Enhancement of the fracture toughness of bulk L1₂-based (Al+12.5 at.% M)₃Zr (M=Cu, Mn) intermetallics synthesized by mechanical alloying

Abstract

The microstructural evolution and the mechanical properties of L1₂-type bulk (Al+12.5 at.% M)₃Zr (M=Cu, Mn) intermetallic compounds with a nanocrystalline structure were investigated. The (Al+12.5 at.% M)₃Zr (M=Cu, Mn) powders synthesized by planetary ball milling (PBM) could be successfully consolidated into nearly pore-free bulk compacts at 580 and 620 °C without taking holding time by spark plasma sintering (SPS). Their grain sizes were in the range from 8 to 10 nm. The micro-hardness of the SPS-processed bulks was measured to be 975.8 and 983.9 Hv, respectively. On the other hand, their fracture toughness was barely ∼2 MPam^1/2. It was lower than those (∼4–6 MPam^1/2) of the coarse-grained (∼100 nm) bulk specimens annealed. This result indicates that a grain refinement towards the nanoscale does not have an appreciable effect on improving fracture toughness in brittle intermetallics. Thus, it was found that the fracture toughness could be enhanced by proper annealing and addition of the boron. Furthermore, the effect of grain size on the fracture toughness in nano-sized level was investigated in the bulk specimen prepared by arc melting, using mechanical alloying powders with ball-milling.

Introduction

The L1₂-ordered cubic zirconium trialuminides are derived from tetragonal D0₂₃-ordered Al₃Zr by alloying with fourth-period transition elements such as Cr, Mn, Fe, Ni, Cu, and Zn. They have been considered as advanced high temperature structural materials because of their high melting temperature, low density and good oxidation resistance [1], [2], [3], [4], [5], [6]. The major problems limiting the practical use of this compound are, however, the low ductility and fracture toughness at ambient temperature and poor strength at ambient and elevated temperatures [7]. Recently, research efforts have been made to improve the ductility of Al₃X intermetallic compounds. Nanocrystallization is expected to improve their ductilities by enhancing grain boundary sliding and diffusion creep at room temperatures [4]. The modification of crystal structure from the tetragonal D0₂₂/D0₂₃ to the more symmetric cubic L1₂ structure is also known to improve their ductilities to some extent. It is, therefore, expected that the ductility of nanocrystalline L1₂ Al₃X intermetallic compound is considerably enhanced [8]. However, L1₂ structure does not guarantee a ductile behavior [9] and the experiments showed that ductilities are disappointingly low, typically less than 2% elongation for most nanostructured materials such as L1₂ Al₃Zr and Al₃Ti alloy with grain sizes <25 nm [10]. One of the reasons that L1₂ alloys are brittle might be the antiphase boundary (APB)-coupled dislocation slip. It has been observed that 〈110〉 dislocations in L1₂-stabilized TiAl₃ alloys dissociate into APB coupled with two 〈110〉/2 superpartials [11], [12]. Consequently, the nucleation of dissociations from the crack tip in these alloys is difficult and they should be intrinsically brittle. According to Koch [10], three major limitations to ductility for nanocrystalline materials can be identified: (1) artifacts from processing; (2) force instability in tension; (3) crack nucleation or propagation instability. Therefore, annealing for the L1₂-based bulk zirconium trialuminides with nanocrystalline structure may be necessary to improve fracture toughness by increasing grain size.

In our previous study [13], after annealing the L1₂ bulk (Al+12.5 at.% Cu)₃Zr specimen with nanocrystalline structure in the temperature range, 500–1200 °C for 2 h, the fracture toughness increased up to 4.6 MPam^1/2, accompanied with increase in the grain size (∼100 nm). It is generally known that fracture toughness increases as the grain size decreases down to the micro-sized range [14], [15], but the effect of grain size on fracture toughness is controversial in the nano-sized range. The results of the consolidated bulk specimens do not show an explicit relationship between grain size and fracture toughness in the nano-scale. The micro-hardness increased up to 1010 Hv following the Hall–Petch relationship as the grain size decreased down to a critical value (41.7 nm), but further decrease in the grain size caused a decrease in the hardness. On the other hand, the L1₂ structure of bulk specimen began to transform to the D0₂₃ structure at 900 °C because it is thermally metastable. Furthermore, Pope [16] have reported that Mn can increase the cleavage resistance because Mn elements segregate at the grain boundaries, resulting in an increase of the grain boundary cohesion. In the present study, 12.5 at.% Mn as well as 12.5 at.% Cu added Al₃Zr intermetallic compounds was synthesized by PBM (planetary ball milling) and SPS (spark plasma sintering). Moreover, it is generally known that the micro-alloying with boron sharply increases the ductility (or fracture toughness) and effectively suppresses intergranular or cleavage fracture [17], [18], [19].

Thus, the purpose of the present study is to enhance the fracture toughness by means of annealing and boron addition in bulk (Al+12.5 at.% M)₃Zr (M=Cu, Mn) specimens. Furthermore, to examine the effect of grain size on the fracture toughness in nano-sized level, we tried to synthesize bulk specimen by arc melting milled powders.