Effect of Grain Size and Testing Temperature on Low-Cycle Fatigue Behavior and Plastic Deformation Mode of Ti-2Al-2.5Zr

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on Ti-2Al-2.5Zr samples with different grain sizes (5 and 40 μm) at room temperature (RT) and low temperature (77 K). The results show that the coarse-grained samples of 40 μm exhibit a higher ductility and LCF life than the fine-grained ones at RT. Meanwhile, the fine-grained samples of 5 μm displayed improved ductility and LCF life at 77 K compared with those at RT. Microstructural observations using optical microscopy (OM), scanning electron microscopy, and transmission electron microscopy (TEM) revealed that a transition occurred in the plastic deformation mode, from twinning with slip to slip alone, as the grain size decreased from 40 to 5 μm at RT. Conversely, in the fine-grained samples fatigued at 77 K, twinning was activated and became one of the dominant plastic deformation modes. The improvement in the LCF life of the coarse-grained samples at RT and the fine-grained ones at 77 K could be attributed to the activation of deformation twinning. In addition, the cyclic stress response curves showed that cyclic stress saturation was exhibited in the fine-grained samples at all strain ranges. An initial cyclic hardening followed by cyclic softening was displayed ahead of the cyclic stress saturation in coarse-grained samples at high strain amplitudes. When the testing temperature decreased to 77 K, cyclic stress hardening prior to cyclic stress saturation also appeared in the fine-grained samples. The relation among the grain size, testing temperature, plastic deformation modes, and LCF life in Ti-2Al-2.5Zr was subsequently discussed.     

The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part I. room temperature behavior

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and overaged 2219/TiC/15p and unreinforced 2219 Al were investigated using plastic strain-controlled and stress-controlled testing. In addition, the influence of grain size on the particle-reinforced materials was examined. In both reinforced and unreinforced materials, the naturally aged conditions were cyclically unstable, exhibiting an initial hardening behavior followed by an extended region of cyclic stability and ultimately a softening region. The overaged reinforced material was cyclically stable for the plastic strains examined, while the overaged unreinforced material exhibited cyclic hardening at plastic strains greater than 2.5 × 10⁻⁴. Decreasing grain size of particle-reinforced materials modestly increased the cyclic flow stress of both naturally aged and overaged materials. Reinforced and unreinforced materials exhibited similar fatigue life behaviors; however, the reinforced and unreinforced naturally aged materials had superior fatigue lives in comparison to the overaged materials. Grain size had no effect on the fatigue life behavior of the particle-reinforced materials. The fatigue lives were strongly influenced by the presence of clusters of TiC particles and exogenous Al₃Ti intermetallics. formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994 in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Low-Cycle fatigue of dispersion-strengthened copper

The cyclic deformation behavior of a dispersion-strengthened copper alloy, GlidCop Al-15, has been studied at plastic strain amplitudes in the range 0.1 pct ≤Δε _p/2 ≤ 0.8 pct. Compared to pure polycrystalline copper, the dispersion-strengthened material exhibits a relatively stable cyclic response as a consequence of the dislocation substructures inherited from prior processing and stabilized by the A1₂O₃ particles. These dislocation structures remain largely unaltered during the course of deformation; hence, they do not reveal any of the features classically associated with copper tested in fatigue. At low amplitudes, the fatigue lifetimes of the dispersion-strengthened copper and the base alloy are similar; however, the former is more susceptible to cracking at stress concentrations because of its substantially greater strength. This similarity in fatigue lifetimes is a consequence of the dispersal of both deformation and damage accumulation by the fine grain size and dislocation/particle interactions in the GlidCop alloy. The operation of these mechanisms is reflected in the fine surface slip markings and rough fracture surface features for this material. Formerly Graduate Research Assistant, University of California, Davis, CA     

Low-cycle fatigue of niobium

Commercially pure niobium (CPNb) and a niobium-1 pct zirconium (Nb-lZr) alloy were tested under low-cycle fatigue conditions at plastic strain amplitudes in the range of 0.02 pct ≤Δε_pl/2≤ 0.7 pct. At low temperatures, the cyclic deformation response of body-centered cubic (bcc) metals is strongly dependent on strain rate. Thus, it was necessary to test at slow (2 x 10_su-4 s^-1) and fast (2 x 10_-2 s^-1) strain rates in order to fully characterize the cyclic deformation at ambient temperature. Only cyclic hardening was observed for both metals under all testing conditions. As expected, higher cyclic stresses were recorded at the fast strain rate compared to the slow strain rate. The Nb-lZr alloy was always stronger than CPNb, although both metals had the same cyclic life at equal plastic strain amplitudes. Further, the strain rate had no effect on the cyclic life. At the fast strain rate, intergranular cracking occurred, and a microplastic plateau was observed in the cyclic stress-strain (CSS) curve for CPNb. At the slow strain rate, no definitely intergranular cracks were detected, and a microplastic plateau was not observed for CPNb. The results of these experiments are interpreted in terms of the influence of strain rate and solute content on the relative mobilities of edge and screw dislocations.     

Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100 ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ _t /2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ _t /2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress amplitude was observed at Δɛ _t /2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with an inflection point at about Δɛ _p /2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was discussed.     

Fabrication of bulk ultrafine-grained materials through intense plastic straining

Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 μm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 μm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10⁻² s⁻¹. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Low-Cycle Fatigue Behavior of an As-Extruded AM50 Magnesium Alloy

The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis loop in the compressive direction is greater than that in the tensile direction. At the total strain amplitude of 1.5 pct, a serrated flow can be observed in both tensile and compressive directions of the σ-ε hysteresis loop. This means that dynamic strain aging takes place during fatigue deformation. The relation between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior and can be well described by the Basquin and Coffin–Manson equations, respectively. In addition, crack initiation and propagation modes are suggested, based on scanning electron microscopy observations on the fracture surfaces of fatigued specimens. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

The fatigue of pseudoelastic polycrystalline β-cuznsn

The fatigue of polycrystalline pseudoelastic β-CuZnSn has been studied by cycling specimens to fixed stress. The fatigue life was found to decrease with increasing initial strain and decreasing specimen grain size. In both cases the results gave similar stress -vs - fatigue life curves, indicating that stress is the primary parameter controlling fatigue life. The results fitted a curve of the form △ε.N^B _f = constant, whereβ = 0.32 for the total initial strain, andβ = 0.29 for the initial elastic strain. The fatigue life appeared to be independent of strain rate. Fatigue cracks nucleated in the first cycle at three grain intersections and grew along grain boundaries until adjacent cracks linked up. In the later stages of crack growth, some intergranular cracking occurred when there were no suitably oriented grain boundaries. Both the intergranular and transgranular regions showed somewhat ill-defined fatigue striations.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation.     

Low cycle fatigue behavior of polycrystalline Ni3Al alloys at ambient and elevated temperatures

The low cycle fatigue (LCF) resistance of polycrystalline Ni₃Al has been evaluated at ambient, intermediate (300 °C), and elevated (600 °C) temperatures using strain rates of 10⁻²/s and 10⁻⁴/s. Testing was conducted on a binary and a Cr-containing alloy of similar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and intermediate temperatures, the cyclic constitutive response of both alloys was similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually softened for the remainder of the life. Initial hardening resulted from the accumulation of dislocation debris within the deformed microstructure, whereas softening was related to localized disordering. For these experimental conditions, crack initiation resulted within persistent slip bands (PSBs). At the elevated temperature, diffusion-assisted deformation resulted in a rate-dependent constitutive response and crack-initiation characteristics. At the high strain rate (10⁻²/s), continuous cyclic hardening resulted from the accumulation of dislocation debris. At the low strain rate (10⁻⁴/s), the diffusion of dislocation debris to grain boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constitutive response on the driving force for environmental embrittlement. Chromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven. Formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA     

Influence of Microstructure on the Spall Failure of Aluminum Materials

Laser-shock-induced spall failure is studied in thin aluminum targets at strain rates from 2 to 5 × 10⁶ s⁻¹. Targets were prepared from high-purity aluminum in the recrystallized condition and a low-impurity aluminum alloy containing 3 wt pct magnesium in both recrystallized and cold-rolled conditions. The effects of material and microstructure on spall fracture morphology are investigated. Recrystallized pure aluminum produced spall fracture surfaces characterized by transgranular ductile dimpling. Recrystallized aluminum-magnesium alloy with a 50-μm grain size produced less ductile spall surfaces, which were dominated by transgranular fracture, with some isolated transgranular ductile dimpling at fast strain rates. Transgranular ductile dimpling regions disappeared in recrystallized alloy specimens with a 23-μm grain size tested at faster rates. Cold-rolled alloy material produced spall failure surfaces consisting of brittle intergranular and transgranular fractures. Measured spall strength increases with increasing ductile fracture character. Spall failure preferentially follows grain boundaries, making grain size an important factor in spall fracture surface character. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

Mechanisms of high-temperature fatigue failure in alloy 800H

The damage mechanisms influencing the axial strain-controlled low-cycle fatigue (LCF) behavior of alloy 800H at 850 °C have been evaluated under conditions of equal tension/compression ramp rates (fast-fast (F-F): 4 × 10⁻³ s⁻¹ and slow-slow (S-S): 4 × 10⁻⁵ s⁻¹) and asymmetrical ramp rates (fast-slow (F-S): 4 × 10⁻³ s⁻¹ / 4 × 10⁻⁵ s⁻¹ and slow-fast (S-F): 4 × 10⁻⁵ / 4 × 10⁻³ s⁻¹) in tension and compression. The fatigue life, cyclic stress response, and fracture modes were significantly influenced by the waveform shape. The fatigue lives displayed by different loading conditions were in the following order: F-F > S-S > F-S > S-F. The fracture mode was dictated by the ramp rate adopted in the tensile direction. The fast ramp rate in the tensile direction led to the occurrence of transgranular crack initiation and propagation, whereas the slow ramp rate caused intergranular initiation and propagation. The time-dependent processes and their synergistic interactions, which were at the basis of observed changes in cyclic stress response and fatigue life, were identified. Oxidation, creep damage, dynamic strain aging, massive carbide precipitation, time-dependent creep deformation, and deformation ratcheting were among the several factors influencing cyclic life. Irrespective of the loading condition, the largest effect on life was exerted by oxidation processes. Deformation ratcheting had its greatest influence on life under asymmetrical loading conditions. Creep damage accumulated the greatest amount during the slow tensile ramp under S-F conditions. Formerly USA National Research Council Associate, NASA-Lewis Research Center This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of T.S., October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were performed at a strain amplitude of ±0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of γ′ and carbides); and microstructure B, in a double-aged condition with γ′ of 18-nm diameter and M₂₃C₆ carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered γ′ that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 × 10^-5 to 3 × 10^-3 s^-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.     

Effects of nitrogen implantation on low cycle fatigue behavior of ferritic Fe-24Cr-4Al stainless alloy

The effects of nitrogen implantation on cyclic deformation response, near-surface dislocation sub-structure, surface slip band formation, crack initiation, and fatigue life under low cycle fatigue of the ferritic Fe-24Cr-4Al stainless alloy were investigated. Implantation was carried out at an energy of 65 keV and at a fluence of 2 × 10¹⁷ ions/cm². Nitrogen implantation resulted in a substantial cyclic hardening in the alloy. Homogeneous planar dislocation arrangements were formed in the near-surface region of implanted specimens after fatigue, while dislocation loop debris and patches were developed in the un-implanted specimens. Moreover, formation of persistent slip bands (PSBs) was greatly suppressed in the surface of the implanted specimens. Nitrogen implantation also resulted in an alteration of the crack initiation mode from the grain boundary to the surface penetration of the PSBs nucleated below the surface layer. Fatigue life improvements after nitrogen implantation could only be obtained when the PSBs were not only suppressed but also homogenized in the implanted surface layer.     

Temperature and strain-rate effects on low-cycle fatigue behavior of alloy 800H

The effects of strain rate (4 × 10^-6 to 4 × 10^-3 s^-1) and temperature on the low-cycle fatigue (LCF) behavior of alloy 800H have been evaluated in the range 750 ° to 950 °. Total axial strain controlled LCF tests were conducted in air at a strain amplitude of ± 0.30 pct. Low-cycle fatigue life decreased with decreasing strain rate and increasing temperature. The cyclic stress response behavior showed a marked variation with temperature and strain rate. The time- and temperature-dependent processes which influence the cyclic stress response and life have been identified and their relative importance assessed. Dynamic strain aging, time-dependent deformation, precipitation of parallel platelets of M₂₃C₆on grain boundaries and incoherent ledges of twins, and oxidation were found to operate depending on the test conditions. The largest effect on life was shown by oxidation processes. On leave from the Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India.     

Cyclic deformation of ferritic steel—I. Stress-strain response and structure evolution

The complete CSS-curve has been established for a low alloyed structural steel tested under plastic strain control. The curve which can be divided into three separate regimes has a plateau region between cyclic strain levels of 10⁻⁴ and 8·10⁻⁴. PSBs are formed on the specimen surface when the plastic strain range corresponds to the plateau regime. The PSBs are sites for crack nucleation. The substructure evolution as seen going along the CSS-curve and with accumulating numbers of cycles is documented in detail and includes: dislocation loops, veins, walls including the ladder-like walls usually associated with PSBs, labyrinths, cells, subgrains, banded cells and subgrains. At low and intermediate plastic strain ranges the surface grains contain a more fine-scaled substructure and develop features which appear in the interior at higher plastic strain ranges. At larger cyclic strain levels microbands and noncrystallographic deformation bands become dominating features. Heavily displaced and serrated grain boundaries are observed at intermediate and high plastic strains both in interior- and surface grains containing microbands, banded cells or banded subgrains.     

High-pressure,laser-driven deformation of an aluminum alloy

Recent development of a laser-based experimental platform allows loading materials to high pressures in the solid state while controlling both strain rate and peak pressure. The drive utilizes momentum transfer from a plasma generated by the introduction of a strong shock in a reservoir of low-Z material. This study looks at the response of a commercial aluminum alloy (6061-T6) subjected to pressures of 18 and 40 GPa at strain rates of 10⁷/s and 5 × 10⁷/s, respectively. It was found that the depth of the crater formed on the sample surface is a good indicator of the general yield behavior of the material and that a relatively simple strength model prevails under the loading conditions considered here. Metallographic examination of recovered samples showed no evidence of shear-band formation or significant melting due to plasma-surface interactions. Crystal plasticity-based calculations were used to assess the effects of material texture. Lack of shear-band formation during the laser-based drive is rationalized by considering the strain gradient as compared to grain size and texture. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.