Thermomechanical deformation modeling of Al2xxxT4/SiCp composites

A constitutive model for metal matrix composites is developed and its capabilities for predicting cyclic isothermal and cyclic thermomechanical behavior are demonstrated. The silicon carbide particulate reinforced Al2xxxT4 alloy was studied experimentally and theoretically with the model. Cyclic stress-strain behavior of 15 and 20% reinforced silicon carbide particulate reinforced Al2xxx-T4 were successfully predicted at temperatures of 20, 200 and 300°C at strain rates between 3 × 10⁻⁵s⁻¹ and 3 × 10⁻³s⁻¹. The themomechanical stress-strain behaviors (T_min = 100°C, T_max = 200, 300°C) were studied experimentally and the results were closely predicted when temperature-strain phasing was in-phase and out-of-phase. This study clarifies the influence of mechanical property mismatch in the elastic and in the inelastic ranges vs the thermal property mismatch on composite and the matrix behaviors. The transverse and hydrostatic stresses in the matrix, developed during cyclic loading, are reported for both isothermal and thermomechanical loading conditions.     

High strain rate deformation and failure of A533B steel at various temperatures

A study of the response of A533B steel of high strain rate loading in shear is presented. Experiments have been conducted using a torsional split Hopkinson bar technique at strain rates of 800 s⁻¹ and 5000 s⁻¹, and various temperatures ranging from −150° to 300°C. The results show a sensitivity of the material to both the strain rate and he temperature at which it deforms. The failure process of the material at temperatures of 5° and −150°C is closely examined. The results show a ductile failure at both temperatures even though the stress-strain curves from tests at −150°C show no, or very little, homogeneous plastic deformation beyond the observed yield point. The failure process at both temperatures is associated with nucleation, growth and coalescence of voids.     

The brittle compressive failure of fresh-water columnar ice under biaxial loading

The brittle compressive failure of fresh-water, columnar ice was investigated under biaxial loading at a strain rate of ϵ = 10^⇔2s⁻¹ at temperatures of −10 and −40°C. Tests were performed through proportional loading over the range 0 ⩽R<1 where R is the ration of the major to major comprehensive stress, i.e. R = σ₂/σ₁. Two types of confinement were considered, across the long axis of the columnar grains (type-A) and along the columns (type-B). For both types the major stress was orthogonal to the columns. The results reveal two failure regimes under cross-column loading: the failure stress first increases rapidly with increasing R_A in the range 0 ⩽R_A ⩽ R_t, and then decreases as R_A increases further. The transition ratio, R_t, decreases from ∼0.2 at −10°C to ∼0.1 at −40°C. Correspondingly, the failure mode changes from splitting along the columns along the loading direction at zero confinement to shear faulting in the loading plane at 0 < R_A ⩽ R_t to a combined mode of splitting across the columns and shear faulting out of the loading plane at R_A >R_t. The failure envelope at both temperatures resembles a truncated Coulomb envelope. Under along-column confinement (type-B) neither the failure stress nor the failure mode depends upon the confining stress. High-speed photography and thin-section examinations revealed that wing cracking and localized fragmentation are important elements in the failure process. The observations ae explained in terms of two failure mechanisms; viz. frictional crack sliding and contact tensile fractures.     

The Fracture Characteristics of a Near Eutectic Al-Si Based Alloy Under Compression

The fracture of eutectic Si particles dictates the fracture characteristics of Al-Si based cast alloys. The morphology of these particles is found to play an important role in fracture initiation. In the current study, the effects of strain rate, temperature, strain, and heat treatment on Si particle fracture under compression were investigated. Strain rates ranging from 3 × 10^?4/s to 10²/s and three temperatures RT, 373 K, and 473 K (100 °C and 200 °C) are considered in this study. It is found that the Si particle fracture shows a small increase with increase in strain rate and decreases with increase in temperature at 10 pct strain. The flow stress at 10 pct strain exhibits the trend similar to particle fracture with strain rate and temperature. Particle fracture also increases with increase in strain. Large and elongated particles show a greater tendency for cracking. Most fracture occurs on particles oriented nearly perpendicular to the loading axis, and the cracks are found to occur almost parallel to the loading axis. At any strain rate, temperature, and strain, the Si particle fracture is greater for the heat-treated condition than for the non-heat-treated condition because of higher flow stress in the heat-treated condition. In addition to Si particle fracture, elongated Fe-rich intermetallic particles are also seen to fracture. These particles have specific crystallographic orientations and fracture along their major axis with the cleavage planes for their fracture being (100). Fracture of these particles might also play a role in the overall fracture behavior of this alloy since these particles cleave along their major axis leading to cracks longer than 200 μm.     

Tensile Properties and Fracture Behavior of ATI 718Plus Alloy at Room and Elevated Temperatures

The effect of temperature over the range of ambient to 704 °C and strain rate from 10⁻⁴ to 10⁻² s⁻¹ on the tensile properties and fracture behavior of ATI 718Plus was investigated. The results showed that with increase in temperature at a strain rate 10⁻⁴ s⁻¹, there is a small reduction in the yield strength, but a large drop in ductility at 704 °C. This reduction was accompanied by a change in fracture mode from ductile transgranular to brittle intergranular cracking. Detailed analysis of the microstructure and microchemistry of the areas around the crack using electron microscopy showed that the driving mechanism behind the failure at elevated temperatures and slow strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism. In addition, the results suggest that the δ precipitates on the grain boundaries tend to oxidize and may facilitate the oxygen-induced intergranular cracking. Finally, an increase in strain rate at 704 °C caused a small increase in the yield strength and a huge increase in ductility. This increase in ductility was accompanied by a change in fracture mode from brittle-to-ductile failure. Possible mechanisms for the deformation, failure mechanisms, and strain rate dependence are discussed.

     

Influence of the crystalline microstructure on the diffusivity of hydrogen in palladium galvanostatic permeation and X-ray diffraction measurements

The influence of crystalline microstructure upon the apparent diffusion coefficient of hydrogen in Pd samples are reported. The apparent diffusion coefficient was measured by the galvanostatic permeation method (at 20°C), while the structure was characterized by X-ray diffraction measurements. Pd membrane electrodes of different thicknesses, l = 5 × 10⁻³cm and l = 10⁻²cm, were used. The structure of Pd membrane electrodes was changed by successive annealing and sequences of absorption and desorption of hydrogen accompanied by the α⇌β phase transition. Both the irreversible and reversible traps affect the mobility of hydrogen in Pd. The irreversible traps manifest in the variation of the apparent diffusion coefficient as a function of the average stationary bulk concentration of hydrogen, while the diffusion coefficient of the free hydrogen does not depend on the actual concentration of hydrogen in Pd. This is true provided that the average stationary bulk concentration of hydrogen is higher than the irreversible trap concentration. The diffusion coefficient of free hydrogen depends on the specific internal surface area according to McNabb-Foster-like equation. The value of the diffusion coefficient of hydrogen in single crystal of palladium was estimated, D₀ = (4.2 ± 0.3) × 10⁻⁷ cm²s⁻¹ (20°C).     

Activation parameters of high-temperature creep in polycrystalline nickel at ambient and high pressures

Polycrystalline specimens of pure nickel were deformed in uniaxial compression at temperatures of 1000–1550 K, strain rates of 1×10⁻⁵-3×10⁻³s⁻¹ and pressures of 0.1–1500 MPa, in order to determine the activation parameters of high temperature creep. Experiments at 0.1 MPa were conducted in an MTS apparatus with the specimen immersed in a molten heat-treating salt to prevent oxidation. The data show a decreasing power-law stress exponent with decreasing normalized steady-state flow stress (σ/G), approaching the “natural law” value of n=3 at normalized stresses <10⁻⁴. In contrast, the activation energy is constant over our range of temperatures (T/T_m = 0.55−0.90), and is indistinguishable from the activation energy of self-diffusion (284 kJ/mol). High pressure experiments were conducted in a modified piston-cylinder apparatus using the same molten heat-treating salt for the confining medium. The small activation volume could not be resolved; however, the trend of the high pressure data parallels that of the 0.1 MPa data with a systematic offset, and is consistent with the measured activation volume of self diffusion. Specimens deformed at 0.1 MPa exhibited significant strain-enhanced grain growth; this effect is greatly reduced under hydrostatic pressure, whereas subgrain size was less affected.     

Hot ductility and fracture mechanisms of a C-Mn-Nb-Al steel

Hot-ductility tests of a C-Mn-Nb-Al steel were performed in a tensile machine at different strain rates of 1×10⁻⁴, 3×10⁻⁴, 1×10⁻³, and 3×10⁻³ s⁻¹ and at temperatures of 650 °C, 710 °C, 770 °C, 840 °C, 900 °C, 960 °C, and 1020 °C, which are close to the continuous casting conditions of steel. Fracture surfaces were examined using a scanning electron microscope. It was found that low strain rates and coarse austenitic grains decrease hot ductility. At all test temperatures, when the strain rate decreases, the hot ductility also decreases because the void growth mechanism predominates over void nucleation, giving time for nucleated cracks to grow. This leads, finally, to the catastrophic failure. The minimum hot ductility was found at 900 °C for all strain rates, and the fracture was intergranular. Fractographic evidence showed that the voids formed during the deformation surrounded the austenite grains, indicating that the deformation was concentrated in ferrite bands located in the same places when the testing temperature was in the two-phase field.     

Influence of grain size on the low-cycle fatigue lives of austenitic stainless steels at high temperatures

The influence of grain size on the fatigue lives was investigated for eight kinds of austenitic stainless steels with the grain size numbers from 9 to 1. Fatigue tests were carried out at 600 and 700 °C under triangular wave shapes at strain rates of 6.7 × 10^-3/s and 6.7 × 10^-5/s, respectively, and under truncated wave shape with 30 m;n hold-time at tension side. When a strain rate was 6.7 × 10^-3/s at both 600 and 700 °C, the fracture modes were always transgranular, and the fatigue lives scarcely depended on the type of steels or the grain size. When a strain rate was 6.7 × 10^-5/s at 600 °C, the fracture modes changed from a dominantly transgranular mode to a completely intergranular one and the fatigue lives decreased with decreasing the grain size number. When a strain rate was 6.7 X 10^-5/sVs at 700 °C, grain size dependence of the fatigue lives was divided into two groups of the steels depending on the type of steel. The fracture modes of some types of the steel were completely intergranular, and others mixed. In hold-time tests, the grain size dependence of the fatigue lives was similar to that in the tests of triangular wave shape at a strain rate of 6.7 × 10^-5/s.     

High-temperature deformation of MnO

Steady-state flow stresses have been measured for {100}-oriented MnO single crystals for temperatures from 900 to 1400°C and strain rates from 2 × 10⁻⁶to 2 × 10⁻³s⁻¹. The crystals were equilibrated in oxygen partial pressures ranging from 10¹¹ to 10²Pa, which induced deviations from stoichiometry of 4 × 10⁻⁴to 9 × 10⁻². Deformation rates are controlled by oxygen diffusion. Pipe diffusion along dislocation cores is predominant for T ≤ 1000°C; volume diffusion is predominant for T ≥ 1200dgC. In general, the primary diffusing oxygen species are singly charged vacancies for low Po₂ and neutral interstitials for high Po₂. At 1400°C, ionization states decrease for the oxygen point defects.     

Internal friction peak associated with the enhanced re-orientation of split interstitials of magnesium atoms in close vicinity of the dislocation kinks in aluminium

A new internal friction peak has been observed around 125°C (| = 1 Hz) in AlMg specimens twisted and subsequently annealed at an elevated temperature. This peak appears only when internal friction measurements are taken at descending temperatures. It does not appear in the case of tensile deformation and the peak is suppressed by subsequent tensile deformation. The activation energy is found to be 0.7 ± 0.1 eV and τ₀ is 10⁻⁸ s. The peak is very sensitive to application of a small bias-stress. It is assumed that the mechanism of this peak is the stress-induced re-orientation of an <100 > split interstitials or dumbbell of MgMg atoms enhanced by the stress field of moving dislocation kinks in aluminium.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

Experimental determination of silica/copper interfacial toughness

Experiments designed to measure the fracture toughness of ceramic-metal interfaces over a wide range of phase angles are described, and a simple approach to data analysis accounting for plasticity effects in specifying interfacial toughness is outlined. A modified version of a fixture proposed by Richard and Benitz [Int. J. Fract.22, R55 (1983)] is used to apply mixed-mode loadings to silica/copper sandwich specimens. The experimentally observed crack trajectories depend on the phase angle of loading. In general, the tendency for initial propagation of the crack to occur in the ceramic increases as the magnitude of the phase angle increases. The introduction of a modest amount of mixed-mode loading resulted in a substantial increase in fracture toughness, from approximately 2.2 J/m² at 3° to 6.4 J/m² at 16° and 8.7 J/m² at −10°. The data clearly indicate that plasticity effects become increasingly important as the magnitude of the phase angle increases.     

Flow Behavior and Hot Workability of Nb-15Si-22Ti-5Cr-3Al-2.5Hf Alloy

Constitutive models for flow behaviors of an arc-melted Nb-15Si-22Ti-5Cr-3Al-2.5Hf alloy at temperatures of 1350 °C to 1500 °C and strain rates of 0.001 to 0.1 s⁻¹ have been successfully established during work hardening and dynamic softening stages, respectively, and relatively small average absolute relative errors of the predicted flow stresses are reached (7.7 pct for the work hardening stage and 5.7 pct for the dynamic softening stage). The hot processing map has also been established successfully for this Nb-Si-based ultrahigh temperature alloy. The favorable conditions for hot working of this alloy have been determined as 1350 °C to 1380 °C/0.001 to 0.003 s⁻¹ and 1380 °C to 1440 °C/0.001 to 0.01 s⁻¹. The deformed microstructures under different conditions have been explored and the mechanisms for flow instability of this alloy have been revealed. Flow instability at relatively low temperatures and high strain rates (1350 °C and 1410 °C, 0.1 s⁻¹) is mainly derived from the cracking of Nb₅Si₃ and the debonding of Nbss/Nb₅Si₃ interfaces, while flow instability at higher temperatures (1500 °C) should primarily result from the development of cracks and holes within the Nbss phase because of excessive strain accumulation.

     

Hot workability of nimonic 115 as a function of strain rate

The hot workability of Nimonic 115 was studied by means of very high strain rate stress rupture tests in the temperature interval 1323 to 1473 K (1050 to 1200°C) at strain rates of 10⁻⁴ to 10 per s. Hot plasticity, measured as elongation and reduction of area at fracture, increased generally with decreasing strain rates. Maximum values of about 40 pct elongation and 70 pct reduction of area were obtained between 1398 to 1448 K (1125 to 1175°C) for strain rates below about 1 per s. For higher rates of strain than about 1 per s, ductility at fracture fell sharply. Ductility above 1448 K (1175°C) was poor at all strain rates and fell to a minimum at 1473 K (1200°C) regardness of strain rate. The highest ductility values are associated with intermediate temperatures and intermediate strain rates where conditions are optimum for significant recovery without encountering grain growth. The presence of excess phases leads to severe intergranular embrittlement at the highest temperatures and strain rates.     

Hot workability of nimonic 115 as a function of strain rate

The hot workability of Nimonic 115 was studied by means of very high strain rate stress rupture tests in the temperature interval 1323 to 1473 K (1050 to 1200°C) at strain rates of 10^?4 to 10 per s. Hot plasticity, measured as elongation and reduction of area at fracture, increased generally with decreasing strain rates. Maximum values of about 40 pct elongation and 70 pct reduction of area were obtained between 1398 to 1448 K (1125 to 1175°C) for strain rates below about 1 per s. For higher rates of strain than about 1 per s, ductility at fracture fell sharply. Ductility above 1448 K (1175°C) was poor at all strain rates and fell to a minimum at 1473 K (1200°C) regardness of strain rate. The highest ductility values are associated with intermediate temperatures and intermediate strain rates where conditions are optimum for significant recovery without encountering grain growth. The presence of excess phases leads to severe intergranular embrittlement at the highest temperatures and strain rates.     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Optimization of Hot Workability in Superaustenitic Stainless Steel 654SMO

Hot compression tests were conducted on a Gleeble-3800 machine in a temperature range of 950 to 1200 ℃ and a strain rate range of 0. 001 to 10 s-1 in order to study the hot deformation behaviour of superaustenitic stainless steel 654SMO. The results show that peak stress increases with decreasing temperature and increasing strain rate, and the apparent activation energy of this alloy was determined to be about 494 kJ/mol. The constitutive equation which can be used to relate the peak stress to the absolute temperature and strain rate was obtained. The processing maps for hot working developed on the basis of flow stress data and the dynamic materials model were adopted to op- timize the hot workability. It is found that the features of the maps obtained in the strain range of 0.2 to 1.0 are fun- damentally similar, indicating that the strain does not have a substantial influence on processing map. The combina- tion of processing map and mierostructural observations indicates that the favorable hot deformation conditions are located in two domains of processing map. The first domain occurs in the temperature range of 980 to 1035 ℃ and strain rate range of 0. 001 to 0.01 s-1 with a peak efficiency of 55%. The second domain appears in the temperature range of 1 120 to 1 180 ℃ and strain rate range of 0.3 to 3 s-1 with peak efficiency of 35%. Compared to other stable domains, the specimens deformed in these two domains exhibit full dynamic recrystallization grains with finer and more uniform sizes. An instability domain occurs at temperatures below 1 100 ℃ and strain rate above 0.1 s-1 , and flow instability is manifested in the form of flow localization.     

High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800 °C, 900 °C and 1000 °C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al₂O₃, Cr₂O₃. Other less protective oxide such as spinels (NiCr₂O₄ and CoCr₂O₄) and TiO₂ were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800 °C and an enhanced resistance at 900 °C and 1000 °C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900 °C and 1000 °C and an enhanced resistance at 800 °C. At 1000 °C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800 °C and 900 °C, with k_p values typical of alumina forming alloys (2 × 10⁻¹⁴ to 3.6 × 10⁻¹³ g² cm⁻⁴ s⁻¹). However, Ta modified alloys exhibited superior oxidation resistance at 1000 °C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.