The effect of temperature and strain rate on the deformation behaviour, structure development and properties of biaxially stretched PET-clay nanocomposites

The inclusion of a synthetic fluoromica clay in PET affects its processability via biaxial stretching and stretching temperature (95 °C and 102 °C) and strain rate (1 s⁻¹ and 2 s⁻¹) influence the structuring and properties of the stretched material. The inclusion of clay has little effect on the temperature operating window for the PET-clay but it has a major effect on deformation behaviour which will necessitate the use of much higher forming forces during processing. The strain hardening behaviour of both the filled and unfilled materials is well correlated with tensile strength and tensile modulus. Increasing the stretching temperature to reduce stretching forces has a detrimental effect on clay exfoliation, mechanical and O₂ barrier properties. Increasing strain rate has a lesser effect on the strain hardening behaviour of the PET-clay compared with the pure PET and this is attributed to possible adiabatic heating in the PET-clay sample at the higher strain rate. The Halpin-Tsai model is shown to accurately predict the modulus enhancement of the PET-clay materials when a modified particle modulus rather than nominal clay modulus is used.     

Forming behavior and workability of 6061/B4CP composite during hot deformation

Compression tests of 6061/B₄C_P composite have been performed in the compression temperature range from 300 °C to 500 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300 °C and 400 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹ and the other was situated at the temperature between 425 °C and 500 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹.The estimated apparent activation energies of these two domains, were 129 kJ/mol and 149 kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350 °C - 0.01 s⁻¹ and 500 °C - 0.01 s⁻¹. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite.     

A study on the hot workability of wrought NiTi shape memory alloy

The hot workability of a wrought 49.8 Ni-50.2 Ti (at pct) alloy was assessed using the hot compression tests in temperature range of 700-1000 °C, strain rate of 0.001-1 s⁻¹, and the total strain of 0.7. The constitutive equations of Arrhenius-type hyperbolic-sine function was used to describe the flow stress as a function of strain rate and temperature. The preferable regions for hot workability of the alloy were achieved at Z (Zener-Holloman parameter) values of about 10⁹-10¹³ corresponding to the peak efficiency of 20-30% in the processing map. However, a narrow area in the processing map including the deformation temperature of 1000 °C and strain rate of 1 s⁻¹ is inconsistent with the related Z values. A flow instability region was observed at high Z values. Further instability regions were found at low temperature of 700 °C and low strain rates of 0.01-0.001 s⁻¹ as well as at high temperature of 1000 °C and high strain rate of 1 s⁻¹. The apparent feature of flow curves, the low value of peak efficiency, the similarity between the estimated apparent activation energy of deformation and that of the self diffusion of Ti in Ni, and the stress exponent of higher than 5, suggested that dynamic recovery (DRV) is the dominant restoration phenomenon during the hot working of the alloy.     

Hot deformation behavior of an extruded Mg-Li-Zn-RE alloy

A new Mg-7.8%Li-4.6%Zn-0.96%Ce-0.85%Y-0.30%Zr alloy has been developed. α phase, β phase and RE-containing intermetallics formed in the alloy. It is found that the alloy can easily be extruded at 260 °C with σ_0.2 = 256 MPa, σ_b = 260 MPa and δ = 14%. Hot deformation behavior of the extruded alloy was studied using the processing map technique. Compression tests were conducted in the temperature range of 250-450 °C and strain rate range of 0.001-10 s⁻¹ and the flow stress data obtained from the tests were used to develop the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (1) Domain I occurs in the temperature range of 250-275 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 50% at 250 °C/10 s⁻¹. Incomplete DRX process has occurred in β phase and DRX process hardly occurs in α phase; (2) Domain II occurs in the temperature range of 250-275 ?C and strain rate range of 0.001-0.003 s⁻¹, with a peak efficiency of about 42% at 250 °C/0.001 s ⁻¹. Incomplete DRX process has occurred in β phase and α phase; (3) Domain III occurs in the temperature range of 400-450 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 42% at 450 °C/10 s⁻¹. Complete DRX process has occurred in β phase and α phase. No cracking, cavity and band of flow localization are observed in flow instability region. The optimum parameters for hot working of the alloy are 250 °C/10 s⁻¹ and 250 °C/0.001 s⁻¹, at which fine dynamic recrystallization microstructure will be achieved. RE-containing intermetallics and α phase accelerate the DRX process in β phase. The softer β phase reduces the driving force for DRX process in α phase, so DRX process in α phase is retarded.     

Hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy superalloy

The hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy (P/M) superalloy was studied in the temperature range of 950–1150 °C and strain rate range of 0.0003–1 s⁻¹ using Gleeble-1500 thermal simulator. The dynamic recrystallization-time–temperature (RTT) curve was developed and the constitutive equation of flow stress during hot deformation was established. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate. The flow stress represents as the characteristic of dynamic crystallization with the increasing of strain at the deformation temperatures lower than 1100 °C and strain rates higher than 0.0003 s⁻¹. The beginning time of dynamic crystallization has no linear relationship with deformation temperature in the condition of strain rate lower than 0.01 s⁻¹. Besides, the experiments verify that the hyperbolic sine model including the variable of strain reflects the changing law of flow stress during the hot deformation process.     

Low-temperature spark plasma sintering of NiO nanoparticles

NiO nanoparticles of 20 nm in diameter were spark plasma sintered between 400 °C and 600 °C for 5 and 10 min durations. Application of 100 MPa pressure from room temperature resulted in densities between 75% and 92%. The final grain size was between 26 nm and 68 nm. Lower densities were recorded when 100 MPa was applied at the SPS temperature. Two shrinkage rate maxima of ∼3.4 × 10⁻³ s⁻¹ and ∼2 × 10⁻³ s⁻¹ were observed around 390 ± 10 °C and at the SPS temperature. The two shrinkage rate maxima were related to densification by particle sliding followed by diffusional grain boundary sliding during the heating. The strong effects of the surface and interfacial processes which are active during the SPS were highlighted.     

Hot deformation behavior and microstructural evolution of Ag-containing 2519 aluminum alloy

The hot deformation behaviors of Ag-containing 2519 aluminum alloy were studied by isothermal compression at 300–500 °C with strain rates from 0.01 s⁻¹ to 10 s⁻¹. The microstructural evolution of the alloy was investigated using Polyvar-MET optical microscope and Tecnai G² 20 transmission electron microscope (TEM). It has been shown that the flow stress of the alloy increases with increasing the strain rate and decreasing the deformation temperature. When the strain rate is lower than 10 s⁻¹, the flow stress increases with increasing strain until the stress reached the peak value, after which the flow stress remains almost constant. This result indicates that dynamic recovery happens during deformation. When the strain rate is 10 s⁻¹ and the temperature is higher than 300 °C, serrated flow behavior is generally observed with the stress decreasing with increasing strain, a typical phenomenon of dynamic recrystallization.     

Research on the hot deformation behavior of Ti40 alloy using processing map

The deformation behavior of a Ti40 titanium alloy was investigated with compression tests at different temperatures and strain rates to evaluate the activation energy and to establish the constitutive equation, which reveals the dependence of the flow stress on strain, strain rate and deformation temperature. The tests were carried out in the temperature range between 900 and 1100 °C and at strain rates between 0.01 and 10 s⁻¹. Hot deformation activation energy of the Ti40 alloy was calculated to be about 372.96 kJ/mol. In order to demonstrate the workability of Ti40 alloy further, the processing maps at strain of 0.5 and 0.6 were generated respectively based on the dynamic materials model. It is found that the dynamic recrystallization of Ti40 alloy occurs at the temperatures of 1050-1100 °C and strain rates of 0.01-0.1 s⁻¹, with peak efficiency of power dissipation of 64% occurring at about 1050 °C and 0.01 s⁻¹, indicating that this domain is optimum processing window for hot working. Flow instability domains were noticed at higher stain rate (≥1 s⁻¹) and stain (≥0.6), which located at the upper part of the processing maps. The evidence of deformation in these domains has been identified by the microstructure observations of Ti40 titanium alloy.     

Effect of temperature on deformation behavior and microstructures of TC11 titanium alloy

The isothermal compression deformation behavior of TC11 titanium alloy with beta microstructure was studied between 750 °C and 1100 °C under the strain rate ranging from 0.001 s⁻¹ to 10 s⁻¹ by THERMECMASTOR-Z simulator. In addition, the effect of temperature on microstructure was observed using optical microscope. The results showed that the temperature greatly affected the flow stress and microstructure of TC11 titanium alloy cooled from beta phase region in air. During hot deformation of TC11 titanium alloy, the steady state flow characteristic was observed at higher temperature or lower strain rate. In the α + β phase region, spheroidization fraction of α lamellar decreased with increasing temperature, while in near-β and β phase regions, dynamic recrystallization fraction increased with increasing temperature in all strain rates except at the strain rate of 0.001 s⁻¹.     

Effect of cooling rate on the high strain rate properties of boron steel

In this work, the effect of cooling rate on the high strain rate behavior of hardened boron steel was investigated. A furnace was used to austenize boron sheet metal blanks which were then quenched in various media. The four measured cooling rates during the solid state transformation were: 25 (compressed air quench), 45 (compressed air quench), 250 (oil quench) and 2200 °C/s (water quench). Micro-hardness measurements and optical microscopy verified the expected as-quenched microstructure for the various cooling rates. Miniature dog-bone specimens were machined from the quenched blanks and tested in tension at a quasi-static rate, 0.003 s⁻¹ (Instron) and a high rate, 960 s⁻¹ (split Hopkinson tensile bar). The resulting stress vs. strain curves showed that the UTS increased from 1270 MPa to 1430 MPa as strain rate increased for the specimens cooled at 25 °C/s, while the UTS increased from 1615 MPa to 1635 MPa for the specimens cooled at 2200 °C/s. The high rate tests showed increased ductility for the 25, 45 and 250 °C/s specimens, while the specimens cooled at 2200 °C/s showed a slight decrease. The Hollomon hardening curve was fit to the true stress vs. true strain curves and showed that the mechanical response of the high rate tests exhibited a greater rate of hardening prior to fracture than the quasi-static tests. The hardening rate also increased for the specimens quenched at higher cooling rates. Optical micrographs of the fractured specimens showed that the failure mechanism transformed from a ductile-shear mode at the lower cooling rates to a shear mode at the high cooling rates.     

Tensile deformation behaviour of forged disc of IN 718 superalloy at 650 °C

The tensile properties of forged disc of IN 718 superalloy were evaluated in the strain rate regime between 10⁻⁴ and 10⁻² s⁻¹ at 650 °C. Flow oscillations were observed in stress–strain curves in the strain rate regime investigated. These flow oscillations were identified as strain increments attributed to twining mechanism at all the strain rates. However, presence of well defined serrations, temperature insensitivity of yield strength and increase in strain hardening exponent confirmed the occurrence of dynamic strain aging at 10⁻³–10⁻² s⁻¹ strain rates. Deformation behaviour was observed to be planer in nature. Fracture features remained same (transgranular) in the strain rate regime studied.     

Dynamic properties of pultruded natural fibre reinforced composites using Split Hopkinson Pressure Bar technique

The paper presents results on dynamic mechanical properties of jute, and kenaf fibre reinforced composites at various strain rates using compression Split Hopkinson Pressure Bar technique. The stress–strain curves for both pultruded natural fibre reinforced composites at strain rates of nearly 1400 s⁻¹ are illustrated and then compared with statically determines stress–strain curve (1.0 × 10⁻³ s⁻¹). Results show that the strain rate does affect the value of dynamic compressive properties of both pultruded natural fibre composites. Higher dynamic compression modulus and 2.5% flow stress were recorded for higher strain rates as compared to lower strain rate over the range of strain rates investigated. Under dynamic loading, jute fibre reinforced composites recorded the highest value of dynamic response in terms of compression modulus, 2.5% flow stress and compressive strength than that of kenaf fibre reinforced composites. In addition, kenaf fibre reinforced composites is more severely damaged as compared to jute fibre reinforced composites for all tested strain rate.     

Superplastic behavior of a TiAl alloy in its as-cast state

Superplastic behavior of a TiAl alloy was investigated in a temperature range between 800 and 1000°C and at a strain rate of 1×10⁻⁴ s⁻¹. The results show that the present alloy possesses very good superplasticity even in its as-cast state. A tensile elongation of 628% was obtained at 850°C. The observed superplastic behavior can be explained by the formation of a fine microstructure containing a metastable β-phase during solidification.     

Determination of strain rate dependent through-thickness tensile properties of textile reinforced thermoplastic composites using L-shaped beam specimens

The presented work focuses on a methodology to characterise strain rate dependent strength and elastic properties of textile reinforced composites in laminate through-thickness direction. Here, for the characterisation L-shaped beam specimens are used. The investigated composite is a fabric reinforced thermoplast made of hybrid E-glass/polypropylene yarns. The analytical solution for the determination of the through-thickness tensile strength as proposed by Lekhnitskii and Shivakumar is verified by means of an optical deformation analysis and is extended for thew determination of the through-thickness elastic modulus. Finally, the possibility of the strain rate dependent characterisation is investigated and a Johnson-Cook based modelling approach is used to represent the apparent strain rate dependency of the through-thickness failure onset. The methodology is successfully used to capture the material strain rate effects with the according strength values and model parameters over a strain rate range of 10 ⁻⁴ s⁻¹ to 10 s⁻¹ as well as the elastic modulus.     

Hot working parameters and mechanisms in as-cast Mg-3Sn-1Ca alloy

The hot working characteristics of as-cast Mg-3Sn-1Ca alloy have been studied using processing-map technique and the kinetic rate equation. The map exhibited two domains; one in the lower strain rate range (0.0003-0.01 s^− 1) and the other in the higher strain rate range (0.1-10 s^− 1)—both falling in the same temperature range of 350-550 s^− 1. Hot extrusion at 500 °C and a speed corresponding to an average strain rate of 3 s^− 1 exhibited dynamically recrystallized microstructure. The estimated apparent activation energy values are higher than those for self-diffusion in pure magnesium suggesting that the large volume fraction of MgSnCa intermetallic particles causes significant back stress.     

An aqueous sol-gel synthesis of chromium(III) doped mesoporous titanium dioxide for visible light photocatalysis

Nanoparticles of titanium dioxide doped with Cr³⁺ ions have been prepared through an aqueous sol-gel method. The mesoporous nature of both pure and Cr³⁺ doped TiO₂ powders, with specific surface area of 7.4 and 6.6 m² g⁻¹, respectively, is maintained even at calcination temperature of 800 °C. The transformation of TiO₂ from the anatase to rutile phase is suppressed up to 800 °C by Cr³⁺ ion doping. Even though surface area values are decreased, the doped materials show improved photocatalytic activity, which may be due to increased crystallinity of the anatase phase without the formation of rutile. Doped materials have a red-shift in the band gap energy and hence, photoactivity under visible light. The rate of photodegradation of methylene blue dye for both pure and doped TiO₂ under visible light has been monitored in this study. The 0.25 mol% Cr(III) doped photocatalyst, calcined at 800 °C, shows the highest photocatalytic activity under visible light with a rate constant of ∼15.8 × 10⁻³ min⁻¹, which is nearly three times higher than that of commercially available Degussa P25 titania (5.8 × 10⁻³ min⁻¹).     

Processing map for hot working characteristics of a wrought 2205 duplex stainless steel

Processing map on a wrought 2205 duplex stainless steel under hot compression conditions has been developed based on the dynamic material model theories in the range 1223–1473 K and 0.01–10 s⁻¹. The various domains in the map corresponding to different deformation characteristics have been discussed in combination of microstructural observations. The results show that the power dissipation efficiency (η) depends strongly on the dynamic recrystallization (DRX) of austenite which plays a dominant role in microstructural evolution, while the ferrite phase mainly continues to exhibit relatively well-developed dynamic recovery (DRV) at large strain. The optimum hot working domain of wrought 2205 duplex stainless steel is obtained to be in the temperature range 1373–1473 K and at strain rate of 0.01 s⁻¹, with peak efficiency 50% occurring at about 1423 K, in which more uniform microstructure is developed due to the occurrence of complete DRX of austenite. The unstable hot working regimes are predicted by Prasad instability criterion, in good agreement with the macro-and microstructural observations. As predicted, flow instability, which are manifested as twinning, bands of flow localization and the absence of DRX in austenite are observed at lower temperatures and higher strain rates (1223–1273 K and 1–10 s⁻¹); in other cases, wedge cracking is responsible for instability phenomena observed at the temperature range 1373–1423 K and strain rate of 10 s⁻¹.     

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate (0.001 s⁻¹) hot tension tests at temperature range of 300–500 °C. The results indicated a ductility drop at ∼450 ± 25 °C. This was attributed to the occurrence of eutectic reaction (L → α + γ) and the partial melting of intermetallic γ phase. The ductility was started to improve by increasing the temperature to 500 °C. The latter was explained considering the effect of liquid phase on stress relaxation through accommodation of the grain boundary sliding phenomena. To further investigating the semi-solid tensile deformation behavior of the experimental alloy, the cavitations characteristics of the alloy were also examined.     

Materials modeling and simulation of isothermal forging of rolled AZ31B magnesium alloy: Anisotropy of flow

Isothermal forging of a rib–web shape in AZ31B magnesium alloy in the rolling direction was conducted at speeds of 0.01–10 mm s⁻¹ in the temperature range of 300–500 °C with the purpose of validating the results of materials models involving kinetic analysis and processing map. The process was also simulated using finite element method DEFORM to obtain the local values of strain and strain rate. Forging parallel to the rolling direction in the range 375–550 °C and 0.0003–0.3 s⁻¹ under the conditions of dynamic recrystallization (DRX) resulted in a symmetrical cup-shape while at other conditions an elliptical boat-shape was produced with the major axis coinciding with the transverse direction and the minor axis aligned with the normal direction. This anisotropy of flow has been attributed to the strong basal texture in the rolled plate and the dominance of prismatic slip at lower temperatures. In the DRX domain on the other hand, pyramidal slip dominates along with cross-slip as the recovery mechanism, which destroys the initial texture and restores the symmetry of flow. The grain size variation for forgings done in the DRX domain validated the predictions of the material models.     

Microstructure and mechanical properties of ZA62 Mg alloy by equal-channel angular pressing

The Mg-6Zn-2Al alloy was processed by ECAP and microstructure and mechanical properties of the alloy before and after ECAP were studied. The results revealed that the microstructure of the ZA62 alloy was successfully refined after two-step ECAP (2 passes at 473 K and 2-8 passes at 423 K). The course bulk interphase of Mg₅₁Zn₂₀ was crushed into fine particles and mixed with fine matrix grains forming “stripes” in the microstructure after the second step of ECAP extrusion. A bimodal microstructure of small grains of the matrix with size of ∼0.5 μm in the stripes and large grains of the matrix with size of ∼2 μm out of stripes was observed in the microstructure of samples after 4-8 passes of ECAP extrusion at the second step. The mechanical properties of the alloy studied were significantly improved after ECAP and the highest yield strength and elongation at room temperature were obtained at the samples after 4 and 8 ECAP passes at the second step, respectively. Tensile tests carried out at temperature of 473 K to 573 K and strain rate of 1 × 10⁻³ s⁻¹ to 3 × 10⁻² s⁻¹ revealed that the alloy after 8 ECAP passes at the second step showed superplasticity and the highest elongation and strain rate sensitivity (m-value) reached 520% and 0.45, respectively.