Bonding characters of Al-containing bulk metallic glasses studied by²⁷Al NMR

Abstract

We report very small (27)Al metallic shifts in a series of Cu-Zr-Al bulk metallic glasses. This observation and the Korringa type of spin-lattice relaxation behavior suggest that s-character wavefunctions weakly participate in bonding and opens the possibility of enhanced covalency (pd hybridization) with increasing Al concentration, in good agreement with elastic constants and hardness measurements. Moreover, ab initio calculations show that this bonding character originates from the strong Al 3p band and Zr 4d band hybridization since their atomic energy levels are closer to each other while the Al 3s band is localized far below the Fermi level. This study might provide a chemical view for understanding flow and fracture mechanisms of these bulk glass-forming alloys.

Correlation of mechanical properties in bulk metallic glasses with 27Al NMR characteristics

“…The values of the Knight shift given in Figure 1 (a) and (b) are on the order of ~350 ppm demonstrating that the s-electron contribution at the Fermi energy is quite small as compared to the shift in pure Al metal (~1630 ppm) [20]. This is consistent with previous NMR studies of Al-based BMGs [15,16,22] and recent electronic structure calculations of Al-Zr-Cu [16] and Al-Ca [23] amorphous systems.…”

“…The calculations indicate that the s-electron band is significantly shifted to higher binding energies whereas the valence d-electron band of transition metals remain prominent at the Fermi level [16,24].…”

“…Furthermore, while the 27 Al Knight shifts demonstrate that local electronic structure at Al-sites is strongly coupled to mechanical properties of metallic glass, Figure 3 shows that the d-electron susceptibility provides a strong correlation with hardness and toughness properties. Previous electronic structure studies of Zr-Cu-based metallic glasses [3,16] attribute a decreasing toughness (or an increase in brittleness) to an enhancement of covalent-like bonding, or specifically an increase in Al-TM sp-d hybridization in band structure. While this cannot be confirmed directly here, the implications of the data presented in Table 1 are that both the s and d magnetic susceptibilities of BMGs are highly influential for obtaining favorable mechanical properties.…”

“…To optimize the potential of metallic glasses for engineering applications, an understanding of correlations between electronic structures and mechanical properties can be quite valuable for designing and fine-tuning the properties of BMGs. Previous experimental [15][16][17] and computational studies of transition metal (TM)-based BMGs [16,18,19] have addressed the issue of the relationship between electronic structures and mechanical properties such as strength and ductility. More experimental evidence would be very useful for further establishing and understanding the correlations between structural properties and electronic properties of BMGs.…”

Atomic dynamics in Zr-(Co,Ni)-Al metallic glass-forming liquids

“…An important feature of Al in MGs is its strong chemical interaction with the transition metal (TM) atoms, particularly, late transition metals (LTM) like Cu, Co, and Ni [15][16][17]. This correlates with the drastic change in the compression plasticity as function of Al content [16], or of fracture toughness when substituted with different LTM atoms [17,18], via tuning of the electronic structure as observed in Zr-LTM-Al systems. However, whether and how such chemical interactions affect the properties of MGF liquids is still an open question.…”