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.     

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.     

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.     

Trace Ca alloying enhance simultaneously strength and ductility of squeeze-cast Al-5Cu-0.5Mn-based alloys

The strength-ductility inversion relationship of alloys is a persistent challenge in advanced materials de-sign.Al-Cu series cast aluminum alloys that are considered as an exceptionally high-strength light alloy are not exclusive in structural applications due to their inherently poor plasticity.In this work,we em-ployed a squeeze casting technique and Ca microalloying strategy for microstructure modulation to ef-fectively address this difficulty.The addition of low concentrations of Ca(0.5 wt.％and 1 wt.％)elements to the as-cast Al-5Cu-0.5Mn alloy significantly enhances its plasticity by threefold at room temperature.Unexpectedly,even after T6 treatment,which typically compromises ductility for increased strength,the low-Ca micro-alloyed Al-5Cu-0.5Mn exhibited a further increase in its strength without sacrificing its ductility.The low-Ca addition to the alloy generates an ultrafine eutectic colony with a complex\"core-shell\"structure,which can serve as a carrier for localized stress transfer,effectively distributing the strain uniformly to more grains.Precipitation hardening of α-Al grains and spheroidization of lamellar ultrafine eutectic phases were simultaneously realized in the low-Ca alloy after T6 heat treatment,which resulted in comparable hardness of α-Al grains and eutectic colonies.The synergistic coordination of external strains through extensive strain-hardening induced by slip line and substantial microcrack generation by ultrafine eutectic colonies is evidenced by a series of in situ characterizations of the low-Ca alloys.There-fore,the uniform spreading deformation due to the transfer of strain-hardening effect and the alternating plastic deformation of α-Al grains and ultrafine eutectic colonies are the critical keys to overcoming the strength-plasticity paradox in low-Ca alloys.This study provides a perspective route for Al-Cu system cast aluminum alloys to be utilized as high-strength and tough structural materials.     

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.     

Influence of rolling direction on strength and ductility of aluminium and aluminium alloys produced by accumulative roll bonding

Sheets from commercial purity aluminium AA1050 and aluminium alloy AA6016 were processed by accumulative roll bonding to obtain an ultrafine-grained microstructure. The accumulative roll bonded samples showed a significant increase in specific strength paired with high ductility. Despite a strongly elongated grain structure, tensile testing of samples oriented 45° to the rolling direction revealed considerable improvement in elongation to failure compared to the samples oriented parallel to the rolling direction. From hydraulic bulge tests, it was observed that the accumulative roll bonded samples reached higher burst pressures and slightly lower equivalent strains in comparison to the as-received conventionally grain-sized samples. This behaviour reflects the extraordinary mechanical properties of the ultrafine-grained materials and indicates promising metal sheet formability.     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

Tensile ductility behaviour of fine-grained alumina at elevated temperature

High-temperature tensile ductility behaviour of polycrystalline fine-grained alumina is shown to be classified into four regimes, depending on flow stress: (1) fast-crack growth regime, (2) single-crack growth regime, (3) microcracks growth regime, and (4) superplastic-crack growth regime, in the order of decreasing flow stress. The unique tensile ductility behaviour observed for each fracture regime is related to the type of damage accumulation. A fracture mechanics model is applied to interpret the tensile ductility of alumina in the superplastic-crack growth regime. The model correctly predicts the observed linear decrease in the true fracture strain with an increase in the logarithm of flow stress. In addition, the model is in quantitative agreement with the increase in the true fracture strain with decreasing grain size when compared at a given stress. The enhancement of tensile ductility in alumina by dilute MgO additions is attributed to an increase in the surface energy and/or decrease in the grain-boundary energy which resists the fracture process. On the other hand, the enhancement of tensile ductility in alumina by addition of a second phase of zirconia is attributed to an increase in the amount of alumina–zirconia grain boundaries which have a low grain-boundary energy. © 1998 Chapman & Hall