Microstructure characterisation and CTE study of Fe—42Ni—Nb invar alloys

Abstract

Invar alloy has low coefficient of thermal expansion; however, the week strength always limits its application. To circumvent this problem, the authors have designed Fe—42Ni—Nb invar alloy using midfrequency vacuum induction melting technique. The microstructures of this alloy were characterised by means of XRD, SEM and TEM. The multicomponent composition (NiFeCMnNb) is chosen such that upon chill casting of the alloy the liquid undergoes a metastable reaction, forming a strengthening NbC precipitated phase on γ-(Fe, Ni) solid solution. As a result, the alloy achieves the low coefficient of thermal expansion α_30–100=6·14 × 10⁻⁶ K⁻¹ ranged from room temperature to 100°C. The results show: in NiFe invar alloys, the primary phases are γ-(NiFe), FeNi₃) Ni and Fe₂C; and with the addition of carbon, manganese and rare earth niobium, NiFeCMnNb invar alloy achieves such primary phases as γ-(NiFe) and Nb_yC_x; and TEM structure consists of superfine structure and its matrix is γ-(NiFe).