Microstructure

P. Paillard, E. Bertrand, L. Couturier

As part of a CIFRE thesis (Quentin BOYADJIAN thesis) with a foundry on the influence of the composition of an alloy, here Lead - Antimony alloys, we studied the evolutions of microstructures in connection with the composition of the alloy (Figure 1a). In addition, we have followed the evolution of the mechanical properties of alloys over time. Indeed, lead alloys have the unfortunate tendency to change over time, even at room temperature due to microstructural modification leading to more or less marked modifications of mechanical properties depending on the alloys (Figure 1b).

 

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Figure 1 : evolution of foundry microstructures (a) and mechanical properties (b) for lead - antimony alloys

 

The microstructural characterization means that we master within the team, have enabled us to make very fine characterizations, in particular via EBSD maps (Figure 2).

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Figure 2 : EBSD maps of pure lead foundry microstructures (a) and a Lead - 4% Antimony alloy (b)

 

As indicated above, the Lead - Antimony alloys evolve over time at room temperature either by precipitation or by recrystallization in the case where the alloy has been shaped by strain hardening following the foundry operation. Figure 3 shows the evolution of the microstructure, characterized by EBSD, work hardened over time while Figure 4 gives the changes in mechanical properties of the various lead - Antimony alloys deformed (20 and 40% work hardening) or not deformed over time.

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Figure 3 : EBSD imaging of static recrystallization at room temperature of a Lead - 8% Antimony alloy deformed to 20%, (a) 1 hour after deformation, (b) 4 hours after deformation

 

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Figure 4 : evolution of mechanical properties (Vickers hardness) over time for pure lead and 2 lead - antimony alloys over time and for 2 initial deformation rates: 20% (a) and 40% (b)

 

As part of Said BEROUAL's thesis (co-supervision with the Faculty of Sciences and Technology of the Mohamed Khider University in Biskra, Algeria), the latter characterized the microstructures of castings in aluminum alloy (Aluminum - Silicon - Copper - Magnesium) obtained by two foundry processes: gravity foundry in sand mold and pressure foundry in metal mold. The differences in terms of cooling rate of the two processes made it possible to obtain different microstructures in terms of granular morphology, grain size and state of precipitation. Figure 5 shows the 2 microstructures obtained. Said Béroual was then able to determine the various precipitates present in the parts by various techniques present in the laboratory. Figure 6 shows observations by scanning electron microscopy as well as chemical analyzes by EDX of the various precipitates.

Figure 5 : influence of foundry technology on the microstructures obtained in the case of Aluminum - Silicon - Copper - Magnesium alloys: (a) and (c) sand casting by gravity, (b) and (d) pressure casting. Observations by optical microscopy (a) and (b) and by scanning electron microscopy in chemical contrast (c) and (d)

 

Figure 6 : SEM micrographs and EDX maps of the a-Fe phase in Chinese writing in a sand-cast Al-Si-Cu-Mg alloy. (a) and (c) chemical contrast imaging, (b) topographic contrast imaging, (d) chemical element mapping