Improvement of mechanical properties of low thermal expansion alloys by rapid solidification

Invar 36, an iron–nickel alloy containing approximately 36 wt% Ni, is indispensable to the aviation, defense, and space industries due to its ultra-low coefficient of thermal expansion (CTE). The substance under consideration is utilized in a variety of applications, including, but not limited to, accelerometers, composite molds, hermetic packaging, and opto-mechanical systems. Notwithstanding, the austenitic crystal structure is responsible for the ductile nature of the substance, which consequently results in relatively poor mechanical properties. This, in turn, leads to limitations in its industrial applicability. The alloy is also notoriously difficult and expensive to machine. Conventional strategies to enhance the mechanical response of Invar 36 include precipitation hardening via alloying, grain refinement, and severe plastic deformation (SPD). While these routes can enhance hardness and tensile strength, they invariably compromise the alloy's characteristic low coefficient of thermal expansion (CTE). The addition of alloying elements disrupts the distinctive magnetic-lattice coupling that governs Invar's "magnetostrictive" lattice collapse upon heating. Consequently, the coefficient of thermal expansion (CTE) exhibits a substantial increase. SPD has been demonstrated to enhance strength; however, it does so by compromising the material's stability, resulting in the presence of residual stresses and elevated dislocation densities. This, in turn, engenders an unpredictable thermal expansion during subsequent temperature fluctuations. In this thesis, the production of Invar 36 ribbons is reported for the first time in the open literature. These ribbons were produced by melt-spinning, a rapid-solidification technique that achieves cooling rates on the order of ~106 °C s⁻¹. In comparison with conventionally cast counterparts, the ribbons exhibit more than a two-fold increase in hardness while retaining their low-expansion behavior. The ribbons, with a thickness ranging from 20 to 120 m and a length of approximately 200 mm, were then consolidated into solid rods or ingots through the process of hot extrusion. This method was found to be a viable route for processing ribbons into bulk forms without compromising the enhanced property characteristics that were observed.