The puzzling thermal expansion behavior of invar alloys: a review on process-structure-property relationship

Guillaume stunned the scientific community in 1897 by reporting a very low CTE in Fe36Ni alloy, which also named invar 36. The anomalous CTE feature makes invar 36 an indispensable material in a wide range of critical applications, including high precision optics, laser components, composite molds in aeronautical industry, and various instruments in spacecraft. However, the main drawback of invar 36 is that it is a ductile material, and thus, its machinability is poor. The problem is afore-mentioned applications mostly necessitate invar 36 to be machined into various complex shapes, and thus, mechanical properties of invar 36 should be improved in most cases. Many strategies have been studied to improve the mechanical properties of invar 36, including utilization of different manufacturing methods, introduction of numerous alloying elements, and application of various post-processes. The most common manufacturing methods of invar 36 are casting, powder metallurgy, and additive manufacturing. Casting allows rapid manufacturing and it is relatively cheap, thus, enables mass production of small to large-sized parts. Powder metallurgy is a versatile alternative to casting since it allows preparation of complex shapes from small to large volume that are close to net shape with almost no loss of raw materials. Recently, additive manufacturing is proposed to prepare invar 36 with complex shapes without a severe impurity problem. As one could expect, not only the mechanical properties, but also the puzzling thermal expansion behavior of invar 36 strongly depends on manufacturing methods as well as post-processes. This paper intends to provide a comprehensive and critical review of the research activities on invar 36. Previous studies are discussed in terms of process-structure-property relation, on-going challenges and conflicts are highlighted, and author opinions are provided with an intention of serving as a guide for future studies.