Microstructure And Properties Of Rapidly Solidified Cu-Sn Hypoperitectic Alloys

In this paper, the rapid solidification and microstructural characteristic of Cu-xwt%Sn(x=7, 13.5, 20) hypoperitectic alloys are investigated by melt-spun method, the cooling rate is calculated theoretically through coupling the melt heat conduct equation and Navier-Stokes equation, the phase selection and microstructural evolution of the alloys are studied by XRD,TEM,OM and SEM techniques, meanwhile, the electrical resistivity,microhardness,tensile strength and elongation of alloys are measured experimentally, and the relationships between cooling rate and alloy properties are further analyzed theoretically. It shows that:Under rapid solidification conditions, the single phaseα-Cu solid solution is formed for Cu-7%Sn hypoperitertic alloy;The microstructure of Cu-13.5%Sn alloy consists of main phase Cu_13.7Sn and a few ofα-Cu phase;The peritectic transition and eutectoid transitions of Cu-20%Sn alloy are all suppressed, which results in the formation of microstructures characterized mainly by metastable Cu5.6Sn metallic compounds. The micrstructural morphology of Cu-7%Sn,Cu-13.5%Sn alloy along the direction vertical to wheel surface can be roughly divided into three crystal zones: fine equiaxed zone near roller side, columnar zone in the middle part of ribbon and coarse equiaxed zone at free surface respectively. With the rise of cooling rate, the growth form of Cu5.6Sn phase in Cu-20%Sn transforms from faced to non-faced growth and the microstructure exhibits apparent growth orientation. Thus, the microstructure changes from coarse lath to fine columnar morphology. Bothα-Cu and Cu_13.7Sn phase grow in the manner of columnar crystals. With the increase of temperature gradient, the growth rate of columnar crystals increases linearly. TEM analysis reveals that there exists large quantity of dislocationpile-up and twins in Cu_5.6Sn grains of Cu-20%Sn alloy. Those twins are parallel to each other with an interspacing about 25⁸0μm, and extend to the boundary of Cu5.6Sn branches. With the increase of cooling rate, on one hand, the microstructure is refined and the amount of grain boundary increases, which intensifies the scattering of free electrons, resulting in the rise of alloy resistivity. Provided the value of grain boundary reflection coefficient r is about 0.992, the electrical resistivity of the alloys can be theoretically analyzed by the M-S model. On the other hand, the microhardness and tensile strength of alloy foils increase linearly, but the elongation of alloys decreases within the range of 1.0%⁴.6%.