Order-Disorder Transitions in Ternary Alloys
- PDF / 1,828,407 Bytes
- 10 Pages / 414 x 635.4 pts Page_size
- 75 Downloads / 266 Views
ORDER-DISORDER TRANSITIONS IN TERNARY ALLOYS A Study on the Microstructure of Li 0 and Li 2 Alloys J. Th. M. DE HOSSON Department of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands
ABSTRACT The order-strengthening mechanism in the ternary alloy, Cu-Ni-Zn, is investigated by comparing stress-strain curves and electron micrographs of corresponding dislocation structure in disordered, partly ordered and ordered samples. Around the composition Cu 2 NiZn two different types of ordering are found experimentally: LI 0 and LI 2 . The degree of ordering is determined by measuring the equilibrium distance between superlattice dislocations and by single crystal neutron diffraction. The long-range zinc order parameter thus obtained appeared to be in agreement with calculations using the cluster variation method (CVM) in the tetrahedron approximation. Electron microscopic observations show that even in the disordered phase, when short range order exists, superlattice dislocations are created by FrankRead sources, rather than the generation of single dislocations.
INTRODUCTION The mechanical behaviour of solid solutions which develop long-range order is a topic of considerable scientific and engineering interest. The formation of long-range order in concentrated alloys affects the configuration of dislocations and hence results in a change of the dislocation microstructure. For instance, some LI 2 alloys exhibit an increase in flow stress with increasing deformation temperature, whereas disordered f.c.c. alloys exhibit a substantial decrease in flow stress. Due to the great variety of possible dislocation processes which occur during plastic flow, it will never be possible on a purely theoretical basis to predict the evolution of the dislocation structure under different deformation conditions and to calculate the macroscopic properties of an alloy with sufficient accuracy. Rather, for an understanding of mechanical properties, the common procedure is to define phenomenological relationships experimentally, to correlate these with transmission electron microscopic observations of the evolution of the dislocation structure and hence to deduce the physical nature of the controlling dislocation mechanisms. In this work Cu-Ni-Zn alloys which show ordering as well as clustering are investigated along these lines. Around the composition Cu2NiZn two different types of ordering are found experimentally: LI 0 and L1 2 , that might have consequences for the mechanical behaviour. The interpretation of the mechanical properties of L12 alloys is still a controversial matter. Liang and Pope [1] divide the Li 2 alloys into two categories: Type I alloys (e.g. Cu 3 Au, Ni3Fe, Ni 3 Mn) having a critical temperature below their melting point and type II alloys (e.g. Ni 3 Si, Ni 3 Ga, Ni 3 AI, Ni 3 Ge) which melt before the critical temperature is reached. Type II alloys, which have a large APB energy on 1111) planes behave similarly regarding their mechanical properties. For t
Data Loading...
