Fabrication And Mechanical Behavior Of Copper-Based And Nickel-Based Laminated Composites

Nowadays,the load-bearing components of micro-electro-mechanical systems(MEMS)devices working at high temperature require materials having sufficient strength and ductility at high temperature.However,existing LIGA Ni and nanograined Ni-W alloy cannot meet the actual needs of materials for high-temperature MEMS components because of their poor strength and ductility at elevated temperatures.Thus,designing and fabricating materials with superior synergy of strength and ductility at high temperature and a solid fundamental understanding of the mechanisms of strengthening/toughening and damage/fracture would be of great importance in the development of high-performance MEMS.Based on the above application background and key scientific issues,the design concept of heterostructured materials,especially heterogeneous laminated composites,was considered.In this thesis,Cu-based and Nibased nanograined laminated composites with varied characteristics of interfaces and length scales were firstly fabricated and the corresponding mechanical behavior and damage mechanisms at ambient temperature were studied.Secondly,a kind of Ni/NiW laminated composites for high-temperature MEMS components was fabricated and the corresponding mechanical properties at elevated temperature were studied.Our results are expected to deepen our understanding of the strengthening/toughening mechanisms in the heterogeneous materials,provide ways to obtain the optimum microstructures and demonstrate the possible applications of the heterogeneous materials in the MEMS devices at elevated temperature.The main research results are shown as follows:1.The sandwich-structured Ni/Cu/Ni laminated composites with varied initial dislocation density were fabricated and the effect of dislocation density contrast on the interface coupling behavior was studied.The strength of laminated composites decreases first and then increases with the decreasing degree of mechanical incompatibility across the Cu/Ni interface.Because the extra tensile yield strength caused by the interface coupling effect decreases with the decreased hindering effect of geometrical necessary dislocations(GNDs)on the subsequent dislocation slip.Meanwhile,two competitive strengthening mechanisms were found to be GND strengthening introduced by the interface coupling effect and cyclic hardening arising from dislocation veins,which is revealed as the decreased width of the interfaceaffected zone with decreasing initial dislocation density.2.The sandwich-structured Ni/Cu/Ni laminated composites with varied thickenss ratios of the Cu layer to the Ni layer were fabricated and the effect of thickness ratio on the necking behavior of the soft layer was studied.There is an evident transition of the fracture behavior from the necking-delayed ductile mode with evident deflection of the Ni layers to the necking-inhibited brittle mode with interface delamination occurred with decreasing thickness ratio.A beam deflection model was established to evaluate the critical thickness ratio corresponding to the optimum synergy of high strength and ductile fracture.3.The laminated Ni with varied grain size difference across the interface were fabricated by tuning the thickness ratio of the hard layer to the soft layer and the effect of grain size difference across the interface on the propagation behavior of shear bands in hard layers was studied.The strength/ductility synergy is enhanced with increasing the grain size difference across the interface.Because the resistance of layer interfaces to the propagation of micro SBs in hard layers gradually increases and the formation of dispersive micro shear bands instead of a few shear bands lead to strain delocalization.Considering the transition of the fracture mode from necking fracture to shear fracture with decreasing the grain size of soft layers,the mechanical model about intergranular dislocation emission assisted by local shear stress was evaluated and the critical grain size of soft layers corresponding to the optimum synergy of high strength and ductile fracture was also obtained.4.The nanosized Cu/W laminated composites with varied layer thickness were fabricated and the effect of grain boundary arrangement on the fatigue cracking behavior was studied.With decreasing layer thickness,there is a transiton of fatigue cracking behavior from intergranular normal fracture of the W layers and dislocationdominant shear fracture of the Cu layers to the interface-dominated damage such as grain boundary and Cu/W interface cracking for the nanosized Cu/W laminated composites.Due to the appearance of periodic wavy structure,the straight crack path becomes prevalent when layer thickness decreases from 20 to 5 nm.A stochastic model was established to reveal that the quantitative relation between the fatigue cracking path and the grain size of constituent layers and grain boundary arrangement deviation.5.Nanograined Ni/Ni-W laminated composites were designed and fabricated and the effect of thickness ratio and interface spacing on the tensile behavior at ambient temperature was studied.The strength/ductility synergy of Ni/Ni-W laminated composites is enhanced and become superior to that of monolithic nanograined Ni with increasing thickness ratio of the Ni layers to Ni-W layers.Because the contribution of stable GB sliding under interface constraint to the plastic flow of Ni-W layers with small layer thickness is magnified,which leads to the increased thinning degree during the deformation process.Considering the transition of the fracture behavior from coexistence of channel cracks and micro SBs to micro SBs for the Ni-W layers,a model based on the energy criterion was proposed and the deformation mode of Ni-W layers dominated by the length scale and the flow stress level was clarified.The strength and ductility of Ni/Ni-W laminated composites are simultaneously enhanced with decreasing interface spacing.When the tensile strength increases to 2 GPa,the elongation to failure could reach 5.5%.Because the contribution of shear-stressdominated stable grain boundary migration to the grain growth in the Ni layers with small layer thickness is magnified.The recovered strain hardening ability with grain growth eventually leads to the strain delocalization process which consists of the transition of micro shear bands to local strain region.6.Ni/Ni-W laminated composites suitable for high temperature MEMS devices were fabricated.The tensile strength of Ni/Ni-W laminated composites at 400℃ could reach 400 MPa-620 MPa which is 2 to 3 times more than that of the traditional LIG A Ni.The elongation to failure is larger than 10%at the same time.The superior properties could be attributed to the interface-constrained grain growth in the Ni layers and enhancement of thermal stability and strength of Ni-W layers caused by grain boundary relaxation.It could provide a new method for developing and fabricating materials with superior mechanical properties at elevated temperature for hightemperature MEMS applications.