Preparation Of Cu/Nb Laminated Nano-composites And In-situ TEM Study Of The Deformation Mechanism

Laminated composite materials can be applied to extreme conditions due to the excellent properties such as high strength,good thermal stability and radiation resistance,and thus have attracted extensive research interest.It is known that the properties of materials are closely related to their microstructures.Laminated composites are important structural materials and their mechanical properties can be optimized through the regulation of microstructure parameters,such as sub-layer thickness,proportion and crystal structure matching rate of component materials.Researchers have prepared different laminated composite materials with different structures by means of large plastic deformation,physical vapor deposition,etc.,and explored deformation mechanism of laminated composite materials through mechanical testing,microstructure characterization as well as simulations.They brought forward different models including grain boundary pile-up,confined layer slip,and interfacial strength under different sub-layer thicknesses.However,the validity of the models is controversial due to lack of direct experimental evidence.Therefore,it is very important to select appropriate methods to prepare laminated composite materials,accurately measure their mechanical properties,and analyze their deformation mechanisms in combination with in-situ deformation experiments.In this paper,Cu/Nb laminated composites with different sub-layer thicknesses were prepared by magnetron sputtering and accumulative roll bonding?ARB?.Mechanical properties of bulk materials prepared by ARB were tested.Microstructures of the materials were analyzed by transmission electron microscopy?TEM?and scanning electron microscopy?SEM?.Nucleation,motion behavior of the dislocations and fracture behavior were studied by TEM double-tilt in-situ deformation technique.This work conducts a deep understanding of the deformation mechanism in laminated composite materials,it provides a direct experimental basis and lays a theoretical foundation for the design of new laminated composite materials.The main research results are as follows:?1?Cu/Nb nano-laminated composite with sub-layer thickness of 70 nm was prepared by magnetron sputtering.1)Cu and Nb phases are alternately distributed,and the interface is clear and nano-level straight.Cu grain is equiaxed crystal with a lateral size of 60 nm to 200 nm,and Nb grain is columnar crystal with a diameter of 20 nm.Cu?111?texture and Nb?110?texture formed along the growth direction;2)The interfaces are still the major sources of dislocations.Dislocations emitted by phase boundary and slided along the Nb columnar crystal towards other phase boundary;There are two dislocation slip modes in the Cu layer:Dislocation obey confined layer slip model and pile-up at interface model;3)Primary twin with an average layer thickness of 10 nm formed in the Cu layer.There is no dislocation at twin boundaries,so it will only restrict other dislocation slips.When the nucleation occurs in the twin,the twin can also become a dislocation slip channel;4)The microcracks are easily initiated at the phase boundary and the columnar grain boundaries of Nb grain.The microcracks in Nb rapidly extend along the columnar grain boundaries to the entire layer,and Nb layer undergoes brittle inter-granular fracture.If there is a Cu grain boundary where the microcracks occur,the cracks will longitudinally extend along the grain boundaries within Cu layer,and Cu layer will crack along the crystal ductility;if there is no Cu grain boundary at the microcracks initiation site,the lateral expansion will result in two phase separation,nanowire-type ductile shear fractures occur in Cu layer.?2?Bulk Cu/Nb laminated composites were prepared by ARB.With the increase of rolling deformation,the sub-layer thickness decreases from 20?m to 110 nm and the minimum layer thickness is 50 nm,the yield strength increases from 230 MPa to693 MPa,the strength change is consistent with the calculation of the grain boundary pile-up model,and the plasticity decreased from 17.2%to 3.2%.1)With strong combination of two phases,bulk Cu/Nb composite has clear and straight K-S interface of<110>Cu||{110}<111>Nb interface without diffusion.For the sample with the 4^th cycle,small amount of Cu layer fractured due to nonuniform deformation,spheroidized during the heat treatment and thus bend the interface;2)The interface is the main dislocation source during the plastic deformation process.Two types of dislocation slipping processes were observed in the Cu layer when the sub-layer thickness is in the range of 120 nm to 70 nm:Dislocation piled up in the interface;Dislocation confined slip within the Cu layer;3)Deformation twins formed by stacking faults arraying along?111?plane was founded in 120 nm Cu layer during the in-situ tensile deformation.Shockley dislocations present on twin boundaries pin the dislocations in Cu crystal grains;4)Dislocations pile-up at the interface will cause stress concentration and form micro cracks.In the bulk Cu/Nb,when the layer thickness of 200 nm or more,the fracture mode is inter-granular fracture dominated by dislocations pile-up at interfaces;When the layer thickness is between 200 nm and 100 nm,the fracture mode is trans-granular shear fracture dominated by dislocations pile-up at the phase boundary;Curved phase boundary in Cu layer accelerates the crack.