Abstract
Hot deformation behavior of powder metallurgical high tungsten alloy steel was investigated by Gleeble thermal simulator in a temperature range of 900~1100 °C and a strain rate range of 0.001~1
Science Press
High-density alloys have been widely applied in the industrial, aerospace, and defense fields such as balance weight and kinetic energy projectil
Although the P/M process has a series of advantages, it is generally considered that the subsequent high temperature deformation process is necessary to further improve the microstructure and mechanical properties of alloys. Processing map theory, developed by Prasad based on the dynamic material model (DMM) theor
High-tungsten steel has been recently developed using powder metallurgy. Firstly, tungsten, molybdenum, and 18Ni maraging steel (18Ni8.5Co5Mo0.4Ti0.1Al, wt%) powders were mechanically mixed with a mass ratio of 22:4:74. Secondly, the mixed powders were milled in a planetary ball mill using a ball-to-powder mass ratio of 5:1 for 12 h at a milling speed of 300 r/min. Thirdly, the milled powders were cold isostatically pressed (CIP) for 40 min at a pressure of 250 MPa. Finally, the sample was hot isostatically pressed (HIP) at 1300 °C and 130 MPa for 4 h in argon atmosphere. The microstructure of this high-tungsten steel, as shown in

Fig.1 Optical microstructure of as-sintered high-tungsten steel
Hot compression tests were conducted on a Gleeble 3500 thermal simulator at different temperatures and strain rates under a vacuum of about 1 Pa. Cylindrical specimens with an aspect ratio of 1.5 (6 mm in diameter and 9 mm in length) were machined from HIPed alloys. A pair of highly sensitive thermocouples were welded at the mid-length of the specimen to measure and control the temperature during hot compression. The specimens were heated to 1200 °C at a rate of 10 °C/s, held for 3 min to eliminate temperature gradient, and then cooled at a rate of 5 °C/s to the deformation tempe-ratures. The isothermal compression was conducted in the range of 900~1100 °C with intervals of 50 °C. The strain rate was set as 0.001, 0.01, 0.1, and 1
The deformed specimens were cut along the compression axis in the center and mounted in the resin for metallographic observation. The mounted specimens were ground and polished, followed by etching in 4% HNO3 alcohol solution. The microstructures were characterized by optical microscope (OM, Zeiss ProgRes C5) and scanning electron microscope (SEM, FEI Quanta 650) equipped with energy backscatter diffraction (EBSD) detector. The crystalline structure was identified by X-ray diffraction (XRD) using Bruker D8 diffractometer with Co Kα radiation operated at 35 kV/40 mA with a scanning range from 30° to 100°. The foils for transmission electron microscopy (TEM) were prepared by grinding to the thickness of 40 μm and thinning by the ion beam thinning method. The TEM observation was performed on JEM-2100 TEM with an acceleration voltage of 200 kV.
The true stress-true strain curves during isothermal compression are shown in Fig.2. The flow curves correspond to the deformation temperatures of 900, 950, 1000, 1050, and 1100 °C at different strain rates of 0.001, 0.01, 0.1, and 1

At the initial stage of deformation, the work hardening effect is the dominant reason for the increase of flow stress. The propagation and accumulation of dislocation occur with an increase in deformation strain, leading to an increase in strain hardening effect. Meanwhile, the thermal activation promotes the nucleation and growth of sub-grains. Thus, dynamic recovery also occurs at the same time as strain hardening. With increasing the deformation temperature, dynamic recovery develops faster, and it takes less time to reach the peak stres
The flow stress evolution at high deformation temperature is related to the strain rate (), deformation degree (ε), and temperature (T). The relationship between flow stress, strain rate, and deformation temperature is generally described in the form of Arrhenius equation, as follows:
(1) |
(2) |
(3) |
where Q (kJ/mol) is the activation energy of deformation, R is the gas constant (8.314 J/(mol·K)), A1, A2, A, n1, n, β and α (≈β/n1) are material constants determined by deformation, and is the flow stress.
The effects of temperature and strain rate under hot working process on deformation behavior can be analyzed using the hyperbolic sine function incorporated in the Zener-Hollomon parameter (Z). It can be expressed as follows:
(4) |
The values of n1 and β can be calculated by linear regression of equations obtained by taking logarithm on each side of
The plots of the linear regression relationship between ln[sinh(ασ)] and are shown in Fig.3c, which can determine the value of stress exponent, n. The plot of ln[sinh(ασ)] and 1/T is used for calculating the value of activation energy Q, as shown in Fig.3d.
(5) |
(6) |
From
(7) |

The values of α and n were determined as 0.003 546 and 5.8525, respectively. The activation energy for isothermal deformation at a constant strain rate is determined by
(8) |
The average activation energy is 377 kJ/mol which is calculated by the sinh type equation. By substituting the values α, n, Q and A into
(9) |
Fig.3e shows the variation of the Zener-Hollomon parameter as a function of peak flow stress, and relationship of lnZ versus ln[sinh(ασ)] is listed in
lnZ=5.7541ln[sinh(ασ)]+31.231 | (10) |
The Q value of powder metallurgy high-tungsten alloy is determined to be 377 kJ/mol, which is higher than 355 kJ/mol of 18Ni maraging stee
For the powder metallurgy alloy, it is easy to induce microvoids, cracks, and even macro damage during deformation. The mechanical behavior of alloys is related to the flow stress (σ), strain rate (), strain (ε), and temperature (T). Therefore the processing map, consisting of power dissipation map and instability map, can be a promising method to instruct the hot deformation process. According to the principle of DMM, the power (P) absorbed by the samples during hot deformation can be described using two complementary functions, G and J, as follows:
(11) |
where G represents the power dissipated during plastic deformation, mostly losing in the form of deformation heat, while the co-content J is related to the microstructural evolution.
The power partition between J and G can be expressed by
(12) |
where m is connected with and :
(13) |
From
(14) |
For an ideal linear dissipation process, m = 1; the value of J reaches its maximum, Jmax=/2. The efficiency of the power dissipation η can be deduced by:
(15) |
Dharmendr
(16) |
The value of instability parameter becomes negative when the specimen is in the region of flow instability. The cubic spline interpolation method was used to calculate the flow stress values at a finer interval of temperatures and strain rates, using the experimental data points obtained by the Gleeble test as knots. Therefore, the value of strain rate sensitivity, the efficiency of power dissipation, and the instability parameter can be calculated through the equations mentioned above.
Based on the data obtained in hot deformation experiments at different strain rates, temperatures, and strains, the processing maps of studied alloy are constructed by dissipation efficiency map and instability map at strains of 0.2, 0.3, 0.4, and 0.5, as shown in Fig.4. The contour numbers represent the power dissipation efficiency (η), and the shaded area represents the instable domain judged by instability parameter . It shows a high-efficiency region at lower strain rates and higher temperatures. Besides, the power dissipation maps exhibit similar shape at strains ranging from 0.2 to 0.5. With increasing the strain, the region with a large value of dissipation efficiency becomes larger. In general, the value of η in the DRV domination region is about 0.3, while it is 0.3~0.5 in the DRX regio
Fig.4 Processing maps established from hot compression data at strains of 0.2 (a), 0.3 (b), 0.4 (c) and 0.5 (d) (the contour numbers represent the efficiency of power dissipation, while the shaded area represents the instability area)


Fig.5 OM microstructures and EBSD images of the region along the compression axial at different temperatures and strain rates: (a) 950 °C/1
The magnified images, focusing on the homogenous deformation region at different strain rates and temperatures, are shown in

Fig.6 SEM images of the center region of deformed samples (a~c) and circum swelling region (d): (a) 1000 °C/1

As shown in
1) The flow stress of P/M high-tungsten steel increases with increasing the strain rate and decreasing the deformation temperature. The activation energy of the studied steel is 377 kJ/mol. The constitutive equation of the Zener-Hollomon parameter is successfully established, namely
2) The processing map is successfully established, indicating that the domain at the strain rate of 0.001~0.01
3) The area of instability domains in the processing map increases with increasing the strain. The processing map shows an instability domain that occurs at the temperature below 1000 °C and the strain rate above 0.1
this domain should be avoided in the hot working process.
4) The addition of tungsten and precipitation of the μ phase are the main sources of crack propagation during the hot deformation process.
References
Rao A S, Manda P, Mohan M K et al. Journal of Alloys and Compounds[J], 2018, 742: 937 [Baidu Scholar]
Ding L, Xiang D P, Li Y Y et al. Materials & Design[J], 2012, 37: 8 [Baidu Scholar]
Kiran U R, Panchal A, Sankaranarayana M et al. International Journal of Refractory Metals and Hard Materials[J], 2013, 37: 1 [Baidu Scholar]
Pathak A, Panchal A, Nandy T K et al. International Journal of Refractory Metals & Hard Materials[J], 2018, 75: 43 [Baidu Scholar]
Wu Y C, Hou Q Q, Luo L M et al. Journal of Alloys and Compounds[J], 2019, 776: 926 [Baidu Scholar]
Upadhyaya G S. Powder Metallurgy Technology[M]. Cambridge: Cambridge Int Science Publishing, 1997 [Baidu Scholar]
Güral A, Tekeli S. Materials & Design[J], 2007, 28: 1224 [Baidu Scholar]
Prasad Y, Gegel H. Metallurgical and Materials Transactions A[J], 1984, 15: 1883 [Baidu Scholar]
Momeni A, Dehghani K. Materials Science and Engineering A[J], 2010, 527: 5467 [Baidu Scholar]
Liu G W, Han Y, Shi Z Q et al. Materials & Design[J], 2014, 53: 662 [Baidu Scholar]
Sun C Y, Li Y M, Xiang Y et al. Rare Metal Materials and Engineering[J], 2016, 45(3): 688 (in Chinese) [Baidu Scholar]
Wen D X, Lin Y C, Li H B et al. Materials Science and Engineering A[J], 2014, 591: 183 [Baidu Scholar]
Wang Y X, Zhao G Q, Xu X et al. Journal of Alloys and Compounds[J], 2019, 779: 735 [Baidu Scholar]
Zhu Y C, Zeng W D, Feng F et al. Materials Science and Engineering A[J], 2011, 528: 1757 [Baidu Scholar]
Li C W, Xie H, Mao X N et al. Rare Metal Materials and Engineering[J], 2017, 46(2): 326 [Baidu Scholar]
Rajput S K, Chaudhari G P, Nath S K. Journal of Materials Processing Technology[J], 2016, 237: 113 [Baidu Scholar]
Lu L, Hou L G, Cui H et al. Journal of Iron and Steel Research International[J], 2016, 23(5): 501 [Baidu Scholar]
Wang J, Zhao G, Li M. Materials & Design[J], 2016, 103: 268 [Baidu Scholar]
Mirzadeh H, Cabrera J M, Prado J M et al. Materials Science and Engineering A[J], 2011, 528: 3876 [Baidu Scholar]
Sun Y, Wan Z P, Hu L X et al. Materials & Design[J], 2015, 86: 922 [Baidu Scholar]
Tan L M, Li Y P, He G A et al. Materials Characterization[J], 2019, 147: 340 [Baidu Scholar]
Lin Y C, Li L T, Xia Y C et al. Journal of Alloys and Compounds[J], 2013, 550: 438 [Baidu Scholar]
Mirzadeh H, Cabrera J M, Prado J M et al. Materials Science and Engineering A[J], 2011, 528: 3876 [Baidu Scholar]
Avadhani G S. Journal of Materials Engineering & Performance[J], 2003, 12: 609 [Baidu Scholar]
Jacob A, Schmetterer C, Singheiser L et al. Calphad: Computer Coupling of Phase Diagrams & Thermochemistry[J], 2015, 50: 92 [Baidu Scholar]
Dharmendra C, Rao K P, Jain M K et al. Journal of Alloys and Compounds[J], 2018, 744: 289 [Baidu Scholar]
Liu Y H, Ning Y Q, Yao Z K et al. Journal of Alloys and Compounds[J], 2014, 612: 56 [Baidu Scholar]
Huang L J, Zhang Y Z, Geng L et al. Materials Science and Engineering A[J], 2013, 580: 242 [Baidu Scholar]
Detrois M, Antonov S, Tin S et al. Materials Characterization[J], 2019, 157: 109 915 [Baidu Scholar]
Song Z G, Xu C Z, Pu E X et al. Rare Metal Materials and Engineering[J], 2018, 47(3): 895 [Baidu Scholar]
Gao S, Liu Z Q, Li C F et al. Acta Materialia[J], 2016, 110: 268 [Baidu Scholar]
Wang L X, Jiang X, Dai F M et al. Hot Working Technology[J], 2016, 45: 153 [Baidu Scholar]