Strength,deformation, fracture behaviour and ductility of aluminium-lithium alloys

The ever increasing need for high strength, improved performance, lightweight and cost-effective materials has resulted in significant improvements and development of new aluminium alloys for structural applications. Lithium additions to aluminium have the potential for providing a class of high strength alloys with exceptional properties suitable for weight-critical applications. In this paper, published studies of composition-processing-microstructure relationships are discussed. Contributions to strength of the solid solution are discussed with reference to the presence of lithium in solid solution, the presence of coherent, ordered precipitates in the matrix and the co-precipitation of binary, ternary and more complex strengthening phases. Microstructural influences on strength are discussed with reference to metallurgical variables. These variables include the intrinsic microstructural features; the presence of dispersoids, the nature and type of matrix strengthening precipitates and the presence of denuded zones adjacent to grain boundaries. The extrinsic and intrinsic micromechanisms governing the deformation characteristics and fracture behaviour are critically examined with specific reference to ageing condition of the alloy, the matrix slip characteristics, and the nature, volume fraction and distribution of strengthening precipitates. The deleterious effects of strain localization and the exacerbating effect of precipitate-free zones are also highlighted. The micromechanics governing the fracture processes are examined and the sequence of events in the fracture process is reviewed in light of the specific role of several concurrent factors involving nature and volume fraction of second-phase particles, deformation mode, and dislocation-microstructure interactions. Past attempts made to improve the tensile ductility and mechanical response of these alloys are also examined so as to provide a better basis for understanding processing-microstructure-deformation interactions.     

Criteria of the contact strength,ductility, and fracture of materials in shear with buckling

A model is presented of the origination and suppression of the fracture of a contact surface during shear of its layers. The process is examined as a function of the configuration of projections on the rigid body, the thickness of the layers undergoing deformation, and the distribution of the contact stresses. Two mechanisms of fracture suppression on the contact surface are found: an unstable mechanism dependent on the thickness of the layer undergoing shear and the properties, roughness, and condition of the surface of the material being deformed; a stable mechanism independent of these factors. Both mechanisms are found to be of practical value in the solution of problems which involve increasing the strength of contact layers and resistance welds in various structures and optimizing regimes of plastic deformation and resistance welding. Criteria of contact strength are also presented for ductile and low-ductility materials.Translated from Problemy Prochnosti, No. 6, pp. 26–32, June, 1992.     

Tensile ductility and necking of metallic glass

Metallic glasses have a very high strength, hardness and elastic limit. However, they rarely show tensile ductility at room temperature and are considered quasi-brittle materials. Although these amorphous metals are capable of shear flow, severe plastic instability sets in at the onset of plastic deformation, which seems to be exclusively localized in extremely narrow shear bands approximately 10 nm in thickness. Using in situ tensile tests in a transmission electron microscope, we demonstrate radically different deformation behaviour for monolithic metallic-glass samples with dimensions of the order of 100 nm. Large tensile ductility in the range of 23-45% was observed, including significant uniform elongation and extensive necking or stable growth of the shear offset. This large plasticity in small-volume metallic-glass samples did not result from the branching/deflection of shear bands or nanocrystallization. These observations suggest that metallic glasses can plastically deform in a manner similar to their crystalline counterparts, via homogeneous and inhomogeneous flow without catastrophic failure. The sample-size effect discovered has implications for the application of metallic glasses in thin films and micro-devices, as well as for understanding the fundamental mechanical response of amorphous metals.     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

Tensile strength and fracture surface characterisation of sized and unsized glass fibers

The tensile strength of commercial glass fibers is examined by single fiber tensile tests. The fibers are analysed as received from the manufacturer (sized) and after a heat treatment at 500C (unsized). Weibull plots of the two series are used for comparison of the strengths of the sized and unsized fibers. It is shown that large sample sizes (over 60 tests) are required to lead to a reliable two-parameter Weibull distribution. The experimental tests clearly indicated that the unsized fibers were weaker in the low strength range, but had similar strength in the high strength range. An investigation of the fracture surfaces in the SEM showed distinct differences in the fracture patterns for high and low strength fibers. Fracture mechanics were applied to estimate the original flaw size and relate the observed fracture mirror surface to the fiber strength. Based on the observation of surface flaws, a healing mechanism by the sizing is considered likely for this type of fiber and sizing, thereby effectively increasing the strength of the fiber in the presence of larger surface flaws.     

A tuning nano-precipitation approach for achieving enhanced strength and good ductility in Al alloys

A new strategy through controlling the aggregation states of alloying elements before cold-rolling is proposed to tune the nano-precipitation in Al alloys processed by combining deformation and aging. A considerable improvement in strength and good ductility is achieved in Al–Mg–Si–Cu alloy by our approach, which is applicable to many precipitation-hardened alloy systems and suitable for industrial application. A bi-modal distribution of precipitates, discrete lath-like Q″-type precipitate and continuous precipitate with the composition identical to Q′-type phase, occur in the annealed sample pre-treated by natural aging. A short-term artificial aging which induces a dispersion of nano-sized coherent particles, mainly GP zone of the monoclinic β″-type phase, before cold-rolling can lead to the formation of finely and homogeneously distributed Q″-type precipitates, which are in favor of an enhancement in strengthening potential for the final aging. Our finding suggests that the solutes aggregation state before deformation affects significantly the re-precipitation of solutes during the subsequent annealing.     

Simultaneous enhancement of strength and ductility of body-centered cubic TiZrNb multi-principal element alloys via boron-doping

Body-centered cubic(BCC)multi-principal element alloys(MPEAs)have intrinsic high strength but poor ductility,which greatly limits their potential applications.Here we present the boron-doping strategy to enhance the strength and ductility of TiZrNb MPEAs simultaneously.The yield strength and ductility of the TiZrNb MPEA with boron addition of 500 ppm are increased by 19.0％and 48.7％compared to the boron-free TiZrNb MPEA,respectively.Boron-doping induced high efficiency in grain refinement from～96.0 pm to～16.2 pm is the main factor for strengthening.Dislocation dominated deformation mechanism involving cross slip and dislocation pining in the TiZrNb containing 500 ppm boron serves to enhance the strain-hardening capacity,resultant the enhancement of ductility from 7.8％to 11.6％.While the planar slip of dislocations is the dominated deformation mechanism for the boron-free TiZrNb.     

Comparison of shear and tensile fracture in high strength aluminum alloys

A comparison was made between tensile (mode I) and shear (mode II) fracture characteristics in high strength aluminium alloys (7075-T6 and 6061-T651) using a relatively new mode II fracture specimen to evaluate the critical stress intensity factor. The enlarged plastic zone during mode II fracture required that an increased specimen thickness be used for determining K _Hc under a purely plane strain condition. Plane stress conditions prevailed in the mode II fracture of 7075-T6 with a specimen thickness less than 10 mm, while plane strain controlled mode II fracture at a thickness of 10 mm or greater. Fractographic analysis revealed a distinctive difference in the micromechanisms responsible for crack extension. Small dimples were observed only on the mode II fracture surfaces, resulting from a microvoid nucleation fracture mechanism. The mode I fracture surfaces showed a mixed distribution of dimple sizes resulting from a void growth fracture mechanism. Comparing the critical stress intensity factors, the shear mode of failure exhibited a substantially higher value than the tensile mode, resulting from the effect of the sign and magnitude of the hydrostatic stress state on the microvoid nucleation event. Zero hydrostatic tension in the mode II loading configuration helps delay microvoid nucleation, increasing the apparent toughness. The high hydrostatic tension resulting from a mode I loading configuration enhances microvoid nucleation which promotes crack propagation at relatively lower stress intensity factors.     

热处理对粉末热挤压法制备6061铝合金强度和塑性的影响

研究了单级固溶及峰值时效处理对粉末热挤压法制备6061铝合金显微组织及室温力学性能的影响,观察了合金挤压态、固溶态及时效处理后的显微组织,并对其力学性能进行了测试.结果表明:挤压态材料的晶粒均匀细小,基体合金中存在大量的第二相颗粒,主要为Mg2Si相;热挤压后的6061铝合金经固溶时效处理(530℃×1 h水冷+170℃×6 h)后,晶粒内部析出大量的β″(Mg5Si6)相,并伴有少量棒状的β’(Mg2Si)相析出,拉伸强度和延伸率分别为311 MPa和10%,相比挤压态铝合金,其拉伸强度提高了近160%.