Molybdenum brittleness varies under the influence of two major factors: temperature, which affects the pattern of electron distribution; and the interaction of certain impurities.
Molybdenum is a transition metal—its atoms contain an unsaturated, secondary outer electron layer with an asymmetric d-orbit, and an outermost electron layer with a spherically symmetrical s-orbit.
The asymmetric d-orbit causes molybdenum atoms to form covalent bonds, while the symmetrical s-orbit causes the formation of metallic bonds. Consequently, at different temperatures, molybdenum exhibits either higher brittleness imparted by its covalent bonds, or lower brittleness imparted by its metallic bonds.
In addition, certain impurities can increase molybdenum’s brittleness by weakening the strength of grain boundaries.
Temperature-Driven Bonding Dynamics Affecting Molybdenum Brittleness
The predominant atomic bonds among molybdenum atoms differ as surrounding temperature changes, manifesting varying degrees of ductile deformation or brittle fracture.
In high-temperature environments, metallic bonds play the primary role, and molybdenum exhibits significant ductility. As the temperature drops, the bonds between the outer electrons gradually shift from metallic to covalent, increasing signs of brittle fracture.
Below the ductile-to-brittle transition temperature (DBTT), metallic bonds are completely converted to covalent bonds, dramatically increasing the resistance between molybdenum’s crystal lattices, reducing the mobility of dislocations, and making cross-slip more challenging. Such dislocation phenomena concentrate stress at the metal’s grain boundaries, leading to intergranular brittle fracture.
Impurity Elements and Their Role in Molybdenum Brittleness
Impurities (mainly carbon, nitrogen, and oxygen) in pure molybdenum can significantly increase brittleness, but their relative composition is important: even pure molybdenum contains large amounts of carbon and oxygen, yet at a specific ratio, the metal’s grain boundary strength can nevertheless remain high.
Compared to C and N impurities, O most strongly increases brittleness. A mere 6×10-6 % of O can cause molybdenum to evince intergranular brittle fracture. This is mainly because O can readily combine with Mo to MoO2. The monolayer-molecular of MoO2 and its segregating reactions at the molybdenum grain boundaries significantly reduces the metal’s intergranular bonding strength.
Precipitated nitrogen can also segregate on molybdenum grain boundaries, inducing stress concentration and causing intergranular brittle fracture. Under very specific conditions, nitrogen may enhance the bonding energy of molybdenum, but in most cases, it will increase the ductile-to-brittle transition temperature, thus increasing brittleness. Annealing of molybdenum alloys can usually control nitrogen content.
While carbon is actually beneficial to the ductility of molybdenum, in excess it can also produce coarse carbide precipitates at the grain boundaries, significantly reducing the ductility.
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