Effect of test temperature on the dynamic torsional deformation behavior of two aluminum-lithium alloys

The objective of the present study is to investigate the effect of test temperature on the dynamic torsional deformation behavior of two Al-Li alloys, i.e., 2090 and 8090 alloys. Dynamic torsional tests were conducted using a torsional Kolsky bar at room temperature and a low temperature (−196 °C), and the torsionally deformed regions and the fracture surfaces of the tested specimens were examined. The dynamic properties of the two Al-Li alloys at the low temperature were improved, owing to the modification of the deformation behavior. The dynamic deformation behavior at room temperature was dominated by intergranular cracks due to planar slips and by crack propagation along the grain boundaries. At the low temperature, plastic deformation proceeded more homogeneously as planar slip was prevented. These results indicated that the overall deformation mode of both the Al-Li alloys changed from planar slip to homogeneous deformation with decreasing temperature, resulting in the improvement of cryogenic properties under dynamic torsional loading.     

Mechanical behavior of aluminum-lithium alloys at cryogenic temperatures

The cryogenic mechanical properties of aluminum-lithium alloys are of interest because these alloys are attractive candidate materials for cryogenic tankage. Previous work indicates that the strength-toughness relationship for alloy 2090-T81 (Al-2.7Cu-2.2Li-0.12Zr by weight) improves significantly as temperature decreases. The subject of this investigation is the mechanism of this improvement. Deformation behavior was studied since the fracture morphology did not change with temperature. Tensile failures in 2090-T81 and -T4 occur at plastic instability. In contrast, in the binary aluminum-lithium alloy studied here they occur well before plastic instability. For all three materials, the strain hardening rate in the longitudinal direction increases as temperature decreases. This increase is associated with an improvement in tensile elongation at low temperatures. In alloy 2090-T4, these results correlate with a decrease in planar slip at low temperatures. The improved toughness at low temperatures is believed to be due to increased stable deformation prior to fracture.     

Mechanisms influencing the cryogenic fracture-toughness behavior of aluminum-lithium alloys

Cryogenic strength-toughness relationships for advanced aluminum-lithium alloys 2090, 8090, 8091 and 2091 are examined as a function of microstructure, plate orientation and wrought-product form (plate vs sheet), with specific emphasis on the underlying micro-mechanisms associated with crack advance. It is found that, with decrease in temperature from 298 K to 77 and 4 K, strength, tensile elongation and strain-hardening exponent are increased for all alloy chemistries, microstructures and product forms; however, the longitudinal (L-T, T-L) fracture toughness may increase or decrease depending upon the prevailing microscopic mechanism (microvoid coalescence vs transgranular shear) and macroscopic mode (plane strain vs plane stress) of fracture. In general, alloy microstructures that exhibit changes in either the fracture mechanism or mode at low temperatures show a decrease in L-T toughness. Conversely, when the fracture mechanism is unchanged between ambient and 4 K, observed variations in toughness with temperature are a strong function of the degree of local stress-triaxiality that develops at the crack tip. In very thin sheets, where the fracture mode remains one of plane stress (“slant” fracture), the elevation in toughness at low temperatures is associated with the concurrent increase in tensile strength and ductility; conversely, in thick plate, the increased occurrence of through-thickness delaminations (due to the weak short-transverse properties) at low temperatures locally promotes plane-stress conditions, thereby enhancing toughness by relaxing triaxial constraint. In sheets of intermediate thickness, however, the absence of such through-thickness delaminations permits the expected transition from plane-stress to plane-strain conditions, with the result that the toughness now decreases with reduction in temperature.     

Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys

The effect of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys was investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were compared via microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased as sintering temperature and time were increased, thereby deteriorating fracture toughness. The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregularly shaped, cleavage fracture occurred predominantly with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of the in situ fracture test results, i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size because the cleavage fracture mode would be beneficial for the “self-sharpening” of the tungsten heavy alloys.     

The effect of low melting point impurities on the properties of aluminum-lithium alloys

The effect of low melting point impurities, such as sodium and potassium, on the properties and microstructure of Al-Li alloys has been investigated. These impurities have been shown to occur as discrete lenticular particles along grain boundaries. In underaged and peak-aged conditions, the impurity particles are liquid and the toughness and ductility of alloys in this condition were found to be improved by lowering the test temperature below the freezing point of the impurity. In the overaged condition, the sodium and potassium occur as a solid phase and materials in this condition were found to follow the pattern of most aluminum alloys and show a decrease in Charpy impact toughness as the testing temperature was lowered. The addition of sodium and potassium to Al-Li alloys reduced toughness and ductility at room temperature in the range of compositions investigated (up to 476 ppm Na and up to 23 ppm K). Low melting point impurities were found to have little effect on the toughness or ductility of high purity aluminum. The detrimental effects of sodium and potassium on toughness appear to exist only in alloys containing substantial amounts of lithium or magnesium, which break down the relatively harmless solid NaAlSi_x phase which exists in conventional alloys. The loss of toughness due to low melting point impurities was found to be about the amount expected from the calculated reduction in surface energy of a grain boundary crack containing a liquid phase.     

The solubility of hydrogen in solid binary aluminum-lithium alloys

The solubility of hydrogen in solid binary aluminum alloys with 1,2, and 3 wt pet lithium has been determined for the temperature range of 473 to 873 K and the pressure range of 26,700 to 113,300 Pa (0.26 to 1.12 atm), using an absorption/quench/desorption technique. The results fit the Van't Hoff isobar and Sieverts' isotherm and are given by log (S/S°)- log (p/p°) =-A/T/K +B whereS, S° are, respectively, the solubility and a standard value of solubility equal to 1 cm3 of diatomic hydrogen measured at 273 K and 101,325 Pa/100 g of metal;p, p° are, respectively, the pressure and a standard value of pressure equal to 101,325 Pa (1 atm). The values of the constantsA andB are Al-1 pet Li 473T/K 680:A = 358;B = 0.576 680T/K 873:A = 604;B = 0.620 Al-2 pet Li 473T/K 740:A = 273;B = 0.597 740T/K 873:A = 676;B = 0.767 Al-3 pet Li 523T/K770:A = 615;B = 1.272 770T/K 873:A = 830;B = 1.166 The solubility of hydrogen is one or two orders of magnitude greater in the alloys than in the pure metal and increases with lithium content. The results are consistent with the dissociative dissolution of a diatomic gas. The isobars are discontinuous at critical temperatures which lie within the a-phase field,i.e., 680, 740, and 770 K for Al-1 pet Li, Al-2 pet Li, and Al-3 pet Li, respectively, indicating a change in the character of the solution at lower temperatures due to an increase in the solute binding energy, which is attributed provisionally to an unidentified change in thea matrix. P. N. Anyalebechi, formerly with the Department of Materials Technology at Brunei, The University of West London, United Kingdom     

Correlation of microstructure with dynamic deformation behavior and penetration performance of tungsten heavy alloys fabricated by mechanical alloying

In this study, tungsten heavy alloy specimens were fabricated by mechanical alloying (MA), and their dynamic torsional properties and penetration performance were investigated. Dynamic torsional tests were conducted on the specimens fabricated with different sintering temperatures after MA, and then the test data were compared with those of a conventionally processed specimen. Refinement of tungsten particles was obtained after MA, but contiguity was seriously increased, thereby leading to low ductility and impact energy. Specimens in which both particle size and contiguity were simultaneously reduced by MA and two-step sintering and those having higher matrix fraction by partial MA were successfully fabricated. The dynamic test results indicated that the formation of adiabatic shear bands was expected because of the plastic localization at the central area of the gage section. Upon highspeed impact testing of these specimens, self-sharpening was promoted by the adiabatic shear band formation, but their penetration performance did not improve since much of kinetic energy of the penetrators was consumed for the microcrack formation due to interfacial debonding and cleavage fracture of tungsten particles. In order to improve penetration performance as well as to achieve selfsharpening by applying MA, conditions of MA and sintering process should be established so that alloy densification, particle refinement, and contiguity reduction can be simultaneously achieved.     

Development of adiabatic shear bands in annealed 316L stainless steel: Part I. Correlation between evolving microstructure and mechanical behavior

Adiabatic shear localization in an annealed AISI 316L stainless steel was examined through a forced shear technique using a split Hopkinson pressure bar and hat-shaped specimens. A well-controlled forced shear technique provided the possibility of correlating the microstructural evolution of adiabatic shear localization to its transient mechanical behavior. The initiation of adiabatic shear bands occurred when the shear stress peaked after substantial work hardening. The work-hardening rate was found to play a dominant role in the formation of adiabatic shear localization. The stress drop presupposed the development of the localized deformation. A core structure of shear bands was generated within the shear band, which characterized a narrow-down process in the early stage of the shear band evolution. The continuous expansion of the shear band core to the entire width of the band was seen to correlate with the full development of shear localization.     

Deformation behavior of submicrocrystalline aluminum alloys during dynamic loading

The structure and the mechanical properties of aluminum V95 and AMts alloys with various grain sizes (from micron to submicron) are studied in a wide range of strain rates (from 10^–3 to 10⁵ s^–1). Submicrocrystalline (200–600 nm) materials are formed by dynamic channel-angular pressing at a strain rate of 10⁵ s^–1 using a pulsed power source.     

The study of adiabatic shear band instability in a pearlitic 4340 steel using a dynamic punch test

The formation of adiabatic shear band instabilities in a pearlitic 4340 steel using a dynamic punch test has been studied. The dynamic punch-impact test produced white-etching adiabatic shear bands. The average strain of 0.5 was sufficient to produce adiabatic shear bands in this steel at an average strain rate of 18,000 s⁻¹. Nanohardness variations found across the adiabatic shear band are thought to be caused by the fragmentation and spheroidization of the Fe₃C and the overall deformation and work hardening of the pearlitic microstructure. The cracks formed at the termination of the adiabatic shear band caused the sample to fracture in a ductile mode.     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

Mechanical behavior and properties of mechanically alloyed aluminum alloys

The fracture and deformation behaviors of several product forms produced from mechanically alloyed (MA) aluminum alloys 9052 and 905XL were studied. The main operative strengthening mechanism is strengthening due to the submicron grain size. Ductility and toughness were found to be controlled by the morphology of the prior particle boundaries. We propose that the work-hardening behavior of these MA alloys is similar to the behavior exhibited by a deformed fcc alloy that (a) contains rigid barriers to dislocation motion, (b) deforms by wavy slip, and (c) forms a cell substructure upon deformation.     

The structure of adiabatic shear bands in a titanium alloy

A study has been made of the structure of adiabatic shear zones in Ti-6% Al-4% V with different parent microstructures, resulting from ballistic impact of steel spheres. Metallographic examination of well-developed shear bands showed that they consisted of zones of intense shearing distortion of the original microstructure, modified by the effects of elevated temperature. An analogy is made between their structure and that of the white-etching shear zones observed in steels. Unlike steels, however, there was no clear evidence in this alloy to suggest that the shear bands in the α + β microstructures had undergone a martensitic phase transformation. The structure of the shear zones in an α' martensite parent alloy appeared to be a tempered form of the original microstructure.     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

Interaction of deformation shear bands with nanoparticles in amorphous-nanocrystalline alloys

The interaction of shear bands with crystalline nanoparticles in amorphous-nanocrystalline Fe-Ni-B alloys is analyzed theoretically and experimentally. The following five interaction mechanisms are revealed: absorption, bypassing, cutting, retardation, and accommodation. The nanocrystal size is shown to be the key factor of this interaction. The experimental results obtained are satisfactorily described by the proposed theoretical models.     

Control of surface carburization and improvement of dynamic fracture behavior in tungsten heavy alloys

A carburization technique using a Cr powder layer has been developed to control the diffusion depth of carbon in W-Ni-Fe heavy alloys. The aged heavy alloy samples were covered with a Cr powder layer of about 1-mm thickness and then packed with carbon black powder. The packed samples were heat-treated at 1150 °C for 10 minutes in H₂ and then for 50 minutes in N₂. The carburization treatment resulted in the formation of Cr₇C₃ and Fe₃W₃C around the tungsten grains from the sample surface with a thickness of 40 to 50 μm. This carburized layer was much thinner than that formed without a Cr powder layer under the same experimental conditions. With the surface carburization, the surface hardness increased by ∼75 pct, from 508 to 888 VHN, and the impact energy decreased by ∼31 pct, from 123 to 85 J. After the carburization treatment, the main fracture behavior in a dynamic torsional test changed from smearing of the matrix to cleavage of the tungsten grains. A high-speed impact test showed that the surface carburization of penetrators induced the formation of many cracks around the penetrator surface, enhanced the self-sharpening, and improved the penetration performance. It appears that the developed technique provides an easy method of carburization without serious deterioration of the toughness of the material.     

Effects of texture gradients on yield loci and forming limit diagrams in various aluminum-lithium sheet alloys

Marked through-thickness variations of preferred crystallographic orientations in aluminum-lithium (Al-Li) sheet alloys have been observed and documented. These metallurgical features could have an effect on the way in which these materials distribute strain during plastic deformation. From a theoretical or a practical point of view, it is important to investigate these texture effects on plastic-deformation properties and particularly on forming limit strains. In this work, quantitative texture data, which were determined by X-ray and neutron diffraction techniques, were used with a polycrystal model to predict the yield locus of recrystallized and unrecrystallized AA8090 and AA2090 Al-Li sheets. The conventional AA2024 alloy in the annealed condition was also investigated as a reference material. Subsequently, these yield loci were used to calculate forming limit diagrams (FLDs) in the stretching range, using the Marciniak-Kuczynski (M-K) approach with strain rate potentials to describe the constitutive properties of the sheets. A simple critical-thickness-strain criterion was used to predict the FLD in the drawing range. The predicted FLDs were found to be in fair agreement with experimental curves obtained under punch-stretching conditions. In general, experimental trends were accounted for by the results predicted using the average texture data. However, the texture gradients do not completely explain the large scatter observed in the experimental forming limits and the high average limit strain of the recrystallized AA8090.     

The structure of adiabatic shear bands in metals: A critical review

A review of the structure of adiabatic shear bands in metals is presented. Shear bands are redefined as being either “transformed” or “deformed” according to how the prior shear deformation is partitioned between two discrete zones in metallographic section. Metals are then classified by their general tendency to form these two types of shear zone during adiabatic shear deformation, based on available literature. Metals of low thermal diffusivity and of low resistance to adiabatic shear localization tend more readily to form “transformed” shear bands; these metals are also capable of transforming to other phases at elevated temperature (and pressure), and forming martensite on rapid cooling to room température. Shear bands of “transformed” appearance can also form in other metals during extremely localized adiabatic shear deformation resulting from the effect of localized plastic flow and elevated temperature alone. The role of phase transformations themselves in promoting the formation of “transformed” shear bands cannot be isolated using the arguments presented in this work, and may even by incidental.