单晶γ-TiAl合金微裂纹扩展行为的分子动力学模拟

为了从微观原子结构探索γ-Ti Al合金裂纹扩展的机理,研究了恒定加载速度下温度对γ-Ti Al合金中裂纹扩展的影响。采用分子动力学方法对单晶γ-Ti Al合金中预置微裂纹的扩展过程进行模拟,研究表明,室温下裂纹呈脆性解理扩展,中、高温时,裂纹在扩展过程中发射位错,裂尖钝化并伴有偏转;随温度的升高,微裂纹由脆性解理扩展向韧性扩展转化,裂纹扩展速率减慢,材料塑性增加;裂尖发射的位错堆积在边界附近,使得位错堆积处萌生空洞缺陷,随着加载的继续,空洞最终长大形成微裂纹,出现边界开裂的现象。     

Ni-36at%Al合金室温和高温韧性的改进

一、前言 NiAl金属间化合物具有比重轻,熔点高,抗氧化性好等优点,是潜在的高温结构材料,但室温脆性和高温强度低的问题阻碍了这一系列合金的发展和应用。NiAl的脆性归结于室温下仅开动<110>方向的滑移系,该系统仅存在三个独立的滑移系,无法满足多晶体范性形变的Von,Mises准则,即至少存在五个独立滑移系的要求。通过合金化改变滑移系以改善NiAl合金塑性的努力没有取得成功。通过控制成分和细化晶粒的途径也未明显地改善NiAl的室温塑性。NiAl高温强度低往往归结于具晶界结合强度低,通过合金化的途径有可能改善其高温强度。 富Ni的NiAl合金中存在马氏体相变,马氏体相转变点在273k附近的NiAl合金通过产生应力感生马氏体和马氏体再取向,在室温表现出显著的塑性。但是由于合金晶界强度低和马氏体转变过程引起的微裂纹,合金仍显得十分脆弱。     

Fe—3%Si 单晶体裂纹尖端位错行为及氢效应的 TEM 动态观察~*

在透射电镜中对 Fe-3%Si 单晶体薄膜进行拉伸试验,观察了在受力状态下裂纹尖端的位错行为及固溶氢的影响。结果表明,在受力时裂尖会自动发射位错。位错一旦从裂尖发射,便很快离开裂纹尖端以反塞积群形式存在于塑性区中。在裂纹尖端与塑性区之间存在一无位错区(DFZ)。裂纹扩展包括裂尖发射位错的塑性过程以及 DFZ 解理的脆性过程。固溶的氢改变了裂纹尖端的位错分布,促进了位错从裂尖发射,致使 DFZ 的长度减小甚至消失。     

温度对单晶γ-TiAl合金裂纹扩展的影响的分子动力学模拟

用分子动力学方法从原子尺度对单晶γ-TiAl合金中心裂纹的扩展机理进行了研究,模拟了不同温度下预制中心裂纹的扩展过程。结果表明:随着温度升高,裂纹的启裂时间变长,启裂应力值分别为5.64GPa、4.58GPa和4.27GPa;裂尖和边界发射的位错数目随温度的升高而增多;温度为300K时,裂纹先脆性扩展,出现分枝后,裂纹通过裂尖发射位错向前扩展,扩展过程为塑性扩展;温度达750K时裂纹出现分枝,扩展过程为塑性扩展,此时的裂纹扩展速率慢于300K时的裂纹扩展速率;950K时裂纹没有出现分枝,扩展过程为塑性扩展且扩展速率最快;三种温度下裂纹扩展过程均出现裂尖钝化与偏折现象。     

DD6单晶高温合金振动疲劳性能及断裂机理

研究[001]取向的DD6单晶高温合金的室温振动疲劳S-N曲线,并获得了其室温振动疲劳极限。利用体视显微镜、扫描电子显微镜、背散射衍射等手段对DD6单晶高温合金振动疲劳断裂机制进行分析。结果表明:采用S-N法估算得到的[001]取向的DD6单晶高温合金室温振动疲劳极限约为337.5MPa。振动疲劳裂纹断口呈现单个或多个沿{111}晶体学扩展平面组成的形貌特征,断口上分为疲劳源区和疲劳扩展区两个阶段,裂纹在应力最大截面处的表面或内部缺陷处萌生,呈单源特征,疲劳扩展区呈现类解理断裂特征,未出现典型的疲劳条带特征。说明沿{111}晶面滑移是DD6单晶高温合金室温振动疲劳断裂的主要变形机制,断口上的类解理扩展平面以及微观上类解理花样是DD6单晶高温合金室温振动疲劳断裂的主要特征。     

单晶γ-TiAl中孔洞位置对裂纹扩展影响的分子动力学模拟

运用分子动力学方法对单晶γ-TiAl合金的裂纹扩展过程进行了研究,分析了不同孔洞位置对裂纹扩展的影响,得到相应的原子轨迹、能量演化以及应力-应变关系。结果表明无孔洞时,裂纹以脆性解理方式快速扩展至材料断裂,能量曲线只有一个波峰;L=1.6nm时裂纹先以脆性解理的方式扩展,孔洞抑制裂纹扩展,孔洞周围发射位错,裂纹以尖端空洞形核、长大成微裂纹,最终微裂纹与主裂纹连接的方式扩展,能量曲线有多个峰值;L=4.8nm时,裂纹初始扩展过程与L=1.6nm时相似,后期未出现空洞形核、长大成微裂纹并与主裂纹结合的现象;另外孔洞距裂尖距离不同,发射第一个位错的方向不同。     

单晶γ-TiAl合金中裂纹沿[111]晶向扩展的分子动力学研究

为了从微观角度探索γ-TiAl合金中特定晶向的裂纹扩展机理,研究了γ-TiAl合金中[111]晶向微裂纹扩展的过程及其断裂机理。首先在单晶γ-TiAl合金中预置[111]晶向的微裂纹,然后通过分子动力学方法模拟该裂纹的扩展过程,最终分析了裂尖原子组态变化、微裂纹扩展路径以及应力-应变情况。研究表明,该晶向的微裂纹不是沿直线扩展,而是启裂时裂尖发生偏转,表现出明显的取向效应;微裂纹以裂尖发射滑移位错以及裂尖上形成孪晶的方式进行扩展;受边界的影响,微裂纹扩展到一定阶段会在边界位错堆积处萌生子裂纹,且扩展机制与主裂纹类似;在两个裂纹尖端发射滑移位错的相互作用下,在主裂尖前端再次萌生子裂纹,最终主、子裂纹相连导致断裂;微裂纹扩展过程中的应力分布主要集中于裂尖和扩展过程中形成的孪晶面上,并且随着微裂纹的扩展,裂尖应力值随时间的增大而减小。     

NiAl合金的超塑性行为及其变形机制

研究了等原子比NiAl合金的NiAl-25Cr,NiAl-20.4Fe-Y,Ce,NiAl-30Fe-Y合金的超塑性行为及其变形机制,结果表明,几种合金在一定条件下均表现出超塑性行为,单相NiAl超塑性变形源于变形过程中所发生的动态回夏及再结晶,两相及多相NiAl合金的超塑性变形机制则是晶粒的转动和界面的滑动。     

TiAl基合金层状组织中裂纹扩展区的塑性变形

本文通过观察ＴｉＡｌ合金层片组织四点弯曲试样侧面裂纹扩展与滑移迹线之间的关系，分析讨论了层状组织中塑性变形对裂纹扩展的影响。发现不论层片相对裂纹处于何种取向，沿层片方向都有一定程度的塑性变形，这种变形对于平行，倾斜，垂直于片层扩展的主裂纹都可引起裂尖钝化，从而引发构成各种韧化机制。     

TiAl基合金层状组织中裂纹扩展区的塑性变形

本文通过观察ＴｉＡｌ合金层片组织四点弯曲试样侧面裂纹扩展与滑移迹线之间的关系，分析讨论了层状组织中塑性变形对裂纹扩展的影响。发现不论层片相对于裂纹处于何种取向，沿层片方向都有一定程度的塑性变形，这种变形对于平行、倾斜、垂直于片层扩展的主裂纹都可引起裂尖钝化，从而引发构成各种韧化机制     

Effect of martensite transformation on fracture behavior of shape memory alloy NiTi in a notched specimen

In this paper, the finite element calculation of the stress–strain distribution in front of a notch tip were carried out for two materials. One is a shape memory alloy NiTi with the stress-induced martensite transformation, and another is a fully transformed martensite NiTi without the transformation. Based on the results obtained, and combining a model of the fracture process zone, effect of martensite transformation on the fracture behavior of the shape memory alloy NiTi in a notched specimen of plane stress state is comparably analyzed. The results show that the martensite transformation increases the load to produce plastic deformation in the transformed martensite at the notch tip and decreases the maximum normal stress and plastic strain near the notch tip, and tends to suspend the crack nucleation and propagation in the fully transformed martensite in front of the notch tip, and thus increases the fracture load and improves the toughness. A quantitative analysis based on the model of the fracture process zone shows that the martensite transformation in the SMA NiTi causes about 47% increase in the apparent fracture toughness.     

High temperature,high strain rate embrittlement of Al-Mg-Mn alloy: evidence of cleavage of an fcc alloy

Abstract

The fracture behaviour in tension of an Al-Mg -Mn alloy has been investigated. At high temperatures and strain rates, intergranular brittle fracture is observed along with cleavage fracture. Intergranular fracture is related to local melting at the grain boundaries. Cleavage occurs in equal proportions on the {100} and {110} crystallographic planes. The area fraction of cleavage facets on the fracture surface has been quantified. Their initiation is shown to be related both to the liquid metal embrittlement of the grain boundaries and to the presence of brittle Mn containing particles at the grain boundaries. Cleavage fracture in an aluminium alloy also requires an inhibition of plastic flow which prevents plastic blunting at the crack tip. It is proposed that this modification of the plastic behaviour is provided by the decrease in stacking fault energy at high temperatures in Al-Mg alloys.     

Structure, texture and mechanical properties of AlZnMgCuZr alloy rolled after heat treatments

The aluminium alloy containing 6.7 wt.% Zn, 2.6 wt.% Mg, 1.6 wt.% Cu and 0.1 wt.% Zr was continuously cast and either quenched from 465°C, or furnace cooled down to 100°C to find the best ductility for further cold plastic deformation. The alloys were then cold rolled down to the highest possible degree of deformation. The initial texture in both alloys can be described by (211)[111], (321)[346] and (110)[112] ideal orientations. With increasing deformation other orientations like {110}001 and cubic {100}001 appear after both types of treatments. TEM studies revealed increase of subgrain misorientation up to approx. 9° after 75% of deformation by rolling. On ageing at 120°C for 24 h the maximum hardness of 210 HV was reached. The alloys deformed prior to ageing at 120°C attained 230 HV. Very small GP zones, up to a few nanometers in size, grow after several days of ageing giving diffused diffraction effects. After ageing for 1 day at 120°C, precipitates grow and were identified as η′.     

Deformation structure in ductile B2-type Zr–Co–Ni alloys with martensitic transformation

Microstructural evolution during tensile deformation in ternary Zr–Co–Ni alloys were investigated using transmission electron microscopy to clarify the mechanism of the enhancement of ductility observed in these alloys. In Zr₅₀Co₃₉Ni₁₁ alloy deformed at room temperature, lenticular martensite is observed in the B2 parent phase immediately after yielding, in addition to dislocations with the <100>_B2-type Burgers vector. The orientation relationship between the B2 parent phase and B33 martensite is determined to be [001]_B2//[100]_B33, (010)_B2//(021)_B33, and (110)_B2//(010)_B33. A midrib-like contrast is observed at the center of the lenticular martensite variant, and it is found to be a (021)_B33 twin. A trace analysis indicates that this contrast is nearly parallel to the {100}_B2, which may correspond to the habit plane of the martensite. The martensite variants grow into the B2 parent phase along the {100}_B2 with increasing tensile loading, and then grid-shaped martensite variants are formed at the failure of the specimen. The martensite would be dominantly formed and grow in the regions where the stress concentration occurs during tensile deformation. It is likely that the plastic deformation mainly proceeds in the untransformed B2 parent phase because this martensite is harder than the B2 parent phase. Consequently, the authors conclude that the remarkable enhancement of ductility can be attributed to a transformation-induced plasticity associated with deformation-induced martensite.     

Fatigue Crack Propagation Behavior of TiNi50.6Shape Memory Alloy

The fatigue crack propagation behavior of TiNi50.6 shape memory alloy was studied. The experiment results showed that the crack propagation properties of this alloy display difference and similarity in comparison with common metallic materials. Because of the stress concentration there was stress induced martensite transformation (SIMT) near the crack tip though thenominal stress was lower than the threshold stress of SIMT. The position and the amount of SIMT was in situ observed by a quester remote measurement system (QRMS). The observationresults showed that the position of SIMT was beside the crack tip and was not in the plastic zone of common     

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

Effect of rotationally accelerated shot peening on the microstructure and mechanical behavior of a metastable β titanium alloy

Microstructural evolution and deformation mechanism of a metastable β alloy(Ti-10V-2Fe-3Al)pro-cessed by rotationally accelerated shot peening(RASP)were systematically investigated with optical microscopy,X-ray diffraction,electron backscatter diffraction and transmission electron microscopy.Different gradient hierarchical microstructures(gradients in α"martensite and β phase,and hierarchical twins range from the nanoscale to microscale)can be fabricated by RASP via changing the shot peening time.The hardening behavior and tensile mechanical properties of gradient hierarchical microstructure were systematically explored.Novel deformation twinning systems of{112}α:and{130}〈310〉α"in the kinked α"martensite were revealed during the tensile deformation.It was found that stress-induced martensitic transformation,twinned α"martensite and the related dynamic grain refinement contribute to hardness and work hardening ability.Simultaneous improvement of strength and ductility of the metastable β titanium alloy can be achieved by introducing a gradient hierarchical microstructure.     

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].     

Crack growth process in Ni-Single crystal with hydrogen cathodic charging

Notched tensile tests {orientation tensile axis [001] and ${[\bar{1}\bar{1}1]}$ , direction of notching: [010], [011], [112]} were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. It was investigated that the crack growth direction tends to be <011> on {001} with any tensile direction. Y-shaped hillocks and striation-like pattern were observed on the fracture surfaces of hydrogen embrittled specimens using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The striation-like patterns were not matched on both fracture surfaces of specimens, though the Y-shaped hillocks were exactly matched. Moreover, it was indicated that the striation-like pattern is perpendicularly formed to the crack growth direction on the fracture surface behind the crack tip and its shape results in an obtuse angle with respect to the fracture surface. The Y-shaped hillock was formed between micro-cracks located at crack tip by shear fracture. Furthermore, we proposed a crack growth model in Ni-single crystal at hydrogen atmosphere from the observation of the fracture surfaces.     

Fatigue damage in nickel and copper single crystals at nanoscale

Nanoscale fatigue damage simulations using molecular dynamics were performed in nickel and copper single crystals. Cyclic stress–strain curves and fatigue crack growth were investigated using a middle-tension (MT) specimen with the lateral sides allowing periodic boundary conditions to simulate a small region of material as a part of a larger component. The specimen dimensions were in the range of nanometers, and the fatigue loading was strain controlled under constant and variable amplitude. Four crystal orientations, [111], [100], [110] and [101] were analyzed, and the results indicated that the plastic deformation and fatigue crack growth rates vary widely from one orientation to another. Under increasing strain amplitude loading, nickel nanocrystals experienced a large amount of plastic deformation causing at least in one orientation, [101], out-of-plane crack deviation in a mixed mode I+ II growth. Under constant amplitude loading, the fatigue cracks were a planar mode I type. Double slip is observed for some orientations, while for others, many more slip systems were activated causing a more evenly distributed plastic region around the crack tip. A comparative analysis revealed that small cracks grow more rapidly in copper than in nickel single crystals.