Lattice Parameter of Ferrite in Silicon Cast Irons

Abstract

While the density of materials is important for their mechanical and service properties, it is striking that so little is known in the case of silicon cast irons, which are essentially Fe–C–Si alloys with graphite precipitates in an iron-rich matrix. In this work, a review of literature data was used to establish an expression for the lattice parameter of ferrite based on that of pure iron, to which were added the individual effects of carbon, silicon, manganese, phosphorus, and copper. The expression thus obtained is valid for a homogeneous material, and a correction is proposed to take into account of the presence of graphite in Fe–C–Si ferritic alloys. Next, the ferrite lattice parameter of a series of 12 alloys containing carbon between 1.0 and 4.1 wt pct, silicon between 0.5 and 4.3 wt pct, up to 0.7 wt pct Mn and up to 0.9 wt pct Cu, was measured by XRD from room temperature to a temperature of 1150 °C. These recordings showed that the mirror polishing used to prepare the samples resulted in surface alteration and a significant overestimation of the ferrite lattice parameter for temperatures below 500 °C. We therefore carried out high-energy XRD at room temperature, which was free of any surface effects and validated our analysis. Valid measurements at room temperature and XRD values obtained at temperatures above 500 °C were then successfully compared with the predictions given by the proposed expression for the lattice parameter of ferrite as a function of temperature and alloying elements. This work provides the necessary information to calculate the theoretical density of ferritic cast irons.

Appendix A

Considering poor grain statistics with laboratory X-rays or partial ordering/disordering of the matrix could not explain the discrepancies observed in the room-temperature lattice parameters before and after the XRD heating cycle. This discrepancy is again illustrated in Figure A1 in the case of alloy L. It was then considered that strains developed during polishing could be the reason for this. The sample corresponding to alloy L was again submitted to mechanical polishing after the XRD cycle and the lattice parameter was again measured in this condition. The results at room temperature in Figure A1 and Table AI show that the value of the lattice parameter decreased by 0.004 Å from before to after the XRD cycle, and increased by 0.002 Å by polishing again the cycled XRD sample.

Table AI Change in the Room-Temperature Lattice Parameter Obtained by XRD for Sample L Before and After the Heating Cycle, Taking into Consideration the Effect of Polishing

Fig. A1

Evolution of the lattice parameter of alloy L upon heating (disks) and room- temperature values after cooling down in the non-polished (square) and repolished (diamond) condition

The effect of plastic deformation associated with polishing on XRD records has been known since a long time and is still regularly reported. Quite recently again, Vashista and Paul,^[25] Jiang et al.,^[26] and Gadallah et al.^[27] noticed a close relation between the full width at half-maximum (FWHM) of X-ray peaks with the strain or stress state, and Jiang et al. also emphasized the possible errors this implies on XRD data. All records of the XRD samples investigated during this work were checked and showed high FWHM at temperature lower than 400 °C when compared to higher temperature, as illustrated in Figure A2 for alloy L. This effect was found more prominent for the (211) peak than for the (110) peak. For A, B, and K alloys, it was observed that the FWHM increased again at high temperature after going through a minimum at temperature between 600 °C and 700 °C, which could well be related to the fact that ferrite coexisted with austenite in these samples for temperature higher than 800 °C.

Fig. A2

Evolution with temperature of the FWHM of (110) and (211) XRD peaks for sample L. Solid and empty symbols are for (110) and (211) peaks, respectively, circles for the heating cycle, squares for the values after the cycle, and diamonds for repolished sample

According to these observations, it was concluded that the discrepancies found in the room-temperature lattice parameter of ferrite are mainly due to a high sensitivity to the polishing procedure. This effect vanishes as the sample temperature goes beyond 500 °C, and does not appear again after cooling back to room temperature. Check of sample type and preparation used in literature works reporting room-temperature lattice parameter for pure Fe showed most of them used powder or fillings that had been stress relieved before experiments.