Stability Of Nanograined Metals Prepared By Using Plastic Deformation

Strength and hardness of metals increase significantly with a dec rease of grain size.As grain sizes become smaller and smaller,more and more grain boundaries are introduced into the material,which strongly hinder dislocation motion and make the material more difficult to deform.After decades of intensive study of nanograined(or sometimes called "nanocrystalline")materials,it is realized that in addition to strengthening materials,the high density grain boundaries also provide a strong driving force for grain boundary migration accompanied by property degradation when the materials are subject to thermal or mechanical stimuli.Grain growth in nanograined materials occurs at much lower temperatures than their coarse-grained counterparts,thus losing their excellent properties at relative low temperatures.In some nanograined metals,such as Cu,Ag,Al,grain coarsening occurs even at ambient temperature.Mechanically-induced grain growth is also easy to occur during plastic deformation of nanograined metals,such as tension,compression,torsion,and fatigue,which causes strain localization and early necking of nanograined metals.It also makes it difficult to refine grain sizes below 100 nm by means of plastic deformation.The intrinsic thermal and mechanical instability of nanograined materials becomes their Achilles heels,limiting their preparation and technological applications.In this work,.the grain size dependence of thermal stability and mechanical stability of several pure metals,including Cu,Ni,Ag,are investigated by using gradient nanograined(GNG)samples.The GNG samples were prepared by using surface mechanical grinding treatment(SMGT).The thermal stability of nanograins or ultrafine grains in Cu and Ni with different average grain sizes during isothermal annealing is systematically studied by comparing microstructures of the samples before and after annealing.The mechanical stability of nanograins in Cu,Ni,Ag are analyzed by comparing microstructures of the samples before and after tension.The main results are as follows:GNG structures are obtained in the surface layer of Cu samples treated by using SMGT.A series of isothermal annealing experiments show that the apparent grain coarsening temperature of nanograined copper decreased with a decrease of grain size firstly,which is consistent with the trend of "smaller less stable "as usually observed in nanograined materials.But below a critical size(70-80 nm for copper),thermal stability of nanograined copper is greatly enhanced.The apparent grain coarsening temperature rises substantially at smaller grain sizes.Extremely fine nanograins in the top surface layer keep stable even up to 623 K,which is higher than the recrystallization temperature of deformed coarse grains.The differential scanning calorimetry(DSC)measurements and TEM observations indicated that the inherent thermal stability of the nanograins below the critical size originates from an autonomous grain boundary evolution to low-energy states via activation of partial dislocations in plastic deformation processing.The average grain boundary energy decreases rapidly when the average grain size is smaller than?100 nm.For nanograins with an average size of 50 nm,the grain boundary energy is about 0.23-0.27 mJ/m2,about half of the normal grain boundary energy in copper.TEM observations of the GNG samples showed that the fraction of grains containing twins or stacking faults in the smaller nanograins in the top surface layer is much higher than that in the grains in deeper layers.A similar trend of thermal stability dependence on grain size is found in pure Ni with a higher stacking fault energy.The apparent grain coarsening temperature can be as high as 1173 K for the nanograins in the top surface layer in the GNG sample,which is even higher than that of some wrought superalloys.Grain size dependences of mechanically-induced grain boundary migration in nanograined Cu,Ni,and Ag under tension are investigated quantitatively in a wide size range.Tensile tests of gradient nanograined Cu are carried out at 300 K and 77 K,respectively.Microstructures of the as-prepared samples and the uniform deformation section of samples after tension were analyzed comparatively.It is found that mechanical stability of nanograined copper prepared by using SMGT shows a turning point at about 75 nm.Above this size,the grain coarsening amplitude increases with a decrease of grain size at room temperature.While below this size,the grain coarsening amplitude drops with a decrease of grain size.The abnormal mechanical stability of nanograins below the critical size is related to the strain-induced relaxation of grain boundaries during preparation,which leads to formation of low-energy and faceted grain boundaries.The suppression of the mechanically induced grain boundary migration with grain boundary relaxation below a critical size is attributed to inhibition of full dislocations and nucleation of partial dislocations to form twins or stacking faults in the nanograins.The smaller the grain size is,the smaller the grain growth is.At 77 K,the partial dislocation motion is more significant,and the grain growth is smaller.Similar grain size-dependent mechanical stability is verified in gradient nanograined pure Ag and Ni prepared by using SMGT.The GB migration peaks at about 80 and 38 nm in Ag and Ni,respectively.The tensile test results also show that work hardening rate of gradient grained Cu at 77 K is almost identical to that of the coarse-grained Cu.The main reasons for the enhanced work hardening at cryogenic temperatures include that the mechanical-induced grain growth behavior of nanograins in the surface layer is inhibited to a small extent at cryogenic temperature as well as the high stress level at low temperatures makes it easier for nanograins to initiate deformation twinning.This provides a certain extent of work hardening,which inhibits strain softening of surface gradient nanograins during tensile process at 77 K.The overall gradient nanograined copper sample shows almost the same work hardening rate as the coarse-grained copper.The discovery of the marked thermal stability and mechanical stability of very fine nanograins in pure metals is important for understanding the nature of GBs at the nanoscale and their response to external thermal and mechanical stimuli.Thus,structures of metals could be further refined and stabilized below the critical sizes with plastic deformation.Stable nanostructured metals and alloys for high-temperature applications can be expected.