Short transverse fracture of Al–Li–Cu–Mg–Zr extrudate

Abstract

The short transverse fracture toughness of an Al–Li–Cu–Mg–Zr extrudate was determined as a function of aging condition and testing temperature. To elucidate the underlying micromechanisms, the short transverse fracture surfaces of the extrudate were characterised via scanning electron microscopy, grain boundary precipitates and precipitation free zones were identified via transmission electron microscopy, and segregation of elements to grain boundaries was analysed using secondary ion mass spectrometry. Three principal observations were made as follows. First, with increasing aging time, the short transverse toughness of the extrudate increased when tested at room temperature, but decreased at liquid N₂ temperature, whereas with decreasing testing temperature, it remained essentially constant for the underaged condition, and decreased sharply for the peak aged and overaged tempers. Second, in addition to regions exhibiting shallow dimples, smooth ‘featureless’ zones were revealed on the short transverse fracture surfaces, which are intergranular in nature for all the specimens tested. The area fraction of the featureless regions decreased noticeably with increasing aging time when tested at room temperature, and increased markedly with decreasing testing temperature for the peak aged and overaged conditions. Third, segregation of Li, Si, Na, and H was detected for both the underaged and overaged specimens, and also of K for the underaged specimens only. In general, the enhancement of the room temperature short transverse toughness with aging and the negative effect of cryogenic temperature on fracture toughness are in obvious contrast to the in plane toughness behaviour reported in the literature, the featureless character of the short transverse fracture and its connection with poor toughness seldom having been emphasised. Based upon the present study, segregation induced brittleness is proposed as the critical micromechanism responsible for grain boundary weakness, and thus for the poor short transverse fracture toughness.

MST/1829     

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) K_ρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.     

TEM study of microstructure development during low-cycle fatigue of an overaged Al-Mg-Si alloy

The evolution of dislocation structure during room-temperature, uniaxial, low-cycle fatigue of an overaged Al-Mg-Si alloy is studied. Ageing at 450°C produces a fine dispersion of Mg₂Si precipitate particles. During fully reversed strain-controlled cyclic tests, these fine particles restrict deformation to local regions and a stable dislocation substructure is developed early in fatigue life. Substructural observations of hardening and saturation by transmission electron microscopy reveal extensive dislocation band formation on Mg₂Si precipitate rods. Various microstructural features such as configuration of tangled dislocations, dislocation cells, precipitate morphologies, sizes, precipitate-free zones, etc., have been examined during cyclic hardening and saturation. The results have been analysed in terms of kinematic and isotropictype microstructural mechanisms.     

Fracture behavior of thin ring Cu-2.5 wt.%Co alloy specimens at high strain rates

The mechanisms of fracture in precipitation hardenable Cu-2.5 wt.%Co alloys subjected to strain rates of 10³-10⁴s⁻¹ have been investigated and compared with the fracture behavior accompanying quasi-static loading. Transmission electron microscopy (TEM) was used to examine the role assumed by deformation substructure in the fracture process. An increase in macroscopic ductility and a decrease in the extent of necking were observed at high strain rates. Specimens were prepared and tested in the solution treated, peak aged, and overaged conditions; specimens in the solution-treated and peak-aged conditions exhibited heavy slip band ridges on the fracture surfaces with a limited number of voids, whereas the overaged specimens showed high void densities. Only minor differences in fracture surface features were noted between the dynamic and static-loaded specimens. TEM studies of the solution-treated specimens deformed at high strain rates revealed random arrays of dislocations without a well-defined cell structure. TEM also revealed uniform coherency losses between the precipitate and the matrix in the peak-aged alloys and microvoids at the precipitate/ matrix interfaces in the overaged alloys after deformation.     

Shear yielding modes of polycarbonate

The modes of shear yielding in edge-notched sheets of polycarbonate have been studied under slow tensile loading. Optical microscope techniques were used to characterize the flow lines through the thickness of the plastically deformed region. Three modes are observed, namely core yielding, hinge shear and intersecting shear. Core yielding consists of two families of shear flow lines contained in the centre region where the stress is highest. In the nearly plane strain condition, the dominating shear mode is hinge shear which is through-thickness yielding on inclined planes above and below the notch. Intersecting shear dominates in the nearly plane stress condition. In this case, yielding occurs through the entire thickness by slip along planes parallel to the width direction that make an angle with the plane of the sheet. It produces a necking effect in front of the notch. The thickness dependent transition from hinge shear to intersecting shear follows conditions suggested by Hahn and Rosenfield.     

Hydrogen embrittlement susceptibility of laser-hardened 4140 steel

Slow strain rate tensile (SSRT) tests were performed to investigate the susceptibility to hydrogen embrittlement of laser-hardened AISI 4140 specimens in air, gaseous hydrogen and saturated H₂S solution. Experimental results indicated that round bar specimens with two parallel hardened bands on opposite sides along the loading axis (i.e. the PH specimens), exhibited a huge reduction in tensile ductility for all test environments. While circular-hardened (CH) specimens with 1 mm hardened depth and 6 mm wide within the gauge length were resistant to gaseous hydrogen embrittlement. However, fully hardened CH specimens became susceptible to hydrogen embrittlement for testing in air at a lower strain rate. The strength of CH specimens increased with decreasing the depth of hardened zones in a saturated H₂S solution. The premature failure of hardened zones in a susceptible environment caused the formation of brittle intergranular fracture and the decrease in tensile ductility.     

Effect of hybridization of liquid rubber and nanosilica particles on the morphology, mechanical properties, and fracture toughness of epoxy composites

A liquid carboxyl-terminated butadiene–acrylonitrile copolymer (CTBN) and SiO₂ particles in nanosize were used to modify epoxy, and binary CTBN/epoxy composites and ternary CTBN/SiO₂/epoxy composites were prepared using piperidine as curing agent. The morphologies of the composites were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), and it is indicated that the size of CTBN particles increases with CTBN content in the binary composites, however, the CTBN particle size decreases with the content of nanosilica in the ternary composites. The effects of CTBN and nanosilica particles on the mechanical and fracture toughness of the composites were also investigated, it is shown that the tensile mechanical properties of the binary CTBN-modified epoxy composites can be further improved by addition of nanosilica particles, moreover, obvious improvement in fracture toughness of epoxy can be achieved by hybridization of liquid CTBN rubber and nanosilica particles. The morphologies of the fractured surface of the composites in compact tension tests were explored attentively by field emission SEM (FE-SEM), it is found that different zones (pre-crack, stable crack propagation, and fast crack zones) on the fractured surface can be obviously discriminated, and the toughening mechanism is mainly related to the stable crack propagation zone. The cavitation of the rubber particles and subsequent void growth by matrix shear deformation are the main toughening mechanisms in both binary and ternary composites.     

Effect of heat treatment on microstructure and mechanical properties of Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloy

ABSTRACT

The effect of heat treatment on the mechanical properties and microstructures of Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloys was investigated. The results show that the microstructure of the as-cast alloy mainly consists of an alpha-copper matrix and γ-phase Ni₃Al particles. The microstructure of the alloy after solution treatment at 950°C for 2?h is a single-phase alpha-copper supersaturated solid solution and the second-phase strengthening disappears. After ageing treatment at 550°C for 6?h, the γ-phase particles are fully precipitated, and the mechanical properties of the alloy are significantly improved. The tensile strength is increased from 305 to 588?MPa. Quasi-cleavage fracture with shallow dimples appeared in the Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloy aged at 550°C for 6?h.     

Structures and magnetic properties of overaged Fe-AI-Mn-C alloys

Abstract

Microstructures and magnetic properties in the aged hardened Fe-9AI-30Mn-x(C,Si) alloy, during overaging at 823 Kfor 48 h to 313 days, have been investigated by transmission electron microscopy, X-ray diffraction line profiles, and vibrating sample magnetometry. The results reveal that the precipitate k phase ((Fe,Mn)₃AIC) decomposition in this alloy, overaged at 823 k for one week, proceeded by two separate mechanisms; (a) wetting of the½α_o′<100> antiphase boundary segment of D0₃ ((Fe/Mn)₃Al) domains by B2 ((Fe/Mn)Al) phase, and (b) precipitation of B2 ((Fe/Mn)Al) structure within the domain. A similar superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for 120-313 days. The super soft magnetic properties were mainly attributed toferromagnetic B2 ((Fe/Mn)Al) domains, D0₃, and ;α′-Mn phases.     

Ductile fracture before localized necking in a strip under tension

Unexpected cracking with a 22° opening angle grew out of a rough-sheared edge before appreciable necking in an 0.79 by 37 mm mild steel strip, which normally fractures after diffuse and then localized oblique necking. From the crack tip, two shear bands formed at 55° to the load direction, consistent with isotropic plane stress characteristics (53° was predicted from anisotropy, but necking in thin strips occurred at 67°). Photomicrographs showed that the 22° crack growth occurred by first tunnelling at mid-thickness, and then spreading along through-thickness shear planes. Springback on unloading caused a 0.038 mm crack closure and local buckling. This form of cracking illustrates a size effect in fracture under macroscopically plane stress. It also gives an example of a local mechanism triggering a fracture mode that can require more total work than an alternative.Analysis of isotropic localized necking shows the equivalent strain at fracture in thin strips to be uniquely related to the Reduction in Squared Thickness (RST). With smooth edges, width and thickness strains before and during necking differed by factors of 1.4 and 1.7; such measures of anisotropy should be routinely found and reported for strips.     

High Strength AA7050 Al alloy processed by ECAP: Microstructure and mechanical properties

Commercial AA7050 aluminium alloy in the solution heat-treated condition was processed by ECAP through routes A and B_C. Samples were processed in both room temperature and 150 °C, with 1, 3, and 6 passes. The resulting microstructure was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM). Only one pass was possible at room temperature due to the low ductility of the alloy under this condition. In all cases, the microstructure was refined by the formation of deformation bands, with dislocation cells and subgrains inside these bands. The increase of the ECAP temperature led to the formation of more defined subgrain boundaries and intense precipitation of spherical-like particles, identified as η′ and η phases. After the first pass, an increase in the hardness was observed, when compared with the initial condition. After 3 passes the hardness reached a maximum value, higher than the values typically observed for this alloy in the overaged condition. The samples processed by route B_C evolved to a more refined microstructure. ECAP also resulted in significant strength improvement, compared to the alloy in the commercial overaged condition.     

AA5052薄板磁脉冲胶焊复合接头动态力学性能研究

目的 研究AA5052铝合金薄板在高速冲击载荷下的磁脉冲胶焊复合接头的动态力学性能,探究不同载荷速率对该胶焊复合接头力学和失效行为的影响规律.方法 利用磁脉冲焊接系统成功制备了胶焊复合连接试件.采用万能拉伸试验机、高速拉伸试验系统,结合全场应变测量系统,获得胶焊复合接头的力学性能规律,以及渐进失效过程和搭接区应变变化....     

Analysis of mechanical properties and its associated fracture surfaces in dual‐phase austempered ductile iron

This work aims at evaluating the fracture surfaces of tensile samples taken from a new kind of ductile iron referred to as ‘dual‐phase Austempered Ductile Iron (ADI)’, a material composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. The tensile fracture surface characteristics and tensile properties of eight dual‐phase ADI microstructures, containing different relative quantities of ferrite and ausferrite, were studied in an alloyed ductile cast iron. Additionally, samples with fully ferritic and fully ausferritic (ADI) matrices were produced to be used as reference. Ferritic–pearlitic ductile irons (DI) were evaluated as well. For dual‐phase ADI microstructures, when the amount of ausferrite increases, tensile strength, yield stress and hardness do so too. Interesting combinations of strength and elongation until failure were found. The mechanisms of fracture that characterise DI under static uniaxial loading at room temperature are nucleation, growth and coalescence of microvoids. The fracture surface of fully ferritic DI exhibited an irregular topography with dimples and large deformation of the nodular cavities, characteristic of ductile fracture. Microstructures with small percentages of ausferrite (less than 20%) yielded better mechanical properties in relation to fully ferritic matrices. These microstructures presented regions of quasi‐cleavage fracture around last‐to‐freeze zones, related to the presence of ausferrite in those areas. As the amount of ausferrite increased, a decrease in nodular cavities deformation and a flatter fracture surface topography were noticed, which were ascribed to a higher amount of quasi‐cleavage zones. By means of a special thermal cycle, microstructures with pearlitic matrices containing a continuous and well‐defined net of allotriomorphic ferrite, located at the grain boundaries of recrystallised austenite, were obtained. The results of the mechanical tests leading to these microstructures revealed a significant enhancement of mechanical properties with respect to completely pearlitic matrices. The topographies of the fracture surfaces revealed a flat aspect and slightly or undeformed nodular cavities, as a result of high amount of pearlite. Still isolated dimple patterns associated to ferritic regions were observed.     

Microstructural changes taking place during dynamic compaction of aluminium powders

The dynamic compaction of powders is characterized by a complex sequence of pressure loading and shear deformation, adiabatic temperature rise and structural annealing. The relative effects of these factors have been examined by studying the hardness and microstructure of compacted aluminium samples. Particle interiors are considerably shock-hardened, and then partially softened by dislocation recovery. The recovered structures appear to be relatively stable against further softening and recrystallization. The melted zones which occur as pore-filling between powder particles takes place are extremely hard and thermally stable. These zones possess a fine-grained structure after rapid resolidification, contain an oxide dispersion from the oxide films on the original powder surfaces, and are hardened by microvoids from the shrinkage occurring during solidification. The final material structures and properties are therefore variable throughout a given compacted samples, and depend sensitively on many aspects of the material, the powder and the consolidation conditions.     

Ductile fracture in sheets under transverse strain gradients

The fully plastic fracture of a metal sheet subjected to a small transverse gradient of tensile strain near a reinforcement is modeled as mode I fracture under transverse plane strain (TPS). Necking and fracture were analyzed by assuming that they were set by prior uniform strains and then necking displacements. Equations for the spreading of TPS necking and fracture were thus derived for a sheet with strain gradient. Experiments on tapered specimens confirmed the expected fracture displacements within 12 percent, but Moiré studies suggest the agreement may be fortuitous. In any event, in-plane transverse displacements and normal strains in the crack growth direction, as well as shear strains, were negligible. This should simplify any future numerical analysis.     

On the impact toughness of Al-15 vol.% B4C metal matrix composites

The present work was performed on ten metal matrix composites (MMCs) produced using the new powder injection technique. These MMCs were divided into two series in which pure aluminum was the matrix for one series, while an experimental 6063 alloy was the matrix for the second series. Small amounts of Ti, Zr and Sc were added to those composites, either individually or combined. In all cases the volume fraction of the reinforced B₄C particles was in the range 12–15 vol. %. The molten metal was cast in an L-shaped metallic mold preheated at 350°C. Unnotched rectangular impact samples (1 cm × 1 cm × 5 cm) were prepared from these castings and heat treated. Samples were tested using instrumental impact testing machine. Microstructure and fracture surface were examined using Hitachi SU-8000 FESEM. The results show that the presence of Ti improves the wettability of the B₄C particles and their adherence to the matrix. Repeated remelting at 730 °C applying vigorous mechanical stirring could lead to fragmentation of some of the B₄C particles. Aluminum based composites exhibited better toughness compared to those obtained from 6063 based composites in all the studied conditions. The composite impact toughness was controlled by the precipitation and coarsening of hardening phase particles namely Mg₂Si, Al₃Zr and/or Al₃Sc. Cracks in the fracture surface were observed to be initiated at the particle/matrix interfaces and propagate either through the B₄C particles or through the protective layers. No complete debonding was reported due the presence of Zr/Ti/Sc rich layers which improved the particle/matrix adhesion.     

Mechanical properties of 304L stainless steel SMAW joints under dynamic impact loading

The impact properties of 304L Stainless Steel Shielded Metal Arc Welded (SMAW) joints are studied at strain rates between 10^{− 3} and 7.5× 10³ s^{− 1} using a compressive split-Hopkinson bar. The effects of strain rate on the flow response and fracture characteristics are fully evaluated. The results show that the tested weldments exhibit a pronounced strain rate sensitivity, and that changes in the strain rate result in a difference in the flow stress, fracture strain, and work hardening rate. Furthermore, it is noted that the strain rate sensitivity and activation volume vary with the magnitude of the strain rate, and are related to different work hardening stress levels. At all values of strain rate, the tested weldments fail as a result of adiabatic shearing, in which cracks initiate within the shear band and then propagate along this shear band until failure occurs. Observation of the fractured specimens reveals that the fracture surfaces of the fusion zone and base metal regions are characterized by the presence of elongated dimples. The variation in the observed dimple features with strain rate is consistent with the results of the impact stress-strain curves.     

Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

In this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.     

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.     

Microcosmic mechanism of material softening and fracture in primary shear zone during high speed machining of hardened steel

The metallurgical observations of microstructure and fracture characteristics of the adiabatic shear bands, within the primary shear zones of the serrated chips produced during high speed machining 30CrNi3MoV hardened high strength steel, have been performed using optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure observations showed that the material softening occurred in the primary shear zones during high speed machining. A microcosmic model of microstructure development and rotational dynamic recrystallisation in the primary shear zone during high speed machining high strength steel was suggested by analysis of material softening mechanism. The fracture observations showed that adiabatic shear fracture was the primary reason of serrated chip formation during high speed machining. A microcosmic fracture model during adiabatic shear was proposed.