Thermal expansion properties of cast invar-type alloys

摘要

Invar-type alloys are iron-nickel-based alloys and are known generally for their low thermal expansion coefficients. In these alloys, the magnitude of the thermal expansion coefficient in a temperature range can be controlled by varying thechemical composition and processing variables, such as cold working and annealing. These alloys are used in applications requiring a controlled thermal expansion coefficient.Conventionally, invar alloys are processed into wrought products, and components are fabricated from these. The alloys are optimized for production by wrought processes. However, for some complex shapes such as molds for curing net-shape polymer matrixcomposite components, it is desirable to use the casting process due to its inherent superiority for shape complexity and economy of production. This paper examines the thermal expansion behavior of cast invar alloys.Alloys of various compositions were melted, and Y-blocks with section thicknesses varying between 0.25 and 2 in. (0.64 and 5.1 cm) were poured. The thermal expansion behavior of these was studied between ambient temperature and 850F (454C). Analysis ofthe data shows that the primary variable determining the thermal expansion properties was the chemical composition, which can be represented by a single parameter, the nickel equivalent.The thermal expansion behavior is characterized by a low thermal expansion coefficient between the ambient temperature and an inflection temperature, and a high thermal expansion coefficient beyond this. These parameters are quantitatively related to thenickel equivalent. The expansion characteristics of cast alloys are similar to those of the wrought alloys within the same composition ranges.