膨胀合金分为低膨胀合金与定膨胀合金两大类。所谓定膨胀合金,即在一定温度范围内膨胀系数几乎不变。定膨胀合金大部分用来和硬玻璃、软玻璃、陶瓷及石英焊接,是电子管、整流管、高频发射管、x-射线管以及真空系统中玻璃和金属封接的部件,是电真空工业中必不可少的。可伐合金是定膨胀合金中一种极为重要的合金,是电真空工业中广泛使用的封接结构材料。可伐合金是Fe-Ni-Co系合金,其典型化学成分是27.5～29.5％Ni,17～19％Co,其余为Fe,并含有微量夹杂。从 Expansion alloy is divided into low-expansion alloy and fixed expansion alloy two categories. The so-called fixed-expansion alloy, that is, within a certain temperature range almost unchanged coefficient of expansion. Most of the expansive alloy is used for welding with hard glass, soft glass, ceramic and quartz. It is a component of sealed glass and metal in electron tubes, rectifiers, high-frequency launch tubes, x-ray tubes and vacuum systems. Indispensable Kovar is an extremely important alloy in fixed-expansion alloys and is a widely used sealing material in the vacuum industry. Kovar is a Fe-Ni-Co based alloy with a typical chemical composition of 27.5-29.5% Ni, 17-19% Co and the balance of Fe with trace inclusions. From