Enhancing the strength and plasticity of Kovar alloy without sacrificing thermal expansion properties

Abstract

Enhancing the strengths of most constant-expansion alloys, including Kovar alloys, typically results in reduced plasticities and increased coefficients of thermal expansion (CTEs). In this study, we systematically investigated the effects of carbon on the microstructure, mechanical properties, and CTE of a 4J29 Kovar alloy prepared via metal injection molding (MIM). When the carbon content was in the appropriate range, the strength and plasticity could be improved without compromising the CTE. MIM samples prepared by adding 1000 ppm graphite and oxalic acid for degreasing exhibited increases of 28.2 % and 17.3 % in tensile strength and elongation, respectively, compared to those of samples with low carbon contents of 46 ppm. The average CTE remained stable at 4.92 × 10⁻⁶ ℃–1 (25–400 ℃). The reduced number of pores at the grain boundaries was the main cause of the improvement in plasticity. The low CTE is attributed to the combined effect of the grain boundary volume fraction and porosity. This study provides significant theoretical and practical guidance for use in producing high-performance 4J29 Kovar alloys that may be used in complex components with constant-expansion properties.