Synthesis,Mechanical Properties And Thermal Stability Of Spinodoid Nanomaterials

The introduction of high-density interfaces in nanostructured materials results in greatly improved strength and hardness.However,grain refinement towards the nanometer scale leads to two intrinsic defects:ⅰ)High-density interfaces strengthen materials at the expense of loss of ductility,for the reason that extremely fine grains lack the ability to work-hardening and thus are not effective in accommodating plastic strain.ⅱ)The nano-grains are thermally unstable and have an increased tendency to structure coarsening via interface migration,driven by the high-density grain boundaries and triple junctions.Coherent interfaces are stable against structural coarsening due to the quite low excess surface energy,meanwhile,act as effective barriers to dislocation transmission across the interfaces.The three-dimensional minimal-interface structures in polycrystalline copper with extremely fine grains make it stable against grain coarsening and atomic diffusion.Spinodal structures,consisting of two interwoven nanophases with large-scale crystal coherency and periodically modulated composition,combine the structural features of coherent interface and zero-mean-curvature 3D bi-continuous topology.However,its viable alloy systems and the range of composition modulation are very limited.In the present thesis,a similar "spinodoid" structure was formed by selective etching and electrochemical refilling instead,operating in undecomposable alloys.The influences of the 3D bi-continuous(semi-)coherent interfaces on mechanical property and thermal stability were examined.The main results are summarized as follows:1.The synthesis of spinodoid alloys via electrochemical dealloying and refilling.(1)The electrodeposition of metals on the highly curved surfaces of ligaments in nanoporous Au differs from the case of planar substrate.Two typical refilling morphologies were observed when the electrodeposition was carried out in nanoporous Au:ⅰ)"Nucleation and growth".Deposited Cu forms full-density island-like clusters which grow via the progression of deposition front in space.Deposited Ag shows the same refilling morphology.ⅱ)"Conformal coating".Deposited Ni covers all the surfaces of ligaments,resulting in a uniform Ni coating which thickens with more deposited Ni.(2)Full-density bulk materials can be realized by the "nucleation and growth"-type electrodeposition.The nucleation potential of Cu deposited on the highly curved ligament surfaces is more positive than that on an Au planar substrate,and the positive-shift is more pronounced as the ligament sizes decrease to nanoscale.The size of island-like cluster of deposited Cu increases with decreasing current density(at more positive potential),which is in favor of skipping-free refilling of nanoporous Au.2.The microstructures of spinodoid alloys.The main features of spinodoid alloys are summarized as follows:ⅰ)a bi-continuous dual-phase structure;ⅱ)the cube-on-cube orientation relationship between two phases;ⅲ)the large-scale crystal coherency,i.e.,large grain size(of both phases);ⅳ)nanoscale feature size;ⅴ)3D gyroid-like interfaces,certainly including regions with minimal interfaces of near-zero mean curvature;ⅵ)the coherent or semi-coherent interface,depending on lattice mismatch,where Ag/Au surface(S(?)0.2%)is coherent but Cu/Au surface(δ(?)11.5%)is semi-coherent;ⅶ)square-modulated composition,which can be further tuned by post-annealing;ⅷ)tunable feature size,grain size and phase volume fraction,depending on dealloying precursor.3.The mechanical properties of Cu/Au spinodoid alloys.(1)The strength of Cu/Au spinodoid alloys increases continuously with decreasing fea ture sizes(λ),showing no tendency of level-off or softening,and approaches theoretical strength at λ=5 nm.The critical stress required for the bow-out of misfit dislocation segments pinned at the Cu/Au interface defines upper-limit strength,which is approximately 1.40 GPa at λ(?)5 nm.(2)The deformation behavior of Cu/Au spinodoid alloys undergoes a composite-like to single-phase-like transition with decreasing λ.In high-λ samples(>50 nm),dislocations in Cu matrix can bow out easily between two Au ligaments at a low stress,and thus two phases deform on their own as in other composites.In low-λ samples(<50 nm),the slip of either perfect dislocations or partial dislocations(twinning)across the Cu/Au interfaces lead to the formation of micro deformation bands,which run across entire grains and terminate at grain boundaries.Cu/Au spinodoid alloys show an evident grain-size effect on strength.(3)Cu/Au spinodoid alloy demonstrates superior mechanical properties such as near theoretical strength and single-phase-like behavior owing to:ⅰ)the fine composition modulation,which leads to the differences between two phases across the interface in lattice parameters,shear modulus,and stacking fault energy,and the generation of a 2D array of residual dislocation loops on the plane of dislocation glide;ⅱ)the large-scale coherence of crystal lattice,which effectively hinder dislocation motion but provide room for dislocation accumulation and storage;ⅲ)the smoothly shaped three-dimensional interface morphology,which suppress interface sliding and migration and resultant softening.4.The thermal stability of Cu/Au and Ag/Au spinodoid alloys.(1)The hardness and strength of both Cu/Au and Ag/Au spinodoid alloys do not decrease monotonically,but increase and then decrease with annealing time.Such transient hardening is more pronounced in Cu/Au then in Ag/Au.The incremental hardness at the very early stage of annealing(t=30 s)is inversely proportional to the square root of λ,suggesting that the annealing-induced transient hardening is related to the barrier strength of interfaces.The transient hardening is more pronounced in samples with a coarser λ,resulted from the increase of barrier strength in more-diffuse interfaces at longer annealing time.(2)The bi-continuous dual-phase nanostructure is stable against coarsening,but transforms into more-stable single-phase solid solutions during annealing.The transformation of Cu/Au is initiated by the intermixing at all Cu/Au interfaces,followed by a recrystallization-like discontinuous reaction from grain boundaries.Unlike Cu/Au,the transformation of Ag/Au is dominated by interfacial intermixing.The lattice mismatch and interface structure are decisive to their annealing behavior.(3)Transient hardening is associated with the formation of a thick diffuse interface between two nanophases.Solute hardening alone cannot account for this observation.Instead,the increasing barrier strength of interface to dislocation transmission might have led to the hardening of Cu/Au and Ag/Au spinodoid alloys.What’s more,the dissociation of misfit dislocations during annealing partly explains the transient hardening of Cu/Au.