Analysis of Liquid Phase Sintering from Dilatometry Curves
Figure 1(a) shows the curves obtained from dilatometry, with time along the X-axis and dilatation (dL/L
o) as well as temperature along the Y-axis for the two samples: one is the PM compact with boron-containing master alloy (Fe+2.5MA+C) and another one is the PM compact without master alloy addition (Fe+C), sintered at 1513 K (1240 °C) for 30 minutes. In Figure 1(a), 1-2 represents a heating stage, 2-3 dwell or sintering stage, and 3-4 is the cooling stage. Sintering of PM compact with boron-containing master alloy (Fe+2.5MA+C) clearly exhibits strong shrinkage as compared to the reference PM compact (Fe+C), see Figure 1(a). Figures 1(b) through (d) show the dilatometric curves with temperature along X-axis and dilatation as well as dilatation rate (dL/dt) along the Y-axis for various samples. Figure 1(b) shows the dilatometric and dL/dt curves during the heating stage for Fe+2.5MA+C and Fe+2.5MA in order to distinguish the difference in the dilation of the samples containing boron-based master alloy with and without carbon addition. Figures 1(c) and (d) show the dilatometric curves of Fe-Mo+2.5MA and Fe-Mo+1.5MA samples with and without carbon addition. Table II shows the analyzed data from the dilatometry sintering curves as seen in Figures 1(b) through (d).
Observations from the dilatometry curves for Fe+2.5MA, Fe-Mo+2.5MA, and Fe-Mo+1.5MA without carbon show that α-γ iron transition start and end temperatures are higher than for carbon-containing samples. After the α-γ transition, there is a continuous linear expansion until the samples reach the maximum expansion (peak) as indicated with the LP-1 segment in Figures 1(b) through (d). The shrinkage that begins right after the peak temperature continues throughout the sintering cycle as well as during cooling cycle. The overall shrinkage values, as shown in Table II, indicate that the major influence is from the amount of master alloy and carbon addition. The Fe-Mo+2.5MA sample shows higher linear shrinkage compared to the Fe-Mo+1.5MA and Fe+2.5MA samples. The Fe+2.5MA+C sample shows the initiation of shrinkage before reaching 1513 K (1240 °C), indicating that the formation of the liquid phase is well below the sintering temperature. Figure 1(b) clearly shows that the liquid formation occurs in two stages (boxes in the figure), the first stage (LP-1) and the second stage (LP-2). The LP-1 stage shows the shrinkage which is associated with the liquid phase formation due to the master alloy melting well below 1373 K (1100 °C), which spreads rapidly due to capillary action and excellent particle wetting resulting in shrinkage above 1373 K (1100 °C).
The LP-1 temperature in Table II is taken from the onset of the shrinkage portion in the dilatometry curve. A continuous shrinkage is observed from the dL/L
o curve till the final cooling. However, analysis of the dL/dt curve during shrinkage stage clearly shows a wide peak with multiple slopes in all the samples containing master alloys, indicating that secondary densification processes. The initial portion of the dL/dt curve exhibits a rapid downward slope which can be correlated to the rapid shrinkage associated from the master alloy melting which enables the initial densification, and the later part of the curve shows a deviation in the slope with a sudden deflection with a positive slope. This can be related to the formation of the secondary liquid phase from the eutectic reaction.
The shrinkage, i.e., LP-1 stage, begins at a lower temperature in carbon-containing samples as compared to carbon-free samples. Shrinkage also starts earlier for Mo-containing samples, see Table II. The dL/dt curve shows a sharp increase in the shrinkage rate, reaching a maximum value for carbon-containing samples in the range between ~1393 K and ~1403 K (1120 °C to 1130 °C) and at about 20 K higher for the carbon-free materials. It is important to note the deviation of the peak in the case of dL/dt curve, indicating two overlapping processes of densification, most probably related to the generation of the second liquid phase. It can be seen from Table II that the shrinkage obtained after ~1373 K (1100 °C) until the sintering temperature of 1513 K (1240 °C) is greatest for the Fe-Mo+2.5MA+C sample, containing the highest amount of boron and carbon. Hence, it corroborates to the shrinkage in this region and is associated with the amount of the liquid generated from the master alloy melting and from further eutectic reaction with iron. The shrinkage during sintering-holding is similar for all the samples admixed with 2.5 MA and is slightly lower for the 1.5 MA samples. The overall shrinkage is significantly influenced by alloying elements Mo, C, and the amount of MA addition.
Eutectic Liquid Formation (LP-2) Analysis from DSC Curves
Figure 2 shows the DSC curves for all six samples. Invariably, all samples reveal the curie temperature of around 1046 K (773 °C) during the heating stage. A clear transition peak from α→γ is observed around 1185 K (912 °C) for the samples without carbon addition. Also, the effect of Mo as a ferrite stabilizer is clear from the slight shift in the α→γ transformation to a higher temperature. The peak from master alloy melting, that is around 1292 K (1019 °C) for the MA powder, is not observed due to the low master alloy content. The endothermic peak is observed at a higher temperature, between 1373 K and 1473 K (1100 °C and 1200 °C) for all the samples. This results in the liquid phase formation and can be correlated to the second stage of densification due to eutectic liquid generation, marked as an LP-2 stage from the dilatometry curves and DSC peaks, see Figures 1 and 2. The presence of Mo shifts the LP-2 onset slightly to lower temperature by ~10 K and ~20 K with the addition of carbon see Table III. In the case of an Fe-based system, adding carbon reduces onset temperature by ~10 K. From analyzing the onset and end points of DSC peak from Figure 2, it is evident that the alloying elements Mo and C have a significant influence on the eutectic liquid formation temperature.
Estimation of Eutectic Liquid Formation (LP-2) from Simulations
Reaching higher density is correlated to the volume fraction of liquid phase generated through both master alloy melting and eutectic formation. Using Thermo-Calc with TCFE8 database, simulations were performed to predict the equilibrium phases formed at different temperatures as well as the amount of the eutectic liquid phase. This can be correlated further to the liquid phase formation during sintering. The predicted molar volume fraction of liquid at the sintering temperature of 1513 K (1240 °C) is presented in Table IV. Figure 3(a) shows the comparison between onset eutectic temperatures estimated from DSC experiments and Thermo-Calc simulations.
For Mo-containing samples, Thermo-Calc predicts a shift in onset temperature to a slightly higher temperature, see Figure 3(a), resulting in more liquid molar volume at the sintering temperature, see Figure 3(b). The carbon addition clearly lowers the onset temperature significantly in all cases to ~30 K and contributes to higher molar volume fraction of liquid, see Figure 3(b). The molar volume fraction of liquid formed at the sintering temperature is increased by ~1.1 vol pct for 1.5 MA with and without carbon, whereas for 2.5 MA with and without carbon the increase in LP is ~1.75 vol pct from the onset. Both carbon and molybdenum accelerate sintering by promoting the formation of liquid at a much lower temperature. Simulations showed a delay in LP-2 formation, as seen in Figure 3(b), for Mo-containing systems, and the total molar volume fraction of liquid after sintering in the case of Mo-prealloyed systems is larger than in the case of pure Fe.
Microstructure and Properties
Figure 4 shows the optical micrographs of the samples sintered in the dilatometer at 1273 K and 1373 K (1000 °C and 1100 °C) for 1 minute and of samples sintered at 1513 K (1240 °C) for 30 minutes. The different stages of the liquid phase sintering can be clearly observed in the optical micrographs. Until the temperature of 1273 K (1000 °C), only initial stage of the master alloy melting is evident, see Figure 4, that is in accordance with the DSC of the MA powder. The micrographs of the samples at 1273 K (1000 °C) show the pore formation inside MA particles in connection to the partial melting of the master alloy particles; however, overall MA particle was not completely melted. It can also be observed that there is no spreading of this liquid through the particle boundaries. Liquid phase along the particle and grain boundaries is seen after sintering at 1373 K (1100 °C) Figures 4 (b) and (e). The optical micrographs show the presence of the spherical pore in the sites of former MA-particle location as well as the spreading of the liquid phase along the particles and grain boundaries. This is in good agreement with the beginning of shrinkage stage associated with the liquid phase formation as shown in LP-1 in Figure 1 and Table II.
The secondary (LP-2) liquid formation occurs once eutectic reaction begins, which accelerates the shrinkage further until sintering as seen from the dilatometry curves, see Figure 1, and results in a typical eutectic microstructure as seen from optical micrographs of the samples sintered at 1513 K (1240 °C). Hence, the optical micrographs in Figure 4 show a typical characteristic behavior of the liquid phase sintering through the formation of the liquid phase and following the liquid spreading and the rearrangement in this case with two distinct melting regimes, LP-1 and LP-2.
Table V shows the results of the density, hardness, and impact energy values of the samples sintered at 1513 K (1240 °C) for 30 minutes. The impact energy (IE) values significantly decreased from 82 to 24 J with carbon addition, whereas the density and hardness increased for Fe+2.5MA+C as compared to Fe+2.5MA. The presence of Mo also shows a drastic decrease in impact energy, but also enhanced density and hardness values, when changing from Fe+2.5MA to Fe-Mo+2.5MA and from Fe+2.5MA+C to Fe-Mo+2.5MA+C. Addition of molybdenum strengthen the matrix, as observed from increased hardness values, but the impact energy values drops due to the microstructural embrittlement. In addition, when carbon is added, these effects are more pronounced and decrease the impact energy but increase the hardness and density. It is clearly seen from Table IV that with increasing MA content and addition of C and Mo, a higher volume fraction of liquid is formed that contributes significantly to densification. The impact energy values drastically decrease with the addition of Mo and C due to embrittlement. When the MA content is reduced from 2.5 to 1.5 wt pct, impact energy is improved for 1.5 wt pct MA samples with and without carbon as compared to 2.5 wt pct MA samples.
The fracture surface of the samples after sintering at 1513 K (1240 °C) for 30 minutes reveals well-sintered microstructures and displays predominantly transparticle cleavage failure, see Figures 5(a), (b), and (d). In case of Fe-Mo+1.5MA, the failure is a mixture of transparticle cleavage and ductile failure with large dimples initiated by secondary phases of pores shown as magnified insets in Figure 5(c) in red. Samples with carbon, Figures 5(b) and (d), show brittle failure with predominantly transgranular cleavage fracture, and the Fe-Mo+1.5MA showing some regions of ductile dimple failure.