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Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy
Author’s Accepted Manuscript Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)30576-1 http://dx.doi.org/10.1016/j.msea.2015.10.113 MSA32964
To appear in: Materials Science & Engineering A Received date: 6 August 2015 Revised date: 28 October 2015 Accepted date: 29 October 2015 Cite this article as: N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo and H.Z. Fu, Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy N.N. Guo, L. Wang*, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su*, J.J. Guo, H.Z. Fu National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and
，
Engineering Harbin Institute of Technology, Harbin 150001, China Corresponding author: Tel. :+86 0451 86413910; fax: +86 0451 86413910, Email: [email protected] (L. Wang), [email protected] (Y.Q. Su) Abstract: The hot deformation characteristics of dynamic recrystallization (DRX) of the MoNbHfZrTi refractory high-entropy alloy (HEA) was investigated using the isothermal compression tests in the strain rate of 0.001-0.1 s-1 at the temperature range of 800-1200 oC. Scanning electron microscope (SEM) with the electron backscatter diffraction (EBSD) technique was used to study the effect of deformation temperature and strain rate on the stress-strain behavior, microstructure evolution and dynamic recrystallization (DRX) during hot deformation. At 800 oC, the stress-strain curve exhibits a work-hardening stage until fracture at the strain rate of 0.1 s-1 and 0.01 s-1. Under other deformation conditions, the stress-strain curves exhibit the typical DRX characteristics. On the whole, the stress decreases with the increase of deformation temperature and decrease of the strain rate. The initial dendritic structure gradually disappears and more dynamic recrystallized grains form with the decrease of strain rate and the increase of the deformation temperature. The nucleation mechanism of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occurred simultaneously for the MoNbHfZrTi alloy during hot deformation. The effect of CDRX was weakened with the increase of deformation temperature and with the decrease of strain rate. Keywords:
high-entropy
alloy;
hot
deformation,
microstructure
evolution,
dynamic
recrystallization
1. Introduction High-entropy alloys (HEAs) are a new research frontier in the metal materials filed which containing five or more principle elements with equiatomic or near equiatomic concentration [1-3]. HEAs have attracted much attention due to their simple phase structures and many unique properties that are mainly attributed to the four effects of high mixing entropy, serious lattice
distortion, sluggish diffusion and cocktail effects[4-7]. Several refractory HEAs, that most composing elements are refractory metals such as W, Ta, Hf, Nb, Mo, have been developed for elevated temperature applications adopting the design concept of HEAs[8-10]. Most refractory HEAs are mainly composed of simple body-centered cubic (BCC) solid solution phases and exhibit excellent elevated temperature properties [10-15]. For example, WTaNbMo and WTaNbMoV alloys are composed of a single BCC phase and have the yield strength of 405 MPa and 477 MPa at 1600 oC [16]. AlMo0.5NbTa0.5TiZr alloy consists of two BCC phases and its Vickers hardness reaches 5.8 GPa[17]. Refractory HEAs may be one promising candidate for elevated temperature applications. By replacing Ta in the TaNbHfZrTi with Mo, a lower density and cost refractory alloy is developed and investigated. The MoNbHfZrTi alloy is composed of a single BCC phase with high structural stability and has compressive yield strength of 1719 MPa at room temperature, 825 MPa at 800 oC, 728 MPa at 900 oC, 635 MPa at 1000 oC, 397 MPa at 1100 oC and 187 MPa at 1200 oC in the cast state[18-20]. However, it is well known that most cast alloys are not used directly due to the factors such as shrinkage porosity, coarse dendritic structure, the chemical heterogeneity and so on. In order to achieve the desired the mechanical properties of the final products, thermo-mechanical processing is a useful approach. It is essential to understand the hot deformation behavior during thermo-mechanical processing since it can significantly refine the microstructure and enhance the mechanical properties. For example, isothermal hot forging transformed the dendritic structure into a fine multiphase equiaxed structure with the average grain/particle size of ~2.1 µm for AlCoCrCuFeNi alloy[21]. Extensive multistep forging at 950 °C for the cast AlCuCrFeNiCo alloy refines the microstructure with the average grain/particle size of ∼1.5 ± 0.9 µm and the yield stress, ultimate tensile strength, and tensile ductility of the forged condition were strengthened and improved compared with that of the as-cast condition[22]. However, the study on the hot deformation behavior of refractory HEAs especially on the dynamic recrystallization behavior (DRX) is very few [23]. In this study, the hot deformation behavior of NbMoHfZrTi HEA under different deformation conditions including the different deformation temperatures and different strain rates was studied using the hot compression tests. The associated microstructure evolution with the flow stress and the dynamic recrystallization are analyzed. The aim of this study is to evaluate the effects of
deformation temperature and strain rate on flow stress, microstructure evolution and DRX in the MoNbHfZrTi alloy.
2. Experimental procedures The MoNbHfZrTi alloy was prepared by arc melting in high purity argon inside a water-cooled copper crucible. The purity of the alloying elements was above 99.6%. To ensure the chemical homogeneity, all ingots were flipped over and re-melted for five times. The produced ingots had dimensions of φ35 mm × 15 mm. Compressive tests were performed on cylindrical specimen with dimensions of φ6 mm × 8 mm using the dynamic thermal simulation testing machine (Gleeble-1500D) and the furnace chamber was evacuated to 10-3 torr before each test. Testing was performed at temperatures of 800oC, 900oC, 1000oC, 1100oC and 1200oC. The strain rate was 0.1s-1, 0.01 s-1 and 0.001-1. Under the same testing condition, there are two specimens tested. The microstructure after deformation was analyzed with the use of a scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) detectors. Samples were carefully mechanically polished and then underwent electropolishing in mixture of perchloric acid, n-butanol and methanol. TSL OIM Analysis 6.1 was used to process EBSD data.
3. Results and Discussion 3.1 Stress-strain behavior The stress-strain curves of the MoNbHfZrTi alloy obtained during compression testing at different strain rates and different temperatures are shown in Fig. 1. Obviously, there is a work-hardening stage before the specimen fractured at 800 oC when the strain rate (ߝሶ) is 0.1 s-1 and 0.01 s-1. At the high strain rate of 0.1 s-1, the stress increases to a peak with increasing strain, and then the stress decreases as the strain is further increased at 900 oC and above 900 oC. While at the low strain rate of 0.01 s-1 and 0.001 s-1, the stress increases to a peak with increasing strain and then decreases and gradually reaches a steady value as the strain increases. The features of the flow curves for this alloy belong to those of the normal behavior for the alloys with low stacking fault energy, which indicate the occurrence of DRX phenomenon during hot deformation[24-27]. In the initial stage of hot deformation, the initial rapid rise in stress is associated with the work
hardening which was caused by the increase in dislocation density and the formation of poorly developed subgrain boundaries[28-30]. The stress decreases with the increasing strain since the effect of work hardening can be neutralized or partially neutralized by the occurrence of the dynamic softening mechanism such as dynamic recovery and DRX. Moreover, a steady-state flow stress can be reached when the work hardening and dynamic softening reach a dynamic equilibrium. In general, the hot deformation is a dynamically competing process of work hardening and dynamic softening. It can be also found from the true stress-strain curves that the hot deformation temperature and the strain rate have significant effect on the stress. The stress decreases with the decrease of the strain rate and with the increase of the deformation temperature. It is well known that the adiabatic heating has also an effect on the stress softening. It is stated that more than 90% of the input energy during deformation is converted to the heat due to adiabatic heating and a large fraction of deformation heating is dissipated from the specimen to the surrounding environment. At a higher strain rate, the heat generated by plastic deformation cannot be extracted away from the specimen because the available time is too short. At the same strain rate, the higher deformation temperature can provide more thermal energy which can leads to a higher mobility of dislocation, vacancy and grain boundaries[31]. In addition, the stress-strain curve is peculiar at 900 oC and ߝሶ=0.1 S-1. The stress rapidly reaches a peak and then decreases to a certain value. During the next compressive deformation, the stress increases to other higher peak again and then gradually decreases until the strain reaches to 70%. To ensure the accuracy of results, the tests were conducted for three times at the same conditions. The results show three tests exhibit the same characteristics. This characteristic at this condition may be attributed to the following reason: At the initial deformation stage, the work-hardening effect is significant which induces the stress to the first peak. The dynamic recovery may occur in the following deformation which leads to the decrease of the stress. The work-hardening effect may become the dominant factor, leading to the second increase of the stress in the next deformation. There may be some other reasons for the deformation characteristics which will be investigated and discussed by analyzing the microstructure of the deformed specimen with different strains in other paper.
Fig. 1 Stress-strain curves at 800 oC, 900 oC, 1000 oC, 1100 oC, 1200 oC with different strain rates (εሶ ): (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1. 3.2 Microstructure evolution Fig. 2 and Fig. 3 show the microstructure of the MoNbHfZrTi alloy after deformation at 900 oC and 1100 oC with different strain rates. At 900 oC, the microstructure tends to transform from the cast dendritic structure to the elongated equiaxed structure while part of the cast dendritic structure still remains inside the equiaxed grains at the higher strain rate of 0.1 S-1. With the decrease of the strain rate at the same temperature, the volume of the dendritic structure also decrease and the cast dendritic structure disappeared completely when the strain rate is 0.001 S-1. And at εሶ =0.001 S-1, there are some fine particles or grains are formed along grain boundaries, which will be determined to be the DRX grains according to the EBSD results. Thus it can be inferred that the decrease of the strain rate will promote the formation of DRX grains leading to the decrease of the stress, which is also consistent with the result of the stress. At 1100 oC, the microstructure exhibits the same trend that the lower strain rate is favorable for the formation of DRX. Compared with the microstructure after deformation at 900 oC, it can be found that more DRX grains at the higher deformation temperature with the same strain rate. Thus, the microstructure evolution of this alloy during hot deformation depended closely on the
strain rate and the deformation temperature. With decrease of strain rate and the increase of the deformation temperature, the initial dendritic structure gradually disappears and more dynamic recrystallized grains form. In other words, the volume fraction of the dynamic recrystallized grains increases with the decreasing strain rate and increasing the deformation temperature. The condition for the growth of the dynamic recrystallized grains is thought to be dependent on the distribution and density of dislocations [31]. The decrease of the strain rate or increase of the deformation temperature would lead to a decrease in the critical dislocation density and thus lower the critical strain for the occurrence of DRX [31]. Moreover, the lower strain rate and the higher deformation temperature can provide more time and energy for grain boundary migration which can promote the development of the DRX process.
Fig. 2. Microstructure of the MoNbHfZrTi alloy after deformation at 900 oC with different strain rates: (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1.
Fig. 3. Microstructure of the MoNbHfZrTi alloy after deformation at 1100 oC with different strain rates: (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1. 3.3 Dynamic recrystallization It is well known that the desirable specific properties of alloys are usually determined by the microstructure evolution during hot deformation which is closely related with the DRX process[32]. The occurrence of DRX can play a significant role in grain refinement and the reduction of deformation resistance, which will improve the workability and mechanical properties. DRX is a kind of dynamic softening mechanism which can produce new grains with high angle grain boundaries (HAGBs)[33]. The presence of a large fraction of HABGs is advantageous since the boundaries can contribute to the strength and the toughness[34]. There are two main nucleation mechanisms for DRX, which are the conventional discontinuous dynamic recrystallization (DDRX) and in situ or continuous dynamic recrystallization (CDRX) [32, 35]. The DDRX is characterized by the bulging of corrugated grain boundaries, which involves the nucleation of new grains and subsequent grain growth[32]. On the other hand, CDRX is considered as a recovery process and proceeds by continuous absorption of dislocations in subgrain boundaries, which will result in the formation of HAGBs and new grains[32]. So more
detailed information on the effect of deformation temperature and strain rate on the DRX of the MoNbHfZrTi alloy can be obtained using the EBSD methods[36]. 3.3.1 The effect of deformation temperature on DRX Fig. 4 shows the orientation imaging microscopy (OIM) maps and the fractions of grain boundaries with different misorientation angles at different temperatures with a strain rate of 0.001 s-1, in which HAGBs and low angle grain boundaries (LAGBs) are represented by black lines and gray lines, respectively. Moreover, the grain boudaries with misorientation angles between 10° and 15° are defined as medium angle grain boundaries (MAGBs), which are represented by red lines. At 800 oC, extensively bulging grain boundaries with high angles which is closely related with the strain induced grain boundary migration can be observed. Such grain boundaries characteristics indicate that the nucleation mechanism of DRX during hot deformation belongs to the DDRX. However, a large number of subgrains boundaries with LAGBs can be also observed, which are the typical characteristics of CDRX. Thus, DDRX and CDRX occur simultaneously during hot deformation for the MoNbHfZrTi alloy. With the increasing of the deformation temperature to 1000 oC, the original grain boundaries become more serrated and more bulging. At 1200 o C, some DRX grains along the original grain boundaries can be observed. With the increase of the deformation temperature, the subgrain boundaries with LAGB gradually shift from the interior of the original grains to the vicinity of the original grain boundaries. Fig. 4 (d) shows the the fractions of grain boundaries with different misorientation angles. It can be found that the fraction of LAGBs decreases and the fraction of HAGBs increases with the increase of the deformation temperature, which suggests that the effect of CDRX becomes weaker and DDRX becomes the main operatiing nucleation mechanism of DRX at higher deformation temperature. The MAGBs are defined as the transition zone from low angle to high angle boundaries[37]. The low content of MAGBs indicates that the rapid migration rate from low angle to high angle boundaries. It is well known that the mechanism of CDRX can be operated through progressive subgrain rotation [27, 38]. In order to understand the effect of deformation temperature on CDRX, misorientation analysis was performed along the lines in Fig. 4. As shown in Fig. 5, the point-to-point misorientations (blue lines) both near the original grain boundaries and in the grains are low. While the point-to-origin misorientations (red lines) are high, indicating that the misorientation accumulates and the progressive subgrain rotation has been well developed [26].
With the increase of the deformation temperature, the level of cumulative misorientations decreases gradually, which is related with the weakened effect of progressive subgrain rotation or the accelerated subgrain transformed from low angle to high angle boundaries by rotation at higher deformation temperature[37]. Thus, it can be confirmed that the effect of CDRX is weakened with the increase of the deformation temperature.
Fig. 4. OIM maps of the MoNbHfZrTi alloy after deformation at different temperatures with a strain rate of 0.001 s-1: (a) 800 oC; (b) 1000 oC; (c) 1200 oC; (d) the fractions of grain boundaries with different misorientation angles at the strain rate of 0.001 s-1.
Fig. 5. Misorientations measured along the lines marked in Fig. 4: (a) a, (b) b, (c) c, (d) d, (e) e, (f) f. 3.3.2 The effect of strain rate on DRX Fig. 6 shows the OIM maps and the fractions of grain boundaries with different misorientation angles with different strain rates at 1200 oC. It can be observed that there are some DRX grains at 1200 oC with all studied strain rates. The fraction of LAGBs decreases and the fraction of HAGBs
increases with the increase of the strain rate, which indicates that the effect of the CDRX is strengthened with the decrease of the strain rate. It can be confirmed from the OIM maps since there are hardly any subgrains boundaries at the higher strain rate of 0.1 s-1 and 0.01 s-1, while there are some subgrain boundaries observed at 0.001 s-1. It is well known that MAGBs are necessary for the nucleation of CDRX operated through progressive subgrain rotation. Hence, the fraction of MAGBs can directly reflect the effect of CDRX during hot deformation with the strain rate. It can be found that the fraction of MAGBs decreases with the increase of the strain rate. This result also confirms that the effect of CDRX is strengtened with the decrease of the strain rate. The misorientation analysis was performed along the lines in Fig. 6 to understand the effect of the strain rate on CDRX as shown in Fig. 7. It can be found that the cumulative misorientation (point-to-origin misorientation) and local misorientation (point-to-point) are relatively small. Comparing the results in Fig. 5 and Fig. 7, the misorientation is more sensitive to the deformation temperature than to the strain rate. Higher deformation temperature has a more pronounced weakened effect on the CDRX.
Fig. 6. OIM maps of the MoNbHfZrTi alloy after deformation with different strain rates at 1200 o
C: (a) 0.1 s-1; (b) 0.01 s-1; (c) 0.001 s-1; (d) the fractions of grain boundaries with different misorientation angles at 1200 oC.
Fig. 7. Misorientations measured along the lines marked in Fig. 6: (a) a, (b) b, (c) c, (d) d, (e) e, (f) f. Fig. 8 shows the grain size distributions. It can be observed that these DRX grains have a distinct grain size difference with the deformed original grains. And the DRX grains can be partitioned using the grain size difference. The DRX grain size firstly increases and then decreases with the decrease of the strain rate. At the medium strain rate of 0.01 s-1, the DRX grain size is the maximum, which also accounts for the result that the stress is the lowest at 1200 oC with the strain rate of 0.01 s-1.
Fig. 8. Grain size distributions after deforamtion at 1200 oC with different strain rates: (a) 0.1 s-1; (b) 0.01 s-1; (c) 0.001 s-1.
4. Conslusions The stress-strain behavior, microstructure evolution and DRX behavior with the deformation temperature range of 800-1200 oC and strain rate range of 0.001-0.1 s-1 in the refractory MoNbHfZrTi HEA through isothermal compression tests in a thermomechanical simulator were studied. The primary outcomes are as follows: (1) At 800 oC, the stress-strain curve of MoNbHfZrTi alloy exhibits a work-hardening stage with the increase of the strain until fracture at the strain rate of 0.1 s-1 and 0.01 s-1. The stress-strain curves exhibit the typical DRX characteriscs at above 900 oC with the strain rate of 0.1 s-1 and 0.01 s-1 and at deformation temperature of 800-1200 oC with the strain rate of 0.001 s-1. (2) The stress decreases with the increase of deformation temperature and with the decrease of the strain rate except at 1200 oC. (3) The microstructure evolution depends closely on the deformation temperature and strain rate. With decrease of strain rate and the increase of the deformation temperature, the initial dendritic structure gradually disappears and more dynamic recrystallized grains form. (4) The nucleation mechanism of DDRX and CDRX occurred simultaneously for the MoNbHfZrTi alloy during hot deformation and DDRX is the primary mechanism and CDRX
is the assistant mechanism. The effect of CDRX was weakened with the increase of deformation temperature and with the decrease of strain rate.
Acknowledgements We acknowledge financial support of 973 project (2011CB610406) and Natural Science Foundation of China (51425402).
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PII: DOI: Reference:
S0921-5093(15)30576-1 http://dx.doi.org/10.1016/j.msea.2015.10.113 MSA32964
To appear in: Materials Science & Engineering A Received date: 6 August 2015 Revised date: 28 October 2015 Accepted date: 29 October 2015 Cite this article as: N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo and H.Z. Fu, Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot deformation characteristics and dynamic recrystallization of the MoNbHfZrTi refractory high-entropy alloy N.N. Guo, L. Wang*, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su*, J.J. Guo, H.Z. Fu National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and
，
Engineering Harbin Institute of Technology, Harbin 150001, China Corresponding author: Tel. :+86 0451 86413910; fax: +86 0451 86413910, Email: [email protected] (L. Wang), [email protected] (Y.Q. Su) Abstract: The hot deformation characteristics of dynamic recrystallization (DRX) of the MoNbHfZrTi refractory high-entropy alloy (HEA) was investigated using the isothermal compression tests in the strain rate of 0.001-0.1 s-1 at the temperature range of 800-1200 oC. Scanning electron microscope (SEM) with the electron backscatter diffraction (EBSD) technique was used to study the effect of deformation temperature and strain rate on the stress-strain behavior, microstructure evolution and dynamic recrystallization (DRX) during hot deformation. At 800 oC, the stress-strain curve exhibits a work-hardening stage until fracture at the strain rate of 0.1 s-1 and 0.01 s-1. Under other deformation conditions, the stress-strain curves exhibit the typical DRX characteristics. On the whole, the stress decreases with the increase of deformation temperature and decrease of the strain rate. The initial dendritic structure gradually disappears and more dynamic recrystallized grains form with the decrease of strain rate and the increase of the deformation temperature. The nucleation mechanism of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occurred simultaneously for the MoNbHfZrTi alloy during hot deformation. The effect of CDRX was weakened with the increase of deformation temperature and with the decrease of strain rate. Keywords:
high-entropy
alloy;
hot
deformation,
microstructure
evolution,
dynamic
recrystallization
1. Introduction High-entropy alloys (HEAs) are a new research frontier in the metal materials filed which containing five or more principle elements with equiatomic or near equiatomic concentration [1-3]. HEAs have attracted much attention due to their simple phase structures and many unique properties that are mainly attributed to the four effects of high mixing entropy, serious lattice
distortion, sluggish diffusion and cocktail effects[4-7]. Several refractory HEAs, that most composing elements are refractory metals such as W, Ta, Hf, Nb, Mo, have been developed for elevated temperature applications adopting the design concept of HEAs[8-10]. Most refractory HEAs are mainly composed of simple body-centered cubic (BCC) solid solution phases and exhibit excellent elevated temperature properties [10-15]. For example, WTaNbMo and WTaNbMoV alloys are composed of a single BCC phase and have the yield strength of 405 MPa and 477 MPa at 1600 oC [16]. AlMo0.5NbTa0.5TiZr alloy consists of two BCC phases and its Vickers hardness reaches 5.8 GPa[17]. Refractory HEAs may be one promising candidate for elevated temperature applications. By replacing Ta in the TaNbHfZrTi with Mo, a lower density and cost refractory alloy is developed and investigated. The MoNbHfZrTi alloy is composed of a single BCC phase with high structural stability and has compressive yield strength of 1719 MPa at room temperature, 825 MPa at 800 oC, 728 MPa at 900 oC, 635 MPa at 1000 oC, 397 MPa at 1100 oC and 187 MPa at 1200 oC in the cast state[18-20]. However, it is well known that most cast alloys are not used directly due to the factors such as shrinkage porosity, coarse dendritic structure, the chemical heterogeneity and so on. In order to achieve the desired the mechanical properties of the final products, thermo-mechanical processing is a useful approach. It is essential to understand the hot deformation behavior during thermo-mechanical processing since it can significantly refine the microstructure and enhance the mechanical properties. For example, isothermal hot forging transformed the dendritic structure into a fine multiphase equiaxed structure with the average grain/particle size of ~2.1 µm for AlCoCrCuFeNi alloy[21]. Extensive multistep forging at 950 °C for the cast AlCuCrFeNiCo alloy refines the microstructure with the average grain/particle size of ∼1.5 ± 0.9 µm and the yield stress, ultimate tensile strength, and tensile ductility of the forged condition were strengthened and improved compared with that of the as-cast condition[22]. However, the study on the hot deformation behavior of refractory HEAs especially on the dynamic recrystallization behavior (DRX) is very few [23]. In this study, the hot deformation behavior of NbMoHfZrTi HEA under different deformation conditions including the different deformation temperatures and different strain rates was studied using the hot compression tests. The associated microstructure evolution with the flow stress and the dynamic recrystallization are analyzed. The aim of this study is to evaluate the effects of
deformation temperature and strain rate on flow stress, microstructure evolution and DRX in the MoNbHfZrTi alloy.
2. Experimental procedures The MoNbHfZrTi alloy was prepared by arc melting in high purity argon inside a water-cooled copper crucible. The purity of the alloying elements was above 99.6%. To ensure the chemical homogeneity, all ingots were flipped over and re-melted for five times. The produced ingots had dimensions of φ35 mm × 15 mm. Compressive tests were performed on cylindrical specimen with dimensions of φ6 mm × 8 mm using the dynamic thermal simulation testing machine (Gleeble-1500D) and the furnace chamber was evacuated to 10-3 torr before each test. Testing was performed at temperatures of 800oC, 900oC, 1000oC, 1100oC and 1200oC. The strain rate was 0.1s-1, 0.01 s-1 and 0.001-1. Under the same testing condition, there are two specimens tested. The microstructure after deformation was analyzed with the use of a scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) detectors. Samples were carefully mechanically polished and then underwent electropolishing in mixture of perchloric acid, n-butanol and methanol. TSL OIM Analysis 6.1 was used to process EBSD data.
3. Results and Discussion 3.1 Stress-strain behavior The stress-strain curves of the MoNbHfZrTi alloy obtained during compression testing at different strain rates and different temperatures are shown in Fig. 1. Obviously, there is a work-hardening stage before the specimen fractured at 800 oC when the strain rate (ߝሶ) is 0.1 s-1 and 0.01 s-1. At the high strain rate of 0.1 s-1, the stress increases to a peak with increasing strain, and then the stress decreases as the strain is further increased at 900 oC and above 900 oC. While at the low strain rate of 0.01 s-1 and 0.001 s-1, the stress increases to a peak with increasing strain and then decreases and gradually reaches a steady value as the strain increases. The features of the flow curves for this alloy belong to those of the normal behavior for the alloys with low stacking fault energy, which indicate the occurrence of DRX phenomenon during hot deformation[24-27]. In the initial stage of hot deformation, the initial rapid rise in stress is associated with the work
hardening which was caused by the increase in dislocation density and the formation of poorly developed subgrain boundaries[28-30]. The stress decreases with the increasing strain since the effect of work hardening can be neutralized or partially neutralized by the occurrence of the dynamic softening mechanism such as dynamic recovery and DRX. Moreover, a steady-state flow stress can be reached when the work hardening and dynamic softening reach a dynamic equilibrium. In general, the hot deformation is a dynamically competing process of work hardening and dynamic softening. It can be also found from the true stress-strain curves that the hot deformation temperature and the strain rate have significant effect on the stress. The stress decreases with the decrease of the strain rate and with the increase of the deformation temperature. It is well known that the adiabatic heating has also an effect on the stress softening. It is stated that more than 90% of the input energy during deformation is converted to the heat due to adiabatic heating and a large fraction of deformation heating is dissipated from the specimen to the surrounding environment. At a higher strain rate, the heat generated by plastic deformation cannot be extracted away from the specimen because the available time is too short. At the same strain rate, the higher deformation temperature can provide more thermal energy which can leads to a higher mobility of dislocation, vacancy and grain boundaries[31]. In addition, the stress-strain curve is peculiar at 900 oC and ߝሶ=0.1 S-1. The stress rapidly reaches a peak and then decreases to a certain value. During the next compressive deformation, the stress increases to other higher peak again and then gradually decreases until the strain reaches to 70%. To ensure the accuracy of results, the tests were conducted for three times at the same conditions. The results show three tests exhibit the same characteristics. This characteristic at this condition may be attributed to the following reason: At the initial deformation stage, the work-hardening effect is significant which induces the stress to the first peak. The dynamic recovery may occur in the following deformation which leads to the decrease of the stress. The work-hardening effect may become the dominant factor, leading to the second increase of the stress in the next deformation. There may be some other reasons for the deformation characteristics which will be investigated and discussed by analyzing the microstructure of the deformed specimen with different strains in other paper.
Fig. 1 Stress-strain curves at 800 oC, 900 oC, 1000 oC, 1100 oC, 1200 oC with different strain rates (εሶ ): (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1. 3.2 Microstructure evolution Fig. 2 and Fig. 3 show the microstructure of the MoNbHfZrTi alloy after deformation at 900 oC and 1100 oC with different strain rates. At 900 oC, the microstructure tends to transform from the cast dendritic structure to the elongated equiaxed structure while part of the cast dendritic structure still remains inside the equiaxed grains at the higher strain rate of 0.1 S-1. With the decrease of the strain rate at the same temperature, the volume of the dendritic structure also decrease and the cast dendritic structure disappeared completely when the strain rate is 0.001 S-1. And at εሶ =0.001 S-1, there are some fine particles or grains are formed along grain boundaries, which will be determined to be the DRX grains according to the EBSD results. Thus it can be inferred that the decrease of the strain rate will promote the formation of DRX grains leading to the decrease of the stress, which is also consistent with the result of the stress. At 1100 oC, the microstructure exhibits the same trend that the lower strain rate is favorable for the formation of DRX. Compared with the microstructure after deformation at 900 oC, it can be found that more DRX grains at the higher deformation temperature with the same strain rate. Thus, the microstructure evolution of this alloy during hot deformation depended closely on the
strain rate and the deformation temperature. With decrease of strain rate and the increase of the deformation temperature, the initial dendritic structure gradually disappears and more dynamic recrystallized grains form. In other words, the volume fraction of the dynamic recrystallized grains increases with the decreasing strain rate and increasing the deformation temperature. The condition for the growth of the dynamic recrystallized grains is thought to be dependent on the distribution and density of dislocations [31]. The decrease of the strain rate or increase of the deformation temperature would lead to a decrease in the critical dislocation density and thus lower the critical strain for the occurrence of DRX [31]. Moreover, the lower strain rate and the higher deformation temperature can provide more time and energy for grain boundary migration which can promote the development of the DRX process.
Fig. 2. Microstructure of the MoNbHfZrTi alloy after deformation at 900 oC with different strain rates: (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1.
Fig. 3. Microstructure of the MoNbHfZrTi alloy after deformation at 1100 oC with different strain rates: (a) εሶ =0.1 S-1, (b) εሶ =0.01 S-1, (c) εሶ =0.001 S-1. 3.3 Dynamic recrystallization It is well known that the desirable specific properties of alloys are usually determined by the microstructure evolution during hot deformation which is closely related with the DRX process[32]. The occurrence of DRX can play a significant role in grain refinement and the reduction of deformation resistance, which will improve the workability and mechanical properties. DRX is a kind of dynamic softening mechanism which can produce new grains with high angle grain boundaries (HAGBs)[33]. The presence of a large fraction of HABGs is advantageous since the boundaries can contribute to the strength and the toughness[34]. There are two main nucleation mechanisms for DRX, which are the conventional discontinuous dynamic recrystallization (DDRX) and in situ or continuous dynamic recrystallization (CDRX) [32, 35]. The DDRX is characterized by the bulging of corrugated grain boundaries, which involves the nucleation of new grains and subsequent grain growth[32]. On the other hand, CDRX is considered as a recovery process and proceeds by continuous absorption of dislocations in subgrain boundaries, which will result in the formation of HAGBs and new grains[32]. So more
detailed information on the effect of deformation temperature and strain rate on the DRX of the MoNbHfZrTi alloy can be obtained using the EBSD methods[36]. 3.3.1 The effect of deformation temperature on DRX Fig. 4 shows the orientation imaging microscopy (OIM) maps and the fractions of grain boundaries with different misorientation angles at different temperatures with a strain rate of 0.001 s-1, in which HAGBs and low angle grain boundaries (LAGBs) are represented by black lines and gray lines, respectively. Moreover, the grain boudaries with misorientation angles between 10° and 15° are defined as medium angle grain boundaries (MAGBs), which are represented by red lines. At 800 oC, extensively bulging grain boundaries with high angles which is closely related with the strain induced grain boundary migration can be observed. Such grain boundaries characteristics indicate that the nucleation mechanism of DRX during hot deformation belongs to the DDRX. However, a large number of subgrains boundaries with LAGBs can be also observed, which are the typical characteristics of CDRX. Thus, DDRX and CDRX occur simultaneously during hot deformation for the MoNbHfZrTi alloy. With the increasing of the deformation temperature to 1000 oC, the original grain boundaries become more serrated and more bulging. At 1200 o C, some DRX grains along the original grain boundaries can be observed. With the increase of the deformation temperature, the subgrain boundaries with LAGB gradually shift from the interior of the original grains to the vicinity of the original grain boundaries. Fig. 4 (d) shows the the fractions of grain boundaries with different misorientation angles. It can be found that the fraction of LAGBs decreases and the fraction of HAGBs increases with the increase of the deformation temperature, which suggests that the effect of CDRX becomes weaker and DDRX becomes the main operatiing nucleation mechanism of DRX at higher deformation temperature. The MAGBs are defined as the transition zone from low angle to high angle boundaries[37]. The low content of MAGBs indicates that the rapid migration rate from low angle to high angle boundaries. It is well known that the mechanism of CDRX can be operated through progressive subgrain rotation [27, 38]. In order to understand the effect of deformation temperature on CDRX, misorientation analysis was performed along the lines in Fig. 4. As shown in Fig. 5, the point-to-point misorientations (blue lines) both near the original grain boundaries and in the grains are low. While the point-to-origin misorientations (red lines) are high, indicating that the misorientation accumulates and the progressive subgrain rotation has been well developed [26].
With the increase of the deformation temperature, the level of cumulative misorientations decreases gradually, which is related with the weakened effect of progressive subgrain rotation or the accelerated subgrain transformed from low angle to high angle boundaries by rotation at higher deformation temperature[37]. Thus, it can be confirmed that the effect of CDRX is weakened with the increase of the deformation temperature.
Fig. 4. OIM maps of the MoNbHfZrTi alloy after deformation at different temperatures with a strain rate of 0.001 s-1: (a) 800 oC; (b) 1000 oC; (c) 1200 oC; (d) the fractions of grain boundaries with different misorientation angles at the strain rate of 0.001 s-1.
Fig. 5. Misorientations measured along the lines marked in Fig. 4: (a) a, (b) b, (c) c, (d) d, (e) e, (f) f. 3.3.2 The effect of strain rate on DRX Fig. 6 shows the OIM maps and the fractions of grain boundaries with different misorientation angles with different strain rates at 1200 oC. It can be observed that there are some DRX grains at 1200 oC with all studied strain rates. The fraction of LAGBs decreases and the fraction of HAGBs
increases with the increase of the strain rate, which indicates that the effect of the CDRX is strengthened with the decrease of the strain rate. It can be confirmed from the OIM maps since there are hardly any subgrains boundaries at the higher strain rate of 0.1 s-1 and 0.01 s-1, while there are some subgrain boundaries observed at 0.001 s-1. It is well known that MAGBs are necessary for the nucleation of CDRX operated through progressive subgrain rotation. Hence, the fraction of MAGBs can directly reflect the effect of CDRX during hot deformation with the strain rate. It can be found that the fraction of MAGBs decreases with the increase of the strain rate. This result also confirms that the effect of CDRX is strengtened with the decrease of the strain rate. The misorientation analysis was performed along the lines in Fig. 6 to understand the effect of the strain rate on CDRX as shown in Fig. 7. It can be found that the cumulative misorientation (point-to-origin misorientation) and local misorientation (point-to-point) are relatively small. Comparing the results in Fig. 5 and Fig. 7, the misorientation is more sensitive to the deformation temperature than to the strain rate. Higher deformation temperature has a more pronounced weakened effect on the CDRX.
Fig. 6. OIM maps of the MoNbHfZrTi alloy after deformation with different strain rates at 1200 o
C: (a) 0.1 s-1; (b) 0.01 s-1; (c) 0.001 s-1; (d) the fractions of grain boundaries with different misorientation angles at 1200 oC.
Fig. 7. Misorientations measured along the lines marked in Fig. 6: (a) a, (b) b, (c) c, (d) d, (e) e, (f) f. Fig. 8 shows the grain size distributions. It can be observed that these DRX grains have a distinct grain size difference with the deformed original grains. And the DRX grains can be partitioned using the grain size difference. The DRX grain size firstly increases and then decreases with the decrease of the strain rate. At the medium strain rate of 0.01 s-1, the DRX grain size is the maximum, which also accounts for the result that the stress is the lowest at 1200 oC with the strain rate of 0.01 s-1.
Fig. 8. Grain size distributions after deforamtion at 1200 oC with different strain rates: (a) 0.1 s-1; (b) 0.01 s-1; (c) 0.001 s-1.
4. Conslusions The stress-strain behavior, microstructure evolution and DRX behavior with the deformation temperature range of 800-1200 oC and strain rate range of 0.001-0.1 s-1 in the refractory MoNbHfZrTi HEA through isothermal compression tests in a thermomechanical simulator were studied. The primary outcomes are as follows: (1) At 800 oC, the stress-strain curve of MoNbHfZrTi alloy exhibits a work-hardening stage with the increase of the strain until fracture at the strain rate of 0.1 s-1 and 0.01 s-1. The stress-strain curves exhibit the typical DRX characteriscs at above 900 oC with the strain rate of 0.1 s-1 and 0.01 s-1 and at deformation temperature of 800-1200 oC with the strain rate of 0.001 s-1. (2) The stress decreases with the increase of deformation temperature and with the decrease of the strain rate except at 1200 oC. (3) The microstructure evolution depends closely on the deformation temperature and strain rate. With decrease of strain rate and the increase of the deformation temperature, the initial dendritic structure gradually disappears and more dynamic recrystallized grains form. (4) The nucleation mechanism of DDRX and CDRX occurred simultaneously for the MoNbHfZrTi alloy during hot deformation and DDRX is the primary mechanism and CDRX
is the assistant mechanism. The effect of CDRX was weakened with the increase of deformation temperature and with the decrease of strain rate.
Acknowledgements We acknowledge financial support of 973 project (2011CB610406) and Natural Science Foundation of China (51425402).
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