Formation Of Nano-scale Low Energy Boundary Structures And Related Property Exploration In Al And Its Alloys

Refining grains of metals into the nanometer scale may greatly elevate their strength and hardness.But the introduced high density of grain boundaries(GBs)provides a strong driving force for grain boundary migration and grain coarsening under thermal and mechanical stimuli.The intrinsic instability of nanograined metals against grain coarsening is believed to be the 'Achilles' heel',which not only deteriorates their property but also makes further grain refinement difficult to process.These phenomenon is particularly significant in Al and its alloy whose melting points are low.Nano-sized grains in Al are thermally unstable with a much tendency of coarsening even at room temperature.The present study aims to introduce nano-scale low energy boundaries in Al and its alloy and investigate the distribution of solutes and its effect on the formation and stability of the low energy boundaries.In this work,the minimum structural size and microstructural characteristics of pure Al processed with three different deformation techniques,i.e.,cold rolling,dynamic plastic deformation and surface mechanical grinding treatment(SMGT),have been investigated by means of transmission electron microscope and orientation mapping.The effects of deformation parameters on structural refinement was studied.The distribution of Cu and Mg in nanograined Al alloy as well as their effect on the formation and properties of low energy boundary structures were investigated.The main results are as follows:The microstructures of pure Al produced by means of cold rolling and dynamic plastic deformation at large strains exhibit lamellar morphology with a minimum structural size of 250 nm and 180 nm,respectively.Quantitative analysis of the boundary characters shows that the grain boundaries of cold rolling sample are mainly high angle GBs with a fraction of 75%,while dynamic plastic deformation samples are mainly of low angle GBs with a fraction of 80%.This difference could be attributed to the combined effect of low deformation temperature and high strain gradient of dynamic plastic deformation on the suppression of thermally activated GB migration and the acceleration of dislocation density storage in the form of low angle GBs.The Al sample prepared with SMGT exhibit a nanolaminated structures with a transversal size of about 68 nm and a large fraction of low angle GBs(?65.5%).The hardness of nanolaminated Al is as high as 780 MPa.A gradient nanostructured surface layer was synthesized on an Al-4Cu alloy sample by means of SMGT at liquid nitrogen temperature.Within the deformed surface layer,laminated structures with a wide range of thickness are formed.With a decreasing depth from the treated surface,lamellae thickness decreases accompanied by an increased fraction of high angle grain boundaries from 10%to 70%.In the topmost surface layer of the treated sample,nanolaminated structures with an average lamellae thickness of 28 nm are formed with a strong shear texture,i.e.,{111}<112>.Deformation twinning was frequently observed inside the nanolaminates.Composition analysis indicated that Cu atoms segregate at nanolaminates boundaries in the as-prepared sample,Cu concentration is about 3-4 times higher than that in the lattice.The obvious GB segregation of Cu induced by cryogenic plastic deformation is attributed dynamic interaction between solute atoms with gliding dislocations.GB segregation of Cu is responsible for the stabilization of the nanolaminated structures with a much finer structural size than that in pure Al,resulting in higher hardness of 2570 MPa.The Hall-Petch slope of the nanostructured Al-4Cu sample is much higher than that in the pure Al,indicating a stronger boundary resistance to dislocation glide in the nanostructured Al-4Cu alloy.The deformation-induced GB segregation provides an alternative strategy to achieving stable high strength nanostructures in Al alloys.By means of SMGT at liquid nitrogen temperature,extremely high density of relaxed grain boundaries depleted with Mg were generated in an Al-5Mg alloy.It was found that Mg exhibited a completely different segregation behavior from Cu in nanograined Al.For the submicro-grains,no difference in Mg concentration was detected inside grains and at grain boundaries.However,the concentration of Mg at GBs decreases evidently as the grain size decreases to the nano-scale.For the nanograins of 31 nm,Mg concentration is about one half of that in the lattice.The discovered Mg depletion at grain boundary is originated from a fundamental change in structure of relaxed GBs in metals in the nanometer scale.The relaxed low-? GBs make it thermodynamically difficult for Mg to segregation at the GB;the high compressive strain may repel Mg atoms from the relaxed low-? GBs;and then,Mg atoms can be carried by dislocations away from the relaxed low-? GBs.The nanostructured Al alloy exhibits an ultrahigh hardness of 2.9 GPa,harder than the high-strength Al alloys such as 7075 and 2024.The coarsening temperature of the 31 nm grains is 488 K,80-95 K higher than that of submicro-grains.Meanwhile,its corrosion rate in 3.5 wt%NaCl solution is only a fraction of these hard Al alloys,without detectable localized corrosion due to the suppressing of precipitation at low-? GBs during aging.