Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Cover Picture: Ductile‐to‐Brittle Transition in Nanocrystalline Metals (Adv. Mater. 16/2005)

Uniaxial tensile studies concerning electrodeposited nanocrystalline face‐centered cubic Ni and Ni–Fe alloys are reported on p. 1969 by Ebrahimi and Li. The nanograined metals display a transition in the deformation mechanism at a critical grain size. The cover shows that their fracture surfaces exhibited a ductile‐to‐brittle transition from the “cup–cup” (intragranular, ductile failure, dislocation controlled) (top panels) to “cup–cone” (intergranular, brittle fracture, probably due to breaking of atomic bonds) (bottom panels) characteristics at room temperature across this critical grain size value.     

Plastic behavior of steel and iron in high strain rate regime

The shock response of anti-hydrogen steel (HR-2) and iron was studied in a series of laser-driven shock wave experiments. A line-imaging optical recording velocity interferometer system for any reflector was used to record the free surface velocity histories of shock loaded samples, 100–300 \(\upmu \hbox {m}\) thick and with an initial temperature ranging from 296 to 1073 K. Based on the recorded free surface velocity profiles, the elastic precursors, dynamic yield and tensile (spall) strengths of HR-2 and iron were calculated. The dependence of the measured HEL stresses on the propagation distance for HR-2 and polycrystalline iron is approximated by a power law relationship.But, that for the single crystal iron with orientation of (110) seems to be constant. Spall strengths \((\upsigma _{\mathrm{sp}})\) of HR-2 estimated from the magnitude of the pull-back signal show that the spall strength dependence on the strain rate \((\dot{\upvarepsilon })\) is approximated by a power law relationship \(\upsigma _{\mathrm{sp}} =0.24\left( \dot{\upvarepsilon } \right) ^{0.24}\,\left( {\hbox {GPa}} \right) \). The spall strength of HR-2 and single crystal iron at the initial temperatures of 296–1073 K decreases slightly with increasing temperature and that of poly crystal iron abnormally increases at a temperature of 873 K. The X-ray diffraction results on the recovered poly crystal samples indicate significant changes in the relative peak intensity and the change in the crystal orientation may be the reason for the abnormal increasing at 873 K. The spall fracture surfaces of HR-2 were observed using a 3D laser scanning confocal microscope. The spall surface contains many dimples, suggesting that the fracture mode is that of ductile fracture. At ambient temperatures, the dimples and crowns were evenly distributed at the fracture surface. At high temperatures, many large crowns appeared and were unevenly distributed at the fracture surface.     

Role of Poisson’s ratio mismatch on the crack path in glass matrix particulate composites

The hydrogen-related fracture propagation process in martensitic steel was investigated through crystallographic orientation and fracture surface topography analyses. The hydrogen-related fracture surface consisted of three typical surfaces, namely smooth surfaces, surfaces with serrated markings, and surfaces with dimples. Crystallographic orientation analysis suggested that the smooth surface was generated by intergranular fracture at prior austenite grain boundaries, and the surface with serrated markings originated from quasi-cleavage fracture propagated along \(\{011\}\) planes. According to the reconstructed fracture propagation process by fracture surface topography analysis, the intergranular fracture at prior austenite grain boundaries initiated and propagated suddenly at the early stages of fracture. The quasi-cleavage fracture along \(\{011\}\) planes then gradually propagated within the prior austenite grains. At the final stages of fracture, ductile fracture accompanied by dimples occurred around the edge of the specimen. The results clearly indicated that the fracture propagation path changed with the proceeding fracture from the prior austenite grain boundaries to along \(\{011\}\) planes within the prior austenite grains.     

Dynamic response and microstructure evolution of the finite steel target subjected to high velocity impact by copper explosively formed projectile

The dynamic deformation of the finite steel target subjected to high velocity impact of copper explosively formed projectile is investigated by optical, scanning, and transmission electron microscopy. Morphology analysis of fracture surfaces indicates that the copper remainder plated to the crater wall shows extremely plastic deformation, which consists of elongated parabolic dimples, and the mild carbon steel target exhibits excellent brittle features that material fails mainly along the cleavage facets on the rear surface of target under strong impact of explosively formed projectile. In the surface of crater, the whole part of copper remainder and partial material of steel target undergoes completely dynamic recrystallization. The layer thickness of dynamic recrystallization zone, which displays an extreme plastic flow in solid state, is about 21.3 μm in steel target, and the average size of the refined grains significantly decreases to approximately 200 nm. Theoretically calculated results indicate that the temperature increase is associated with shock wave and plastic deformation of steel target and can reach 1352 K, which is 0.75T_m (where T_m is the melting temperature of steel target). The change in microhardness from the crater wall to the matrix of target is consistent with micro‐deformation of grains, and maximum microhardness is observed on the interface between dynamic recrystallization and severe plastic deformation zones of steel target.     

The ductile to brittle transition for C-Mn steel with an ultrafine grain ferrite/cementite microstructure

The Charpy impact energy was measured as a function of temperature for plain C-Mn steel with an ultrafine grain ferrite/cementite (UGF/C) microstructure and with a coarse-grain ferrite/pearlite (CGF/P) microstructure. Instrumented Charpy impact tests were carried out at 273 K and 77 K for the UGF/C microstructure. The steel with the UGF/C microstructure exhibited an upper shelf energy lower than the steel with the CGF/P microstructure, a higher lower shelf energy and the transition in impact energy occurred more gradually at a lower temperature. The ductile to brittle transition in the steel with the UGF/C microstructure was caused by a decrease in energy absorbed by ductile fracture rather than a change of fracture mode from ductile fracture to cleavage as occurred for the CGF/P microstructure. The fracture surface of the UGF/C steel in the upper shelf region contained large dimples and numerous small dimples whereas the fracture surface in the lower shelf region contained small dimples and cleavage facets. A lower amount of energy was absorbed in the transition region and the lower shelf region due to the decrease of dimple diameter and depth. Nevertheless, some energy was absorbed in the lower shelf because there were some small dimples even at the liquid helium temperature. In contrast, essentially no energy was absorbed in the lower shelf for the CGF/P steel because of typical cleavage fracture. The UGF/C microstructure has a high cleavage fracture stress.     

Effect of equal channel angular pressing on fracture toughness of Al-7075

In this paper, influence of equal channel angular pressing (ECAP) on the fracture behavior of Al-7075 alloy is experimentally investigated. The specimens are successfully processed by ECAP methodology up to four passes using different routes. Transmission electron microscope (TEM) images showed that after four passes of ECAP, the average grain size is refined from 40 μm to less than about 500 nm. The percentage increase in yield strength, ultimate strength and microhardness of the specimens after four ECAP passes was 230, 90 and 110 respectively. Standard tests on the disk-shaped compact DC(T) specimens showed that fracture toughness is decreased up to 8% at the first ECAP pass while after four passes, this parameter roused to 17% higher than that of annealed condition. Furthermore, scanning electron microscope (SEM) micrographs demonstrated that ductile fracture mechanism with large dimples occurred in the annealed samples, changed to limited ductile fracture with fine dimples after ECAP process. This research provides new insights into the effect of ECAP and grain refinement on the fracture behavior of materials.     

Fracture In Polycrystalline Iridium Coating

Iridium coating was prepared by the double glow plasma. The phase identification and the microstructure observation of the fracture surface of the coating were examined by X-ray diffraction and scanning electron microscopy, respectively. The deposition rate of the coating was up to 20μm/h. The iridium coating failed predominantly by grain boundary brittle fracture at room temperatures. Brittle intergranular fracture does not depend on grain size. Intergranular fracture in iridium coating has been considered to arise from low cohesive strength of the grain boundaries.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Effect of hydrogenation on mechanical properties and tensile fracture mechanism of a high-nitrogen austenitic steel

Austenitic stainless steels are frequently used for hydrogen applications due to their high ductility at low temperatures and lower hydrogen environment embrittlement compared to ferritic steels. We study the effect of electrochemical hydrogen saturation up to 40 h on tensile behavior and fracture mechanisms in high-nitrogen austenitic 17Cr–24Mn–1.3V–0.2C–1.3N steel. Hydrogen saturation weakly influences the characteristic of stress–strain curves, but decreases steel ductility, yield stress, and ultimate tensile stress. Hydrogenation provides a change in steel fracture peculiarities—a hydrogen-assisted thin brittle surface layer of ≈5 μm and ductile subsurface layer of 50–150 μm in width in hydrogen-saturated specimens. The subsurface layer shows ductile transgranular fracture with elongated dimples and flat facets. The central parts of fracture surfaces for hydrogenated specimens show ductile fracture mode similar to hydrogen-free state, but they include numerous secondary cracks both for central part and for transition zone between ductile central part and subsurface layer associated with highest hydrogen saturation. The possible reasons of decrease in hydrogen-associated ductility and change in fracture character are discussed.     

A grain-scale study of spall in brittle materials

The evolution of spall for a brittle material is investigated under variance of anisotropy, grain boundary fracture energy, and loading. Because spall occurs in the interior of the specimen, fundamental studies of crack nucleation and growth are needed to better understand surface velocity measurements. Within a cohesive approach to fracture, we illustrate that for anisotropic materials, increases in the fracture energy cause a transition in crack nucleation from triple-points to entire grain boundary facets. Analysis of idealized flaws reveals that while crack initiation and acceleration are strong functions of the fracture energy, flaws soon reach speeds on the order of the Rayleigh wave speed. Finally, simulated surface velocities of spalled configurations are correlated with microstructural evolution. These fundamental studies of nucleation, growth, and spall attempt to link atomic separation to the macroscopic spall strength and provide a computational framework to examine the evolution of spall and the impact on the simulated surface velocity field.     

Change of fracture mode in Charpy toughness transition temperature range

Abstract

A transition layer of width 5 - 10 μm was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and flow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.     

Effect of stress state and microstructural parameters on impact damage of alumina-based ceramics

Alumina discs of two grain sizes (4 and 24m), and three compositions (99.4% purity, 85% purity, alumina + partially stabilized zirconia) were subjected to planar normal impact in a gas gun at a nominal pressure of 4.6GPa. The alumina discs were confined in copper and aluminium capsules, which provided solely compressive and compressive plus tensile pulses in the ceramic, respectively. These experiments were conducted at different pulse durations (controlled by the thickness of the flyer plates). The surface area of cracks per unit volume was measured in order to estimate the impact damage. Compression followed by tension produced significantly more damage than compression alone. The small grain-sized discs exhibited more damage than the large grain-sized discs. The amount of damage increased with the duration of the tensile stress pulse. The addition of partially stabilized zirconia ( 14%) did not enhance the resistance to fragmentation of the discs; X-ray diffraction did not reveal an impact-induced phase transformation. Although the pressures generated were below the Hugoniot elastic limit of alumina, considerable fracturing of the specimens took place. Scanning electron microscopy revealed that the fracture was intercrystalline in regions away from the spall plane. In the spall plane energy was sufficient to comminute the grains, producing considerable grain debris and transgranular fracture. Transmission electron microscopy revealed the onset of damage to the structure, in the form of dislocations (present in only a small fraction of grains), microcracks nucleating at voids, and intergranular microcracks.     

Effects of temperature and grain size on deformation and fracture in recrystallized Ni3Al doped with boron

The effects of temperature and grain size on the deformation and fracture behaviour of recrystallized Ni₃Al doped with boron were investigated by tensile tests at temperatures up to 973 K as a function of grain sizes from 1.6 to 105m. The yield stress showed a positive temperature dependence to a peak temperature in somewhat different manners depending on the grain size. For coarse-grained specimens, a rapid drop in elongation was observed with increasing temperature. The predominant fracture mode changed with temperature from the transgranular fracture of {1 1 1} cracking to brittle intergranular fracture. This embrittlement at elevated temperatures was considered to occur by a high stress concentration at grain boundaries arising from increased flow stress level and the occurrence of grain boundary sliding (GBS). In contrast, the elongation was not so markedly decreased with temperature for intermediate- and fine-grained specimens which exhibited ductile intergranular fracture and cavitation fracture, respectively, at elevated temperatures, and a slant-type fracture and cup-cone fracture, respectively, at low temperatures. The suppression of serious high-temperature embrittlement for intermediate-grained specimens was explained in terms of the slow propagation of a crack formed by GBS, owing to stress relaxation by dynamic recrystallization (DR) and plastic deformation. In the case of ultra-fine-grained specimens a large elongation was developed at elevated temperatures, which was interpreted as that the further occurrence of DR with increasing volume fraction of grain boundaries reduces the cavitation promoted by GBS, and that the limited sliding length due to extremely small grain diameter raises the stress for cavity formation.     

Zur Zähigkeit von Vergütungsstählen

On Toughness of Quenched and Tempered Steels Toughness as consumed fracture energy is dependent on fracture mechanism. Grain size and loading conditions influence the transition from ductile dimple fracture to brittle cleavage fracture. In quenched and tempered steels packet size and particle distribution are of importance as well as brittle intergranular fracture modes by grain boundary segregation of impurities in ferrite (temper embrittlement) or precipitates in austenite. Anisotropy of toughness arises from banded structures.     

Dynamic fracture of binary Al–Li alloy in torsion

Abstract

To investigate the mechanical properties of a binary Al–2·8Li alloy at high strain rates in as received (solution treated) and aged conditions, quasistatic and dynamic torsion tests were carried out. With increasing strain rate, the strength and ductility of the as received alloy increased and a change of the fracture mode from intergranular to transgranular was also observed. However, in the aged alloy, the intergranular fracture mode was predominant at both quasistatic and dynamic rates. At the grain boundary offsets on the fracture surface of the aged alloy, a large number of very fine dimples were observed. This evidence suggests that a ductile precipitate free zone may be present there.

MST/1043     

The effect of heat treatments on the creep–rupture properties of a wrought Ni–Cr heat-resistant alloy at 973 K

The effect of heat treatments on the creep–rupture properties was investigated on a wrought Ni–Cr heat-resistant alloy at 973 K. Short-time aging (aging for 3.6 ks (1 h) at 973 K) was made on the solution-treated specimens with different grain sizes. The fine-grained specimen (the grain diameter, d = 45.2 μm) produced by short-time solution treatment exhibited almost the same rupture life and superior creep ductility as those of the medium-grained specimen (d = 108 μm) produced by normal solution treatment. The fine-grained specimen and medium-grained specimen showed the longer rupture life compared with the specimen with recommended aging. The principal strengthening of specimens was attributed to the precipitation hardening by γ′ phase particles. The fine-grained specimen had the highest hardness, and the increase of the hardness was observed in both the fine-grained and the medium-grained specimens during creep at 973 K. However, coarse-grained specimen (d = 286 μm) with high-temperature long-time solution treatment exhibited significantly short rupture life owing to insufficient precipitation hardening after the short-time aging and during creep. Ductile intergranular fracture with dimples occurred in the fine-grained specimen, while brittle intergranular fracture was observed in the medium-grained specimen and in the specimen with recommended aging. Both transgranular fracture and brittle intergranular fracture were observed in the coarse-grained specimen. A simple heat treatment composed of short-time solution treatment and short-time aging is applicable to high-temperature components of wrought Ni–Cr alloys.     

Dynamic fracture (spalling) of metals

The fundamental mechanical aspects of dynamic fracture in metals are presented, with emphasis on spalling produced by the interactions of shock and reflected tensile waves. The major research efforts conducted in this area are reviewed; the process has been successfully described as a sequence of nucleation—growth—coalescence of voids or cracks. Quantitative models predicting the extent of damage have been successfully compared with experimental observations, by incorporating them into computer codes.A number of metallurgical aspects of importance are discussed: failure initiation sites, crack propagation paths, strain-rate-dependent ductile to brittle transition, grain size effect, intergranular versus transgranular spalling. Of particular importance in iron and steels is the change in spall morphology when the 13 GPa stress is exceeded. This change is documented and interpreted in terms of the α(BCC)→?(HCP) phase transformation undergone at that pressure. Micromechanical models describing the growth of voids in terms of dislocation motion are discussed.Areas requiring additional research effort are identified.     

A study aimed at determining and understanding the fracture behaviour of an Al–Li–Cu–Mg–Zr alloy 8090

The S-L fracture toughness of aluminium–lithium based alloys is generally poor and this has limited their applicability, particularly in aerospace where good damage tolerance is required. The low S-L toughness has been attributed variously to grain boundary precipitation, planar slip and lithium segregation. This work examined the S-L fracture behaviour of an 8090 Al–Li-based alloy in 34–45mm plate. The results confirmed that the S-L fracture toughness decreases from the centre to the surface of plate material and increases with double ageing. Fracture is principally intergranular, with both ductile and brittle components occurring, but some transgranular fracture also occurs and this produces steps in the fracture plane. Changes in the relative proportions of brittle and ductile intergranular fracture, as well as in the amount of transgranular fracture, accompany the changes in toughness. However, the decrease in fracture toughness across the plate is accompanied principally by an increase in the relative proportion of brittle intergranular fracture, while the toughening produced by double ageing is accompanied principally by an increase in the amount of transgranular fracture. Evidence of coarse slip, indicative of slip planarity, was seen from slip steps and dislocation structures. However, planar slip was seen only towards the centre of the plate and not towards the surface. The level of planar slip was not reduced markedly by double ageing. Texture varied with position across the plate. There were also a larger number of low angle boundaries towards the centre of the plate than towards the surface. The hardness did not change across the plate. The results could not be explained fully in terms of either the grain boundary precipitate theory or the planar slip model, but were generally consistent with the lithium segregation model. However, the basic tenet of this model is that the level of embrittlement is influenced by the grain boundary structure, and the results did not indicate a substantial difference between the boundaries that failed in a ductile manner and those which failed by brittle fracture. This suggests that the factors which affect lithium segregation may be more complex than originally envisaged.     

The effect of nickel content on the fracture behaviour of Cu-Al-Niβ-phase alloys

The fracture behaviour of Cu-14 wt% Al alloys has been studied as a function of nickel content varying from 0 to 10 wt%. It was found that the presence of a brittle phase ₂ at the grain boundaries is responsible for intergranular fracture in low nickel alloys. Severe intergranular embrittlement exhibited by high nickel alloys in not associated with any precipitate at the grain boundaries. In fact when high nickel alloys are cooled slowly, a ductile phase () forms along the grain boundaries that resists the propagation of crack through grain boundaries and the fracture is transgranular.