Understanding the role of Ca segregation on thermal stability, electrical resistivity and mechanical strength of nanostructured aluminum

Abstract

Achieving a combination of high mechanical strength and high electrical conductivity in lowweight Al alloys requires a full understanding of the relationships between nanoscaled features and physical properties. Grain boundary strengthening through grain size reduction offers some interesting possibilities but is limited by thermal stability issues. Zener pinning by stable nanoscaled particles or grain boundary segregation are well-known strategies for stabilizing grain boundaries. In this study, the Al-Ca system has been selected to investigate the way segregation affects the combination of mechanical strength and electrical resistivity. For this purpose, an Al-Ca composite material was severely deformed by high-pressure torsion to achieve a nanoscaled structure with a mean grain size of only 25 nm. X-ray diffraction, transmission electron microscopy and atom probe tomography data revealed that the fcc Ca phase was dissolved for large levels of plastic deformation leading mainly to Ca segregations along crystalline defects. The resulting microhardness of about 300 HV is much higher than predictions based on Hall and Petch Law and is attributed to limited grain boundary mediated plasticity due to Ca segregation. The electrical resistivity is also much higher than that expected for nanostructured Al. The main contribution comes from Ca segregations that lead to a fraction of electrons reflected or trapped by grain boundaries twice larger than in pure Al. The two-phase state was investigated by in-situ and ex-situ microscopy after annealing at 200°C for 30 min, where precipitation of nanoscaled Al4Ca particles occurred and the mean grain size reached 35 nm. Annealing also significantly decreased electrical resistivity, but it remained much higher than that of nanostructured pure Al, due to Al/Al4Ca interfaces that reflect or trap more than 85% of electrons.

Al–Ca, Al–Ce, and Al–La Eutectic Aluminum Alloys Processed by High‐Pressure Torsion

“…The absence of solubility in the Al-8% Ca binary alloy during HPT through five turns was previously shown using synchrotron radiation X-ray method. [14] At the same time, when studying the Al-12% Ca alloy, [20] it was shown that at a high number of turns (N ¼ 100), calcium are formed along the crystallite boundaries, causing additional alloy hardening. It should be noted that the Al-12% Ca alloy was obtained not by casting, [20] but by mixing Al and Ca powders, and the initial structure was not analyzed.…”

“…[14] At the same time, when studying the Al-12% Ca alloy, [20] it was shown that at a high number of turns (N ¼ 100), calcium are formed along the crystallite boundaries, causing additional alloy hardening. It should be noted that the Al-12% Ca alloy was obtained not by casting, [20] but by mixing Al and Ca powders, and the initial structure was not analyzed. As noted in the Introduction Section, the behavior of two-phase eutectic aluminum alloys both during casting and deformation differs significantly from the behavior of complex eutectic aluminum alloys due to the effect of chemical elements on mutual solubility.…”

“…The softening of the Al-12% Ca alloy with an increase in the number of turns was observed. [20] Thus, the change in the microhardness of the Al-8% Ca, Al-10% Ce, and Al-10% La alloys with an increase in the number of turns is apparently associated with a change in the ratio of dispersion hardening (from eutectic particles), strain hardening (characterized by an increased density of crystalline defects), and softening from the formation of a recrystallized grain structure (characterized by a low density of crystalline defects) during the HPT deformation. The initial morphology of the eutectic structure of the alloys should also influence the development of these processes.…”

Superfunctional Materials by Ultra-Severe Plastic Deformation

“…Currently, ultra-SPD is considered an effective tool to produce a wide range of materials such as binary and ternary alloys and intermetallics [ 16 , 17 ], high-entropy alloys [ 30 ], metal hydrides [ 31 , 32 ], and high-entropy ceramics [ 33 ]. Table 1 summarizes the results of the application of ultra-SPD to different systems in the author’s group and summarizes the main achievements of each system [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ]. As can be seen in Table 1 , the application of ultra-SPD is not limited to the synthesis of conventional metallic alloys, and it has been recently extended to ceramic materials.…”

“…The microstructure can be stabilized by second-phase particles of an immiscible element, but due to the immiscibility effect, such alloys show negligible solid solution hardening and poor age hardening [ 69 , 70 , 71 , 72 ]. Ultra-SPD provides an effective path to achieve supersaturation in various Al-based alloys such as Al-Ca [ 55 ], Al-Fe [ 49 ], Al-Zr [ 26 ], and Al-La-Ce [ 56 ]. Such a supersaturation can lead to an enhanced solution hardening effect in these alloys.…”