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Effect of relative density on microstructure and mechanical properties of Fe-12Mn-0.2C alloy fabricated by powder metallurgy
Powder Technology 298 (2016) 106–111
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Effect of relative density on microstructure and mechanical properties of Fe-12Mn-0.2C alloy fabricated by powder metallurgy Suchul Yoon a,1, Singon Kang b,1, Young Choi c, Hyunjoo Choi d, Seok-Jae Lee a,⁎ a
Division of Advanced Materials Engineering, Research Center for Advanced Materials Development, Chonbuk National University, Jeonju 561-756, Republic of Korea Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401, USA Convergence Components and Agricultural Machinery Application Group, Jeonbuk Regional Division, KITECH, 222 Palokro, Jeonju 561-202, Republic of Korea d School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 14 August 2015 Received in revised form 15 January 2016 Accepted 18 May 2016 Available online 19 May 2016 Keywords: Relative density Compressive strength Strain-induced martensite Sintered Fe-Mn-C alloy
a b s t r a c t The microstructure features and compressive strength of sintered Fe-12 wt.% Mn-0.2 wt.% C alloy steel have been investigated by considering the effect of relative density. Both the relative density and the strength increased linearly irrespective of the initial relative density during the room-temperature compressive test. The formation of the strain-induced martensite during deformation affected additional work hardening and brought about a higher actual compressive stress. The strain-induced martensite transformation from austenite was observed by means of optical micrograph and was distinguished by EDS line scan analysis with hardness measurement. The volume fraction of austenite during deformation was significantly decreased depending on the initial relative density of sintered sample. The rate of the strain-induced martensite transformation kinetics was accelerated and resulted in the relatively high work hardening when the initial relative density was higher. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The recent research for advanced automotive high-strength steel (AHSS) focuses on the design of new steel alloys and processes to achieve high strength with high ductility for fuel efficiency and passenger safety. The concept of third-generation AHSS such as medium Mn transformation-induced plasticity (TRIP) steel [1,2] and quenching and partitioning (Q&P) steel [3] has been significantly investigated. The key feature of third-generation AHSS is a microstructure consisting of ferrite as a matrix and retained austenite at room temperature. The retained austenite with proper stability and volume fraction can be transformed to strain-induced martensite during plastic deformation, resulting in high strength and ductility [4,5]. The optimal kinetics of the strain-induced martensite transformation are directly influenced by the stability of metastable austenite at room temperature. Three main factors related to austenite stability have been reported [6]. First is the addition of austenite stabilizing elements, such Mn and Ni [7,8]. Second, a decrease in austenite grain size lowers the martensite start temperature and increases the volume fraction of austenite with higher austenite stability [9,10]. Finally, the austenite transformation can be suppressed in accordance with the amount of dislocations. Thus, control of the volume fraction and stability of austenite is critical. It has been
reported that the mechanical properties of steel can be improved by increasing the volume fraction of austenite at room temperature [1,11,12]. However, insufficient or excessive austenite stability cannot maximize the effect of the strain-induced martensite transformation for mechanical property improvement. One recent study, for the purpose of weight reduction in steels related to fuel efficiency, aimed to produce high-strength steels through a powder metallurgy method. In general, many studies have reported improved strength according to the relative density of steels manufactured by powder metallurgy because both strength and ductility deteriorate with decreasing relative density [13–23]. The kind of alloying element added to most sintered alloy steels is very limited, such as Cr and Mo but not Mn or Si. To date, few studies on sintered alloy steels containing high Mn content have been reported [24–26]. Therefore, in the present work, we investigated the effect of relative density on the microstructure and mechanical properties of sintered Fe-Mn-C alloy steel manufactured using a powder metallurgy method. In particular, strain-induced martensite transformed from austenite during room temperature deformation was observed, which corresponded to relative density and was discussed with the consideration of austenite stability. 2. Experimental
⁎ Corresponding author. E-mail address: [email protected] (S.-J. Lee). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.powtec.2016.05.026 0032-5910/© 2016 Elsevier B.V. All rights reserved.
A steel composition of Fe-12 wt.% Mn-0.2 wt.% C was designed using Thermo-Calc for the co-existence of ferrite and austenite at room
S. Yoon et al. / Powder Technology 298 (2016) 106–111
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temperature after sintering. The equilibrium volume fraction of each phase is expressed according to temperature in Fig. 1. The calculated volume fractions of austenite, ferrite, and cementite are approximately 0.15, 0.82, and 0.03, respectively. The designed mass of commercial Fe, Mn, and graphite powders (purity ≥ 99.9%) were precisely weighed and then added to 0.7 wt.% zinc stearate as a lubricant. The powder was milled using a planetary high-energy milling machine for 4 h to homogenize the powder mixture. The powders were placed in a sealed stainless steel jar under an argon atmosphere with a stainless ball to powder ratio of 10:1. Stabilization treatment was conducted for 24 h in an Ar atmosphere to prevent oxidation of the powder surface after milling. The homogeneously mixed powders were compacted with pressures of 250, 500, and 900 MPa using a hydraulic press. Cylindrical samples of 12 mm diameter and 10 mm height were obtained. These green compacts were sintered at 1150 °C for 30 min in a N2 + 10% H2 gas atmosphere to prevent oxidation and any other surface reactions. The samples were furnace-cooled to room temperature. The relative density was calculated by dividing the measured density by the theoretical density for each sample. Archimedes principle was used to measure the densities of sintered samples. The relative density of the sintered samples with different compact pressures of 250, 500, and 900 MPa was about 78.6%, 83.5%, and 87.3%, respectively. The samples were named according to their relative density as ‘R79’, ‘R84’, and ‘R87’. A room-temperature compressive test was carried out using a universal testing machine at a cross head speed of 1 mm/min. The compressive test was stopped at a known compressive strain in order to observe the variation in microstructure during deformation. The maximum compressive strain was 30%. Each phase was observed using X-ray diffraction with a Cu Kα target at 40 kV and 30 mA. The samples were scanned from 35° to 95° of 2θ with a scan speed of 2°/min. The volume fraction of austenite retained at room temperature was calculated using Miller's equation [27]. The microstructural features of pores in the sintered and deformed samples were observed using optical microscope (OM). 2% nital etchant was used to reveal microstructure of polished sample surface. Energy dispersive spectroscopy (EDS) analysis with a scanning electron microscope (SEM) was performed to confirm the concentration of alloying elements in each phase.
Fig. 2. (a) Compressive stress-strain values and (b) the increase in relative density during the compression test.
3. Results and discussion
Fig. 1. Equilibrium phase volume fractions of an Fe-12.0 wt.% Mn-0.2 wt.% C alloy. A black solid line represents austenite, a red dashed line is ferrite, and a blue dot line is cementite.
Fig. 2 shows the variation in compressive stress and relative density according to compressive strain. Compressive strength increases linearly with strain regardless of the initial relative density. The R87 sample showed a higher work hardening value than other samples, whereas the R79 sample had the lowest strain hardening value. Allison et al. [28] reported the relation between a porosity and work hardening rate for the sintered FC-0205 steel. The work hardening rate decreased as decreasing the initial density of the sintered sample. In addition, a linear increase in relative density was observed with increasing compressive strain. The rate of relative density increase is accelerated as initial relative density decreases. Fig. 3 shows the relationship between relative density and compressive stress during compressive deformation. The compressive stress increases steeply despite a slight change in relative density as initial density increases. It is well known that the mechanical properties including tensile strength, ductility, and others commonly deteriorate as the material density is decreased for various steel alloys: austenitic stainless steel [15,16,19,20], duplex stainless steel [17], Fe-Mn-Si alloy [13], Fe-Ni-Mo-C alloy [14], Fe-Cr-Mo alloy [18,21,22], and Fe-Ni-Cu-Mo-C [23]. Similar to the effect of porosity, the microstructure is an important factor to determine the mechanical
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Fig. 3. Relationship between relative density and compressive stress depending on initial sample density.
properties of sintered steel alloys. As increasing the volume fraction of harder phase, e.g., bainite or martensite, the strength and hardness increased. The bainitic or martensitic microstructure of sintered steel alloys can be obtained by suppressing the ferrite or pearlite transformation as increasing sintering temperature and time [13], increasing carbon content [18,22], adding/removing alloying element that influence the phase transformation kinetics during cooling [23, 29]. The cooling rate after sintering is also a strong factor to affect the mechanical properties which relate to a microstructure controlled by the phase transformation kinetics during cooling. Depending on the cooling rate, different volume fractions of phases and precipitates were obtained, resulting in different mechanical properties for various sintered stainless steels [30]. However, considering only the change in relative density in this work, the R79 sample was expected to show the most rapid compressive stress increase due to the highest rate of relative density increase. However, the relatively high work hardening value of the R87 sample indicates that not only the increase in relative density, but also other factors contribute to the increased strength. Das et al. [31] investigated the effect of prestrain on tensile properties of a tungsten heavy alloy. They found that the work hardening value decreased by increasing dislocation density due to prestrain applied to the sample. The dislocation density was reduced after annealing heat treatment for recovery and recrystallization, resulting in the improvement in ductility as increasing the work hardening capacity. In the present work, no prestrain was applied, thus the different work hardening value is influenced by other factors. Fig. 4 compares the actual compressive stress calculated by considering the reduction of pores during sample deformation, i.e., the compressive stress values in Fig. 2(a) are divided by the cross-sectional area excluding pores. It is assumed that the relative density in a given volume is identical to that in the cross-sectional area since the sintered sample is homogenous. Similar to Fig. 2(a), the R87 sample showed a higher compressive strength level. This infers that the other factor acts to increase strength with a decrease in porosity and increasing relative density e.g., the TRIP effect related to strain-induced martensite formation from austenite during compressive deformation. Menapace and coworkers [32] reported the extended ductility by the TRIP effect during tensile test for the nanostructured complex steel containing retained austenite which was fabricated by spark plasma sintering. In the present study, it is thought that the formation of strain-induced martensite during compressive deformation produced additional work hardening or
Fig. 4. Variation in actual compressive stress with compressive strain. The actual compressive stress value was obtained by considering the reduction of pores during sample deformation.
dislocation behavior associated with the pores, resulting in a higher actual compressive stress. Fig. 5 shows the optical micrographs of the as-sintered R87 sample and the 30% deformed R87 sample. The entire sample area is divided into distinguishable dark and bright regions. An acicular-typed microstructure is observed at the boundary of the distinguishable regions for the 30% deformed sample. A SEM-EDS line scan was used to identify the phase. Fig. 6 compares a single location in the as-sintered R87
Fig. 5. Optical microstructure of the as-sintered R87 samples for (a) as-sintered and (b) 30% deformed. An acicular-typed microstructure is partially observed in the deformed sample.
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Fig. 6. (a) Optical micrograph of the as-sintered R87 sample. A white line from A to A′ in the fig. is an austenite-ferrite region for EDS line scanning which was marked by four indents. (b) The concentration of Mn for the white line measured by EDS.
sample using OM and SEM. Four indentation marks of a Vickers hardness tester were used as standard points for measurement with the different microscopes. The concentration of Mn in dark areas was higher than that in bright areas. It is known that Mn is an austenite stabilizing element and is highly soluble in austenite. The equilibrium Mn content in ferrite is 0.009 mol fraction, whereas the equilibrium Mn content in austenite is 0.607 mol fraction for the Fe-12 wt.% Mn-0.2 wt.% C alloy, as calculated by Thermo-Calc. In addition, the hardness values of the two areas were measured. The average hardness in the bright area was about 381 HV, while that in the dark area was about 211 HV. The hardness values are clearly separated. Dakhlaoui et al. [33] reported that the hardness in ferrite is greater than the hardness in austenite for duplex stainless steel. Also Gilmas and coworkers [23] confirmed that the hardness in austenite is lower than the hardness in bainite or martensite for sintered Ni-Cu-Mo alloys. The EDS line scan analysis is in concordance with the hardness measurement results. Thus, we conclude that the bright area is ferrite, and the dark area is austenite. Fig. 7 demonstrates the X-ray diffraction patterns of as-sintered and deformed samples according to initial relative density. It was confirmed that both ferrite (α) and austenite (γ) phases are detected in all samples. The volume fraction of austenite was calculated using these peaks. Small peaks related to MnO are present but ignored for the phase calculation. The variation in measured volume fraction of austenite is shown in Fig. 8, indicating that the initial relative density does not affect the austenite volume fraction in the sintered samples when considering the measurement error. However, the volume fraction of austenite is dramatically decreased with increasing compressive strain. The rate of decrease in austenite is accelerated as the initial relative density is increased. In other words, a higher initial relative density promotes the formation of strain-induced martensite. It is thought that the rapid strain-induced martensite transformation kinetics is responsible for the relatively high work hardening value in the R87 sample.
Fig. 7. X-ray diffraction patterns of the as-sintered and deformed samples influenced by the initial density of the samples: (a) R79, (b) R84, and (c) R87.
Sugimoto et al. [11] evaluated the stability of strain-induced austenite during deformation using the following equation: logf ¼ logf 0 −k ε
ð1Þ
where f is the volume fraction of austenite, f0 is the initial volume fraction of austenite, k is the stability coefficient, and ε is the strain. A
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line scan analysis with hardness measurement. The austenite volume fractions before deformation were almost identical irrespective of the initial relative density, but the austenite volume fraction was significantly decreased depending on the initial relative density when the samples were deformed. The rate of the strain-induced martensite transformation kinetics was accelerated and resulted in the relatively high work hardening in the R87 sample with higher initial relative density.
References
Fig. 8. Effect of initial sample density on strain-induced martensite transformation kinetics during compressive deformation.
smaller k value represents a higher resistance to strain-induced martensite transformation from austenite due to high austenite stability. The k value is 0.454, 0.631, and 1.048 for R79, R84, and R87, respectively. The calculated volume fraction of austenite using Eq. (1) with the k value of each sintered sample is compared with the experimental volume fraction of austenite in Fig. 8. The austenite stability in the R87 sample is obviously lower than in the other samples, which causes the high actual compressive stress in the R87 sample, as seen in Fig. 4. If the pores act as free surfaces and the dislocations generated during deformation are removed near the pores rather than accumulated, dislocation slip and annihilation occur in the R79 sample. On the other hand, it is thought that fewer pores in the R87 sample hinders dislocation annihilation and provide nucleation sites for strain-induced martensite by forming shear bands due to the interaction of accumulated dislocations [34]. Shi et al. [35] derived the equation for the enhanced work hardening rate (dσ/dε) combined with Eq. (1) as follows: dσ ∝k f 0 σ M −σ γ expð−k ε Þ dε
ð2Þ
This equation indicates that the work hardening rate is proportional to the stability coefficient (k) and the initial volume fraction of austenite (f0). As the three samples used in this work have the almost same volume fraction of retained austenite before deformation, the stability coefficient only reveals the work hardening rate. The higher work hardening value of the R87 sample in Fig. 4 is represented by the higher k value. Thus, the higher compressive strength is attributed to the TRIP effect with a sufficient amount of retained austenite and a lower k value indicating less austenite stability for deformation. 4. Conclusions The effect of relative density on the microstructural features and compressive strength of a sintered Fe-12 wt.% Mn-0.2 wt.% C sample was investigated. During the room-temperature compressive test, the relative density and strength increased linearly regardless of the initial relative density. The compressive stress increased steeply despite a slight change in relative density in the R87 sample because the formation of the strain-induced martensite during deformation affected additional work hardening and resulted in a higher actual compressive stress. The strain-induced martensite transformation from austenite was observed by optical micrograph and was distinguished by EDS
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Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Effect of relative density on microstructure and mechanical properties of Fe-12Mn-0.2C alloy fabricated by powder metallurgy Suchul Yoon a,1, Singon Kang b,1, Young Choi c, Hyunjoo Choi d, Seok-Jae Lee a,⁎ a
Division of Advanced Materials Engineering, Research Center for Advanced Materials Development, Chonbuk National University, Jeonju 561-756, Republic of Korea Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401, USA Convergence Components and Agricultural Machinery Application Group, Jeonbuk Regional Division, KITECH, 222 Palokro, Jeonju 561-202, Republic of Korea d School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 14 August 2015 Received in revised form 15 January 2016 Accepted 18 May 2016 Available online 19 May 2016 Keywords: Relative density Compressive strength Strain-induced martensite Sintered Fe-Mn-C alloy
a b s t r a c t The microstructure features and compressive strength of sintered Fe-12 wt.% Mn-0.2 wt.% C alloy steel have been investigated by considering the effect of relative density. Both the relative density and the strength increased linearly irrespective of the initial relative density during the room-temperature compressive test. The formation of the strain-induced martensite during deformation affected additional work hardening and brought about a higher actual compressive stress. The strain-induced martensite transformation from austenite was observed by means of optical micrograph and was distinguished by EDS line scan analysis with hardness measurement. The volume fraction of austenite during deformation was significantly decreased depending on the initial relative density of sintered sample. The rate of the strain-induced martensite transformation kinetics was accelerated and resulted in the relatively high work hardening when the initial relative density was higher. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The recent research for advanced automotive high-strength steel (AHSS) focuses on the design of new steel alloys and processes to achieve high strength with high ductility for fuel efficiency and passenger safety. The concept of third-generation AHSS such as medium Mn transformation-induced plasticity (TRIP) steel [1,2] and quenching and partitioning (Q&P) steel [3] has been significantly investigated. The key feature of third-generation AHSS is a microstructure consisting of ferrite as a matrix and retained austenite at room temperature. The retained austenite with proper stability and volume fraction can be transformed to strain-induced martensite during plastic deformation, resulting in high strength and ductility [4,5]. The optimal kinetics of the strain-induced martensite transformation are directly influenced by the stability of metastable austenite at room temperature. Three main factors related to austenite stability have been reported [6]. First is the addition of austenite stabilizing elements, such Mn and Ni [7,8]. Second, a decrease in austenite grain size lowers the martensite start temperature and increases the volume fraction of austenite with higher austenite stability [9,10]. Finally, the austenite transformation can be suppressed in accordance with the amount of dislocations. Thus, control of the volume fraction and stability of austenite is critical. It has been
reported that the mechanical properties of steel can be improved by increasing the volume fraction of austenite at room temperature [1,11,12]. However, insufficient or excessive austenite stability cannot maximize the effect of the strain-induced martensite transformation for mechanical property improvement. One recent study, for the purpose of weight reduction in steels related to fuel efficiency, aimed to produce high-strength steels through a powder metallurgy method. In general, many studies have reported improved strength according to the relative density of steels manufactured by powder metallurgy because both strength and ductility deteriorate with decreasing relative density [13–23]. The kind of alloying element added to most sintered alloy steels is very limited, such as Cr and Mo but not Mn or Si. To date, few studies on sintered alloy steels containing high Mn content have been reported [24–26]. Therefore, in the present work, we investigated the effect of relative density on the microstructure and mechanical properties of sintered Fe-Mn-C alloy steel manufactured using a powder metallurgy method. In particular, strain-induced martensite transformed from austenite during room temperature deformation was observed, which corresponded to relative density and was discussed with the consideration of austenite stability. 2. Experimental
⁎ Corresponding author. E-mail address: [email protected] (S.-J. Lee). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.powtec.2016.05.026 0032-5910/© 2016 Elsevier B.V. All rights reserved.
A steel composition of Fe-12 wt.% Mn-0.2 wt.% C was designed using Thermo-Calc for the co-existence of ferrite and austenite at room
S. Yoon et al. / Powder Technology 298 (2016) 106–111
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temperature after sintering. The equilibrium volume fraction of each phase is expressed according to temperature in Fig. 1. The calculated volume fractions of austenite, ferrite, and cementite are approximately 0.15, 0.82, and 0.03, respectively. The designed mass of commercial Fe, Mn, and graphite powders (purity ≥ 99.9%) were precisely weighed and then added to 0.7 wt.% zinc stearate as a lubricant. The powder was milled using a planetary high-energy milling machine for 4 h to homogenize the powder mixture. The powders were placed in a sealed stainless steel jar under an argon atmosphere with a stainless ball to powder ratio of 10:1. Stabilization treatment was conducted for 24 h in an Ar atmosphere to prevent oxidation of the powder surface after milling. The homogeneously mixed powders were compacted with pressures of 250, 500, and 900 MPa using a hydraulic press. Cylindrical samples of 12 mm diameter and 10 mm height were obtained. These green compacts were sintered at 1150 °C for 30 min in a N2 + 10% H2 gas atmosphere to prevent oxidation and any other surface reactions. The samples were furnace-cooled to room temperature. The relative density was calculated by dividing the measured density by the theoretical density for each sample. Archimedes principle was used to measure the densities of sintered samples. The relative density of the sintered samples with different compact pressures of 250, 500, and 900 MPa was about 78.6%, 83.5%, and 87.3%, respectively. The samples were named according to their relative density as ‘R79’, ‘R84’, and ‘R87’. A room-temperature compressive test was carried out using a universal testing machine at a cross head speed of 1 mm/min. The compressive test was stopped at a known compressive strain in order to observe the variation in microstructure during deformation. The maximum compressive strain was 30%. Each phase was observed using X-ray diffraction with a Cu Kα target at 40 kV and 30 mA. The samples were scanned from 35° to 95° of 2θ with a scan speed of 2°/min. The volume fraction of austenite retained at room temperature was calculated using Miller's equation [27]. The microstructural features of pores in the sintered and deformed samples were observed using optical microscope (OM). 2% nital etchant was used to reveal microstructure of polished sample surface. Energy dispersive spectroscopy (EDS) analysis with a scanning electron microscope (SEM) was performed to confirm the concentration of alloying elements in each phase.
Fig. 2. (a) Compressive stress-strain values and (b) the increase in relative density during the compression test.
3. Results and discussion
Fig. 1. Equilibrium phase volume fractions of an Fe-12.0 wt.% Mn-0.2 wt.% C alloy. A black solid line represents austenite, a red dashed line is ferrite, and a blue dot line is cementite.
Fig. 2 shows the variation in compressive stress and relative density according to compressive strain. Compressive strength increases linearly with strain regardless of the initial relative density. The R87 sample showed a higher work hardening value than other samples, whereas the R79 sample had the lowest strain hardening value. Allison et al. [28] reported the relation between a porosity and work hardening rate for the sintered FC-0205 steel. The work hardening rate decreased as decreasing the initial density of the sintered sample. In addition, a linear increase in relative density was observed with increasing compressive strain. The rate of relative density increase is accelerated as initial relative density decreases. Fig. 3 shows the relationship between relative density and compressive stress during compressive deformation. The compressive stress increases steeply despite a slight change in relative density as initial density increases. It is well known that the mechanical properties including tensile strength, ductility, and others commonly deteriorate as the material density is decreased for various steel alloys: austenitic stainless steel [15,16,19,20], duplex stainless steel [17], Fe-Mn-Si alloy [13], Fe-Ni-Mo-C alloy [14], Fe-Cr-Mo alloy [18,21,22], and Fe-Ni-Cu-Mo-C [23]. Similar to the effect of porosity, the microstructure is an important factor to determine the mechanical
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Fig. 3. Relationship between relative density and compressive stress depending on initial sample density.
properties of sintered steel alloys. As increasing the volume fraction of harder phase, e.g., bainite or martensite, the strength and hardness increased. The bainitic or martensitic microstructure of sintered steel alloys can be obtained by suppressing the ferrite or pearlite transformation as increasing sintering temperature and time [13], increasing carbon content [18,22], adding/removing alloying element that influence the phase transformation kinetics during cooling [23, 29]. The cooling rate after sintering is also a strong factor to affect the mechanical properties which relate to a microstructure controlled by the phase transformation kinetics during cooling. Depending on the cooling rate, different volume fractions of phases and precipitates were obtained, resulting in different mechanical properties for various sintered stainless steels [30]. However, considering only the change in relative density in this work, the R79 sample was expected to show the most rapid compressive stress increase due to the highest rate of relative density increase. However, the relatively high work hardening value of the R87 sample indicates that not only the increase in relative density, but also other factors contribute to the increased strength. Das et al. [31] investigated the effect of prestrain on tensile properties of a tungsten heavy alloy. They found that the work hardening value decreased by increasing dislocation density due to prestrain applied to the sample. The dislocation density was reduced after annealing heat treatment for recovery and recrystallization, resulting in the improvement in ductility as increasing the work hardening capacity. In the present work, no prestrain was applied, thus the different work hardening value is influenced by other factors. Fig. 4 compares the actual compressive stress calculated by considering the reduction of pores during sample deformation, i.e., the compressive stress values in Fig. 2(a) are divided by the cross-sectional area excluding pores. It is assumed that the relative density in a given volume is identical to that in the cross-sectional area since the sintered sample is homogenous. Similar to Fig. 2(a), the R87 sample showed a higher compressive strength level. This infers that the other factor acts to increase strength with a decrease in porosity and increasing relative density e.g., the TRIP effect related to strain-induced martensite formation from austenite during compressive deformation. Menapace and coworkers [32] reported the extended ductility by the TRIP effect during tensile test for the nanostructured complex steel containing retained austenite which was fabricated by spark plasma sintering. In the present study, it is thought that the formation of strain-induced martensite during compressive deformation produced additional work hardening or
Fig. 4. Variation in actual compressive stress with compressive strain. The actual compressive stress value was obtained by considering the reduction of pores during sample deformation.
dislocation behavior associated with the pores, resulting in a higher actual compressive stress. Fig. 5 shows the optical micrographs of the as-sintered R87 sample and the 30% deformed R87 sample. The entire sample area is divided into distinguishable dark and bright regions. An acicular-typed microstructure is observed at the boundary of the distinguishable regions for the 30% deformed sample. A SEM-EDS line scan was used to identify the phase. Fig. 6 compares a single location in the as-sintered R87
Fig. 5. Optical microstructure of the as-sintered R87 samples for (a) as-sintered and (b) 30% deformed. An acicular-typed microstructure is partially observed in the deformed sample.
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Fig. 6. (a) Optical micrograph of the as-sintered R87 sample. A white line from A to A′ in the fig. is an austenite-ferrite region for EDS line scanning which was marked by four indents. (b) The concentration of Mn for the white line measured by EDS.
sample using OM and SEM. Four indentation marks of a Vickers hardness tester were used as standard points for measurement with the different microscopes. The concentration of Mn in dark areas was higher than that in bright areas. It is known that Mn is an austenite stabilizing element and is highly soluble in austenite. The equilibrium Mn content in ferrite is 0.009 mol fraction, whereas the equilibrium Mn content in austenite is 0.607 mol fraction for the Fe-12 wt.% Mn-0.2 wt.% C alloy, as calculated by Thermo-Calc. In addition, the hardness values of the two areas were measured. The average hardness in the bright area was about 381 HV, while that in the dark area was about 211 HV. The hardness values are clearly separated. Dakhlaoui et al. [33] reported that the hardness in ferrite is greater than the hardness in austenite for duplex stainless steel. Also Gilmas and coworkers [23] confirmed that the hardness in austenite is lower than the hardness in bainite or martensite for sintered Ni-Cu-Mo alloys. The EDS line scan analysis is in concordance with the hardness measurement results. Thus, we conclude that the bright area is ferrite, and the dark area is austenite. Fig. 7 demonstrates the X-ray diffraction patterns of as-sintered and deformed samples according to initial relative density. It was confirmed that both ferrite (α) and austenite (γ) phases are detected in all samples. The volume fraction of austenite was calculated using these peaks. Small peaks related to MnO are present but ignored for the phase calculation. The variation in measured volume fraction of austenite is shown in Fig. 8, indicating that the initial relative density does not affect the austenite volume fraction in the sintered samples when considering the measurement error. However, the volume fraction of austenite is dramatically decreased with increasing compressive strain. The rate of decrease in austenite is accelerated as the initial relative density is increased. In other words, a higher initial relative density promotes the formation of strain-induced martensite. It is thought that the rapid strain-induced martensite transformation kinetics is responsible for the relatively high work hardening value in the R87 sample.
Fig. 7. X-ray diffraction patterns of the as-sintered and deformed samples influenced by the initial density of the samples: (a) R79, (b) R84, and (c) R87.
Sugimoto et al. [11] evaluated the stability of strain-induced austenite during deformation using the following equation: logf ¼ logf 0 −k ε
ð1Þ
where f is the volume fraction of austenite, f0 is the initial volume fraction of austenite, k is the stability coefficient, and ε is the strain. A
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line scan analysis with hardness measurement. The austenite volume fractions before deformation were almost identical irrespective of the initial relative density, but the austenite volume fraction was significantly decreased depending on the initial relative density when the samples were deformed. The rate of the strain-induced martensite transformation kinetics was accelerated and resulted in the relatively high work hardening in the R87 sample with higher initial relative density.
References
Fig. 8. Effect of initial sample density on strain-induced martensite transformation kinetics during compressive deformation.
smaller k value represents a higher resistance to strain-induced martensite transformation from austenite due to high austenite stability. The k value is 0.454, 0.631, and 1.048 for R79, R84, and R87, respectively. The calculated volume fraction of austenite using Eq. (1) with the k value of each sintered sample is compared with the experimental volume fraction of austenite in Fig. 8. The austenite stability in the R87 sample is obviously lower than in the other samples, which causes the high actual compressive stress in the R87 sample, as seen in Fig. 4. If the pores act as free surfaces and the dislocations generated during deformation are removed near the pores rather than accumulated, dislocation slip and annihilation occur in the R79 sample. On the other hand, it is thought that fewer pores in the R87 sample hinders dislocation annihilation and provide nucleation sites for strain-induced martensite by forming shear bands due to the interaction of accumulated dislocations [34]. Shi et al. [35] derived the equation for the enhanced work hardening rate (dσ/dε) combined with Eq. (1) as follows: dσ ∝k f 0 σ M −σ γ expð−k ε Þ dε
ð2Þ
This equation indicates that the work hardening rate is proportional to the stability coefficient (k) and the initial volume fraction of austenite (f0). As the three samples used in this work have the almost same volume fraction of retained austenite before deformation, the stability coefficient only reveals the work hardening rate. The higher work hardening value of the R87 sample in Fig. 4 is represented by the higher k value. Thus, the higher compressive strength is attributed to the TRIP effect with a sufficient amount of retained austenite and a lower k value indicating less austenite stability for deformation. 4. Conclusions The effect of relative density on the microstructural features and compressive strength of a sintered Fe-12 wt.% Mn-0.2 wt.% C sample was investigated. During the room-temperature compressive test, the relative density and strength increased linearly regardless of the initial relative density. The compressive stress increased steeply despite a slight change in relative density in the R87 sample because the formation of the strain-induced martensite during deformation affected additional work hardening and resulted in a higher actual compressive stress. The strain-induced martensite transformation from austenite was observed by optical micrograph and was distinguished by EDS
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