Crack initiation was observed with different micro- structural features. Figure 12 shows a shear band (in an aged alloy) followed by a crack tip that initiated from the outer specimen edge that followed an interdendritic boundary. Edge crack penetration was not observed to be continuous through the specimen. Figure 13 shows shear band formation adjacent to porosity in the solution treated material. The shear bands extend outward in a radial like fashion with nucleated cracks visible in the lower right hand region of the porosity. The crack fol- lows the trace of a shear band.
Nucleation of a shear band occurs in polycrystalline mate- rial by the local reorientation of slip planes within individ- ual grains that aligns each with the macroscopic plane of maximum shear. Whenever another slip plane’s resolved shear stress exceeds the critical shear stress in that plane, slip will shift to that plane propagating the shear band.16 Macroscopically, the slip orientation averages out and shear bands form in the plane 45° to the loading direction. Shear band nucleation at 45° to the loading direction of a 1% silicon, solution treated specimen is visible in Figure 14. The solution treated specimen was loaded at 275 kPa (40 psi); the pressure was insufficient for failure. The 1% silicon alloy shows a resistance to shear localization as evi- dent by the work hardening achieved. A deformed region is highlighted showing the nucleation region of an adiabatic shear band and the rotated crystal structure of adjacent ma- terial conforming to the 45° shear band.
Figure 15 is an image of a shear band in a 1% silicon, 10 hour aged specimen, loaded at 310 kPa (45 psi), which did not fail. The shear band extends nearly the entire specimen. Intense slip and grain rotation is observed adjacent to the shear band. The loading direction (LD) is indicated in the figure’s upper right hand corner.
Figure 16 is an image showing the early stage of shear band formation in the solution treated specimen that was loaded to 345 kPa (50 psi). This specimen did not fracture. Inspec- tion of the deformed microstructure shows multiple shear bands. Planar slip is visible between the shear bands and the planar slip accommodates deformation not associated with adiabatic shear.
Discussion
The 530°C (986°F) age hardening behavior follows closely with work by Hale and Baker23
Fe-30.4Mn-7.6Al-0.81C-0.35Si. Acselrad et al.19
of a similar composition, i.e. investiga-
tion of time temperature transformation behavior suggested a temperature less than 550°C (1022°F) to reduce hetero- geneous precipitation of к-carbide at grain boundaries and formation of B2 or DO3
intermetallic compounds. Further
reductions in ageing temperature (below 530°C [986°F]) would decrease the nucleation and growth of these deleteri- ous phases, but the corresponding time necessary to attain MIL-PRF-32269 hardness requirements were deemed unre- alistic for production foundries.
For both alloys, the higher number density of sulfides and ni- trides is consistent with overall chemistry of the alloy, i.e. ox- ygen was less than 2 ppm. The 1.4% alloy had higher number of non-metallic inclusions as can be seen in Figures 4 and 5, but the chemistries of the inclusions in both alloys are similar
Figure 15. An adiabatic shear band propagated through a 10 hour aged specimen. Shear formation appears indepen- dent of inclusion content and ferrite. The loading direction (LD) is indicated in the upper right hand corner.
International Journal of Metalcasting/Winter 10 15
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