Zhefeng Zhang

Difference in compressive and tensile fracture mechanisms of Zr59CU20Al10Ni8Ti3 bulk metallic glass

The compressive and tensile deformation, as well as the fracture behavior of a Zr59Cu20Al10Ni8Ti3 bulk metallic glass were investigated. It was found that under compressive loading, the metallic glass displays some plasticity before fracture. The fracture is mainly localized on one major shear band and the compressive fracture angle, theta(C), between the stress axis and the fracture plane is 43degrees. Under tensile loading, the material always displays brittle fracture without yielding. The tensile fracture stress, sigma(F)(T), is about 1.58 GPa, which is lower than the compressive fracture stress, sigma(F)(C)( = 1.69 GPa). The tensile fracture angle, sigma(F)(T), between the stress axis and the fracture plane is equal to 54degrees. Therefore, both theta(C) and theta(T) deviate from the maximum shear stress plane (45degrees), indicating that the fracture behavior of the metallic glass under compressive and tensile load does not follow the von Mises criterion. Scanning electron microscope observations reveal that the compressive fracture surfaces of the metallic glass mainly consist of a vein-like structure. A combined feature of veins and some radiate cores was observed on the tensile fracture surfaces. Based on these results, the fracture mechanisms of metallic glass are discussed by taking the effect of normal stress on the fracture process into account. It is proposed that tensile fracture first originates from the radiate cores induced by the normal stress, then propagates mainly driven by shear stress, leading to the formation of the combined fracture feature. In contrast, the compressive fracture of metallic glass is mainly controlled by the shear stress. It is suggested that the deviation of theta(C) and theta(T) from 45degrees can be attributed to a combined effect of the normal and shear stresses on the fracture plane

Dependence of intergranular fatigue cracking on the interactions of persistent slip bands with grain boundaries

Intergranular fatigue cracking mechanisms in various copper crystals with different grain boundaries (GBs) were systematically investigated and summarized. In the present investigation, the GBs are classified into three types, i.e. (1) random large-angle GBs parallel, perpendicular or tilting to the stress axis in various copper bicrystals; (II) low-angle GBs parallel or perpendicular to the stress axis in copper columnar crystals; (III) large-angle Sigma19b GB in a special [(4) over bar 1520]/[18 (2) over bar7]copper bicrystal. The slip planes of the adjacent crystals are coplanar across the later two types of GBs, but the slip directions of the two component grains are different beside the Sigma19b GB. With the help of electron channeling contrast (ECC) technique in scanning electron microscopy (SEM), fatigue cracks and the interactions of dislocations with the GBs in all the fatigued crystals were observed and revealed. The results show that all the large-angle GBs (type I) in copper bicrystals always become the preferential sites to initiate fatigue cracks, independent of the interaction angle between the GB plane and the stress axis. This intergranular fatigue cracking mechanism can be attributed to the piling-up of dislocations at the large-angle GBs. For the columnar crystals containing low-angle GBs (type II), it is observed that persistent slip bands (PSBs), which transfer through low-angle GBs continuously, are the preferential sites for the nucleation of fatigue cracks. However, fatigue cracks were never observed at the low-angle GBs, no matter whether they were perpendicular or parallel to the stress axis. The non-cracking behavior of the low-angle GBs can be explained by the continuity of the dislocations, which led to the disappearance of piling-up of dislocations. For the Sigma19b GB (type III), it is found that the favorable fatigue cracking mechanism is still intergranular type in comparison with PSB cracking even though the two component grains have a coplanar slip system. The corresponding GB cracking mechanism should be attributed to the difference in the slip directions between two component grains, which only allows for partial passing through of dislocations across the Sigma19b GB. Based on the results above, it is suggested that intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. Among all the GBs, only the low-angle GBs are intrinsically strong to resist the nucleation of fatigue cracks under cyclic loading

Wavy cleavage fracture of bulk metallic glass

Dynamic instability is one of the typical cleavage fracture features in brittle materials. The authors find that dynamic instability of metallic glass starts to occur in the mirror region on the fracture surface through a wavy cracking propagation with the formation of periodic nanoscale steps. This kind of dynamic instability is associated with the early crack curving due to the intrinsic isotropic structure of metallic glass. Furthermore, they classify dynamic instabilities of cleavage fracture as crack curving at low velocity and crack branching at high velocity, corresponding to the mirror and hackle regions of metallic glass, respectively.

High strength and utilizable ductility of bulk ultrafine-grained Cu-Al alloys

Lack of plasticity is the main drawback for nearly all ultrafine-grained (UFG) materials, which restricts their practical applications. Bulk UFG Cu–Al alloys have been fabricated by using equal channel angular pressing technique. Its ductility was improved to exceed the criteria for structural utility while maintaining a high strength by designing the microstructure via alloying. Factors resulting in the simultaneously enhanced strength and ductility of UFG Cu–Al alloys are the formation of deformation twins and their extensive intersections facilitating accumulation of dislocations.

Multiplication of shear bands and ductility of metallic glass

The authors find that metallic glass can be controlled to create regularly arrayed fine multiple shear bands under small punch test, indicating that metallic glass essentially has a good plastic deformation ability and thus high ductility under suitable loading condition. The current findings imply that the initiation and propagation of shear bands in metallic glass strongly depends on the stress state and the small punch test can also be regarded as an effective method to characterize the shear deformation ability and distinguish ductile-brittle transition of different metallic glasses.

Controllable fatigue cracking mechanisms of copper bicrystals with a coherent twin boundary

High-angle grain boundaries are always the preferential fatigue cracking sites, while the intrinsic fatigue cracking mechanism of coherent twin boundary remains elusive. Here we systematically investigate the fatigue cracking behaviours of copper bicrystals with a coherent twin boundary as their sole internal boundary. It is found with direct experimental evidence for the first time that, unlike the random grain boundaries, the cracking behaviour of the twin boundary strongly depends on its orientation with respect to the loading direction. When the twin boundary is parallel or perpendicular to the loading direction, the fatigue cracks nucleate along the slip bands preferentially; when it is inclined at an angle to the loading direction, the fatigue crack is especially apt to nucleate along the twin boundary first. The controllable fatigue cracking mechanisms of the twin boundary may provide new and important implications for the optimized interfacial design of the high-performance materials.

Tensile and fatigue fracture mechanisms of a Zr-based bulk metallic glass

The tensile and fatigue fracture behavior of Zr 5 9 Cu 2 0 Al 1 0 Ni 8 Ti 3 bulk metallic glass was investigated. It was found that under tensile load the metallic glass always displays brittle shear fracture and the shear fracture plane makes an angle of θ T (= 54°) with respect to the stress axis, which obviously deviates from the maximum shear stress plane (45°). Under cyclic tension-tension loading, fatigue cracks first initiate along the localized shear bands on the specimen surface, then propagate along a plane basically perpendicular to the stress axis. Tensile fracture surface observations reveal that fracture first originates from some cores, then propagates in a radiate mode, leading to the formation of a veinlike structure and final failure. The fatigue fracture processes of the specimens undergo a propagation stage of fatigue cracks followed by catastrophic failure. Based on these results, a tensile fracture criterion for bulk metallic glasses is proposed by taking the effect of normal stress into account. It is suggested that both normal and shear stresses affect the fracture process of metallic glasses and cause the deviation of the fracture angle away from 45°.

Structure and mechanical properties of naturally occurring lightweight foam-filled cylinder – The peacock’s tail coverts shaft and its components

Feather shaft, which is primarily featured by a cylinder filled with foam, possesses a unique combination of mechanical robustness and flexibility with a low density through natural evolution and selection. Here the hierarchical structures of peacocks tail coverts shaft and its components are systematically characterized from millimeter to nanometer length scales. The variations in constituent and geometry along the length are examined. The mechanical properties under both dry and wet conditions are investigated. The deformation and failure behaviors and involved strengthening, stiffening and toughening mechanisms are analyzed qualitatively and quantitatively and correlated to the structures. It is revealed that the properties of feather shaft and its components have been optimized through various structural adaptations. Synergetic strengthening and stiffening effects can be achieved in overall rachis owing to increased failure resistance. This study is expected to aid in deeper understandings on the ingenious structure-property design strategies developed by nature, and accordingly, provide useful inspiration for the development of high-performance synthetic foams and foam-filled materials.

Improving tensile and fatigue properties of Sn-58Bi/Cu solder joints through alloying substrate

To eliminate the Bi segregation and interfacial embrittlement of the SnBi/Cu joints, we deliberately added some Ag or Zn elements into the Cu substrate. Then, the reliability of the SnBi/Cu–X (X = Ag or Zn) solder joints was evaluated by investigating their interfacial reactions, tensile property, and fatigue life compared with those of the SnBi/Cu and SnAg/Cu joints. The experimental results demonstrate that even after aging for a long time, the addition of the Ag or Zn elements into the Cu substrate can effectively eliminate the interfacial Bi embrittlement of the SnBi/Cu–X joints under tensile or fatigue loadings. Compared with the conventional SnAg/Cu joints, the SnBi/Cu–X joints exhibit higher adhesive strength and comparable fatigue resistance. Finally, the fatigue and fracture mechanisms of the SnBi/Cu–X solder joints were discussed qualitatively. The current findings may pave the new way for the Sn–Bi solder widely used in the electronic interconnection in the future.

Enhanced protective role in materials with gradient structural orientations: Lessons from Nature

UNLABELLEDnLiving organisms are adept at resisting contact deformation and damage by assembling protective surfaces with spatially varied mechanical properties, i.e., by creating functionally graded materials. Such gradients, together with multiple length-scale hierarchical structures, represent the two prime characteristics of many biological materials to be translated into engineering design. Here, we examine one design motif from a variety of biological tissues and materials where site-specific mechanical properties are generated for enhanced protection by adopting gradients in structural orientation over multiple length-scales, without manipulation of composition or microstructural dimension. Quantitative correlations are established between the structural orientations and local mechanical properties, such as stiffness, strength and fracture resistance; based on such gradients, the underlying mechanisms for the enhanced protective role of these materials are clarified. Theoretical analysis is presented and corroborated through numerical simulations of the indentation behavior of composites with distinct orientations. The design strategy of such bioinspired gradients is outlined in terms of the geometry of constituents. This study may offer a feasible approach towards generating functionally graded mechanical properties in synthetic materials for improved contact damage resistance.nnnSTATEMENT OF SIGNIFICANCEnLiving organisms are adept at resisting contact damage by assembling protective surfaces with spatially varied mechanical properties, i.e., by creating functionally-graded materials. Such gradients, together with multiple length-scale hierarchical structures, represent the prime characteristics of many biological materials. Here, we examine one design motif from a variety of biological tissues where site-specific mechanical properties are generated for enhanced protection by adopting gradients in structural orientation at multiple length-scales, without changes in composition or microstructural dimension. The design strategy of such bioinspired gradients is outlined in terms of the geometry of constituents. This study may offer a feasible approach towards generating functionally-graded mechanical properties in synthetic materials for improved damage resistance.