High-rate tensile properties of Si-reduced TRIP sheet steels

There have been efforts to develop Si-reduced TRIP steels to improve the wettability of Zn coatings, since the conventional CMnSi-TRIP steels suffer from poor galvanizability. In addition, for the development of potential applications of Si-reduced TRIP steels in vehicle crash management, a better understanding of high strain rate properties is required. In the present study, the effects of alloying elements, such as Cu, Al, Si, and P, on the high-rate tensile properties of Si-reduced TRIP sheet steels were investigated. Tensile tests were performed with a servo-hydraulic tensile testing machine at strain rates ranging from 10⁻² to 6 × 10² s⁻¹, and the ultimate tensile strength, elongation, strain rate sensitivity, and absorbed energy were evaluated. The retained austenite volume fractions and carbon content of the specimens were measured using neutron diffraction. The UTS was increased with Cu, Al, Si, and P alloying throughout the strain rate range, and the alloying effect on UTS was considerable with Cu and P. The effects of alloying on the microstructure were not significant. All the steels tested in this study exhibited positive strain rate sensitivity, and the m value at strain rates higher than 10 s⁻¹ was at least two times higher than that at lower strain rates.     

The effect of retained austenite stability on high speed deformation behavior of TRIP steels

The safety of passengers is important during an automobile collision. Because a collision event involves high speed deformation, it is necessary to develop property data and understand the applicable deformation mechanisms to aid in the selection of proper materials for crash-related automotive components. Therefore, dynamic mechanical properties of low carbon TRIP steels with varying retained austenite stabilities were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 10² to 6×10² s¹ using standard ASTM E-8 specimens with an elastic strain gage attached to the sample grip end to measure load, and a plastic strain gage mounted onto the gage section to measure strain. Ultimate tensile strengths (UTS), strain rate sensitivities, and strain hardening behaviors are reported. TRIP steel with high stability retained austenite exhibited higher yield stress, lower UTS and lower strain hardening than TRIP steel with low stability retained austenite.     

Superplastic characteristics of fine-grained 7475 aluminum alloy

A 7475-aluminum alloy was subjected to a thermomechanical heat treatment that resulted in a final recrystallized grain size on the order of 10 μm. Tensile specimens of dimensions 10 × 4 × 2.3 mm were machined such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at high temperatures in the range of 773 to 803 K at different cross-head speeds corresponding to initial strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Elongations of several hundred percent were observed at strain rates of <10⁻³ s⁻¹. The correlation between flow stress and strain rate suggests that the strain rate sensitivity m is close to 0.5 at the lower strain rates. The value of m decreases to ≈0.2 at high strain rates. The decrease in m suggests a transition in the rate-controlling process from superplastic deformation (m ≈ 0.5) to dislocation creep (m ≈ 0.2) with increasing strain rate. The calculated activation energies in the two deformation regions are consistent with the suggested rate-controlling processes.     

Development of high strain rate equations for stainless steels

The dynamic response of four types of stainless steel sheet was investigated at different strain rates from 10⁻² up to 10³ s⁻¹. The results from the tensile tests were used to evaluate the parameters in three different multiplicative strain rate equations of the type used in crashworthiness calculations. A new type of sigmoid constitutive equation is proposed for one grade of stainless steel.     

Strain Hardening and Strain-Rate Sensitivity of an Extruded Magnesium Alloy

The strain-hardening behavior and strain-rate sensitivity of an extruded AZ31B magnesium alloy were determined at different strain rates between 10⁻² and 10⁻⁵ s⁻¹ in relation to the thickness of specimens (2.5 and 4.5 mm). Both the common approach and Lindholm’s approach were used to evaluate the strain-rate sensitivity. The yield strength (YS) and the ultimate tensile strength (UTS) increased, the ductility decreased, and the brittle fracture characteristics increased with increasing strain rate. The thinner specimens exhibited a slightly higher UTS, lower ductility, higher strain-hardening exponent, and strain-hardening rate due to smaller grain sizes. The stage III strain-hardening rate linearly decreased with increasing true stress, but increased with increasing strain rate. In comparison to the common approach, the Lindholm’s approach was observed to be more sensitive in characterizing the strain-rate sensitivity due to larger values obtained. The thinner specimens also exhibited higher strain-rate sensitivity. As the true strain increased, the strain-rate sensitivity decreased.     

Superplastic Behavior of Al-4.5Mg-0.46Mn-0.44Sc Alloy Sheet Produced by a Conventional Rolling Process

This article describes the superplastic behavior of the Al-4.5Mg-0.46Mn-0.44Sc alloy. The investigated alloy was produced by casting and was conventionally processed to form a sheet with a thickness of 1.9 mm and an average grain size of 11 μm. The superplastic properties of the alloy were investigated using a uniaxial tensile testing with a constant cross-head speed and with a constant strain rate in the range 1 × 10⁻⁴ to 5 × 10⁻² s⁻¹ at temperatures from 390 to 550 °C. The investigations included determinations of the true-stress, true-strain characteristics, the maximum elongations to failure, the strain-rate sensitivity index m, and the microstructure of the alloy. The m-values determined with the strain-rate jump test varied from 0.35 to 0.70 in the temperature interval from 390 to 550°C and strain rates up to 2 × 10⁻² s⁻¹. The m-values decreased with increased strain during pulling. The elongations to failure were in accordance with the m-values. They increased with the temperature and were over 1000%, up to 1 × 10⁻³ s⁻¹ at 480 °C and up to 1 × 10⁻² s⁻¹ at 550 °C. A maximum elongation of 1969% was achieved at an initial strain rate of 5 × 10⁻³ s⁻¹ and 550 °C. The results show that the addition of about 0.4 wt.% of Sc to the standard Al-Mg-Mn alloy, fabricated by a conventional manufacturing route, including hot and cold rolling with subsequent recrystallization annealing, results in good superplastic ductility.     

Superplastic Behavior of Copper-Modified 5083 Aluminum Alloy

An AA5083 aluminum alloy was modified with two different levels of Cu additions, cast by direct-chill method, and thermo-mechanically processed to sheet gauge. Copper additions reduced sheet grain size, decreased tensile flow stress and significantly increased tensile elongation under most elevated temperature test conditions. The high-Cu (0.8 wt.%) alloy had the finest grain size 5.3 μm, a peak strain-rate sensitivity of 0.6 at a strain-rate of 1 × 10⁻² s⁻¹, and tensile elongation values between 259 and 584% over the temperature range, 400-525 °C, and the strain rate range, 5 × 10⁻⁴ to 1 × 10⁻² s⁻¹, investigated. In biaxial pan forming tests, only the Cu-containing alloys successfully formed pans at the higher strain rate 10⁻² s⁻¹. The high-Cu alloy showed the least die-entry thinning. Comparison of ambient temperature mechanical properties in O-temper state showed the high-Cu alloy to have significantly higher yield strength, ultimate strength, and ductility compared to the base 5083 alloy. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

Tensile behavior of Al−4%Mg−0.4%Sc−0.5% misch metal alloy at room temperature

The effects of strain rate and pre-deformation in Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm (misch metal) alloy on tensile behavior and P-L effect have been investigated. Pre-deformation of Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy clearly enhances the yield strength and ultimate strength, though it decreases the fracture strain. The yield strength of pre-deformed Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy is higher than that of commercially used Al−Mg based alloys. The strength of Al−4 wt.%Mg−0.5 wt.%Sc−0.5 wt.%Mm alloy was changed slightly at a strain rate between 2×10⁻⁵s⁻¹ and 2×10⁻³s⁻¹, but changed significantly when predeformation was introduced. Tensile test results of as-cast Al-4 wt.%Mg-0.4 wt.%Sc-0.5 wt.%Mm alloy show a significant oscillation of serration during deformation at room temperature, and the critical strain (ε _c ), which is the strain at the start of serration, decreases with increasing strain rate. Pre-deformation of Al−4wt%Mg−0.4wt%Sc−0.5wt%Mm also affects the serration oscillation: it decreases the critical strain at lower strain rate and increases it at higher strain rate (>2×10⁻⁴s⁻¹).     

Study on high strain rate superplasticity of A 6061Al alloy composite reinforced with 30 Vol.% AlN particulate

An investigation on the superplastic behavior of 30 vol.% AlNp/6061Al composite prepared by powder metallurgy (PM) techniques was carried out. Superplastic tensile tests of the composite were performed at strain rates ranging from 10° to 10⁻³ s⁻¹ and at temperatures from 823 to 893 K. A fine-grained structure prior to superplastic testing was obtained by hot rolling after extrusion. The highest total elongation to failure of 438% was achieved at a temperature of 863 K and at an initial strain rate of 1.67×10⁻¹ s⁻¹ and the highest value of the strain rate sensitivity index (m) was 0.42 for the composite. Differential thermal analysis (DTA) was used to investigate the possibility of any partial melting in the vicinity of optimum superplastic temperatures. The formation of a liquid phase is attributed to the melting of the Al-Si eutectic phase at the surface of the AlN particulates at elevated temperatures, as determined by electron probe microanalysis (EPMA). The influence of the microstructure and the fracture behavior on the superplastic behavior of the composite was studied by transmission electronic microscopy (TEM) and scanning electron microscopy (SEM). A large number of matrix filaments were present at the fracture surfaces of the specimens when superplastic deformation of the composite was performed under the optimum superplastic test conditions. The presence of dislocations and fine recrystallized grains in the test specimens suggested that they play an important role in the high-strain-rate superplasticity for this composite.     

High-temperature deformation behavior and processing map of 7050 aluminum alloy re]20101008

The high-temperature deformation behavior and processing map of 7050 aluminum alloy were investigated by tensile tests conducted at various temperatures (340, 380, 420, and 460 °C) with various strain rates of 10⁻⁴, 10⁻³, 10⁻², and 0.1 s⁻¹. The results show that the instability region with a peak power dissipation efficiency of 100 % occurs at the low deformation temperature region of 340 °C to 380 °C and high strain rates (>10⁻³ s⁻¹). The 7050 aluminum alloy exhibited a continuous dynamic recrystallization domain with power dissipation efficiency of 35% to 60 % in the deformation temperature range of 410 °C to 460 °C and the strain rate range of 10⁻⁴–10⁻³ s⁻¹. The domain with a power dissipation efficiency of 35 % to 50 % occurring at high deformation temperatures and strain rates was interpreted to represent dynamic recovery. Dynamic recovery and continuous dynamic recrystallization provide chosen domains for excellent hot workability.     

Optimization of process parameters for the manufacturing of rocket casings: A study using processing maps

Maraging steels possess ultrahigh strength combined with ductility and toughness and could be easily fabricated and heat-treated. Bulk metalworking of maraging steels is an important step in the component manufacture. To optimize the hot-working parameters (temperature and strain rate) for the ring rolling process of maraging steel used for the manufacture of rocket casings, a systematic study was conducted to characterize the hot working behavior by developing processing maps for γ-iron and an indigenous 250 grade maraging steel. The hot deformation behavior of binary alloys of iron with Ni, Co, and Mo, which are major constituents of maraging steel, is also studied. Results from the investigation suggest that all the materials tested exhibit a domain of dynamic recrystallization (DRX). From the instability maps, it was revealed that strain rates above 10 s⁻¹ are not suitable for hot working of these materials. An important result from the stress-strain behavior is that while Co strengthens γ-iron, Ni and Mo cause flow softening. Temperatures around 1125 °C and strain rate range between 0.001 and 0.1 s⁻¹ are suitable for the hot working of maraging steel in the DRX domain. Also, higher strain rates may be used in the meta-dynamic recrystallization domain above 1075 °C for high strain rate applications such as ring rolling. The microstructural mechanisms identified from the processing maps along with grain size analyses and hot ductility measurements could be used to design hot-working schedules for maraging steel.     

A study of the impact deformation and fracture behaviour of austenitic manganese steel

This study uses the split-Hopkinson pressure bar to investigate the impact deformation and fracture behaviour of austenitic manganese steel at strain rates ranging from 2.0×10³ s⁻¹ to 8.0×10³ s⁻¹ at room temperature. The experimental results indicate that strain rate exerts a significant influence on the mechanical properties of austenitic manganese steel. With an increasing strain rate, the impact flow stress, work hardening rate, and strain rate sensitivity increase, while the activation volume decreases. The variations of strain rate sensitivity and activation volume are closely related to the work hardening stress. The results of this study show that the observed flow behaviour is described accurately by the Zerilli-Armstrong constitutive law. Fractographic analysis reveals that the specimen fracture is dominated by the formation of an adiabatic shear band formation. Furthermore, dimple characteristics and cleavage facets are observed on the fracture surface, indicating a relatively ductile fracture mode. The cleavage fracture is found to be associated with the increasing strain rate, which gives rise to a loss of deformability.     

Structural evolution and superplastic formability of friction stir welded AA 2095 sheets

Friction stir welding was used to join superplastic AA 2095 sheets. The effect of welding rate on the grain size distribution and grain boundary misorientations in the stir zone was investigated. The superplastic behavior of the weld nugget parallel to the welding direction was also characterized at 495 °C and strain rates from 10⁻⁴s⁻¹ to 10⁻²s⁻¹. Increasing the welding rate during friction stir welding augmented the formation of a fine-equiaxed high-angle grain boundary structure within the stir zone. Increasing intensity of plastic straining during friction stir welding resulted in enhanced properties during subsequent superplastic formation. The maximum strain-to-failure was obtained for the weld made at a tool speed of 1000 rpm and a weld rate of 4.2 mm/s when tested at a superplastic forming strain rate of 10⁻³s⁻¹.     

Tensile characterization and constitutive modeling of AZ31B magnesium alloy sheet over wide range of strain rates and temperatures

Magnesium alloys are an ideal candidate due to their low density in comparison to aluminum and steel alloys when designing a vehicle with lower weight and therefore, reduced fuel consumption. It is important to characterize the strain rate sensitivity of any material that will be used in a structure which can undergo high rate deformation (as in an automobile crash) as well as during high velocity forming processes such as electromagnetic or electrohydraulic forming. Tensile tests for AZ31B magnesium alloy sheet at different strain rates were carried out using different testing techniques: (i) quasi-static strain rates tests were conducted in a range between 10⁻³ and 10⁻¹ s⁻¹ using a conventional electro-mechanical tensile testing apparatus; (ii) intermediate strain rates tests at 4.0 × 10¹ to 10² s⁻¹ using an instrumented falling weight apparatus; and (iii) high strain rates at 0.5 × 10³ to 1.5 × 10³ s⁻¹ using a tensile split Hopkinson bar. Furthermore, quasi-static and high strain rate tests were also performed for different temperatures, from room temperature up to 250 °C. Strain rate and temperature effects are also discussed for rolling and transverse direction, to identify the variation of sheet properties with loading direction. Finally, the constitutive fitting of the stress-strain curves to the widely employed Johnson-Cook material model equation is evaluated and also a new model is proposed based on a modified J-C model to account for the variation of strain hardening with strain rate.     

Effect of stress state on the high temperature workability of AZ31 Mg alloy

The influence of stress state on the high temperature workability of rolled AZ31 Mg alloy was investigated on the basis of a processing map. To construct the processing map, high temperature compression tests were carried out on samples oriented parallel to the rolling direction at various temperatures (25 °C∼450 °C) and strain rates (10⁻³ s⁻¹∼5s⁻¹), and then the results were compared with those of a torsion test. The overall efficiency profiles of both the compression and torsion processing maps were similar to each other, but the index of dissipation efficiency in the torsion was somewhat lower than that in the compression. The microstructure of the compressed specimens revealed much finer grained structure than that of the torsion specimens. Such microstructural differences were attributed to the different tendencies of twin formation and texture evolution depending on the stress state.     

The hot formability of an Al-Cu-Mg-Fe-Ni forging disk

The high temperature formability of AA2618-T61 forged disk was studied by means of tensile test over temperatures and strain rates ranging from 100 to 400°C and 3 × 10⁻⁵ −3 × 10⁻³ s⁻¹, respectively. The constitutive equations of the material were calculated based on an Arrhenius-type equation and the ductility of the material was evaluated considering elongation and percent reduction of area. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during high temperature deformation of the alloy. Strain rate sensitivity of the material was evaluated in all the deformation conditions and the obtained values were used to calculate the apparent activation energy.     

Effect of strain rate on deformation behavior of TRIP steels

Tensile deformation behavior of Si–Mn TRIP (TRansformation Induced Plasticity) steel with vanadium and without vanadium and the DP (Dual Phase) steel of the same composition were studied in a large range of strain rate (0.001–2000 s^?1) by routine material testing machine, rotation disk bar–bar tensile impact apparatus and high-speed material testing machine of servo-hydraulic type. In situ measurement of the transformation of retained austenite was performed by means of X-ray stress apparatus in order to have detailed knowledge about the transformation of retained austenite at quasi-static tensile. Microstructure of steels before and after tensile were observed by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that there is no yield plateau observed on the stress–strain curve at quasi-static condition for TRIP steel containing vanadium because the vanadium carbide suppress the formation of Cottrell atmosphere in matrix. Retained austenite of Si–Mn TRIP steel containing vanadium transforms to martensite at loading stress of 502 MPa (its yielding strength is 486 MPa), while the transformation of retained austenite in matrix of Si–Mn TRIP steel without vanadium happens when its yielding process is finished at quasi-static tensile. It is confirmed that phase transformation of retained austenite in TRIP steel is strain induced phase transformation. It is noted that tensile elongation of TRIP steel at dynamic tensile is always lower than that at quasi-static tensile. That is because gradually strain induced phase transformation of retained austenite in TRIP steel is suppressed by deformation localization at dynamic tensile.     

A journey with prasad’s processing maps

The constitutive flow behavior of austenitic stainless steel types AISI 304L, 316L, and 304 in the temperature range of 873 K (600 °C) to 1473 K (1200 °C) and strain-rate range of 0.001 s⁻¹–100 s⁻¹ has been evaluated with a view to establishing processing-microstructure-property relationships during hot working. The technique adopted for the study of constitutive behavior is through establishing processing maps and instability maps, and interpreting them on the basis of dynamic materials model (DMM). The processing maps for 304L have revealed a domain of dynamic recrystallization (DRX) occurring at 1423 K (1150 °C) at 0.1 s⁻¹, which is the optimum condition for hot working of this material. The processing maps of 304 predict DRX domain at 1373 K (1100 °C) and 0.1 s⁻¹. Stainless steel type 316L undergoes DRX at 1523 K (1250 °C) and 0.05 s⁻¹. At 1173 K (900 °C) and 0.001 s⁻¹ this material undergoes dynamic recovery (DRY). In the temperature and strain rate regimes other than DRX and DRY domains, austenitic stainless steels exhibit flow localization. Large-scale experiments using rolling, forging, and extrusion processes were conducted with a view to validating the conclusions arrived at from the processing maps. The “safe” processing regime predicted by processing maps has been further refined using the values of apparent activation energy during deformation. The validity and the merit of this refining procedure have been demonstrated with an example of press forging trials on stainless steel 316L. The usefulness of this approach for manufacturing stainless steel tubes and hot rolled plates has been demonstrated.     

Yield Stress of 21-6-9 Stainless Steel Over Very Wide Ranges of Strain Rates and Temperatures

The yield strength of solution-annealed 21-6-9 austenitic stainless steel was determined over a wider temperature range (−195 to 1100 °C) and strain rate (10⁻⁴ to 10⁴ s⁻¹) than has been previously reported. The most noteworthy characteristic of the variation of yield stress with temperature was the dramatic decrease in yield strength from −195 to 300 °C. The strain-rate sensitivity exponent, m, was determined using strain-rate change tests. m dramatically increases at about 850 °C with increasing temperature and m is approximately independent of strain (structure). Hopkinson split-bar tests from ambient temperature to 750 °C indicate that the strain-rate sensitivity of 21-6-9 is not strongly influenced by the over eight orders of magnitude change in strain rate. This suggests that the mechanism(s) of plastic flow at the higher rates is similar to that at lower rates. This contention was corroborated by transmission electron microscopy. The yield stress shows grain-size dependency.     

Beta phase superplasticity in titanium alloys by boron modification

Addition of boron to titanium alloys produces fine TiB whiskers in situ with excellent thermal stability and good chemical compatibility with the matrix. These whiskers stabilize a fine-grain microstructure by restricting grain growth at high temperatures in the β phase field. The hot deformation behavior in the β phase field (temperature range 1050–1200 °C) of Ti-6Al-4V alloys modified with two different levels of B additions (1.6 and 2.9 wt.%) produced by powder metallurgy was investigated using hot compression tests in the strain rate range of 10⁻³ to 10⁻¹ s⁻¹ and hot tensile tests at a nominal strain rate of 6×10⁻⁴ s⁻¹. The β phase exhibits superplasticity, which occurs due to stabilization of a fine-grain microstructure by the TiB. Matrix grain boundary sliding and β/TiB interface sliding appear to contribute to the β superplasticity. The ability to achieve superplasticity at higher temperatures enable lower flow stresses, improved chemical homogeneity, and high strain rate capability due to enhanced accommodation processes. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.