Effects of micro- and nano-structure on the deformation response of a Ag60Cu40 lamellar and rod-in-matrix eutectic alloy.

摘要

The presented work investigates the mechanical response of the silver-copper eutectic system (Ag60Cu40, subscripts indicating atomic percent) linking material deformation to microstructural properties. The Ag60Cu40 material system can be produced as either a multidirectional lamellar or unidirectional reinforcement-in-matrix micro-structure. Specimens of each micro-structure type were studied under quasi-static and dynamic loading conditions.;The first part of this work focuses on the study of the material with a multidirectional lamellar structure. Materials produced with this structure primarily consist of eutectic colonies of alternating layers of silver and copper with layer thicknesses between 35 nm -- 200 nm. The orientations of the eutectic colonies are randomly distributed throughout the material resulting in the formation of boundaries between neighboring eutectic colonies which have different orientations with respect to each other. The strength of this material is shown to be strain rate insensitive over the strain rates studied (10-3 s-1 to 103 s-1). Comparisons are made between the Ag-Cu stress-strain response and literature stress-strain responses of nano-structured silver and nano-structured copper demonstrating the high strength of the multidirectional Ag-Cu system. Three primary deformation mechanisms that occur at increasing levels of strain at the specimen radial surface are identified: kinking, brooming, and interfacial delamination. At the specimen interior kinking is the only mechanism observed.;The second portion of this work examines the unidirectional reinforcement in matrix structure again for Ag60Cu40. This structure has a common ⟨101⟩ crystallographic direction matching the axial direction of the cast material. From a single casting specimens are machined such that loading along three directions oriented 1) parallel to, 2) at 45° to and 3) perpendicular to the ⟨101⟩ can occur using dynamic loading. Through alterations in the solidification rate of the unidirectional cast material the micro-structure nominal feature size can be regulated obtaining castings with either 200 nm, 500 nm, 800 nm or 1.2 mum thick reinforcements. For each loading orientation the dynamic material response is presented with the observed internal and external deformation mechanisms. Comparisons of the recorded elastic modulus, yield strength, and strain hardening exponent are made over the loading orientations and nominal micro-structure feature sizes. Crystal anisotropy is used to account for variation in the observed elastic modulus of each loading orientation. Dislocation deformation mechanisms are used to explain the differences in the yield strength and strain hardening. The mechanical properties of the multidirectional lamellar structure are compared to the unidirectional material structure. The multidirectional material is shown to have a higher yield strength. The unidirectional material is shown to have greater strain hardening when microstructure features sizes are greater than 500 nm under certain loading directions.