Decrease In Flow Stress Research Articles

A molecular dynamics investigation into deformation mechanism of nanotwinned Cu/high entropy alloy FeCoCrNi nanolaminates

High-entropy alloys (HEA) present high hardness, but low tensile ductility. Here, deformation behaviour of the nanotwinned Cu/HEA FeCoCrNi nanolaminates prepared by the previous experiment is investigated using atomic simulations. The result shows the nucleation and glide of lattice dislocations in the Cu layers dominate the initial plastic deformation in Cu/HEA multilayers, and these gliding dislocations are deposited at the twinning boundary. With the increasing strain, the dislocations are activated in HEA layer, and deposited in the Cu-HEA interface, finally facilitate slip transmission from the HEA layer to the Cu layer. The flow stress decreases and keeps a constant value with increasing nanotwinned Cu/HEA FeCoCrNi layer number, which depends upon the slip transmission and interfacial dislocation to govern strain hardening.

Hot Deformation Behavior of Hydrogenated 0.21 wt.%H Ti-0.3Mo-0.8Ni Alloy Welded Joints

The effects of height reductions and microstructure inhomogeneity on hot deformation behavior of the hydrogenated 0.21 wt.%H Ti-0.3Mo-0.8Ni alloy welded joints were investigated. The hydrogen-induced evolution mechanisms of flow stress were discussed. The results show that fully lamellar microstructures were stretched or bent with increasing height reductions, the inhomogeneity had no significant effect on microstructure evolution at a small height reduction. The hydrides had occurred local fracture and a large angle bending at height reduction of 60%. The morphology of DRX grains tended to change from approximate equiaxed to large lamellar with increasing hydrogen content to the 0.21 wt.%H. Flow stress decreased significantly with increasing hydrogen content, but height reduction had a limited effect on flow stress evolution. Flow stress of the hydrogenated 0.21 wt.%H weld zone was larger than that of base metal at height reduction of 60%, the 0.21 wt.%H could not significantly improve hot working performance of the weld zone. The hydrogen-induced dynamic recrystallization of alpha lamellas and improved dislocation movement resulted in the rapid reduction of flow stress with the 0.21 wt.%H. The limited increase of beta phase and local stress concentration also finitely led to a slight decrease of flow stress.

Modeling and investigation for atomic diffusion and mechanical properties of TiAl/Ti3Al interface: temperature effect

The effect of temperature on the diffusivity and mechanical property of the TiAl/Ti3Al interface at nanoscale was investigated using molecular dynamics (MD). The diffusion mechanism is atomic self-diffusion in the TiAl or Ti3Al blocks at the temperatures of 673 ~ 1273 K. However, self-diffusion and asymmetric counter-diffusion determine the diffusion process. The self-diffusion of atoms in the Ti3Al block is much better than that in the TiAl block under the same temperature. The average atomic diffusive distance in the TiAl block is significantly farther than that in the Ti3Al block owing to its lower melting point and greater numbers of slipping system. The yield strength of the joint after diffusion welding at the temperatures of 673 ~ 1373 K is about 1.2 GPa higher than that at 1473 K. The temperature has little effect on the mechanical properties of elastic modulus, yield strength, yield strain and flow stress of the diffusion joint at the temperatures of 673 ~ 1273 K. However, the elastic modulus, yield strength and flow stress decrease gradually with the increase in temperature at 1273 ~ 1473 K.

Deformation field evolution and shear banding of an <italic>in-situ</italic> crystal reinforced amorphous alloy composite

The strain field and shear band behaviors of an in - situ crystal reinfored amorphous alloy composite at microscale were experimentally characterized by digital image correlation method. Combined with the finite element analysis, the deformation of the amorphous alloy composite was simulated, and the evolution of stress and strain fields was given out. The role of each phase during the deformation was analysed. The results show that in the macroscopically elastic range, the crystal phase first deformed plastically, then strain concentration formed at the phase interface, and gradually expanded to the surrounding. As the applied strain increased, strain localization occured inside the material. It is found that, the plastic deformation of the composites is accommodated by shear bands, and the presence of crystal phase in the amorphous alloy composite promotes the formation of multiple shear bands. The load of the composite is mainly sustained by the continuous amorphous matrix, and the shear band forms along with a rapid increase of free volume, which leads to a decrease of flow stress and subsequent a reduced loadbearing capacity of the composite.

The Effect of Strain Rate and Temperature on the Mechanical Behavior of Al/Fe Interface Under Compressive Loading

Molecular dynamics (MD) is employed to simulate the mechanical response of Al/Fe interface under compression at extreme conditions of seven temperatures and four strain rates ranging between 150 K and 900 K and 5.0 × 107 s−1 and 1.0 × 1010 s−1, respectively. Stress–strain histories show two distinct yield stress points for simulations at temperatures below 500 K, which tend to merge into one as the temperature increases. Microstructural simulations show nucleation of dislocations, which occur in the bulk of the aluminum region, is associated with the first yield point. In the iron region, dislocations nucleate at the Al/Fe interface and are associated with the second yield point. The incoherent interface employed in these simulations contributes to the heterogeneous nucleation in iron by creating a defected area favorable for this nucleation from the aluminum side. MD generated data show that the two yield stresses and the consequent flow stress decrease with increasing temperature for all strain rates and fit a thermally activated model function of strain rate. The competing mechanisms between dislocation motion and phonon drag driven deformation are also simulated and modeled. The flow stress of the interface was found to fall midway between Zerilli–Armstrong models of the two materials constituting it, whereas the relaxation in Fe followed similar trend to what is reported in the literature.

Quasi-static and dynamic response, and texture evolution of two overaged Al 7056 alloy plates in T761 and T721 tempers: Experiments and modeling

The thermo-mechanical behavior and texture evolution of two overaged Al 7056 alloy plates, in T761 and T721 tempers, are measured over a wide range of strain rates (10−4 – 3 × 103 s−1) and temperatures (22–300 °C) under uniaxial tension and compression along the thickness direction, i.e. normal to the plate surface. A detailed study of the initial microstructure reveals an increase in precipitate size and decrease in density of precipitates, as the alloy is aged from the T761 to T721 temper; which in turn affects the flow stress and strain hardening behavior. Differences in flow strength and strain hardening rate, as well as tension-compression asymmetry in the two tempers, are apparent at the lower temperatures (22 °C & 100 °C) and decrease significantly at the higher temperatures (200 °C & 300 °C). Furthermore, initial texture measurements show a strong texture gradient along the normal direction (ND) of the plate. This texture gradient affects the ultimate stress insignificantly. However, it does have a considerable effect on the failure strains of specimens taken from different locations through the thickness. A transition from shear fracture at and below 200 °C to cup and cone fracture mode above 200 °C is observed in tension. Both tempers exhibit a positive strain rate sensitivity (SRS) that is dependent on temperature and strain rate. A sharp decrease in flow stress is found at 300 °C. The Khan-Liu (KL) model is modified to correlate with the measured thermo-mechanical responses of the two tempers over the studied, wide range of strain rates and temperatures. There is a close correlation between simulated and observed results.

Bio-Inspired Functional Surface Fabricated by Electrically Assisted Micro-Embossing of AZ31 Magnesium Alloy.

Developing bio-inspired functional surfaces on engineering metals is of extreme importance, involving different industrial sectors, like automotive or aeronautics. In particular, micro-embossing is one of the efficient and large-scale processes for manufacturing bio-inspired textures on metallic surfaces. However, this process faces some problems, such as filling defects and die breakage due to size effect, which restrict this technology for some components. Electrically assisted micro-forming has demonstrated the ability of reducing size effects, improving formability and decreasing flow stress, making it a promising hybrid process to control the filling quality of micro-scale features. This research focuses on the use of different current densities to perform embossed micro-channels of 7 μm and sharklet patterns of 10 μm in textured bulk metallic glass dies. These dies are prepared by thermoplastic forming based on the compression of photolithographic silicon molds. The results show that large areas of bio-inspired textures could be fabricated on magnesium alloy when current densities higher than 6 A/mm2 (threshold) are used. The optimal surface quality scenario is obtained for a current density of 13 A/mm2. Additionally, filling depth and depth–width ratio nonlinearly increases when higher current densities are used, where the temperature is a key parameter to control, keeping it below the temperature of the glass transition to avoid melting or an early breakage of the die.

Hot Deformation Behavior of AA2024 with and without In Situ Titanium Diboride Dispersoids

AA2024 is known for its good combination of mechanical properties and is widely used in aircraft fuselage and other aerospace applications. However, because of its relatively lower yield strength, it has limited application in high-stress regions. In alloy AA2024, when titanium diboride particulates are embedded uniformly, it is expected to improve the strength of alloy by working as particulate dispersed composite. However, deformation of such metal matrix composite (MMC) is likely to be difficult and different from the base alloy. In this work, the deformation behavior of AA2024 alloy and its composite with titanium diboride particles developed in situ through salt-metal reaction have been studied. Hot deformation behavior was studied through hot isothermal compression tests over a temperature range of 300°C–450°C and strain rate from 0.01–10 s−1. The results show that there is an increase in flow stress with an increase in strain rate and a decrease in flow stress with an increase in temperature. Processing maps were generated based on the dynamic material model to identify the stable and unstable regions for hot working. The strain rate sensitivity of the composite has been compared to base alloy AA2024. Deformation parameters were calculated from the stress-strain data, and constitutive equations have been generated. Softening in alloy and in MMC is found to be caused by dynamic recrystallization. Microstructural modifications that are caused by titanium diboride reinforcement and its impact during hot deformation are reported. The safe zones for the hot working of both base alloy and MMC were found to be in the range of 380°C to 450°C in a strain rate of 0.001–10 s−1.

Dislocation density based modeling of three-stage work hardening behaviour of type 316LN SS with varying nitrogen content and its finite element implementation for different notch radii

Dislocation density based modified Kocks-Mecking approach has been employed to understand the influence of nitrogen on kinetics of dislocation storage and annihilation processes during tensile deformation of type 316LN stainless steel at 300 K. As compared to original Kocks-Mecking approach, the modified version based on the evolution of forest dislocation density and mean free path of dislocations provides better description of three-stage hardening behaviour of the steel at all the nitrogen content. The effect of increase in nitrogen on work hardening behaviour is clearly reflected in systematic increase in flow stress, mobile and forest dislocation densities and decrease in mean free path at a given plastic strain. The predicted decrease in mean free path with increasing nitrogen has not only influenced the enhancement of dislocation-dislocation interactions leading to higher storage of dislocation but also provides a large driving force for higher dynamic recovery in the steel. The material model is implemented into finite element code via user-defined material subroutine using implicit algorithm to study the influence of notch radius on tensile properties of type 316LN stainless steel for different nitrogen content. Detailed analysis showed that notch-strengthening effect increases with increase in nitrogen and decrease in notch radius in the steel.

Effects of alloying on deformation twinning in high entropy alloys

In the current work, molecular dynamics (MD) simulations were employed to model the compressive property of FCC (face centered cubic) Al0.25CoFeNiCu0.75 high entropy alloy (HEA) nanopillars. For comparison the binary, ternary and quaternary derivatives based on Ni–Cu–Fe–Co–Al system along with pure Ni were investigated. The twin nucleation and migration stress, atomic strain and stress and generalized planar faults energy (GPFE) of the compositions were calculated. The simulation results suggest that plastic deformation of all nanopillars is mediated by deformation twinning, but both the yield strength and flow stress decrease with the increase in the number of alloying elements, implying the decrease in the twin nucleation and migration stresses, respectively. The atomic strain and stress, increasing with the addition of alloying elements, provide direct evidences at atomic scale for the severe lattice distortion resulting in the decrease in stacking fault energy (SFE) and twin boundary energy as shown in the GPFE curves. The SFE of Al0.25CoFeNiCu0.75 HEA is 16mJ/m2. The twinnability of the studied compositions also increases with the addition of alloying elements, so twinning deformation dominates in the current nanocrystals.

Establishment of Constitutive Equation of Hot Deformation of Ti-6Al-4V-0.1B alloy with Widmannstätten structure

In order to study the hot deformation behavior of the Ti-6Al-4V-0.1B alloy with Widmannstätten structure and obtain its constitutive equation,the Ti-6Al-4V-0.1B alloy cylindrical specimens with size of Φ8 × 12mm were prepared for the hot compression test on the Gleeble-1500 simulation machine,the deformation temperature from 850 to 980°Cand strain rates of 0.01 to 1s-1.The true stress-strain data and curves were obtained from the hot compression test.The results show that the true stress-strain curves of Ti-6Al-4V-0.1B alloy with Widmannstätten structure have the characteristics of flow stress softening and the flow stress is sensitive to temperature and strain rate.At the same temperature, flow stress increase with the strain rate.At the same strain rate, flow stress decrease with the temperature.Through regression analysis of true stress and strain data by Arrhenius hyperbolic sine function, the activation energy Q is calculated, 890.49kJ/mol.Under the range of the experimental deformation temperatures and strain rates, the constitutive equation is σ=11.36ln{(Z/e88.47)1/4.4107+[(Z/e88.47)2/4.4107+1]1/2}.

Grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper

ABSTRACTThis work illustrates the grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper through experimental characterisations and numerical calculations. The as-received and annealed samples with different grain sizes are prepared. Mechanical properties at high strain rates are experimentally attained by using a split Hopkinson pressure bar device. With the increase of grain size, dynamic flow stress decreases, but strain-rate dependence of flow stress increases. Johnson–Cook constitutive model is applied to carry out a numerical analysis about the grain size effect on strain-rate dependence of mechanical data and the quantificational illustrations are given.

Physical Simulation and Processing Map of Aluminum 7068 Alloy

Elevated temperature deformation of as-cast aluminum 7068 alloy was done to optimize its workability by physical simulation using Gleeble 3800. The microstructural evolution was traced using electron microscopy and electron back-scattered diffraction studies. Hot deformation involving uniaxial isothermal compression was done in the 300°C–475°C temperature range and 10−3–100 s−1 strain rate up to a true strain of 0.69. From the true stress–true strain plot it is observed that flow stress initially increases sharply because of the multiplication of dislocations during the initial phase of deformation; i.e., only work hardening is predominant during this phase. Subsequently, after attaining the peak value the flow stress increases, decreases, or remains constant. Because of dynamic restoration processes such as dynamic recovery and recrystallization, the flow stress decreases or remains constant. During this phase, the competition between work hardening and dynamic softening governs the slope of true stress–strain curve. The decrease in the Zener-Hollomon parameter with increasing temperature and decreasing strain rate follows the same trend as flow stress. Activation energy for this alloy is calculated as 206 kJ mol−1. The deformed microstructure shows serrated grains, and also well-formed subgrains are observed mostly inside the grains. Moreover, substructural strengthening is observed in this alloy, due to the presence of the high density of precipitates and subgrains. Microstructural analysis confirms the high power dissipation of the stable region may be mainly due to dynamic recovery. In the unstable region, flow instabilities may be due to adiabatic shear band formation, particle cracking, and debonding. The optimized working region is determined from the developed processing map.

Characterization of hot deformation behavior of TC32 titanium alloy

The hot deformation behavior of TC32 alloy is investigated in the temperature range of 873~948°C and strain rate range of 0.01, 0.1, 1, and 10s−1 by hot compression experiments, and the constitutive relationship of TC32 alloy is established on the base of the Arrhenius equations. It is found that, with an increase in deformation temperature and the strain rate reduction, the flow stress decreases, and the deformation mechanisms of TC32 alloy exhibit dynamic recovery feature within the high-temperature region and recrystallization feature within the low-temperature one. Deformation activation energy values are 404.7 kJ/mol in the alpha-beta region and 526.6 kJ/mol in the beta region. Finally, the constitutive equation of TC32 titanium was established.

Strain Softening of Aluminum in Shear at Elevated Temperature

Large-strain deformation of aluminum in shear consistently evinces strain softening of roughly 15-20%. Most researchers have suggested that this flow stress decrease is a consequence of a decrease in the average Taylor factor as a consequence of a shear-texture. The authors also consider, now, the possibility that changes in the dislocation climb stress induced by the texture could rationalize the softening. This work reports on an analysis of large strain deformation of aluminum single crystals in the softest orientation {111} <110>. Here softening is not observed. However, this result and other in earlier publications are consistent with dislocation climb being the rate-controlling process that also explains the observed stress versus strain behavior.

Constitutive modeling, processing map establishment and microstructure analysis of spray deposited Al-Cu-Li alloy 2195

The hot compression tests for spray deposited alloy 2195 were performed on Gleeble-1500 thermomechanical simulator at different conditions. The constitutive models and processing map of the alloy were established. The deformation mechanisms at different areas of the processing map were revealed via metallographic and electron backscatter diffraction examinations of the compressed samples. The results showed that dynamic precipitation of Al2CuLi phase occurred at low deformation temperatures (400–450 °C). The precipitation caused significant reduction in solution strengthening effect, resulting in rapid decrease of flow stress. The instability domain at low temperature was induced by the decreasing flow stress. Moreover, the stacking faults with the precipitation of Al2CuLi caused the discontinuous dynamic recrystallization. The precipitation and recrystallization led to the high dissipation efficiency at low temperature and low strain rate. At high temperatures (500–550 °C), the deformation mechanisms varied with the imposed strain rates. The dynamic recovery and recrystallization occurred at strain rate of 0.1–1 s−1, which led to the high dissipation efficiency. At low strain rate (0.01–0.1 s−1), the alloy exhibited strong dynamic recovery characteristic, which caused the decrease in dissipation efficiency. The deformation instability occurred when the strain rate is about 10 s−1, and intergranular cracking was the main instability mechanism.

New insights on the relationship between flow stress softening and dynamic recrystallization behavior of magnesium alloy AZ31B

The dynamic recrystallization (DRX) behavior of magnesium alloy AZ31B with large initial grain size was investigated. Two methods were applied to establish the DRX kinetics model. One is based on the values of DRX volume fraction evaluated by true stress-strain curves (Model 1). The other is based on the values of DRX volume fraction evaluated using optical micrographs (Model 2). It is found that the predicted DRX volume fraction by Model 1 is significantly larger than that by Model 2. It indicates that the value of DRX volume fraction (Xdrx) cannot be calculated by the true stress-strain curves for the magnesium alloy AZ31B with large initial grain size. To understand the phenomenon, the reason was analyzed and discussed. We found that the main reason can be attributed to the strong fine-grain strengthening (FGS) effect of magnesium alloy AZ31B. For the studied material, the flow behavior will be mainly affected by two factors during DRX: 1) the decrease of average dislocation density which gives rise to the decrease of flow stress; 2) the decrease of average grain size which results in the increase of flow stress. The latter causes that the DRX volume fraction evaluated by the true stress-strain curves is obviously larger than the real one. The research results will be good references for the evaluation of DRX volume fraction of other FGS materials with large initial grain sizes.

Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System.

A novel approach for conducting normal and/or combined pressure-shear plate impact experiments at test temperatures up to 1000 °C is presented. The method enables elevated temperature plate-impact experiments aimed towards probing dynamic behavior of materials under thermomechanical extremes, while mitigating several special experimental challenges faced while performing similar experiments using the conventional plate impact approach. Custom adaptations are made to the breech-end of a single-stage gas-gun at Case Western Reserve University; these adaptations include a precision-machined extension piece made from SAE 4340 steel, which is strategically designed to mate the existing gun-barrel while providing a high tolerance match to the bore and keyway. The extension piece contains a vertical cylindrical heater-well, which houses a heater assembly. A resistive coil heater-head, capable of reaching temperatures of up 1200 °C, is attached to a vertical stem with axial/rotational degrees of freedoms; this enables thin metal specimens held at the front-end of a heat-resistant sabot to be heated uniformly across the diameter to the desired test temperatures. By heating the flyer plate (in this case, the sample) at the breech-end of the gun-barrel instead of at the target-end, several critical experimental challenges can be averted. These include: 1) severe changes in the alignment of the target plate during heating due to the thermal expansion of the several constituents of the target holder assembly; 2) challenges that arise due to the diagnostics elements, (i.e., polymer holographic gratings, and optical probes) being too close to the heated target assembly; 3) challenges that arise for target plates with an optical window, where crucial tolerances between the sample, bond layer, and window become increasingly difficult to maintain at high temperatures; 4) in the case of combined compression-shear plate impact experiments, the need for high-temperature resistant diffraction gratings for the measurement of transverse particle velocity at the free surface of the target; and 5) limitations imposed on the impact velocity necessary for unambiguous interpretation of the measured free surface velocity versus time profile due to thermal softening and possibly yielding of the bounding target plates.By utilizing the adaptations mentioned above, we present results from a series of reverse geometry normal plate impact experiments on commercial purity aluminum at a range of sample temperatures. These experiments show decreasing particle velocities in the impacted state, which are indicative of material softening (decrease in post-yield flow stress) with increasing sample temperatures.

Interactive effects of height-to-diameter ratio and strain on tribological behavior in micro compression of pure nickel cylinder

In micro scale metal forming processes, the plastic deformation behaviors are complicated and difficult to predict because of size effects. In this study, micro cylinder compression tests have been carried out to determine the plastic deformation behaviors in micro bulk metal forming. The effects of specimen height-to-diameter ratio and strain on the plastic deformation behaviors have been investigated extensively by compressing pure nickel micro cylinders. It is found that both the flow stress and friction coefficient decrease with the increase of the specimen height-to-diameter ratio. Friction coefficient decreases with the increase of compression ratio for specimens with the same height or decreases with the increase of the specimen height-to-diameter ratio under the same compression ratio. A relationship between normal pressure of lubricant friction, strain and height-to-diameter ratio (H/D) is established. A modified friction model is proposed based on the assumption of open-close lubricant pockets by introducing the specimen geometrical parameters. The calculated friction coefficients by the developed model and their tendencies are in good accordance with the experimental ones. It is also found that the ratio of the real contact areas (RCAs) decreases with increase of the specimen height-to-diameter ratio under the same compression ratio which can explain the decrease of the equivalent friction stress between the tool and the specimen. The new developed friction model enables more flexible modeling of friction behaviors in microforming processes.

Effect of strain rate and temperature on dynamic mechanical behavior and microstructure evolution of ultra-high strength aluminum alloy

The dynamic mechanical response of Al-Zn-Mg-Cu alloy with a high content of Zinc was studied using split Hopkinson pressure bar (SHPB) at strain rate of 1778 s−1–6516 s−1 and temperature of 25–400 °C. The microstructure evolution and fracture characteristics of this alloy were revealed from these studies. The obtained results show that the strain rate sensitivity is slightly positive below 3136 s−1, but becomes negative at higher strain rates because of the development of adiabatic shear bands and cracks. From 25 to 400 °C, the flow stress decreases and there is an obvious decline above 200 °C due to occurrence of athermal softening. The dislocation microbands and geometrically necessary dislocation contribute to grain fragmentation. The fracture develops due to a combination of ductile and shear failure. This study in general provides a significant understanding on the relationship between microstructure evolution and mechanical behavior of high strength aluminum alloy under dynamic loading.