TEM study of microstructural development during heating in a nanolaminated amorphous ZrAlCuFe/crystalline CuCoFeNi composite structure

A nanolaminated amorphous/crystalline composite structure with a mean lamellar thickness of around 10 nm was fabricated on a Cu plate. The crystalline phase was a multicomponent non-equilibrium face-centered cubic (fcc) Cu(CoFeNi) solid solution, and the amorphous phase was a Zr-based compound containing Al, Cu, and Fe. The composite's thermal stability and microstructural transformation was studied over the temperature range of 200-900 °C. The lamellae maintained their shape during heating up to 600 °C. Transformation of the structure began with separation of the elements inside the crystalline lamellae. In early stages of the transformation, hardening occurred. At 600 °C, an interconnected CoFe phase started to appear with an ordered body-centered cubic (bcc) crystal structure. When the temperature was increased further, the nanolaminated structure degraded and the bcc CoFe phase grew. At 750 °C, the bcc CoFe phase formed a complex network that surrounded the formerly amorphous regions, and the bcc CoFe phase started transforming to the fcc configuration. The Cu atoms segregated to the grain boundaries of the fcc CoFe(Ni) phase. The amorphous phase gradually crystallized into nanometer-sized polycrystalline grains that were attributed to the Zr(Al)O2 phase. As a result of these transformations, heating at 900 °C produced a morphologically complex nanocomposite structure consisting of branched grains of Zr(Al)O2 and fcc CoFe(Ni) with Cu inclusions. When the nanolaminated structure had completely transformed, the layer was softer than it had been in the initial annealing steps, but was still almost five times as hard as the initial Cu plate.