Biaxial Zero Creep Measurements of Interface Energies in Ni/Ag Multilayers
- PDF / 1,684,590 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 26 Downloads / 164 Views
ABSTRACT Biaxial zero creep experiments were performed on Ni/Ag multilayer films on sapphire substrates. The equilibrium curvature was measured using a scanning laser and position sensitive photodetector. The experiments were designed to measure the free energy of Ni/Ag interfaces and to investigate their effect on the structural stability of multilayered materials. For the Ni/Ag multilayers studied, significant plastic straining occurs at temperatures above 400'C, enabling the growth stresses and thermal stresses in the multilayers to decay to zero. After a long time at elevated temperatures, the equilibrium curvature is reached for the film/substrate couple. This curvature is determined by the number and the energy of the Ni/Ag interfaces. Using this equilibrium technique, a free energy of 0.44 ± 0.03 N/m was measured for Ni/Ag interfaces at an equilibrium temperature of 550'C.
INTRODUCTION The energies of solid-solid interfaces such as those at grain boundaries, twin boundaries, and interphase boundaries, affect the physical properties of materials and determine the stability of their microstructures. Grain boundary free energies (Ygb)have been measured for many elements and alloys, but, due to the difficulty of studying interfaces directly, only a few free energies (yi,,) have been quantified for interfaces between dissimilar solid phases. Recent reports [1, 21 have shown that the free energy at the interfaces between two chemically stable solid phases can be measured directly using uniaxial zero creep experiments on multilayer samples. The present work uses equilibrium biaxial zero creep tests on multilayer samples to determine the energy of the interface between chemically distinct layers. Since the technique uses an equilibrium approach, it differs from the uniaxial zero creep techniques, which are kinetic in nature and depend on the dominant deformation mechanisms.
THEORY Grain Boundary Grooving and Stability Current understanding [ 1-4] suggests that the ratio of grain boundary energies and interface energies (Ygt'Yi,,) controls the stability of materials with polycrystalline layers. When the ratio ygJ/Yint is large, significant grooving occurs where grain boundaries meet the interfaces between layers, as illustrated in Figure 1(a). For sufficiently large values of Ygb/yi,,, the grain boundary grooves can extend through layers and the microstructure is unstable. When yg/yi,, is small, as illustrated in Figure l(b), there is little grooving at grain boundary-interface junctions and the layering is stable. The ratio ygd/yn, can be determined directly by measuring the angles of the grooves at the triple junctions shown in Figure 1. An expanded view of one triple junction is shown in Figure 2. Here the interface between the A and the B layers forms a groove at the grain boundary when equilibrium is established. The angle of the groove is determined by the ratio ygJ/yn,, through the relation 2 cos 0 = yg/ynt. 249 Mat. Res. Soc. Symp. Proc. Vol. 586 © 2000 Materials Research Society
(1)
I
1II
I
I
I
II
Data Loading...
