Hot deformation behavior and the processing map of Zr–1.0Be alloy in single α phase

The hot deformation behavior,hot workability and dynamic recrystallization evolution of Zr-1.0(wt%) Be alloy in single a phase were investigated by conducting hot compression tests.The strain rates ranging from 10~(-3) s~(-1) to 10° s~(-1) and testing temperatures varying from 650 ℃to 850℃ were used.Flow stress was found to increase with increasing strain rate and decrease with the increment of the deformation temperature.A constitutive equation of flow behavior was established to describe the dependence of flow stress on strain rate and deformation temperature.The activation energy for deformation of Zr-1.0Be alloy was determined to be Q= 301 kJ/mol.The processing map of Zr-1.0Be alloy was constructed at strain rates ranging from 10~(-3) s~(-1) to 10° s~(-1) and deformation temperatures varying from 650 ℃ to 850 ℃ at the true strain of 0.7.A processing map was used to identify the best domains of thermal processing,including a domain at a temperature of 650 ℃ and strain rate of 10~(-3) s~(-1) as well as another domain at deformation temperatures ranging from 800 ℃ to 850 ℃ and strain rates varying from 10~(-3) s~-~(-1) to 10~(-1) s~(-1).Microscopic analysis of Zr-l.OBe alloy showed that the flow instability and kink were very obvious at low temperatures and high strain rates.At high temperatures and low strain rates,the dynamic recrystallization became the main softening mechanism during hot working.     

High-temperature mechanical properties and deformation behavior of high Nb containing TiAl alloys fabricated by spark plasma sintering

A high Nb containing TiAl alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y (at%) by spark plasma sintering (SPS). Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050℃ and a strain rate range of 0.002 to 0.2 s-1. The results show that the high-temperature mechanical properties of the high Nb containing TiAl alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900℃, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900℃ exhibit obvious flow softening after the peak stress. Under the deformation condition of 900-1050℃ and 0.002-0.2 s-1, the interrelations of peak flow stress, strain rate, and deformation temperature follow the Arrhenius' equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJ·mol-1.     

Hot deformation mechanism and microstructure evolution of an ultra-high nitrogen austenitic steel containing Nb and V

The flow curves of an ultra-high nitrogen austenitic steel containing niobium (Nb) and vanadium (V) were obtained by hot compression deformation at temperatures ranging from 1000℃ to 1200℃ and strain rates ranging from 0.001 s-1 to 10 s-1. The mechanical behavior during hot deformation was discussed on the basis of flow curves and hot processing maps. The microstructures were analyzed via scanning electron microscopy and electron backscatter diffraction. The relationship between deformation conditions and grain size after dynamic recrystallization was obtained. The results show that the flow stress and peak strain both increase with decreasing temperature and increasing strain rate. The hot deformation activation energy is approximately 631 kJ/mol, and a hot deformation equation is proposed. (Nb,V)N precipitates with either round, square, or irregular shapes are observed at the grain boundaries and in the matrix after deformation. According to the discussion, the hot working should be processed in the temperature range of 1050℃ to 1150℃ and in the strain rate range of 0.01 to 1 s-1.     

Hot deformation behavior andfl ow stress modeling of annealed AZ61 Mg alloys

In this study, the hyperbolic-sine type constitutive equation was used to model the flow stress of annealed AZ61 magnesium(Mg) alloys. Hot compression tests were conducted at the temperatures ranging from 250 1C to 450 1C and at the strain rates ranging from 1 10–3s 1to 1 s 1on a Gleeble-3500 thermo-simulation machine. Constitutive equations as a function of strain were established through a simple extension of the hyperbolic sine constitutive relation. The effects of annealing heat treatments on the variations in constitutive parameters with strain were discussed. The hot compressive flow curves exhibited typical features of dynamic recrystallization. Multiple peak flow curves were observed in the annealed specimens upon testing at a strain rate of 1 10 1s–1and at various temperatures. Variations in constitutive parameters with strain were related to flow behavior and dependent on the initial conditions of the test specimens. The flow stresses of annealed AZ61 Mg alloys were predicted well by the strain-dependent constitutive equations of the hyperbolic sine function under the deformation conditions employed in this study.     

Hot compressive deformation behaviors of Mg–10Gd–3Y–0.5Zr alloy

In this paper, the hot compressive deformation characteristics of a Mg–10Gd–3Y–0.5Zr(GW103K) alloy have been investigated by isothermal compression test at the temperature range of 350–450°C and strain rate range of 0.0001–0.1s~(-1). True stress–strain relationships at various strain rates showed the typical strain hardening and softening stage which is indicative of dynamic recrystallization during deformation. The results showed that the peak stress was obviously dependent on temperature and strain rate. A constitutive equation to describe the deformation process was established based on the hyperbolic sine function. The stress exponent n and apparent activation energy Q were determined to be 3.018 and 203.947 k J/mol, respectively. Microstructure investigation showed that dislocation slipping was the dominant deformation mechanism during the hot deformation at all conditions. However, at the temperatures lower than 400 °C and strain rates higher than 0.01 s~(-1), twinning was observed to be activated, which indicated another deformation mechanism. Dynamic recrystallization and dynamic precipitation were found to occur simultaneously under such deformation condition.     

Effects of Deformation Conditions on Dynamic Recrystallization of a Nb Microalloyed Anti-seismic Rebar

The hot deformation behavior of a Nb microalloyed anti-seismic rebar was investigated at deformation temperatures of 950-1 100 ℃ and strain rates of 0. 01-0. 1 s- 1 on a Gleeble-3800 thermo-mechnical simulator. The flow stress-strain curves show the typical dynamic recrystallization with a peak,before reaching the steady state flow at higher deformation temperatures and lower strain rates. The constitutive equation governing the dynamic recrystallization( DRX) was obtained and the average activation energy of deformation was calculated as Q = 389. 5 kJ / mol by the regression analysis. The DRX grain size was also found to decrease with the increasing strain rate and the decreasing deformation temperature. The austenite grain size was refined from 118. 0 μm to 15. 07-40. 01 μm by DRX. The DRX grain size under diverse deformation conditions predicted by mathematical model agrees well with experimental results.     

Study on the thermal deformation behavior and microstructure of FGH96 heat extrusion alloy during two-pass hot deformation

The change rules associated with hot deformation of FGH96 alloy were investigated by isothermal two-pass hot deformation tests in the temperature range 1050–1125°C and at strain rates ranging from 0.001 to 0.1 s~(-1) on a Gleeble 3500 thermo-simulation machine. The results showed that the softening degree of the alloy between passes decreases with increasing temperature and decreasing strain rates. The critical strain of the first-pass is greater than that of the second-pass. The true stress–true strain curves showed that single-peak dynamic recrystallization, multi-peak dynamic recrystallization, and dynamic response occur when the strain rate is 0.1, 0.01, and 0.001 s~(-1), respectively. The alloy contains three different grain structures after hot deformation: partially recrystallized tissue, completely fine recrystallized tissue, coarse-grained grains. The small-angle grain boundaries increase with increasing temperature. Increasing strain rates cause the small-angle grain boundaries to first increase and then decrease.     

A phenomenological constitutive equation for Rene 95 PM alloy and its application to isothermal forging process of turbine disk

The flow behavior of Rene 95 PM alloy was studied from 1050 to 1150℃ with strain rate of 1×10-3, 1×10-2, 1×10-1 and 1s-1. At a given temperature and strain rate, flow curves exhibit a peak followed by flow softening up to a steady state. Moreover, at constant strain, flow stress increases with increasing strain rate and decreasing temperature. An equation relating hyperbolic sine of flow stress to hot working parameters, such as strain, strain rate and temperature, was established by using multiple nonlinear regression method. A very good agreement was found between predicted and experimental flow stress in all the strain range investigated. Application of the constitutive equation in predicting forming loads and flow behavior and temperature distribution in both upper and lower dies in an isothermal forging process of turbine disk of large dimension (about 630mm) by means of a finite element code was systematically analyzed.     

Deformation behavior and microstructure of an Al-Zn-Mg-Cu-Zr alloy during hot deformation

The hot deformation behavior and microstructures of Al-7055 commercial alloy were investigated by axisymmetric hot compression at temperatures ranging from 300℃ to 450℃ and strain rates from 10-2 to 10 s-1, respectively. Microstructures of deformed 7055 alloy were investigated by transmission electron microscopy (TEM). The dependence of peak stress on deformation temperature and strain rate can be expressed by the hyperbolic-sine type equation. The hot deformation activation energy of the alloy is 146 kJ/mol. Moreover, the flow stress curves predicted by the modified constitutive equations are reasonably consistent with the experimental results, which confirms that the proposed deformation constitutive equations can provide evidence for the selection of hot forming parameters. TEM results indicate that dynamic recovery is the main softening mechanism during hot deformation.     

Dynamic recrystallization behavior of the Ti–48Al–2Cr–2Nb alloy during isothermal hot deformation

The hot deformation behavior of the as-cast Ti–48Al–2Cr–2Nb alloy was investigated by isothermal compression tests at deformation temperatures ranging from 1000℃ to 1200℃,and strain rates from 0.001 s~(-1)to 0.1 s~(-1).The single peak stress features common to all flow curves indicate that DRX is the dominating softening mechanism.The calculated values of the hot deformation activation energy Q and stress index n are 296.5 kJ mol~(-1)and 3.97,respectively.Based on this,the Arrhenius type constitutive equation was successfully established.The DRX critical condition model and relationship among DRX volume fractions,deformation temperatures and strain rates were obtained to optimize the process.Combined with microstructure analysis,it's concluded that 1200℃/0.01s~(-1)is the optimization parameter.Besides,both DDRX and CDRX were observed in theγphase evolution.The deformation mechanism from the inter-grain dislocation motion to the grain boundary migration and grain rotation was discussed.     

Hot compression deformation behavior of AISI 321 austenitic stainless steel

The hot compression behavior of AISI 321 austenitic stainless steel was studied at the temperatures of 950–1100℃ and the strain rates of 0.01–1 s?1 using a Baehr DIL-805 deformation dilatometer. The hot deformation equations and the relationship between hot deformation parameters were obtained. It is found that strain rate and deformation temperature significantly influence the flow stress behavior of the steel. The work hardening rate and the peak value of flow stress increase with the decrease of deformation temperature and the increase of strain rate. In addition, the activation energy of deformation (Q) is calculated as 433.343 kJ/mol. The microstructural evolution during deformation indicates that, at the temperature of 950℃ and the strain rate of 0.01 s?1, small circle-like precipitates form along grain boundaries; but at the temperatures above 950℃, the dissolution of such precipitates occurs. Energy-dispersive X-ray analyses indicate that the precipitates are complex carbides of Cr, Fe, Mn, Ni, and Ti.     

Hot deformation behavior of Super304H austenitic heat resistant steel

The hot compression tests of Super304H austenitic heat resistant steel were carried out at 800–1200℃ and 0.005–5 s-1 using a Gleeble 3500 thermal-mechanical simulator, and its deformation behavior was analyzed. The results show that the flow stress of Super304H steel decreases with the decrease of strain rate and the increase of deformation temperature; the hot deformation activation energy of the steel is 485 kJ/mol. The hot deformation equation and the relationship between the peak stress and the deformation temperature and strain rate is obtained. The softening caused by deformation heating cannot be neglected when both the deformation temperature and strain rate are higher.     

Hot deformation characteristics of as-cast high-Cr ultra-super-critical rotor steel with columnar grains

Isothermal hot compression tests of as-cast high-Cr ultra-super-critical (USC) rotor steel with columnar grains perpendicular to the compression direction were carried out in the temperature range from 950 to 1250°C at strain rates ranging from 0.001 to 1 s-1. The softening mechanism was dynamic recovery (DRV) at 950°C and the strain rate of 1 s-1, whereas it was dynamic recrystallization (DRX) under the other conditions. A modified constitutive equation based on the Arrhenius model with strain compensation reasonably predicted the flow stress under various deformation conditions, and the activation energy was calculated to be 643.92 kJ·mol-1. The critical stresses of dynamic recrystallization under different conditions were determined from the work-hardening rate (θ)–flow stress (σ) and -?θ/?σ–σ curves. The optimum processing parameters via analysis of the processing map and the softening mechanism were determined to be a deformation temperature range from 1100 to 1200°C and a strain-rate range from 0.001 to 0.08 s-1, with a power dissipation efficiency η greater than 31%.     

Compressive deformation behavior of an indirect-extruded Mg-8Sn-1Al-1Zn alloy

The medium and warm deformation behaviors of an indirect-extruded Mg-8Sn-1Al-1Zn alloy were investigated by compression tests at temperatures between 298 and 523 K and strain rates of 0.001–10 s?1. It was found that the twinning-slip transition temperature was strain rate dependent, and all the true stress-true strain curves could be divided into two groups: concave and convex curves. Associated microstructural investigations indicated that the dynamic recrystallization (DRX) behavior of the alloy varied with deformation conditions. At high strain rate and low temperature, dynamically recrystallized grains preferentially nucleated and developed in the twinned regions, indicating that twinning-induced DRX was dominant. While, at low strain rate, DRX developed extensively at grain boundaries and twins, and the process of twinning contributed to both oriented nucleation and selective growth. For the studied alloy, cracks mainly initiated from the shear band and twinning lamellar over the ranges of temperature and strain rate currently applied.     

Effect of strain rate on the compressive deformation behaviors of lotus-type porous copper

Lotus-type porous copper was fabricated by unidirectional solidification, and compressive experiments were subsequently conducted in the strain rate range of 10-3–2400 s-1 with the compressive direction parallel to the pores. A GLEEBLE-1500 thermal-mechanical simulation system and a split Hopkinson pressure bar (SHPB) were used to investigate the effect of strain rate on the compressive deformation behaviors of lotus-type porous copper. The influence mechanism of strain rate was also analyzed by the strain-controlling method and by high-speed photography. The results indicated that the stress-strain curves of lotus-typed porous copper consist of a linear elastic stage, a plateau stage, and a densification stage at various strain rates. At low strain rate (< 1.0 s-1), the strain rate had little influence on the stress-strain curves; but when the strain rate exceeded 1.0 s-1, it was observed to strongly affect the plateau stage, showing obvious strain-rate-hardening characteristics. Strain rate also influenced the densification initial strain. The densification initial strain at high strain rate was less than that at low strain rate. No visible inhomogeneous deformation caused by shockwaves was observed in lotus-type porous copper during high-strain-rate deformation. However, at high strain rate, the bending deformation characteristics of the pore walls obviously differed from those at low strain rate, which was the main mechanism by which the plateau stress exhibited strain-rate sensitivity when the strain rate exceeded a certain value and exhibited less densification initial strain at high strain rate.     

Dynamic recrystallization and precipitation in high manganese austenitic stainless steel during hot compression

Dynamic recrystallization and precipitation in a high manganese austenitic stainless steel were investigated by hot compression tests over temperatures of 950–1150℃ at strain rates of 0.001 s?1-1 s?1. All the flow curves within the studied deformation regimes were typical of dynamic recrystallization. A window was constructed to determine the value of apparent activation energy as a function of strain rate and deformation temperature. The kinetics of dynamic recrystallization was analyzed using the Avrami kinetics equation. A range of apparent activation energy for hot deformation from 303 kJ/mol to 477 kJ/mol is obtained at different deformation regimes. Microscopic characterization confirms that under a certain deformation condition (medium Zener-Hollomon parameter (Z) values), dynamic recrystallization appears at first, but large particles can not inhibit the recrystallization. At low or high Z values, dynamic recrystallization may occur before dynamic precipitation and proceeds faster. In both cases, secondary phase precipitation is observed along prior austenite grain boundaries. Stress relaxation tests at the same deformation temperatures also confirm the possibility of dynamic precipitation. Unexpectedly, the Avrami’s exponent value increases with the increase of Z value. It is associated with the priority of dynamic recrystallization to dynamic precipitation at higher Z values.     

Prediction of hot deformation behavior in Ni-based alloy considering the effect of initial microstructure

High temperature heat treatments were conducted for as-cast N08028 alloy to obtain various microstructures with different amounts of σ-phase,and then hot compression tests were carried out using Gleeble-3500 thermo-mechanical simulator in deformation temperature range from 1100 to1200 ℃ and strain rate range from 0.01 to 1 s-1. For the same initial microstructure, the flow stress was observed to increase with increasing the strain rate and decreasing the deformation temperature, while for the same deformation condition, the flow stress was found to increase with increasing the amount of σ-phase in the initial microstructure. Moreover, dynamic recrystallization was found to be the main dynamic soften mechanism. On this basis, Arrhenius-type constitutive equations and artificial neural network(ANN) model with back-propagation learning algorithm were established to predict hot deformation behavior of the alloy. Furthermore, the parameters of constitutive equations were found to be dependent on the initial microstructure, which was also as one of the inputs for the ANN model. Suitability of the two models was evaluated by comparing the accuracy, correlation coefficient and average absolute relative error, of the prediction. It is concluded that the ANN model is more accurately than the constitutive equations.     

Constitutive behavior and processing maps of low-expansion GH909 superalloy

The hot deformation behavior of GH909 superalloy was studied systematically using isothermal hot compression tests in a temperature range of 960 to 1040℃ and at strain rates from 0.02 to 10 s-1 with a height reduction as large as 70%. The relations considering flow stress, temperature, and strain rate were evaluated via power-law, hyperbolic sine, and exponential constitutive equations under different strain conditions. An exponential equation was found to be the most appropriate for process modeling. The processing maps for the superalloy were constructed for strains of 0.2, 0.4, 0.6, and 0.8 on the basis of the dynamic material model, and a total processing map that includes all the investigated strains was proposed. Metallurgical instabilities in the instability domain mainly located at higher strain rates manifested as adiabatic shear bands and cracking. The stability domain occurred at 960-1040℃ and at strain rates less than 0.2 s-1; these conditions are recommended for optimum hot working of GH909 superalloy.     

AISI 321奥氏体不锈钢的应变硬化行为、应变速率敏感性及热变形图

Hot compression tests were performed on AISI 321 austenitic stainless steel in the deformation temperature range of 800–1200°C and constant strain rates of 0.001, 0.01, 0.1, and 1 s?1. Hot flow curves were used to determine the strain hardening exponent and the strain rate sensitivity exponent, and to construct the processing maps. Variations of the strain hardening exponent with strain were used to predict the microstructural evolutions during the hot deformation. Four variations were distinguished reflecting the different microstructural changes. Based on the analysis of the strain hardening exponent versus strain curves, the microstructural evolutions were dynamic recovery, single and multiple peak dynamic recrystallization, and interactions between dynamic recrystallization and precipitation. The strain rate sensitivity variations at an applied strain of 0.8 and strain rate of 0.1 s?1 were compared with the microstructural evolutions. The results demonstrate the existence of a reliable correlation between the strain rate sensitivity values and evolved microstructures. Additionally, the power dissipation map at the applied strain of 0.8 was compared with the resultant microstructures at predetermined deformation conditions. The microstructural evolutions strongly correlated to the power dissipation ratio, and dynamic recrystallization occurred completely at lower power dissipation ratios.     

Hot deformation map and its application of GH4706 alloy

The hot deformation behaviors of GH4706 alloy were investigated using compression tests in a deformation temperature range from 900℃ to 1200℃ with a strain rate range of 0.001–1 s?1. Hot processing maps were developed on the basis of the dynamic material model and compression data. A three-dimensional distribution of power dissipation parameter (η) with strain rate and temperature reveals that η decreases in sensitivity with an increase in strain rate and a decrease in temperature. Microstructure studies show that the grain size of GH4706 alloy increases when η is larger than 0.32, and the microstructure exhibits local deformation when η is smaller than 0.23. The hot processing map at the strain of 0.7 exposes a domain peak at η=0.32 for the temperature between 940℃ and 970℃ with the strain rate from 0.015 s?1 to 0.003 s?1, and these are the optimum parameters for hot working.