Rate Sensitivity Coefficient Research Articles

Warm Tensile Deformation and Fracture Behavior of AZ31 Magnesium Alloy Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP), as a promising severe plastic deformation method suitable for processing sheet metals, was applied to AZ31 magnesium alloy sheets, and ultra-fine grained structure was achieved after two passes of CGP at 473 K. A bimodal structure was also developed due to partial dynamic recrystallization (DRX) and deformation homogeneity. Tensile tests at temperatures from room temperature to 523 K and strain rates from 1 × 10−4 to 1 × 10−1 s−1 were conducted to the processed alloy sheets, and their tensile deformation and fracture behavior were investigated in this work. The maximum elongation to failure of 38.7% is achieved at 1 × 10−4 s−1 and 473 K, and the yield strength and tensile strength are 37.0 and 43.0 MPa, respectively. The strain hardening ability increases gradually with increasing strain rate at elevated temperatures, and its dependence on strain rate is more significant at lower temperatures. The strain rate sensitivity coefficient gradually increases with increasing temperature, and the relatively high values of ~ 0.17 and ~ 0.14 are obtained at 473 and 523 K, respectively, indicating the absence of superplastic behavior. The fracture morphology shows that with increasing temperature, the fracture mode changes from brittle fracture to ductile fracture, which is closely related to the microstructural evolution during tensile deformation. The grain coarsening at 523 K may result in the slight decreases of elongation to failure and strain rate sensitivity coefficient as well as the recovery of strain hardening exponent. The apparent activation energy at 423-523 K is estimated to be 68.8-105.5 kJ/mol. Conclusively, DRX and grain growth should be the dominant mechanism accounting for the warm tensile deformation of AZ31 magnesium alloy sheets processed by CGP, while grain boundary sliding mechanism may contribute little, due to the developed bimodal structure during CGP and the relatively low temperatures for the tensile tests.

Competitive effects in bacterial mRNA decay

In living organisms, the same enzyme catalyses the degradation of thousands of different mRNAs, but the possible influence of competing substrates has been largely ignored so far. We develop a simple mechanistic model of the coupled degradation of all cell mRNAs using the total quasi-steady-state approximation of the Michaelis–Menten framework. Numerical simulations of the model using carefully chosen parameters and analyses of rate sensitivity coefficients show how substrate competition alters mRNA decay. The model predictions reproduce and explain a number of experimental observations on mRNA decay following transcription arrest, such as delays before the onset of degradation, the occurrence of variable degradation profiles with increased non linearities and the negative correlation between mRNA half-life and concentration. The competition acts at different levels, through the initial concentration of cell mRNAs and by modifying the enzyme affinity for its targets. The consequence is a global slow down of mRNA decay due to enzyme titration and the amplification of its apparent affinity. Competition happens to stabilize weakly affine mRNAs and to destabilize the most affine ones. We believe that this mechanistic model is an interesting alternative to the exponential models commonly used for the determination of mRNA half-lives. It allows analysing regulatory mechanisms of mRNA degradation and its predictions are directly comparable to experimental data.

Analysis of the first core of the Indonesian multipurpose research reactor RSG-GAS using the Serpent Monte Carlo code and the ENDF/B-VIII.0 nuclear data library

This paper presents the neutronics benchmark analysis of the first core of the Indonesian multipurpose research reactor RSG-GAS (Reaktor Serba Guna G.A. Siwabessy) calculated by the Serpent Monte Carlo code and the newly released ENDF/B-VIII.0 nuclear data library. RSG-GAS is a 30 MWth pool-type material testing research reactor loaded with plate-type low-enriched uranium fuel using light water as a coolant and moderator and beryllium as a reflector. Two groups of critical benchmark problems are derived on the basis of the criticality and control rod calibration experiments of the first core of RSG-GAS. The calculated results, such as the neutron effective multiplication factor (k) value and the control rod worth are compared with the experimental data. Moreover, additional calculated results, including the neutron spectra in the core, fission rate distribution, burnup calculation, sensitivity coefficients, and kinetics parameters of the first core will be compared with the previous nuclear data libraries (inter-library comparison) such as ENDF/B-VII.1 and JENDL-4.0. The C/E values of ENDF/B-VIII.0 tend to be slightly higher compared with other nuclear data libraries. Furthermore, the neutron reaction cross-sections of 16O, 9Be, 235U, 238U, and S(α,β) of 1H in H2O from ENDF/B-VIII.0 have substantial updates; hence, the k sensitivities against these cross-section changes are relatively higher than other isotopes in RSG-GAS. Other important neutronics parameters such as kinetics parameters, control rod worth, and fission rate distribution are similar and consistent among the nuclear data libraries.

Experimental study of the superplastic deformation mechanisms of high-strength aluminum-based alloy

The role of different deformation mechanisms and the contributing factors behind them needs to be clearly defined to develop materials exhibiting high strain rate superplasticity. It is still not fully understood to what extent the deformation conditions and microstructural parameters affect the mechanisms of superplastic deformation. A novel Al–3.9Zn–4.1Mg–0.8Cu–2.8Ni–0.25Zr alloy is employed to determine what effects the testing conditions, elemental and phase composition and evolution of grain and sub-grain structure make towards deformation mechanisms. To analyze the deformation behavior and microstructure evolution, uniaxial tensile tests are performed following two deformation regimes with the different constant strain rates and temperatures: (1) 2 × 10-3 s-1, 480°С and (2) 2 × 10-2 s-1, 440°С. The elongation to failure exceeds 650% and the strain rate sensitivity coefficient m is near 0.5 at both deformation regimes. Electron microscopy and focused ion beam techniques are employed to analyze mechanisms associated with grain boundary sliding and intragranular strain by using the samples in a stable flow. Surface grids indicated that superplastic flow is accompanied by the formation of striated regions on the surface, whereas grain boundary sliding involves grain neighbor switching and grain rotation. Intragranular deformation with increased dislocation density and dynamic recrystallization are also observed. Weaker intergranular and intense intragranular deformation are revealed at high strain rate deformation regime. Solute effect and the presence of two types of secondary phases, nanoprecipitates of L12-Al3Zr and micron-sized eutectic Al3Ni, are discussed in comparison to conventional AA7475 alloy in different aspects of grain growth, dynamic recrystallization and strain-rate sensitivity.

Influence of heat treatment on the structure evolution and creep deformation behavior of a precipitation hardened B2-(Ni, Fe)Al alloy

The effect of quenching and aging on structural evolution of the Ni27Fe26Al32Cr10Co5 alloy hardened by coherent α-Fe(Cr) precipitates was studied by scanning electron microscopy (SEM) and high-resolution electron microscopy (HRTEM). Heat treatment of the alloy was shown to optimize the morphology and size of hardening α precipitates and give rise to particles of the intermetallic σ-FeCr phase along the grain boundaries.The mechanical properties of the alloy were measured in the temperature range of 273–1573 K. Temperature dependences for the offset yield strength at different strain rates and the Young's modulus values were obtained. Quenching followed by age hardening increased the resistance to viscoplastic strain. The strain rate sensitivity coefficient was evaluated in the temperature range of 873–1573 K. The elastic limit and yield stress values reached under external tensile stress at 973 and 1073 K were determined.The analyzed B2-(Ni, Fe)Al-based alloy was subjected to deformation at stresses of 100, 150, 200, and 300 MPa in the temperature range of 773–1200 K. The steady-state creep strain rate was approximated by the Arrhenius creep equation (stress power being 8.3 and the effective creep activation energy being 335 kJ/mol). Threshold stresses equal to 94, 70, and 33 MPa were observed at 873, 973, and 1073 K, respectively. Calculations of critical stresses for the potential mechanisms controlling creep strain showed that the threshold stresses are caused by local and general dislocation climb over coherent α precipitates. The HRTEM studies of the dislocation substructure of the alloy after high-temperature creep testing confirmed that the creep strain mechanism was controlled by dislocation climb.

Formability prediction of Ti6Al4V titanium alloy sheet deformed at room temperature and 600 °C

Formability predictions of Ti6Al4V titanium alloy sheets deformed at room temperature and 600 °C, using D-Bressan’s shear stress rupture criterion and the critical strain gradient macroscopic modelling are presented and discussed in relation to limit strain results obtained experimentally. Ti6Al4V Forming Limit Strain Curves were predicted and compared with experimental curves at 25 °C and 600 °C and strain rate 0.1 s−1. The analytical models were calibrated by means of tensile tests performed on samples cut at 0°, 45° and 90° to the rolling direction at different temperatures and strain rates to obtain the Lankford coefficients and material strain and strain rate hardening behavior. The applied critical shear stress rupture criterion showed to give a fairly good fit with experimental limit strains outcomes, proving that the shear stress rupture nature of specimens deformed at room and elevated temperatures were well reproduced, despite a not-null strain rate sensitivity coefficient at 600 °C. Fracture occurrence by fast shear stress mechanism through thickness direction was corroborated by experimental fractograph observations close to fracture surfaces. It was shown that predicted limiting major true strain of fracture by shear stress curve is governed by normal anisotropy, strain hardening exponent, pre-strain and normalized critical shear stress parameter, which depends on temperature and strain rate whereas the strain hardening exponent depends largely on temperature. In contrast, the critical strain gradient modelling for onset of localized necking showed poor correlation with the experimental limit strains. Best fit of the critical shear stress criterion with experimental limit strain curves was given by specimens deformed near plane strain or FLCo. Hence, a common feature of various strain-rate independent metals, annealed or cold rolled, is that the main fracture mechanism of thin sheet metals can be considered as fast shear stress through sheet thickness without a visible localized necking in biaxial stretching.

Rate-Dependent Cohesive Zone Model for Fracture Simulation of Soda-Lime Glass Plate

In this paper, rate-dependent cohesive zone model was established to numerical simulate the fracture process of soda-lime glass under impact loading. Soda-lime glass is widely used in architecture and automobile industry due to its transparency. To improve the accuracy of fracture simulation of soda-lime glass under impact loading, strain rate effect was taken into consideration and a rate-dependent cohesive zone model was established. Tensile-shear mixed mode fracture was also taken account. The rate-dependent cohesive zone model was implemented in the commercial finite element code ABAQUS/Explicit with the user subroutine VUMAT. The fracture behavior of a monolithic glass plate impacted by a hemispherical impactor was simulated. The simulation results demonstrated that the rate-dependent cohesive zone model is more suitable to describe the impact failure characteristics of a monolithic glass plate, compared to cohesive zone model without consideration of strain rate. Moreover, the effect of the strain rate sensitivity coefficient C, the mesh size of glass plate and the impact velocity on the fracture characteristics were studied.

Effect of ultrasonic pretreatment on the superplasticity of titanium alloy Ti-6Al-4V processed by ECAP

Improving the superplasticity characteristics of alloys widely used in industry, such as titanium alloy Ti-6Al-4V, is an important practical problem. Processing by the methods of severe plastic deformation can reduce the temperature of superplasticity. However, the material exhibits high flow stresses. In this work we studied the effect of preliminary ultrasonic treatment with an amplitude of oscillating tension-compression stresses of 100 MPa on the characteristics of superplasticity of the Ti-6Al-4V alloy processed by equal channel angular pressing (ECAP). It is shown that high-frequency oscillating stresses resulted in the reduction of the flow stress, the increase in elongation and the strain rate sensitivity coefficient during tensile tests in the conditions of superplasticity.

Hot tensile deformation and fracture behaviors of a typical ultrahigh strength steel

The hot tensile deformation and fracture behaviors of a typical ultrahigh strength steel are investigated by isothermal uniaxial tensile experiments over the tensile temperatures of 900–1150 °C and strain rates of 0.01–10 s-1. By combining the results of tensile tests and microstructural observation, the effects of deformation processing parameter (strain rate and tensile temperature) on the macroscopic plastic deformation behaviors, microscopic fracture morphologies and microstructural evolution are clarified. The studies indicate that the hot tensile behavior of the studied steel is greatly influenced by the competition of damage evolution, work hardening, and dynamic softening. With the increased strain rate, the strain rate sensitivity coefficients decrease, but the value of elongation to fracture increase from 0.38 to 0.50. The variations of microvoids or cracks in amounts and sizes show an increasing trend with the increased strain rate or the decreased tensile temperature. At the low tensile temperature (900 °C), the poor capability of necking diffusion hardly prevents the strain localization, and the mesoscopic damages easily take place. As the strain rate increases from 0.01 to 10 s-1, the neat fracture morphologies gradually change to the serrated. The main ductile fracture mechanisms transform from the intergranular fracture at the high tensile temperature and low strain rate to the transgranular fracture at the low tensile temperature and high strain rate.

Principles of achieving superior superplastic properties in intermetallic alloys based on γ-TiAl+α2-Ti3Al

Superplastic behavior of the Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C alloy (at. %) in a fine grained condition consisted of ultrafine grains and remnant lamellar colonies has been investigated. The fine grained condition was produced via two-stage hot forging with decreasing the forging temperature in the temperature range of T = 1200-950 °C. The produced material was used for preparation of specimens for tensile testing. The tensile tests were performed at T = 800–1000 °C and ε′∼10−4-10−3 s−1 in air without any protection against oxidation. The tests showed incredible superplastic elongations (δ = 1270–2860%) at T = 900–1000 °C and enhanced values of the strain rate sensitivity coefficient, m > 0.3 at ε′∼10−4-10−3 s−1. The obtained superplastic properties and microstructural examination of the tensile strained specimens suggest that the grain boundary sliding was the predominant deformation mechanism during superplastic flow. Superior superplastic properties were attained owing to unusually slow dynamic grain growth during superplastic flow. This was supported by a high content of the α2-Ti3Al phase, high alloying with niobium and dynamic recrystallization/globularization during superplastic flow leading to transformation of remnant lamellar colonies into the fine grains. The activation energy was defined as Q ≈310 kJ/mol suggesting that the predominant deformation mechanism during superplastic flow was grain boundary sliding controlled by volume diffusion of aluminum and titanium in γ-TiAl and α2-Ti3Al. On basis of the performed study, the principles of achieving superior superplastic properties (δ>1000%) in intermetallic alloys based on γ-TiAl+α2-Ti3Al have been proposed.

Tensile Deformation Behaviors of Aluminum Alloy 2219 at High Temperatures from 415°C to 515°C

This study is carried out to provide detailed hot deformation information on aluminum alloy AA2219-O. The uniaxial tensile tests are carried out to study the hot deformation behaviors. The test temperature ranges from 415°C to 515°C, and the strain rates are 0.001s-1 and 0.01s-1. Additionally, the analysis of the strain rate sensitivity coefficient indicates that the AA2219-O exhibits the trend of superplasticity at temperatures above 475°C.

Effect of Ultrasonic Treatment on the Characteristics of Superplasticity of Titanium Alloy Ti-6Al-4V

Effect of ultrasonic treatment (UST) with an amplitude of oscillating tension-compression stresses 100 MPa on the characteristics of superplastic deformation of Ti-6Al-4V alloy with an ultrafine grained (UFG) structure processed by equal-channel angular pressing (ECAP) is studied. During tensile tests at 600°C with initial strain rates in the interval from 10-4 to 10-3 s-1, ultrasonically irradiated samples exhibit a reduced flow stress, higher values of the strain rate sensitivity coefficient and elongation to failure as compared to the samples tested directly after ECAP. Detailed studies of the microstructure of samples subjected to ECAP only and ECAP followed by UST revealed no considerable differences. It is suggested that the UST affected fine structure of the material bringing them to a state with a higher ability of relaxation of deformation-induced defects.

Microstructure and thermomechanical behavior of Heusler phase Ni2AlHf-strengthened NiAl-Cr(Co) alloy produced by HIP of plasma-spheroidized powder

This study demonstrates that homogeneous high-purity micropowders with grain size of 10–45 µm from refractory NiAl-Cr(Co)-Hf alloy can be produced by combustion elemental synthesis and further treated in a thermal plasma flow. The optimal parameters of plasma spheroidization of the synthesized powders are shown at which the resulting spherical alloy particles are characterized by high sphericity (~ 98%) and the absence of satellites or internal porosity. The spherical powders were consolidated by HIP. The alloy structure was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The fine structure and features of segregation of hardening nanoparticles of Ni2AlHf (the Heusler phase) and hafnium solid solution (ssHf) in the NiAl matrix were thoroughly studied. Furthermore, in situ examination of structural phase transformations occurring in the as-HIP NiAl-based alloy was carried out upon heating in the temperature range of 25–850 °C. The α-Cr phase nanoparticles are formed inside the NiAl grains via two different mechanisms: (1) spinodal decomposition of solid solution at 250–450 °C resulting in segregation of the Guinier-Preston zone rich in Cr and its subsequent transformation into α-Cr precipitates 25–170 nm in size; (2) heterogeneous nucleation and growth of α-Cr nanocrystallites 7–40 nm long on prismatic dislocation loops at 750–850 °C.The thermomechanical properties of the alloy were studied in the temperature range of 596–1100 °C on a Gleeble System 3800 testing system. The correlation between the thermal load and the high-temperature creep rate, as well as the temperature dependences of the Young's modulus (E), the offset yield strength (σ0.2), and the rate sensitivity coefficient (m), were ascertained. The Young's modulus (E), the ultimate tensile strength, the offset yield strength σ0.2, and the proportional limit of the alloy being examined during uniaxial compression test of a cylindrical sample at 750 °C and at true strain rate of ~ 0.01 s−1 were as high as 137.8 GPa, 682, 455 and 358 MPa, respectively.

Effect of Various SPD Techniques on Structure and Superplastic Deformation of Two Phase MgLiAl Alloy

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1–3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain e = 5 using MAXStrain Gleeble equipment, both performed at temperature interval 160–200 °C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of α phase from 30 μm down to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10 × up to e = 5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of α and β phases down to about 1 μm. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10−5 to 5 × 10−3 s−1. Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in α phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m = 0.21 was obtained at lower temperature of deformation equal to 160 °C and deformation rate in the range 10−5 to 5 × 10−3. Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase α + β structure.

Dynamic Behavior of AA2519-T8 Aluminum Alloy Under High Strain Rate Loading in Compression

In this study, the effects of strain rate on the dynamic behavior, microstructure evolution and hence, failure of the AA2519-T8 aluminum alloy were investigated under compression at strain rates ranging from 1000 to 3500 s−1. Cylindrical specimens of dimensions 3.3 mm × 3.3 mm (L/D = 1) were tested using the split-Hopkinson pressure bar integrated with a digital image correlation system. The microstructure of the alloy was assessed using optical and scanning electron microscopes. Results showed that the dynamic yield strength of the alloy is strain rate dependent, with the maximum yield strength attained by the material being 500 MPa. The peak flow stress of 562 MPa was attained by the material at 3500 s−1. The alloy also showed a significant rate of strain hardening that is typical of other Al–Cu alloys; the rate of strain hardening, however, decreased with increase in strain rate. It was determined that the strain rate sensitivity coefficient of the alloy within the range of high strain rates used in this study is approximately 0.05 at 0.12 plastic strain; a more significant value than what was reported in literature under quasi-static loading. Micrographs obtained showed potential sites for the evolution of adiabatic shear band at 3500 s−1, with a characteristic circular-shaped surface profile comprising partially dissolved second phase particles in the continuous phase across the incident plane of the deformed specimen. The regions surrounding the site showed little or no change in the size of particles. However, the constituent coarse particles were observed as agglomerations of fractured pieces, thus having a shape factor different from those contained in the as-received alloy. Since the investigated alloy is a choice material for military application where it can be exposed to massive deformation at high strain rates, this study provides information on its microstructural and mechanical responses to such extreme loading condition.

Об испытании сплошных цилиндрических образцов при растяжении с кручением

One of the features of the rheological behavior of metals and alloys at high temperature is their sensitivity to the rate of deformation. This paper proposes a method for studying the dependence between stresses and strain rates based on experimental data for solid cylinder specimen deformed in tension with torsion under non-proportional deformation (complex loading). A strain trajectories with two straight lines is considered. The deformation process is described by two components of the strain rate tensor (axial and shear) in the form of functions of time. At each time, we record the axial force and torque corresponding to them. The function of the force parameters (axial force and torque) recorded in the experiment and the preset kinematic parameters (the rate of tension and the torsional velocity), the strain rate sensitivity coefficient, the strain hardening exponent and the approach angle between the stress vectors and the strain rate on the specimen surface under non-proportional deformation is obtained. The paper also considers particular cases of deformation trajectories of the double-link type, when the second link is uniaxial tension, pure torsion or tension combined with torsion. Analytic formulas for calculating the stress vector and its components, and the approach angle that characterizes the direction of the stress vector in relation to the strain rate vector for points on the specimen cylindrical surface are derived. The method proposed is a generalization of the experimental procedure for plotting shear diagrams from the torsion test data for a material sensitive to the rate and level of strains, based on the Fields Backofen equation. The proposed method was used in processing experimental data obtained after testing the Ti-6Al-4V titanium alloy under non-proportional deformation in tension with torsion under conditions of superplasticity. When processing the test data, the compliance of the “specimen – testing machine” system was taken into account.

Interest Rate Volatility and Stock Returns: A GARCH (1,1) Model

The present study attempts to examine the dual impact of changes in interest rate and interest rate volatility on the mean and variance of portfolio stock returns. The study period is from 1st April 1996 to 30th August 2014 covering a total period of approximately 18 years. Sample used in the study consist of portfolio of financial and non-financial firms listed in the S& P CNX 500 equity index. The effect of interest rate changes and volatility on distribution of stock returns is analyzed using the GARCH (1,1) model. The effect of interest rate changes is found to be higher for financial firms as compared to non-financial firms. Interest rate volatility is found to be the significant factor affecting mean and variance of non-financial firms stock returns. Overall, the effect of interest rate volatility on stock returns and conditional stock returns volatility is evident from the results. If interest rate becomes more volatile it would also increase the volatility of conditional stock returns. When the interest rate volatility is included in the variance equation it is found that in case of those firm's where interest rate sensitivity coefficient is not significant, coefficient of interest rate volatility is significant implying that if changes in interest rate are small then these firm's are able to hedge themselves but if volatility of interest rate increases beyond a limit, it would also make the conditional returns of these firms' more volatile.

SUPERPLASTIC DEFORMATION OF TWO PHASE MgLiAl ALLOY AFTER TCAP PRESSING

Magnesium based alloy containing 9 wt. % Li, 1,5 wt. % Al, composed of  (hcp + bcc) phases was cast under argon atmosphere and extruded at 350oC as bars of cross section 12x12 mm. Twist Channel Angular Pressing TCAP deformation was applied at a temperature of 160oC using BC rotation of samples in the following passes. TCAP tool consisted of helical part in horizontal area of the channel with an angle of lead γ = 30° to simulate back pressure and thus to increase the extrusion force. The initial grain size of the hexagonal  phase estimated at 30 m decreased in the following passes down to 6 m and that of bcc  phase decreased after TCAP from initial 12 m, down to 5 m. TEM studies after TCAP passes showed higher dislocation density in the  region than in the  phase. Crystallographic relationship (001)  || (110)  indicates parallel positioning of slip planes of both phases due to activation of slip. Electron diffraction indicates increase of grain misorientation with a number of TCAP passes. Stress/strain curves measured at temperature of 200oC show superplastic forming after 1st and 3rd TCAP passes with better superplastic properties with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.31 after 1 TCAP pass using flow stress values from step strain increase curves. The sample after 3 TCAP passes showed increase of m value up to 0.47 for the strain rate range 10-5 – 5 x 10-4 s-1. Obtained values are slightly lower than those reported in the literature, however increase of a number of TCAP passes has a positive effect on superplastic properties due to fine grains and increase of their misorientation.

The Effect of Al on the Mechanical Properties and Microstructure of Copper-based Metallic Glass

An investigation was carried out into the effect of different amounts of Al on the mechanical properties, and glass-forming ability, of a three component alloy (Cu–Zr–Al). The mechanical properties were studied using a universal materials tester and a Split-Hopkinson bar tester. A series of amorphous alloys with different Al content were subjected to static compression and dynamic high-speed impact tests at 10−1 s−1, 10−2 s−1, and 10−3 s−1 for the static condition and 2000 s−1, 3000 s−1, and 4000 s−1 for the dynamic condition. Experiment showed that the flow stress value and the strain rate sensitivity coefficient of the copper-based amorphous alloys varied with changes of strain and strain rate. An increase in the rate of strain caused the flow stress value and the sensitivity coefficient of the strain rate to rise. When the Al content in an amorphous alloy Cu47.5Zr45.5Al7 was 7%, both the flow stress value and the strain were improved and effectively upgraded the mechanical properties of the material.

Применение обратного анализа для определения реологических констант материалов по результатам тестовых формовок круглых мембран

The paper is devoted to a problem of determining rheological characteristics of materials from the results of experiments on free forming of sheet blanks to a cylindrical matrix. This information on the material behavior is used in the design of gas forming processes for the production of parts for aerospace industry. Realization of such technologies requires to follow optimum technological regimes, in computer simulations of which one should adequately describe the properties of materials under consideration. A significant disadvantage of the classical experimental method of tensile testing of plate specimens is that an information taken from these tests is related to metal forming in the conditions of uniaxial tension that is not typical for gas forming processes. Experimental method of forming circular membranes considered here allows one to study the behavior of a material in the conditions of biaxial tension. Interpretation of the results is proposed to carry out by an inverse analysis utilizing a special semianalytical model for the solution of the direct task. The key role in this semianalytical model belongs to a dependence of the blank thickness at the dome pole on the dome height, which accounts for the strain rate sensitivity coefficient. This approach was applied for the processing of experimental results obtained by free forming of industrial aluminum alloys. The material characteristics obtained were compared to charecteristics determined by other methods for the same set of experimental data available in the literature. To estimate the efficiency of the method proposed, the characteristics obtained were verified by finite element simulations. For simulations, MSC Nastran software was used. The results of the simulations were found to be in good agreement with the experimental data not only with respect to the dome height but also to the blank thickness at the dome.