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The shock and spall behavior of tempered AF9628, an ultra-high strength (UHSS) martensitic low-alloy carbon steel, is investigated using gun-driven, symmetric, planar impacts. The study utilized laser velocimetry to obtain transmitted wave velocity histories that were analyzed to obtain the Hugoniot Elastic Limit (HEL), the shock Hugoniot, and the spall strength. The work was limited to the stress range below 13 GPa (below the e phase transition), and an HEL of 2.35 GPa and a spall strength of 6.8 GPa are reported. The HEL and spall strength of AF9628 are both above the typical values for inexpensive low-alloy steels, and only slightly below those for a much more expensive Aermet steel. The dynamic compressive yield strength (as calculated from the HEL) is only slightly higher than the quasi-static compressive yield strength when compared using a similar yield assignment convention, indicating a small, but positive, value of strain-rate sensitivity. The Hugoniot for AF9628, US = 0.67uP + 5.01 km/s, is in good agreement with other low-alloy martensitic steels for which data is available, and distinctly different than the Hugoniot for ferritic steels. The Hugoniot extrapolates to a bulk sound speed significantly higher than what is measured at ambient conditions, an incongruity that is unusual among metals and that the authors attribute to the unsteady nature of the shock waves generated in this work. Finally, it is suggested that there may be a relationship between dynamic compressive yield strength (calculated from the HEL) and dynamic tensile failure strength (the spall strength).

The predicted flow behaviors of Aermet 100 steel were analyzed within a wide range of temperatures of 1,073 K–1,473 K and strain rates of 0.01 s–1–50 s–1 based on isothermal compression tests. Using the original Johnson Cook (JC) model and a modified Johnson Cook (MJC) model, the constitutive equations were constructed in the case of elevated temperatures. For both the JC and MJC, and the previously studied (Arrhenius-type model and double-multivariate nonlinear regression (DMNR)) models, their respective predictability levels were evaluated by contrasting both the correlation coefficient R and the average absolute relative error (AARE). The results showed that the prediction from the three models meet the accuracy requirement based on the experimental data, the only exception being the JC model. By comparing the predictability and numbers of material constants involved, the modified Johnson Cook model is regarded as an excellent choice for predicting the flow behaviors of Aermet 100 steel within the range being studied.

The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron microscopy. A microstructure investigation revealed that the samples consist of martensite laths, fine dispersed precipitates, and reverted austenite films at the martensite lath boundaries. The precipitates are enriched with Co and Mo. Because the sample tempered at 486°C has smaller precipitates and a higher number of precipitates per unit area, it exhibits better mechanical properties than the sample tempered at 498°C. Although the 3D-printed samples have the same phase constituents as AerMet 100 steel, the mechanical properties are slightly worse than those of the commercial wrought AerMet 100 steel because of the presence of voids.

Ultra-high strength steels employed in safety-critical applications, such as AerMet®100 used in aircraft landing gear structures, are managed on very conservative rejection criteria for small defects and repair options are limited. A novel repair technique using laser cladding has recently been developed. In the present paper we report a study of the fatigue endurance of AerMet®100 steel components repaired by the laser cladding process, and a fracture mechanics based model to predict the fatigue endurance of repaired components. Three different types of samples were tested; baseline AerMet®100 sample with a small electro-discharge machining notch to initiate a crack, as-clad repaired, and as-clad repaired followed by heat treatment to relieve residual stresses. The specimens were subjected to cyclic loading under a special sequence consisting of constant amplitude segments at two different stress-ratios (ratio of minimum to maximum cyclic stress). The test results showed that the crack propagation lives from a common initial depth of 0.25 mm for the as-clad samples were significantly longer than the baseline samples by a factor of three to four. The longer life is attributed to the beneficial compressive residual stresses resulting from the repair process. The model predictions are found to correlate well with the results of quantitative fractography measurements from samples tested under variable amplitude cyclic loads.

The microstructure and mechanical properties of ultrahigh strength Aermet 100 steel tempered at five different temperatures in the 472–492 °C range have been studied. The results show that the mechanical properties of Aermet 100 steel are sensitive to the tempering temperature; especially at the temperature of 482 °C, in which significant changes can be observed. At 482 °C, the strength of Aermet 100 steel sharply decreases by 70–100 MPa when the tempering temperature increases by only 5 °C. This is mainly due to the growth of M2C carbide at relatively high tempering temperature that can greatly decrease its interface strengthening effect. In addition to strength, the plasticity and fracture toughness of Aermet 100 steel obviously increase at 482 °C when compared with lower tempering temperatures. The rising amplitudes of elongation and fracture toughness can reach 1.7% and 30 MPa m1/2 respectively. This is attributed to the reverted austenite film precipitated at 482 °C. For one respect, the existence of austenite film between lath martensite can reduce the local stress concentration and the chance of microcrack formation in Aermet 100 steel during deformation, which is responsible for the obvious increase of plasticity. For another respect, the excellent softness, toughness and stability of reverted austenite prompt the advancing crack to divert, branch and blunt or appear “zigzag” route, which results in an increase in the fracture toughness. Aermet 100 steel can achieve the optimum combination of strength, plasticity and toughness at the tempering temperature of 482 °C, which has the yield strength of 1845 MPa, the elongation of 12.9% and the fracture toughness of 113 MPa m1/2.

The effect of laser cladding on the fatigue and fracture behavior under variable amplitude loading is a major consideration for the development of laser cladding process to repair high value complex fatigue critical aerospace military components, that otherwise would be replaced. The selected material, AerMet®100, is a widely used ultra-high strength steel in current and next generation aerospace components, such as landing gears. Laser cladding was performed using AerMet® 100 powder on AerMet® 100 fatigue substrate specimens. No micro-cracking and very little porosity were observed in the clad layer. The fatigue tests were performed under variable amplitude loading with a maximum stress of 1000 MPa. Residual stress, microstructure, and hardness, was also evaluated. Both the as-clad and post-heat treated (PHT) samples were compared to a baseline sample with an artificial notch to simulate damaged condition. Results show that laser cladding significantly improves fatigue life, as compared to the baseline sample with a notch. However, the fatigue life of the as-clad sample is lower as compared to a baseline sample without a notch. A compressive residual stress of 300–500 MPa was observed in the clad region and HAZ. The fracture modes in the as-clad specimen consisted mainly of tearing topology surface and some regions of decohesive rupture through the columnar austenite grains. The PHT condition however was not effective in improving the fatigue life. The fracture modes showed mainly decohesive rupture, and as a consequence, reduced the fatigue life.

Isothermal compression tests were conducted on A100 steel using a Gleeble 1500 thermal simulator at a temperature range of 900–1,200°C and strain rate range of 0.001–3 s−1. Results show that the A100 steel has higher strength than the Aermet 100 steel at high temperatures. Constant values, such as A, α, and n, and activate energy Q were obtained through the regression processing of the stress–strain data curves under different strains. A set of constitutive equations for A100 steel was proposed by using an Arrhenius-type equation. The optimum processing craft ranges for A100 steel based on the analysis of the hot working diagram and deformation mechanism are as follows: temperature range of 1,000–1,100°C and strain rate range of 0.01–0.1 s−1. The average grain size within this working range is 7–22.5 μm.

AerMet 100 exhibits excellent mechanical properties with both high yield strength and excellent ductility. Understanding the dynamic mechanical behavior of the material should clear the road for wider application of AerMet 100 as critical mechanical parts serving in extreme mechanical conditions. In this paper, the strain rate dependency of AerMet 100 is investigated by using Split-Hopkinson pressure bar (SHPB) and also compared with its quasi-static counterpart. Based on the experiment results, a simplified Johnson–Cook model is established to describe the strain rate dependent behaviors and is proved to perform better than Cowper–Symonds model. The validation of the suggested constitutive model is embedded in the finite element analysis and can well repeat the strain wave observed from experiment results. Furthermore, the impact fracture toughness of AerMet 100 is also studied both numerically and experimentally. Finally, a thin-walled tube made of AerMet 100 is numerically simulated and the excellent crashworthiness is proven. Results may enlighten the understanding of dynamic mechanical behavior of AerMet 100 and provide fundamental experiment data for its future engineering applications. Keywords: AerMet 100, Strain rate effect, Constitutive behavior, SHPB method, Numerical simulation

The fatigue behaviour of an ultra-high strength steel (>1800 MPa) was evaluated to assess the potential of using laser cladding as a repair tool for such steels in aeronautical structural applications. AISI 4340 and AerMet 100 steel powder were used to clad over a grind-out region in an AISI 4340 steel substrate using a 2.5 kW ND:YAG laser. Post-clad heat treatment (PCHT) was also investigated. Results showed very poor tensile properties and significantly reduced fatigue life of the AISI 4340 as-clad with a very high hardness and brittle fracture in the clad and HAZ zone. Residual stress results showed a compressive residual stress in the clad region and tensile residual stress in the HAZ. Changing the alloy of the clad layer to AerMet 100 steel, as well as applying a PCHT process, showed promising results as the fatigue life was improved from that of the grind-out substrate.

In this paper, the results of a study aimed at understanding the specific role of microstructure on cyclic stress response, cyclic strain resistance, and cyclic stress versus strain response, deformation and fracture behavior of high strength alloy steel AerMet® 100 is presented and discussed. The cyclic strain amplitude-controlled fatigue properties of this ultra-high strength alloy steel revealed a linear trend for the variation of log elastic strain amplitude with log reversals-to-failure, and log plastic strain amplitude with log reversals-to-failure. Cyclic stress response revealed a combination of initial hardening for the first few cycles followed by stability for large portion of fatigue life before culminating in rapid softening to failure at the lower cyclic strain amplitudes and intermediate cyclic strain amplitudes and resultant enhanced cyclic fatigue life. Fracture characteristics of test specimens of this high strength alloy steel were different at both the macroscopic and fine microscopic levels over the entire range of cyclic strain amplitudes examined. Both macroscopic and fine microscopic observations revealed fracture to be essentially ductile with features reminiscent of locally occurring ductile mechanisms. The intrinsic microscopic mechanisms governing stress response, deformation characteristics, fatigue life and final fracture behavior are presented and discussed in light of the competing and mutually interactive influences of intrinsic microstructural effects, deformation characteristics of the microstructural constituents, cyclic strain amplitude and concomitant response stress.

Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130 °C and 1250 °C spanning strain rates from 0.01 s−1 to 4000 s−1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall–Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener–Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000 s−1 where mechanical testing’s results were absent.

A detailed characterization of two dissimilar high-strength steels, SCMV and Aermet 100, joined by inertia friction welding (IFW)—a solid-state welding technique—was undertaken using high energy synchrotron X-ray diffraction and advanced electron microscopy in order to understand the dramatic hardness variation across such a weld. It was found that the severe high-temperature deformation in the thermomechanically affected zones (TMAZs) of the weld, stabilized ordered, and nanosized FeCo zones in Aermet 100 and about 12 to 14 vol pct austenite in SCMV (Ni equivalent 9 wt pct). The ordered FeCo zones in Aermet 100 resulted in exceptionally high hardness values of 700 to 725 HV. Very close to the weld line, the TMAZ of Aermet 100 also displayed a region with about 15 vol pct austenite, while in the parent material, 8 to 9 vol pct was typically observed. No indication of martensite was found in the weld region of Aermet 100. Ferrite texture analysis at different locations within the TMAZs on either side of the weld showed that SCMV develops a very strong α-fiber texture near the weld line and, in addition, a γ-fiber texture toward the heat-affected zone (HAZ), suggesting the presence of ferrite during welding near the weld line and recrystallization further away. The ferrite texture development in the TMAZ of Aermet 100 was relatively weak, suggesting that austenite is a dominant phase in the TMAZ during IFW.

The effects of chromium on the corrosion and the electrochemical behaviors of ultra high strength steels were studied by the salt spray test and electrochemical methods. The results show that ultra high strength steels remain martensite structures and have anodic dissolution characteristic with an increase of chromium content. There is no typical passive region on the polarization curves of an ultra high strength stainless steel, AerMet 100 steel, and 300M steel. However, chromium improves the corrosion resistance of the stainless steel remarkably. It has the slowest corrosion rate in the salt spray test, one order of magnitude less than that of AerMet 100 and 300M steels. With the increase of chromium content, the polarization resistance becomes larger, the corrosion potential shifts towards the positive direction with a value of 545 mV, and the corrosion current density decreases in electrochemical measures in 3.5wt% NaCl solutions. Because of the higher content of chromium, the ultra high strength stainless steel has a better corrosion resistance than AerMet 100 and 300M steels.

The electrochemical behaviors of ultra high strength steels, 300M and AerMet 100, with corrosion products which were formed during salt spray test were studied using potentiodynamic polarization curve, EIS, SEM, scan Kelvin probe (SKP) and Raman spectroscopy. The results show that the corrosion products on the steel 300M are γ-FeOOH and Fe(OH)3, and those on the steel AerMet 100 are β-FeOOH, Fe3O4 and γ-Fe2O3 after salt spray test. Because of the compact characterization, Fe3O4 and γ-Fe2O3 have a better protection property for substrate by hindering the permeation of O2 and Cl � . The corrosion potentials shift negatively and corrosion current densities become larger with increasing salt spray time, meaning a faster rate of anodic dissolution for these two steels. After salt spray test, the values of Kelvin potential for the two steels are 0.5—0.6 V positive than those before tests, and more uneven. For 300M, the corrosion resistance of rust layer becomes larger/smaller with the corrosion productions gathering/drapping on/from the samples. The electrochemical reaction for AerMet 100 is diffusion-controlled process and the value of Warburg impedence which expresses the diffusion-hindering effect becomes larger. The corrosion resistance and protection ability of corrosion products for steel AerMet 100 are better than those of steel 300M.

The welding of dissimilar alloys is seen increasingly as a way forward to improve efficiencies in modern aeroengines, because it allows one to tailor varying material property demands across a component. Dissimilar inertia friction welding (IFW) of two high-strength steels, Aermet 100 and S/CMV, has been identified as a possible joint for rotating gas turbine components and the resulting welds are investigated in this article. In order to understand the impact of the welding process and predict the life expectancy of such structures, a detailed understanding of the residual stress fields present in the welded component is needed. By combining energy-dispersive synchrotron X-ray diffraction (EDSXRD) and neutron diffraction, it has been possible to map the variations in lattice spacing of the ferritic phase on both sides of two tubular Aermet 100-S/CMV inertia friction welds (as-welded and postweld heat-treated condition) with a wall thickness of 37 mm. Laboratory-based XRD measurements were required to take into account the variation in the strain-free d-spacing across the weld region. It was found that, in the heat-affected zone (HAZ) slightly away from the weld line, residual stress fields showed tensile stresses increasing most dramatically in the hoop direction toward the weld line. Closer to the weld line, in the plastically affected zone, a sharp drop in the residual stresses was observed on both sides, although more dramatically in the S/CMV. In addition to residual stress mapping, synchrotron XRD measurements were carried out to map microstructural changes in thin slices cut from the welds. By studying the diffraction peak asymmetry of the 200-α diffraction peak, it was possible to demonstrate that a martensitic phase transformation in the S/CMV is responsible for the significant stress reduction close to the weld line. The postweld heat treatment (PWHT) chosen to avoid any overaging of the Aermet 100 and to temper the S/CMV martensite resulted in little stress relief on the S/CMV side of the weld.

The present study examines the strain-rate sensitivity of four high-strength, high-toughness steels at strain-rates ranging from 0.0002 s −1 to 200 s −1 : AerMet 100, modified 4340, modified HP9-4-20, and a recently developed Eglin AFB steel alloy, ES-1c. A newly developed dynamic servohydraulic method was employed to perform tensile tests over this entire range from quasi-static to near split-Hopkinson or Kolsky bar strain-rates. Each of these alloys exhibits only modest strain-rate sensitivity. Specifically, the semi-logarithmic strain-rate sensitivity factor β was found to be in the range of 14–20 MPa depending on the alloy. This corresponds to a ∼10% increase in the yield strength over the 6-orders of magnitude change in strain-rate. Interestingly, while three of the alloys showed a concomitant ∼3–10% drop in their ductility with increasing strain-rate, the ES-1c alloy actually exhibited a 25% increase in ductility with increasing strain-rate. Fractography suggests the possibility that at higher strain-rates ES-1c evolves towards a more ductile dimple fracture mode associated with microvoid coalescence.

Cylinders and rings fabricated from AerMet ® 100 alloy and AISI 1018 steel have been explosively driven to fragmentation in order to determine the fracture strains for these materials under plane-strain and uniaxial-stress conditions. The phenomena associated with the dynamic expansion and subsequent break up of the cylinders are monitored with high-speed diagnostics. In addition, complementary experiments are performed in which fragments from the explosively driven cylinders are recovered and analyzed to determine the statistical distribution associated with the fragmentation process as well as to determine failure mechanisms. The data are used to determine relevant coefficients for the Hancock–McKenzie (Johnson–Cook) fracture model. Metallurgical analysis of the fragments provides information on damage and failure mechanisms.

Modern precipitation-hardened ultra-high-strength AERMET 100 steel (Fe-Co-Ni-Cr-Mo-C) is susceptible to severe transgranular hydrogen environment–assisted cracking (HEAC) in neutral 3.5 pct NaCl solution. The threshold stress intensity for HEAC, K TH , is reduced to as low as 10 pct of K IC , and the stage II subcritical crack growth rate, da/dt II, is up to 0.5 μm/s. Low K TH and high da/dt II are produced at potentials substantially cathodic, as well as mildly anodic, to free corrosion. However, a range exists at slightly cathodic potentials (–0.625 to –0.700 VSCE), where the crack growth rate is greatly reduced, consistent with reduced crack-tip acidification and low cathodic overpotential for limited H uptake. Short crack size (250 to 1000 μm) does not promote unexpectedly severe HEAC. High-purity AERMET 100 is susceptible to HEAC because martensite boundary trapping and high crack-tip stresses strongly enhance H segregation to sites that form a transgranular crack path. The stage II da/dt is H diffusion rate limited for all potentials examined. A semiquantitative model predicts the applied potential dependence of da/dt II using reasonable input parameters, particularly crack-tip H uptake reverse calculated from measured K TH and a realistic critical distance. Modeling challenges remain.

Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (D L ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (E b ). Solid-solution elements in AERMET 100 reduce D L by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M2C precipitates are the major reversible trap states, with E b of 11.4 to 11.6 kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with E b of 61.3 to 62.2 kJ/mol is likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9 kJ/mol. Severe transgranular hydrogen embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity is due to H repartitioning from a high density of homogeneously distributed and reversible M2C traps to the crack tip under the influence of high hydrostatic tensile stress.

The diffusivity of hydrogen at room temperature in an ultra high strength steel Aermet® 100 was studied using a variety of techniques, such as an electrochemical method, sub-surface microhardness profiling, quantitative depth profiling using nuclear reaction analysis (NRA) and nanohardness profiling. Aermet® 100 samples were cathodically charged under a galvanostatic condition at a current density of 10 A/m2. The electrochemical technique was used to determine the diffusivity of hydrogen in the Aermet® 100 cathode by monitoring the potential as a function of time. These results were compared with diffusivity data extracted from sub-surface microhardness profiling. Independently, nanohardness depth profiling was carried out to validate the hardness data near the surface. Excellent correlations were obtained in the data for hydrogen diffusivity from each of these techniques. The room temperature hydrogen diffusivity in Aermet® 100 was measured at approximately 2.91×10−13 m2/s. NRA indicated that the near-surface concentration of hydrogen under the electrolytic charging conditions utilized was about 5 at.%.

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 °C, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K TH ) as low as 10 pct of the plane-strain fracture toughness (K IC ) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The K TH value decreases from a K IC value of 132 to 143 MPa√m to 12 MPa√m, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing K TH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is ∼1 µm, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,Mo)2C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.

The hydrogen-uptake capacity and mobility in ultrahigh-strength AERMET 100 are characterized for various electrochemical charging and baking conditions. From thermal desorption spectroscopy, the apparent hydrogen diffusivity (D H < 3 × 10−8 cm2/s at 23 °C) is over tenfold less than the values typical of tempered martensitic steels such as AISI 4130. The value of D H decreases with decreasing temperature below 200 °C, with a relatively high apparent activation energy for diffusion of 17.7 to 18.8 ± 0.2 kJ/mol at the 95 pct confidence level. The value of D H also decreases with decreasing diffusible H concentration from less-severe charging or increased baking. Potentiostatic charging in saturated Ca(OH)2 produced total and diffusible H concentrations in AERMET 100 which increase with (H+/H) overpotential and are significantly higher than results for AISI 4130 steel under the same conditions. A significant H concentration was produced by zero overpotential deposition. These characteristics are explained by extensive reversible and irreversible H trapping involving at least three unique trap states in the ultrafine AERMET 100 microstructure. The former likely include coherent M2C carbides, soluble Ni, or precipitated austenite, and the latter include larger incoherent M x C y or martensite lathed-packet interfaces. Baking at 23 °C and 200 °C removes H from the lowest binding-energy sites, but results in reduced D H levels to prolong outgassing time. Additionally, substantial H was retained in stronger trap states. These trapping effects are pertinent to hydrogen embrittlement of AERMET 100 steel.

The purpose of this study is to report well-controlled experiments conducted to determine the fracture resistant properties of AerMet® 100 steels. One of the objectives of this study is to determine the influence of fracture toughness properties on the fracture and fragmentation process. Both sphere impact tests and cylinder expansion test geometry were used to determine the dynamic fracture resistant coefficients. These experiments were conducted at strain rates of ∼ 14 × 10 3 /s for the cylinder expansion tests; the strain rates for the sphere impact tests varied over 50 to 100 × 10 3 /s. Fracture resistant coefficients of 60 MPa √m and 20 MPa √m are obtained from the cylinder test and the sphere impact test, respectively. These measurements do not agree with the static fracture toughness values reported in the literature.

Non-proportional torsion–tension and biaxial-compressive experimental results are presented on tantalum, tantalum alloy with 2.5% tungsten, and AerMet 100 steel. These test results form a comprehensive set of data to show the material behaviors at finite strain and wide strain-rate range. Using the parameter set determined from uniaxial constant strain-rate compressive and tensile tests, the capability of a new constitutive model (Khan, A.S., Liang, R., 1999. Behaviors of three BCC metal over a wide range of strain rates and temperatures: experiments and modeling. International Journal of Plasticity 15, 1089–1109) is shown to accurately predict complex loading paths of current experimental results. Using von Mises equivalent strain, stress, and strain rate, the constitutive model gives excellent predictions of these non-proportional experimental results.

Summary We performed a series of depth-of-penetration experiments using 7.11-mm-diameter, 71.12-mm-long, ogive-nose steel projectiles and 254-mm-diameter, 6061-T6511 aluminum targets. The projectiles were made from vacuum-arc remelted (VAR) 4340 steel (Rc 38) and AerMet 100 steel (Rc 53), had a nominal mass of 0.021 kg, and were launched using a powder gun or a two-stage, light gas gun to striking velocities between 0.5 and 3.0 km/s. Since the tensile yield strength of AerMet 100 (Rc 53) steel is about 1.5 times greater than VAR 4340 (Rc 38) steel, we were able to demonstrate the effect of projectile strength on ballistic performance. Post-test radiographs of the targets showed three different regions of penetrator response as the striking velocity increased: (1) the projectiles remained rigid and visibly undeformed; (2) the projectiles deformed during penetration without nose erosion, deviated from the target centerline, and exited the side of the target or turned severely within the target; and (3) the projectiles eroded during penetration and lost mass. To show the effect of projectile strength, we present depth-of-penetration data as a function of striking velocity for both types of steel projectiles at striking velocities ranging from 0.5 and 3.0 km/s. In addition, we show good agreement between the rigid-projectile penetration data and a cavityexpansion model.

The elastic–plastic behaviors of three body-centered cubic metals, tantalum, tantalum alloy with 2.5% tungsten, and AerMet 100 steel, are presented over a wide range of strains (15%), strain rates (10−6–104 s−1) and temperatures (77–600°F). Johnson-Cook and Zerilli-Armstrong models were found inadequate to describe the observations. A new viscoplastic model is proposed based on these experimental results. The proposed constitutive model gives good correlations with these experimental results and strain-rate jump experiments. In the next paper (Liang, R., Khan A.S., 2000. Behaviors of three BCC metals during non-proportional multi-axial loadings and predictions using a recently proposed model. International Journal of Plasticity, in press), multi-axial loading results on these materials and comparison with the proposed model will be presented.

We conducted depth of penetration experiments in concrete targets with 3.0 caliber-radius-head, steel rod projectiles. The concrete targets with 9.5 mm diameter limestone aggregate had a nominal unconfined compressive strength of 58.4 MPa (8.5 ksi) and density 2320 kg/m3. To explore geometric projectile scales, we conducted two sets of experiments. Projectiles with length-to-diameter ratio of ten were machined from 4340Rc 45 steel, round stock and had diameters and masses of 20.3 mm, 0.478 kg and 30.5 mm, 1.62 kg. Powder guns launched the projectiles to striking velocities between 400 and 1200 m/s. For these experiments, penetration depth increased as striking velocity increased. When depth of penetration data was divided by a length scale determined from our model, the data collapsed on a single curve. Thus, a single dimensionless penetration depth versus striking velocity prediction was in good agreement with the data at two geometric projectile scales for striking velocities between 400 and 1200 m/s. In addition, we conducted experiments with AerMet 100Rc 53 steel projectiles and compared depth of penetration and post-test nose erosion data with results from the 4340Rc 45 steel projectiles.

Fracture mechanics tests were carried out for AerMet 100 in distilled water and NaCl (3.5 and 35 gl−1). The initiation period at higher values of the stress intensity factor indicated that load application in the stress corrosion cracking (SCC) environment is a necessary but not sufficient factor for SCC and that time is needed for some other factor (e.g., the local hydrogen concentration) to reach an appropriate value. The threshold stress intensity factor, KISSC, was found to increase with decreasing NaCl concentration. The plateau stress corrosion crack velocity was 2 × 10−8 m s−1 for NaCl (3.5 and 35 gl−1). The fracture mode was transgranular with small areas of an intergranular nature.

The variation in strength and Charpy impact toughness as a function of tempering temperature in the range of 200 ‡C to 650 ‡C was investigated in AF 1410 and AF 1410 + 1 pct Cr steels produced in a laboratory-scale, and a commercially produced AerMet 100 steel. The tensile test results showed that AF 1410 + 1 pct Cr had lower strength compared to AF 1410, while AerMet 100 had the highest strength of the three steels examined. Transmission electron microscopy (TEM) studies demonstrated that the strength variations among the steels can be attributed to differences in the matrix/carbide coherency strain and the volume fraction of the strengthening M2C carbides. The toughness values of the three steels were comparable when tempered up to 424 ‡C. Tempering at and above 454 ‡C resulted in a relative enhancement of toughness in AF 1410 + 1 pct Cr steel compared to AF 1410. This toughening was attributed to the destabilization of cementite at lath and prior austenite boundaries and the formation of reverted austenite.

The effect of tempering on the microstructure and mechanical properties of ultrahigh strength Aermet 100 steel was examined. In the as-quenched condition, the steel contained a dispersion of relatively fine, undissolved, (CrTiFeMo)C and (CrFeMo)23C6 carbides in a martensitic matrix. Upon tempering at 427 °C, the martensite decomposed to form a high density of cementite particles concomitant with a significant drop in toughness. Tempering at 454 °C resulted in peak strength (yield strength ∼ 1756 MPa) due to the precipitation of coherent zones of fine carbides. The peak in toughness (170 MPa√m), attained at a tempering temperature of 482 °C, was attributed to both the absence of cementite and the formation of reverted, stable austenite. Tempering at higher temperatures resulted in loss of both strength and toughness, which was suggested to be the result of precipitate coarsening and formation of unstable austenite, respectively. The details of the electron microscopy studies and mechanism of strengthening and toughening are discussed in light of the current understanding of this subject.