistance and fatigue crack growth (FCG) behaviour were evaluated. The obtained results are discussed considering the interplay between microstructural features and dominant crack-microstructure interaction and its influence on the operation of given toughening and mechanical fatigue mechanisms for each phase assemblage studied. FCG resistance, under both sustained and cyclic loading, is found to be closely related to the corresponding fracture toughness of each phase assemblage. However, real mechanical fatigue effects are estimated to be, once they are rationalized with respect to particular environmental-assisted cracking behaviours, an exclusive function of crack path type. Finally, different cyclic fatigue mechanisms for Mg-PSZ are pinpointed depending upon the prevalent transgranular or intergranular FCG morphology.     

Microstructure and fatigue properties of linear friction welded TC4 titanium alloy joints

The welded joints are often subjected to cyclic stresses and high-strain-rate deformation resulting in worrisome fatigue failure. Therefore, it is necessary to evaluate the fatigue resistance and cyclic deformation characteristics of these weldments. Ti–6Al–4V–0.3Fe–0.1C–0.05N–0.015H–0.2O (TC4) titanium alloy joints were produced via linear friction welding and a sound joint was obtained. It was observed that fine subgrains formed in the weld zone where the hardness became higher. The strain ratio had a strong effect on the cyclic deformation characteristics of the joint, with hysteresis loops being different at different strain ratios. However, the difference of fatigue life of the joint was small with varying strain ratios. The stress amplitude of linear friction welded TC4 joint showed essentially cyclic softening until failure at all strain ratios. Fatigue cracks initiated from the near-surface of base metal and propagated by the formation of fatigue striations together with secondary cracks.     

TiAl-B合金中TiB_2微观形态的主要存在方式

用XRD、SEM和TEM对原位自生法制备的Ti5 0Al x( % )B合金的金相组成和微观组织进行研究。结果表明 :合金中TiB2主要以片状、板片状、细棒状和块状形式存在 ;TiB2 微观形貌随着合金中B含量的变化而发生显著变化。当合金中B含量由w0 .4%增加到w1.4%时 ,TiB2 的微观形貌由片状依次演变为板片状、细棒状直至块状 ,而基体形貌没有发生明显的变化。根据TiB2 晶体形核与生长条件对TiB形貌演变过程进行了分析。     

Physical Essence of Long Fatigue Crack Growth Behavior for Aluminum－lithium Alloy

ＰｈｙｓｉｃａｌＥｓｓｅｎｃｅｏｆＬｏｎｇＦａｔｉｇｕｅＣｒａｃｋＧｒｏｗｔｈＢｅｈａｖｉｏｒｆｏｒＡｌｕｍｉｎｕｍ－ｌｉｔｈｉｕｍＡｌｌｏｙＣｈｅｎＺｈｅｎｇａｎｄＮｉＹｏｎｇｆｅｎｇ（陈铮）（倪永峰）（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳ...     

Mode of Fatigue Crack Initiation in Ti-55 Alloy

ＭｏｄｅｏｆＦａｔｉｇｕｅＣｒａｃｋＩｎｉｔｉａｔｉｏｎｉｎＴｉ５５ＡｌｏｙＬｅｉＪｉａｆｅｎｇ，ＬｉＤｏｎｇ，ＷａｎｇＺｈｏｎｇｇｕａｎｇａｎｄＨｕＺｈｕａｎｇｑｉ（雷家峰）（李东）（王中光）（胡壮麒）ＫｅｙＳｔａｔｅＬａｂｏｒａｔｏｒｙｏｆＦａ...