Study On Thermal Expansion Of Cu₄₉Hf₄₂Al₉ Metallic Glass

Cu-based metallic glasses have higher glass forming ability than Ni-, Ti-, and Al-based metallic glasses; and Cu-based metallic glasses cost less on elements than Pd-, Pt-, and Y-based metallic glasses; meanwhile Cu-based metallic glasses possess higher fracture strength than the most familiar Zr-based metallic glasses. Consequently, Cu-based metallic glasses have both the potentials to develop large bulks and fine mechanical properties. In the present thesis, Cu49Hf42Al9 metallic glass was chosen because of its lower cost, higher glass forming ability, high strength and high ductility. Thermal expansion of Cu49Hf42Al9 metallic glass was systematically investigated and whose mechanism was discussed, and especially the influence of cooling rate on thermal expansion coefficient was analyzed.In order to obtain large scale of cooling rates, five kinds of Cu-based metallic glasses with different sizes were prepared. The cooling rates are distributed from about 102K/s to 107 K/s. Three kinds of amorphous ribbons were prepared by melt spinning techniques, whose thickness was 0.02 mm,0.04 mm and 0.1 mm respectively. Two kinds of amorphous rods were prepared by copper mould casting, whose thickness was 2 mm and 3 mm respectively.The amorphous structures of as-quenched samples were characterized by X-ray diffraction (XRD). Thermal behaviors were examined by differential scanning calorimetry (DSC). Thermal expansion curves were measured by dilatometer (DIL) respectively for as-quenched, annealed and crystallized samples of Cu49Hf42Al9 metallic glass from room temperature to 100 K above Tx. A new test method was designed so that thermal expansion measurement of amorphous ribbon using traditional dilatometer was achieved. The average thermal expansion coefficientsαaver ranging from room temperature to Tg were calculated. The results show thatαaver of as-quenched samples ranges from 6.14×10-6 K-1 to 9.20×10-6 K-1, and it has a negative correlation with the cooling rate. The valuesαaver of annealed and crystallized samples are closed to each other. Comparison ofαaver between annealed and as-quenched samples shows distinct differences. For annealed ribbons,αaver is increased; while,αaver is decreased for annealed rods.