Hot deformation and processing maps of X-750 nickel-based superalloy

The deformation behavior of X-750 superalloy was investigated using the hot compression test in the temperature range of 850–1050 °C, and strain rate of 0.1–50 s⁻¹. The experimental results show that the flow stress of superalloy is significantly sensitive to the strain, the strain rate and the deformation temperature. Using dynamic materials model the processing maps of X-750 superalloy at strain of 0.1, 0.3 and 0.5 were established respectively. Microstructure observations reveal that the grain size as well as the volume fraction of the recrystallized grains increased at higher deformation temperature or lower strain rate. At strain of 0.5, the flow instability domain mainly located at lower temperature which is associated with shear band formation and flow localization. The optimum parameters for hot working of the alloy are deformation temperature of 1000–1050 °C and strain rate of 0.1–1 s⁻¹ according to the processing map and microstructure at true strain of 0.5.     

Temperature effects on the time dependent viscoelastic behaviour of carbon/epoxy composite materials: Application to T700GC/M21

In this study, strain rate and low temperature dependencies of the viscoelastic behaviour of the T700GC/M21 composite material are characterised and analysed. Dynamic tests for various environmental temperatures are performed on hydraulic jack equipped with an environmental chamber. Three speeds, between 8.33 · 10⁻⁴ m s⁻¹ and 0.5 m s⁻¹, at three temperatures (20 °C, −40 °C and −100 °C) are tested. The increase of the shear modulus with the decrease of the temperature is more pronounced between −40 °C and −100 °C than between 20 °C and −40 °C. Complementary DMA (Dynamic Mechanical Analysis) tests are performed on the M21 epoxy resin to characterise the viscoelastic behaviour of the matrix which contributes to the viscoelastic behaviour of the laminate. DMA tests highlight a low temperature transition called β transition (−67 °C for the 1 Hz test) which is responsible of the larger increase of the storage modulus, for the epoxy matrix, between −40 °C and −100 °C. Consequently the β transition could also be at the origin, for the composite, of the observed larger increase of the shear modulus with respect to the strain rate, for strain rates higher than 10 s⁻¹.     

Effects of strain rate and temperature on the tension behavior of polycarbonate

Tension stress–strain responses of polycarbonate are presented for strain rates of 1 × 10⁻³ s⁻¹–1700 s⁻¹ and temperatures ranging from −60 to 20 °C. The high rate tension tests are performed using a split Hopkinson tension bar apparatus. The influence of strain rate and temperature on the tension behavior of polycarbonate is investigated. Experimental results indicate that the tension behavior of polycarbonate exhibits nonlinear characteristics and rate-temperature sensitivity. The values of yield strength and strain at yield increase with the increase of strain rate and decrease with increasing temperature. A viscoelastic constitutive model consisting of a nonlinear spring and a nonlinear Maxwell element is proposed to characterize the rate and temperature dependent deformation behavior of polycarbonate prior to yielding.     

Characterization of strain-induced martensitic transformation in A301 stainless steel by Barkhausen noise measurement

Kinetics of strain-induced martensitic transformation under biaxial stress state in metastable austenitic AISI 301 stainless steel was characterized by electron back-scattered diffraction and Barkhausen noise measurement. The effect of martensite volume fraction, degree of plastic strain, crystallographic texture and stress state on magnetic properties was evaluated. Increase in Barkhausen noise signal is related both to an increase in the magnetic domains volume fraction due to a transformation of non-magnetic austenite to magnetic martensite and to a reorientation of magnetic domains into 〈1 0 0〉 direction. Up to the saturation of martensite volume fraction, Barkhausen noise is affected by newly created martensite, and subsequently by plastic strain. The intensity of Barkhausen noise signal is strongly angle-dependent as the easy magnetization axis is developed in the transverse direction of pre-strained sheets.     

Effects of initial grain size on hot deformation behavior of commercial pure aluminum

The effects of initial grain size of commercial pure aluminum on hot deformation behavior were investigated using hot compression tests. The hot compression tests were carried out on the pure aluminum samples with the initial grain sizes of 50, 150 and 450 μm using various strains, strain rates and different deformation temperatures. It was found that the hot deformation behavior of used material was sensitive to deformation conditions and initial microstructure. Results indicate that the initial grain size has significant effect on the flow stress. Flow stress decreases when the grain size decreases from 450 to 50 μm and when strain rate is lower than 0.05 s⁻¹. This procedure is reversed at strain rate of 0.5 s⁻¹. Furthermore, effects of other parameters like the strain rates and deformation temperatures on the flow stresses and hardening rates were investigated. It was also found that the inhomogeneity of microstructure distribution at different positions of the deformed specimens depended on the amount of deformation concentration at particular points and other processing parameters such as initial grain sizes, strain rates and deformation temperatures. In addition the geometric dynamic recrystallization (GDRX) was observed in the specimens highly strained (0.7) at elevated temperature (500 °C) using polarized light microscope and sensitive tint (PLM + ST).     

Study on hot workability and optimization of process parameters of a modified 310 austenitic stainless steel using processing maps

To investigate the optimized hot deformation parameters of a modified 310 austenitic stainless steel, the hot compression tests were performed using a Gleeble 3500 thermal simulator. The hot deformation behavior and hot workability characteristics were investigated in a temperature range of 800–1100 °C and a strain rate range of 0.1–10 s⁻¹. The hot processing maps of the tested steel were developed based on the dynamic material model (DMM), from which the safe deformation regions and instable deformation regions were determined. The corresponding microstructural and hardness evolutions during deformation were analyzed in detail. It was found that the deformation in the safe regions was beneficial to dynamic recovery (DRY) and dynamic recrystallization (DRX), while the deformation in unstable region would lead to flow instability, kink boundaries and grain growth. Near 950 °C, the energy dissipation rates were unusually lower, and the hardness of the deformed sample exhibited a significant increase, as a result of strain-induced precipitation. Coupled with the microstructure analysis and processing map technology, the workability map was schematically plotted and the optimal working conditions were determined. Such conditions were: temperatures in the range of 1075–1100 °C and strain rates in the range of 0.5–1.7 s⁻¹. These conditions are critical to attain an excellent homogeneous microstructure with fine grains after deformation for the modified 310 austenitic stainless steel.     

Characterization of hot compression behavior of a new HIPed nickel-based P/M superalloy using processing maps

Isothermal forging was a critical step process to fabricate the high-performance nickel-based superalloy. The temperature and strain rate served the most critical role in determining its microstructure and mechanical properties. In this article, we employed the hot compression to simulate the isothermal forging process upon the temperature ranging from 1000 °C to 1100 °C in combination with a strain rate of 0.001–1.0 s^− 1 for a new P/M nickel-based alloy. The activation energy was determined as 903.58 kJ/mol and the processing maps at a strain range of 0.4–0.7 were developed. The instability domains were more inclined to occur at strain rates higher than 0.1 s^− 1 and manifested in the form of adiabatic shear bands. The map further demonstrated that the regions with peak efficiency of 55% were located at 1080 °C/0.0015 s^− 1 and 1095 °C/0.014 s^− 1, respectively. Obvious dynamic recrystallization could be detected at the strain rate 0.01 s^− 1 leading to a significant flow stress drop and the grain growth was remarkably triggered under 1100 °C. The findings can shed light on the forging processing optimization of the new nickel-based superalloy.     

Quantitative Analysis of Work Hardening and Dynamic Softening Behavior of low carbon alloy Steel Based on the Flow Stress

In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950 °C, 1050 °C, 1150 °C and 1250 °C with strain rate of 0.01 s⁻¹, 0.1 s⁻¹ and 10 s⁻¹ on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V^* of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01 s⁻¹ and 0.1 s⁻¹. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions.     

Strain rate dependent plastic mutation in a bulk metallic glass under compression

This paper reported a strain rate dependent plasticity in a Zr-based bulk metallic glass (BMG) under axial compression over a strain rate range (1.6 × 10⁻⁵–1.6 × 10⁻¹ s⁻¹). The fracture strain decreased with increasing strain rate up to 1.6 × 10⁻³ s⁻¹. A “brittle-to-malleable” mutation occurred at strain rate of 1.6 × 10⁻² s⁻¹, subsequently, the macro plasticity vanished at 1.6 × 10⁻¹ s⁻¹. It is proposed that the result is strongly related to the combined action of the applied strain rate, the compression speed, and the propagating speed of the shear band. When the three factors coordinated in the optimal condition, multiple mature shear bands were initiated simultaneously to accommodate the applied strain, which propagated through the specimen and distributed homogeneously in space, dominating the overall plastic deformation by consuming the entire specimen effectively.     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

Experimental study on the dynamic tensile behavior of a poly-crystal pure titanium at elevated temperatures

A new technique for measuring dynamic tensile behavior of metallic materials at elevated temperatures was developed. This technique employs a rapid contact heating method to obtain a stable and nearly homogenous high temperature field in the testing gage of the specimen. As an application of this new technique, a commercially pure titanium (CP-Ti) was tested in the strain rate range of 300 s⁻¹–1400 s⁻¹ and in a temperature range of 298 K–973 K. Quasi-static experiments (10⁻³ s⁻¹, 10⁻² s⁻¹) were also performed in the same temperature range for comparison. The testing results indicated that both temperature and strain rate have pronounced influence on the mechanical behavior of CP-Ti.     

Additivity hypothesis and effects of stress on phase transformations in steel

Theoretical analysis shows that the fraction of pearlite formed from nucleation is additive and that from growth is not. A modified additivity model is established with two continuous cooling experiments. Calculations of Ae₃ temperature for proeutectoid ferrite formation under stress and nucleation rate as well as incubation period of ferrite or pearlite transformation under stress are successfully made. Kinetics equations for ferrite and pearlite transformations under stress are expressed from modification of J–M–A equation with addition of a stress item. The acceleration effect of stress on bainite formation is mainly attributed to the increase of diffusivity of solute atoms and even iron. By consideration of the grain size effect, Patel and Cohen’s equation expressing the effect of stress on M_s is modified. Calculations of M_s for fcc → bcc(bct) and fcc → hcp under stress are introduced. An equation showing the relationship between strain and nucleation of martensite which can well explain the morphology of martensite formed under stress is mentioned. Appearance and mechanism of mechanical stabilization of austenite in martensitic transformation, i.e., the lowering of M_s, resulted from the work hardening of austenite, are different from retardation of bainite formation under stress, i.e., after B_s raising, occurring the retardation of bainite growth resulted from hindrance by defects.     

The influence of reversion annealing behavior on the formation of nanograined structure in AISI 201L austenitic stainless steel through martensite treatment

Martensite treatment is one of the known thermo-mechanical processes that can be used for the grain refinement of metastable austenitic stainless steels. In this work, the martensite to austenite reversion behavior as well as its effect on the processing of nanocrystalline structure in an as-cast AISI 201L austenitic stainless steel was investigated. The as-cast specimens were first homogenized and then hot forged in order to prepare a suitable microstructure for the subsequent martensite treatment. The cold rolling was carried out to various reductions between 10% and 95% followed by annealing at temperature range of 750–900 °C for different times of 15–1800 s. The microstructure characterization was performed using optical and scanning electron microscopies, X-ray diffraction and Feritscope. Hardness measurements were also used for evaluating the mechanical properties of the experimental material. The results indicated that the specimen which was reversion-annealed at 850 °C for 30 s exhibited the smallest average austenite grain size of 65 nm with more than 86% austenite.     

Low-temperature superplasticity of Al2O3(p)/Ni–Co nanocomposites

Nanocomposites of Al₂O₃/Ni–Co prepared using Al₂O₃ of various particle sizes were fabricated by pulse current electrodeposition. Their superplastic tensile deformation was investigated at strain rates of 8.33 × 10⁻⁴ s⁻¹ and 1.67 × 10⁻³ s⁻¹ and temperatures of 723–823 K. The Al₂O₃ particle sizes and the deformation temperature had significant influence on the elongation of the deposited materials. The optimal superplastic condition and the maximum elongation were determined. A low temperature superplasticity with elongation of 632% was achieved at a strain rate of 1.67 × 10⁻³ s⁻¹ and 823 K. Scanning electron microscopy and transmission electron microscopy were used to examine the microstructures of the deposited and deformed samples. The grains grew to a micrometer dimensions and were elongated along the tensile direction after superplastic deformation. Superplasticity in electrodeposited nanocomposites is related to the presence of S at grain boundaries and to deformation twinning.     

Influence of oxygen on the processing maps for hot working of electrolytic tough pitch copper

Processing maps for the hot deformation of electrolytic tough pitch (ETP) copper (100 ppm oxygen) have been developed in the temperature range 600–950 °C and strain rate range 0.001–100 s^− 1, and compared with those published earlier on ETP copper with higher oxygen contents (180, 220 and 260 ppm). These reveal that dynamic recrystallization (DRX) occurs over a wide temperature and strain rate range and is controlled by different diffusion mechanisms. In ETP copper with 100 and 180 ppm oxygen, the apparent activation energy in the DRX domain occurring in the strain rate range 0.001–10 s^− 1 and temperature range 600–900 °C is about 198 kJ/mol which suggests lattice self-diffusion to be the rate-controlling mechanism. This DRX domain has moved to higher temperatures and lower strain rates in ETP copper with higher oxygen content. In the second domain occurring at strain rates in the range 10–100 s^− 1 and temperatures > 700 °C, the apparent activation energy is 91 kJ/mol and DRX is controlled by grain boundary self-diffusion. This domain is absent in the maps of ETP copper with oxygen content higher than 180 ppm and this is attributed to the pinning of the grain boundaries by the oxide particles preventing their migration.     

A characterization for the flow behavior of 42CrMo steel

In order to evaluate the flow stress and the dynamic softening characteristics of casting 42CrMo steel, isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 1123 K, 1198 K, 1273 K and 1348 K, and the strain rates of 0.01 s⁻¹, 0.1 s⁻¹, 1 s⁻¹ and 10 s⁻¹ on thermal physics simulator Gleeble 1500. The flow behavior of the applied stress as a function of strain, strain rate and temperature exhibits a more pronounced effect of temperature than strain rate, and a typical characteristic of dynamic recrystallization softening. To characterize the flow behavior more factually and accurately, the traditional Fields–Backofen equation was amended, and an innovative mathematical model containing a softening item s, n-value and m-value variable functions was brought forth. The stress–strain curves calculated by the derived flow stress equation are fit with the experimental results well not only at the hardening stage but also at softening stage.     

Grain growth and microstructure evolution based mechanical property predicted by a modified Hall–Petch equation in hot worked Ni76Cr19AlTiCo alloy

The deformation behavior of Ni76Cr19AlTiCo has been investigated under compressive strains by up to 50% at temperatures ranging from 800 °C to 1150 °C, and at strain rates from 0.001 s⁻¹ to 1 s⁻¹. A dramatic change in the mechanical properties of the alloy was observed when the temperature was increased from 850 °C to 950 °C, along with a rapid increase in grain size with increasing the temperature. Recovery and recrystallization processes occurred under deformation at temperatures above 950 °C. The degree of recrystallization was found to increase with increasing temperature or decreasing strain rate. γ′-phase precipitation occurred in the matrix and the particle sizes and the number of the precipitated phases were found to increase with increasing temperature or decreasing strain rate. Grain boundary precipitation of chromium carbides has also been observed, but its influence was negligible because of the small amount of the precipitates present in the matrix.A modified Hall–Petch equation was proposed to predict the mechanical properties of the alloy based on grain growth and microstructure evolution.     

The influence of martensite,bainite and ferrite on the as-quenched constitutive response of simultaneously quenched and deformed boron steel – Experiments and model

This paper examines the relationship between as-formed microstructure and mechanical properties of a hot stamped boron steel used in automotive structural applications. Boron steel sheet metal blanks were austenized and quenched at cooling rates of 30 °C/s, 15 °C/s and 10 °C/s within a Gleeble thermal–mechanical simulator. For each cooling rate condition, the blanks were simultaneously deformed at temperatures of 600 °C and 800 °C. A strain of approximately 0.20 was imposed in the middle of the blanks, from which miniature tensile specimens were extracted. Depending on the cooling rate and deformation temperature imposed on the specimens, some of the as-quenched microstructures consisted of predominantly martensite and bainite, while others consisted of martensite, bainite and ferrite. Optical and SEM metallographraphic techniques were used to quantify the area fractions of the phases present and quasi-static (0.003 s⁻¹) uniaxial tests were conducted on the miniature tensile specimens. The results revealed that an area fraction of ferrite greater than 6% led to an increased uniform elongation and an increase in n-value without affecting the strength of the material for equivalent hardness levels. This finding resulted in improved energy absorption due to the presence of ferrite and showed that a material with a predominantly bainitic microstructure containing 16% ferrite (with 257 HV) resulted in a 28% increase in energy absorption when compared to a material condition that was fully bainitic with a hardness of 268 HV. Elevated strain rate tension tests were also conducted at 10 s⁻¹ and 80 s⁻¹ and the effect of strain rate on the ultimate tensile strength (σ_UTS) and yield strength (σ_Y) was shown to be moderate for all of the conditions. The true stress versus effective plastic strain (flow stress) curves generated from the tensile tests were used to develop the “Tailored Crash Model II” (TCM II) which is a strain rate sensitive constitutive model that is a function of effective plastic strain, true strain rate and area fraction of martensite, bainite and ferrite. The model was shown to accurately capture the hardening behaviour and strain rate sensitivity of the multiphase material conditions examined.     

Hot deformation behavior and workability characteristics of bimodal size SiCp/AZ91 magnesium matrix composite with processing map

The hot deformation behavior of (0.2 um 1.5 vol.% + 10 um8.5 vol.%) bimodal size SiCp/AZ91 magnesium matrix composite fabricated by stir casting was investigated at the temperature of 270–420 °C and strain rate of 0.001–1 S⁻¹. The flow stress at the strain of 0.5 was used for kinetic analysis. Results indicate that dislocation climb is likely to be the main deformation mechanism responsible for the present composite. By evaluating the efficiencies of power dissipation and instability parameters, the processing maps are developed to optimize the hot working processing. Two domains of dynamic recrystallization are found in the processing map. One exists at the temperature of 270–370 °C and strain rate of 0.001–0.01 s⁻¹ with maximum dissipation efficiency of 38%; the other exists at 420 °C and 0.01 s⁻¹ with peak dissipation efficiency of 24%. The instability region of flow behavior can also be recognized at the temperature of 270–320 °C and the strain rate of 0.1–1 s⁻¹. The characteristic microstructures predicted from the processing map agree well with the result of microstructure observations.     

Impact and fracture response of sintered 316L stainless steel subjected to high strain rate loading

This paper describes the use of a material testing system (MTS) and a compressive split-Hopkinson bar to investigate the impact behaviour of sintered 316L stainless steel at strain rates ranging from 10^− 3 s^− 1 to 7.5 × 10³ s^− 1. It is found that the flow stress–strain response of the sintered 316L stainless steel depends strongly on the applied strain rate. The rate of work hardening and the strain rate sensitivity change significantly as the strain rate increases. The flow behaviour of the sintered 316L stainless steel can be accurately predicted using a constitutive law based on Gurson's yield criterion and the flow rule of Khan, Huang and Liang (KHL). Microstructural observations reveal that the degree of localized grain deformation increases at higher strain rates. However, the pore density and the grain size vary as a reversible function of the strain rate. Impacts at strain rates higher than 5.6 × 10³ s^− 1 are found to induce adiabatic shear bands in the specimens. These specimens subsequently fail as a result of crack propagation along the dominant band. The fracture surfaces of the failed specimens are characterized by dimple-like structures, which are indicative of ductile failure. The depth and the density of these dimples are found to decrease with increasing strain rate. This observation indicates a reduction in the fracture resistance and is consistent with the observed macroscopic flow stress–strain response.