Effects of γ/γ interfacial structures on continuous coarsening of lamellar microstructure in TiAl alloy during aging

The effects of γ/γ interfacial structures on continuous coarsening of the fully lamellar microstructure in Ti-48Al alloy aged at 1 150 ℃ were investigated by using transmission electron microscopy(TEM). Continuous lamellar coarsening can be achieved not only by migration of interface faults(such as ledges, edges and curved interfaces) but also by migration and decomposition of perfect γ/γ lamellar interfaces. Thermal grooves, initiative positions of interfacial dissociation, can frequently form at the triple point junctions between the 120°-rotational ordered γ domain boundaries within γ lamellae and the lamellar interfaces. During the early stage of aging at 1 150 ℃, the interface migration and dissociation took place preferentially at the 120°-rotational ordered lamellar interfaces. Comparing the relative thermal stability of the true-twin, pseudo-twin and 120°-rotational ordered γ/γ lamellar interfaces shows that the 120°-rotational ordered lamellar interface is the most unstable. The reason of this phenomenon was analyzed through the comparisons of the interfacial energies and atomic arrangements of the three types of γ/γ lamellar interfaces.     

Influence of columnar grain boundary on fracture behavior of as-cast fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar γ-TiAl alloys depends on the angle between the lamellar orientation and loading axis,but the role of the presentation of grain boundary cannot be ignored.To investigate the influence of the grain boundary on the initiation and propagation of cracks,the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation,respectively.The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior.Results show that the grain boundary has dual influences on the fracture behavior.When the loading axis is parallel to the lamellar orientation,cracks are preferentially initiated at and propagate along the grain boundaries.When the loading axis is perpendicular to the lamellar orientation,the grain boundaries can prevent the propagation of cracks from running across.Additionally,serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries.     

CREEP OF POLYCRYSTALLINE NEAR γ-TiAl WITH A LAMELLAR MICROSTRUCTURE

Creep of a polycrystalline near γ-TiAl alloy in two fully lamellar conditions is presented. A lamellar structure with fine interface spacing and planar grain boundaries provides improved creep resistance. The lamellar structure with wide interface spacing and interlocked grain boundaries has <1/2 the creep life, five times the minimum strain rate and greater tertiary strain.Creep strain is accommodated by dislocation motion in soft grains, but the strain rate is controlled by hard grains. The resistance to fracture is controlled by the grain boundary morphology, with planar boundaries causing intergranular fracture.To maximize the creep resistance of near γ-TiAl with a lamellar microstructure requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Effect of stress ratio on long life fatigue behavior of Ti-Al alloy under flexural loading

A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 10^10 cycles and associated fr＇dcture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy（OM） and scanning electron microscopy（SEM） were used for microstructure characteristic and fracture surface analysis. The S-N curves obtained show that fatigue failure occurs in the range of 10^5-10^10 cycles, and the asymptote of S-N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 10s cycles for R=0.7. Surface roughness （the maximum roughness is no more than 3 μm） has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.     

Numerical modeling of damping capacity of Zn-Al alloys with fully lamellar microstructures

The damping behaviors of Zn-Al alloys with fully lamellar microstructures were simulated with the cell method. The influences of the grain boundary condition, the strain amplitude, the number of the lamellae in the grain （N） and the content ratio of Zn and Al in Zn-Al alloys on the damping capacity were investigated. The results indicate that the grain boundary condition has great influence on the damping capacity of Zn-Al alloys, and also affects the relationship between the damping capacity and the number of lamellae （N）. The variation of damping capacity with the strain amplitude is increasing exponentially with the strain amplitude and the damping capacity increases with the increasing of content of Zn.     

Microstructural evolution in high Nb containing TiAl alloys by reactive hot pressing

High Nb containing TiAl alloys with nominal composition of Ti-45Al-9（Nb, W, B） were fabricated at different hot pressing temperatures. The experimental results show that the microstructure of alloys hot pressed at 1300 ℃ is inhomogeneous, for lots of particulate substances containing Ti3Al phase and Nb powders dispersed in TiAl phase matrix. At 1 350 ℃, only a few lamellar colonies form and the diffusion of Nb element is incomplete. With the increase of hot pressing temperature, the mierostrueture will be more homogeneous. However, borides appear at higher sintering temperature. Meanwhile, the microstructure becomes coarse at 1 500 ℃. 1 400 ℃ will be an appropriate hot pressing temperature for reaction synthesis of high Nb containing TiAl alloys consisting of fully lamellar （FL） microstructure with the colony size of 30-80 μm. Nb element dissolves into the Ti-Al matrix by diffusion. Pore nests form in situ after Nb powders diffusion.     

Transformation mechanism of lamellar microstructure of AZ80 wrought Mg alloy during warm deformation

The microstructure especially the lamellar second phase evolution by a combination of deformation and heat treatment for AZ80 alloy was investigated.The results show that there are finer lamellar Mg_(17)Al_(12) phases after hot compression with the increasing strain,while there are coarse lamellar discontinuous precipitation cells ofβ-Mg_(17)Al_(12) phase spreading from the grain boundaries into the grains after T6 treatment of the compressed samples.The lamellar morphologies especially the lamellar dist...     

Microstructure characterization and tensile properties of a Ni-containing TiAl-based alloy with heat treatment

The effects of Ni addition on solidification micro structure and tensile properties of Ti-48Al-2Cr-2Nb alloy were investigated using differential scanning calorimetry(DSC),X-ray diffraction(XRD),scanning electron microscopy(SEM) equipped with energy-dispersive spectroscope(EDS) and transmission electron microscopy(TEM).Results show that with 3 at%Ni addition,the as-cast micro structure is mainly composed of fine lamellar colonies(~50 μm),γ grains and Ni-ridied τ_3 phase.After heat treatment at 1380℃,the Ni-containing alloy is characterized by fine fully lamellar micro structure(~90 μm).The heat-treated Ni-containing specimen exhibits superior room temperature tensile properties than other specimens.The tensile properties are discussed in light of the microstructure evolution and role of Ni-riched τ_3 phase.     

EFFECT OF NITROGEN ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALLOY HK50

The volume fraction of the lamellar carbide cell in HK50 alloy may be increased with the in-crease of nitrogen content over 0.065%.The habit plane of lmellar carbide is {111}_γ.Thedistribution of nitrogen changes no more before or after the precipitation of lamellar structure.The diffusion activation energy of carbon reduces remarkably with the increase of nitrogencontent.It is believed that the lamellar carbide cell is harmful to the high temperature creepand impact properties of the alloy.     

Research on lamellar structure and micro-hardness of directionally solidified Sn-58Bi eutectic alloy

In this work,the Sn-58Bi(weight percent) eutectic alloy was directionally solidified at a constant temperature gradient(G = 12 K.mm-1) with different growth rates using a Bridgman type directional solidification furnace.A lamellar microstructure was observed in the Sn-58Bi samples.The lamellar spacing and micro-hardness of longitudinal and transversal sections were measured.The values of lamellar spacing of both longitudinal and transversal sections decrease with an increase in growth rate.The microhardness increases with an increase in the growth rate and decreases with an increase in the lamellar spacing.The dependence of lamellar spacing on growth rate,and micro-hardness on both growth rate and lamellar spacing were obtained by linear regression analysis.The relationships between the lamellar spacing and growth rate,microhardness and growth rate,and micro-hardness and lamellar spacing for transversal and longitudinal sections of Sn-58Bi eutectic alloy were given.The fitted exponent values obtained in this work were compared with the previous similar experimental results and a good agreement was obtained.     

全层状TiAl基合金断裂机理原位观察

通过对裂纹与晶内片层和晶界的交互作用的原位观察,研究了全层状组织TiAl基合金的断裂机制。结果表明：裂纹萌发和扩展方式不仅依赖于片层与拉伸轴的相对取向,还受晶界取向的制约。当片层与拉伸轴成较大角度时,沿片层裂纹扩展是通过主裂纹与沿片层微裂纹的连接及剪切的过程;而当拉伸轴近乎平行片层时,跨片层裂纹依靠对界面分离和跨片层微裂纹这两种形式的微裂纹的连接进行扩展的,纵向交叉晶界有利于断裂韧性的提高,而横向晶界则不利于材料断裂韧性。     

The fracture mechanism of a fully lamellar γ-TiAl alloy through in-situ SEM observation

Fracture mechanisms in fully lamellar γ-TiAl alloys were studied by investigating interactions between cracks and lamellae within grain or grain boundaries using in situ SEM technique. The results showed that the fracture mechanisms depended on not only lamellar orientation to loading axis but also the type of grain boundary encountered.     

DD6单晶高温合金低周疲劳断裂特征的研究

对DD6单晶高温合金在高温低周(980℃、760℃)及疲劳/蠕变交互作用的断裂特征进行了研究。结果表明:DD6单晶高温合金高温低周疲劳断口往往呈多源开裂特征,裂纹萌生于试样的表面或亚表面,疲劳裂纹在刚萌生时沿着一定的小平面进行扩展,扩展区主要由垂直于裂纹扩展方向的疲劳条带和河流花样组成,瞬断区为类解理台阶形貌,裂纹扩展初期断口基本与主应力方向垂直,随着疲劳裂纹的扩展,断口呈现与主应力约成45°的平面特征;低周疲劳/蠕变交互作用的断裂特征与相同应变条件下低周疲劳断口总体形貌相似,但也一些不同之处,如断口整体氧化严重、疲劳扩展区面积明显减小。     

Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1B

Investment cast Ti45Al2Mn2Nb1B with fine lamellar microstructures was subject to fatigue crack propagation testing at 650 °C and a stress ratio of R = 0.1. The fracture surfaces were examined under SEM and the observed features are correlated with both stress intensity range (ΔK) and lamellar orientation. Translamellar fracture is primary fracture mode for most of the lamellar orientations. Interlamellar fracture is influenced by a combination of the ΔK and lamellar orientation. At low ΔK only the lamellar colonies with their lamellar interfaces almost perpendicular to the stress axis fractured via interlamellar fracture mode. At high ΔK interlamellar fracture can occur in lamellar colonies with any orientations.     

Dynamic fracture resilience of elk antler: Biomimetic inspiration for improved crashworthiness

The antler of the North American elk has been shown to have impressive fracture resistance under quasi-static loads, but given its viscoelastic behavior and impact nature of loading, questions remain as to its mechanical, and in particular, fracture behavior under dynamic loading. Samples were tested using a unique split-pressure Hopkin-son bar (SPHB) for four-point bending experiments in order to measure the fracture toughness of this material Interestingly, the hierarchical structure of antler had a strong influence on crack propagation characteristics, and cracks tended to propagate along the osteonal growth direction, whether loaded parallel or perpendicular to the osteonal growth direction. This occurred to such a degree so as to stop all crack propagation through the sample on transverse specimens, thus inhibiting the ability to measure a valid crack initiation toughness and demonstrating the extreme resilience of antler to resist dynamic fracture. The high resilience of antler to impact loading may serve as biomimetic inspiration to future material development for crashworthiness and defense applications.     

原位SEM研究LDMD Ti-6Al-4V合金β晶界对拉伸断裂行为的影响

目的研究β晶界对激光直接熔化沉积(LDMD)Ti-6Al-4V合金裂纹形核或传播行为的影响,以澄清合金的断裂机制,为合金性能的改善提供理论依据。方法采用LDMD Ti-6Al-4V合金粉末,在Ti-6Al-4V基板上逐层堆积形成沉积层。沿沉积层扫描方向截取试样,在室温下观察样品的微观组织形貌,并对原位拉伸过程中的微观组织演化进行实时研究。同时研究β晶界对微裂纹萌生、扩展和断裂的影响行为,总结断裂机理。结果 LDMD Ti-6Al-4V合金组织宏观呈现出沿构造方向生长的粗大柱状β晶,β晶内由板条状α晶和整齐排列的具有相同生长取向的α簇组织组成,并有少量孔洞缺陷。采用原位扫描电镜拉伸样品时发现,在横向拉力作用下,样品最初在孔洞周围发生变形,之后裂纹的萌生扩展主要沿β晶界进行,β晶界对拉力起阻碍作用,造成样品的伸长率较低。拉伸过程中,微观组织主要沿着β晶界周围的α相变形,并且孔洞缺陷引起的应力集中使得缺陷周围变形最严重,变形方向与拉力方向呈45°。结论孔洞缺陷决定了样品的初始变形位置,而β晶界则决定了裂纹传播的方向,且由于拉伸试样的截取方向与β晶界相垂直,导致样品的伸长率较低,所以β晶界对样品的力学性能及断裂机理起决定作用。     

带缺口的熔模铸造TiAl合金断裂特性的扫描电镜原位观察(英文)

采用感应凝壳熔炼技术和熔模铸造方法制备TiAl合金。在增量加载情况下,采用扫描电镜原位观察技术观察带缺口的熔模铸造TiAl合金试样的裂纹扩展和断裂特性。在拉伸变形的整个过程中,观察并分析裂纹萌生、扩展直至断裂的全过程。结果表明,TiAl合金的断裂机制不仅对于缺口区域附近的微裂纹敏感,而且与层片方向和加载轴的位向有关。当局部应力大于TiAl合金的断裂韧性时,高的拉伸应力就会导致裂纹萌生、扩展直至断裂失效。因此,TiAl合金的塑性和高的拉伸应力导致带缺口的TiAl合金的断裂失效。     

Micromechanisms of multistage fatigue crack growth in a high-strength aluminum alloy

This study reveals the micromechanisms of fatigue damage formation and evolution with respect to particle topology and grain size and orientation in a rolled 7075-T651 Al alloy. Systematic observations were made of the variations in the fracture surfaces and damage micromechanisms, which were characterized in three fatigue stages: fatigue crack formation, microstructurally/physically small cracks and long cracks. The fatigue crack was formed exclusively at the fractured Fe-rich intermetallic constituent particles preferably located at or near the specimen surface. Large impurities, such as metallic oxides, were also observed to influence the crack nucleation mechanisms. The presence of these impurities close to the nucleation sites was correlated with an approximate 30% reduction in fatigue life. In the microstructurally and physically small crack regimes, the crack front showed a rough localized brittle fatigue fracture along the crack propagation direction in addition to some localized ductile fatigue fracture. Changes in striation size across grain boundaries were clearly observed. In the long crack regime, the fracture surface became rougher but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I fracture. The ramification of the results for a microstructure-based multistage model that comprises crack incubation, small crack growth and long crack growth is discussed in detail.     

TiAl合金750℃下疲劳小裂纹行为研究

研究Ti-46Al-4Nb-1.8Cr-0.2Ta合金在高温下小裂纹的萌生和扩展行为。选用光滑板材试样在原位试验机上进行750 ℃、应力比R=0.1、频率为4 Hz的疲劳试验,并使用扫描电镜对裂纹微观形貌进行观察。结果表明:片层组织TiAl合金的疲劳裂纹最易在片层团界萌生,疲劳过程中伴随较大的微观塑性变形,并在片层团界及片层间萌生大量裂纹,疲劳裂纹扩展试样半圆形缺口根部与水平方向呈45°的应力集中区域最易萌生裂纹,主裂纹通过疲劳过程中产生的片层团界裂纹、片层间裂纹合并而成,并沿与载荷方向垂直的方向扩展。在疲劳裂纹扩展后期,裂纹为穿层扩展断裂。