Abstract
The hot deformation behavior of BFe10-1-2 cupronickel alloy was investigated over wide ranges of deformation temperature and strain rate. The physics-based constitutive model was developed to predict the dynamic recovery (DRV) behavior of BFe10-1-2 cupronickel alloy at elevated temperatures. In order to verify the validity of the developed constitutive equation, the correlation coefficient (R) and average absolute relative error (AARE) were introduced to make statistics. The results indicated that the developed constitutive equation lead a good agreement between the calculated and experimental data and can accurately characterize the hot DRV behaviors for the BFe10-1-2 cupronickel alloy.
Introduction
In the hot working process, the flow behavior of material is complex due to various physical mechanisms occurring such as work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), which are affected by the chemical composition of metals and alloys, deformation temperature, deformation degree and strain rate [1]. The constitutive model and/or equation is significant to explore the flow behavior and the optimization of the deformation process of the alloy as it describes the correlation of dynamic material properties with processing parameters [2]. Furthermore, constitutive equation is used as input to the FEM code for simulating the response of materials under the specified deformation conditions [3]. Therefore, the preciseness of the constitutive relation has positive impact on the accuracy of simulation results.
The constitutive relations can be divided into three categories: the phenomenological, physical and artificial neural network (ANN) models. The phenomenological model provides a definition of the flow stress based on empirical observations and consists of some mathematical functions to describe flow behavior of metal materials (WH, strain rate hardening and temperature softening) [4]. Based on the Arrhenius type of equation [5], a phenomenological model has been extensively used to predict the hot deformation behavior of metal materials, such as 42CrMo steel [6], 9Cr-1Mo steel [7], 316 austenitic stainless steel [8], and so on. However, this model is lack of physical background of materials deformation, and just describes flow stress in a simple mathematical form with macroscopic process parameters. Unlike the regression methods, ANN postulates no mathematical model or identifies its parameters. ANN learns from training data and recognizes patterns in a series of input and output values without any prior assumptions about their nature and interrelations [9]. Therefore, it has powerful ability to predict hot deformation behaviors of metal materials across the whole deformation domain and has exactly predicted the flow stress of precipitation-hardening aluminum alloy [10], as-cast TC21 titanium alloy [11], A357 alloy [12], and AZ81 magnesium alloy [13] during hot working. Different from ANN, physics-based constitutive model accounts for physical aspects of the material behaviors such as thermodynamics, thermally activated dislocation movement, and kinetics of slips. Besides, physics-based constitutive model allows for an accurate definition of material behavior under wide ranges of loading conditions by some physical assumptions [4]. Based on the stress–dislocation relation and the kinetics of DRV and DRX, physics-based constitutive relation has been established to describe the flow behavior during the WH, DRV and DRX periods for 55SiMnMo bainite steel [14], D6ac steel [15], Al-Mg Alloy [16], and so on.
As a kind of a typical Cu-Ni alloy, BFe10-1-2 alloy has been successfully used as cooling-condition material in shipping and seawater desalted industry. The main deformation methods of this alloy include semi-solid casting ingots and hot extrusion, which result in considerable problems, such as long process time, high energy consumption, low product yield and high cost. Therefore, modeling and prediction of the hot deformation behavior of BFe10-1-2 cupronickel alloy is of vital significance for the purpose of improving and accurately controlling the extrusion technology of BFe10-1-2 cupronickel alloy. However, few investigators have focused their research on the physics-based constitutive relation of BFe10-1-2 cupronickel alloy. Therefore, the objective of this investigation is to construct the suitable physics-based constitutive model to characterize the hot DRV behaviors for BFe10-1-2 cupronickel alloy. Toward this end, isothermal hot compression tests were conducted over a wide range of temperature (1,023~1,273 K) and strain rate (0.001~10 s–1). The experimental stress–strain data were then employed to derive the physics-based constitutive equation. Finally, the validity of descriptive results obtained from the developed constitutive model was examined over the entire range of temperatures and strain rates.
Experimental procedure
The material investigated in the present study is a commercial as-cast BFe10-1-2 cupronickel alloy, with the following chemical composition (wt%): Ni = 10, Mn = 0.75, Fe = 1.65, Cu = Bal.
Cylindrical specimens with a diameter of 10 mm and a height of 15 mm were machined to carry out compression testing. In order to minimize the friction, the flat ends of the specimens were recessed a depth of 0.1 mm groove to entrap the lubricant. The specimens prior to isothermal compression were heated to the deformation temperature at a rate of 10 °C/s and held for 3 min at the isothermal conditions so as to obtain a uniform deformation temperature. Then isothermal compression tests were performed on a Gleeble-3800 thermo-simulation simulator in the strain rate range of 0.001~10 s−1 and the temperature range of 1,023~1,273 K. After deformation, the specimens were cooled to room temperature in air. The strain–stress curves were recorded automatically in isothermal compression.
In order to observe the microstructure evolution, the deformed specimens were sectioned parallel to the compression axis, then ground and polished for metallographic examination, and etched with reagent of FCl+HCl+H2O. The microstructure was examined by optical microscopy. In addition, the original microstructure of as received BFe10-1-2 alloy is given in Figure 1.

Microstructure of as received BFe10-1-2 alloy.
Results and discussion
Stress–strain characteristics of BFe10-1-2 cupronickel alloy during hot deformation
The true stress–strain curves obtained from the hot compression tests of BFe10-1-2 cupronickel alloy are shown in Figure 2. Although some noise is shown in the curves, it can be seen from figures that the influence of deformation temperature and strain rate on flow stress is significant. From these figures, it can be seen that the flow stress increases with decrease of deformation temperature and increase of strain rate.

Flow curves of BFe10-1-2 alloy at various strain rates with temperatures of: (a) 1,023 K, (b) 1,073 K, (c) 1,123 K, (d) 1,173 K, (e) 1,223 K, (f) 1,273 K.
The variation of flow stress with the deformation temperature and the strain rate can be expressed quantitatively, and the relationship between deformation temperature, strain rate and dislocation density can be represented as the following formula [1]:
where
where
Meanwhile, the flow stress curves exhibit the DRV characteristics of the material without obvious peak stress. A typical DRV stress–strain curve is given in Figure 3. The flow stress rapidly increases due to WH. When the WH and DRV reach a balance, a saturation flow stress will appear and stay near constant, and the grains continue to elongate with the increase of deformation but the shape and size of subgrains remain unchanged.

A schematic of true stress–strain curve characteristic of DRV behavior.
Figure 4 illustrates the metallographic microstructure of BFe10-1-2 alloy under various deformation conditions (i. e. 1,023 K, 0.001 s–1; 1,123 K, 0.01 s–1; 1,173 K, 1 s–1 and 1,273 K, 10 s–1), where the grains tend to be elongated perpendicular to the compression direction. It can be seen in Figure 2 that flow stresses drop near the strain of 0.6 when strain rate is 0.001 s–1 and 0.01 s–1 (such as 1,023 and 1,073 K). However, no obvious DRX grains can be detected in Figure 4(a) and (b). Similar phenomenon was reported by Park [17] in Ti-6Al-4V alloy that the flow softening at slow strain rates was attributed to the occurrence of dynamic spheroidization of the α grains or DRV rather than DRX. Meanwhile, it is difficult for DRV and DRX to occur simultaneously in hot deformation processes because the distortion energy is released by DRV and is accumulated with difficulty to the level for DRX [18]. The dislocation density, distortion energy and recrystallization locations increase with the increasing of strain rate, which leads to the fact that DRX occurs easily at high strain rates [19, 20]. Therefore, the dominant dynamic softening mechanism of BFe10-1-2 alloy is DRV.

Metallographic microstructure of BFe10-1-2 alloy under the deformation condition of: (a) 1,023 K, 0.001 s–1; (b) 1,123, 0.01 s–1; (c) 1,173, 1 s–1; (d) 1,273, 10 s–1.
Activation energy for hot deformation
The flow stress is mainly influenced by temperature, strain rate and strain during hot working. Zener–Holloman parameter (Z), i. e. temperature modified strain rate, is often used to describe the effects of temperature and strain rate on material deformation behavior:
where
where A, n and α are material constants, and α can be expressed as:
where
where B and C are the material constants, which are independent of the deformed temperatures. The values of

Relationship between: (a) ln(σ) and ln(
For a given strain rate, activation energy Q can be expressed as:
Then the value of Q can be derived from the slopes in the plot of
![Figure 6: Relationship between ln[sinh(ασ)] and 1/T.](/document/doi/10.1515/htmp-2015-0094/asset/graphic/htmp-2015-0094_figure6.jpg)
Relationship between ln[sinh(ασ)] and 1/T.
Constitutive modeling of flow stress
The evolution of dislocation density is a result of the multiplication and annihilation of dislocations due to WH and DRV respectively and can be expressed as [4]:
where
The contribution of the dislocation density to the flow stress can be expressed as:
where
where
The values of

Relationship between
It can be seen from eq. (13) that
The values of R and AARE between the third-order polynomial predicted and measured
It is difficult to determine the accurate initial yield stress
According to eq. (13), Ω can be calculated by the following formula:

Relationship between the yield stress and the Zener–Hollomon parameter.
Using the true stress–strain data, the values of Ω can be determined for the whole deformation conditions. Figure 9 illustrates that a linear relation exists between lnΩ and lnZ. Meanwhile, it can be found from the figure that lnΩ decreases with the increase of lnZ, and Ω is expressed as a function of Z parameter:

Relationship between Ω and the Zener–Hollomon parameter.
Therefore, the physics-based constitutive model of BFe10-1-2 cupronickel alloy during hot working can be summarized as:
Verification of the developed constitutive equations of BFe10-1-2 cupronickel alloy
In order to verify the developed physics-based constitutive equation, a comparison between the experimental and predicted flow stress data at different strains within the range of 0.15~0.75 and the interval of 0.1 (i. e. the testing data that were not used to fitting curves) was carried out in Figure 10. The predicted flow stress data from the constitutive equation could track the experimental data of BFe10-1-2 cupronickel alloy, and there is a good agreement between the experimental and predicted values under most deformation conditions. Only under some processing condition (i. e. at 1,023 K, 1,173 K in 10 s–1 and 1,123 K, 1,273 K in 1 s–1), a remarkable variation between experimental and computed flow stress data could be observed. The main reason of the variation may contribute to the fact that the response of flow behavior of mental materials at evaluated temperatures is highly nonlinear. Most factors affecting the flow stress are nonlinear, which make the accuracy of the predicted flow stress by the constitutive equations low and the applicable range limited [9]. Meanwhile, the fitting of material constants may lead to the variation between experimental and computed flow stress data. For example, experimental data in Figure 5(b) show some deviation. Therefore, some errors may be introduced because of the determination of the values of β and finally affect the accuracy of constitutive equation.

Comparison between the experimental and predicted flow stress at the temperature of: (a) 1,023 K, (b) 1,073 K, (c) 1,123 K, (d) 1,173 K, (e) 1,223 K, (f) 1,273 K.
The predictability of the developed physics-based constitutive equation could be quantified in terms of standard statistical parameters such as R and AARE. These are expressed as following equations [26]:
where E is the experimental flow stress and P is the predicted flow stress obtained from the developed constitutive equation considering strain compensation.
R is a commonly employed statistical parameter and provides information on the strength of the linear relationship between the experimental and predicted data. And the AARE is calculated through a term by term comparison of the relative error and therefore is an unbiased statistical parameter for determining the predictability of the equation [27]. As can be seen from Figure 11, although there are some variation under some deformation conditions, the values of R and AARE are found to be 0.9833 and 6.726 % respectively, which indicate that the developed physics-based constitutive model gives an accurate and precise estimate of the DRV behavior of BFe10-1-2 cupronickel alloy.

Correlation between the experimental and predicted flow stress data from the developed constitutive equation.
Conclusions
A physics-based constitutive model was developed to describe DRV behavior of BFe10-1-2 cupronickel alloy during hot working by performing hot compression tests over a wide range of temperature (1,023~1,273 K) and strain rate (0.001~10 s–1). Based on this study, following conclusions are obtained:
DRV is the main softening process for BFe10-1-2 cupronickel alloy during hot working.
The developed physics-based constitutive equation can precisely predict the flow stress under most deformation conditions. The predictability of developed constitutive equation was quantified in terms of correlation coefficient (R) and average absolute relative error (AARE). The results of R and AARE indicated that the proposed constitutive model could accurately characterize the hot DRV behaviors for BFe10-1-2 cupronickel alloy over a wide range of temperatures and strain rates.
Acknowledgements
The authors gratefully acknowledge the financial support received from Planned Scientific Research Project of Education Department of Shaanxi Provincial Government (15JS056), Innovation Team Project of “Processing and Preparation for High-performance Non-ferrous Metal Materials” of Xi’an University of Architecture and Technology, Fundamental Science Funds of Xi’an University of Architecture and Technology (JC1308) and Talents Science Fund of Xi’an University of Architecture and Technology (RC1369).
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Articles in the same Issue
- Frontmatter
- Research Articles
- Effects of Extrusion–Shear Process Conditions on the Microstructures and Mechanical Properties of AZ31 Magnesium Alloy
- Degradation of TiB2/TiC Composites in Liquid Nd and Molten NdF3–LiF–Nd2O3 System
- Investigation of Oxygen Diffusion in Irradiated UO2 with MD Simulation
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- Short Communication
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