Mechanical Properties And Deformation Mechanism Of Single Crystal Nano-twin Metals

Nano-twinned materials have attracted extensive interests over the past decades,due to their unique structure and performance relationship.Previous studies have demonstrated that the mechanical properties and deformation mechanisms of nano-twinned metals are related to the microstructural parameters including twin spacing,grain size,and twin boundary(TB)orientation.However,these results are based on polycrystalline nano-twinned samples,and consequently,it remains rather elusive how the grain boundaries may affect the mechanical behavior,due to the lack of mechanical tests on single-crystal nanotwinned samples.Carrying out research on the mechanical behavior of single-crystal nano-twinned metals and separating the mechanical contributions of twin boundaries(TBs)and grain boundaries are the key to a better understanding of the excellent mechanical properties of nano twins.In this work,liquid nitrogen temperature dynamic plastic deformation(LNT-DPD)technology was used to treat Cu-Al alloys with low stacking fault energy to obtain coarse-grained nano-twinned Cu-Al alloy samples.Microscale single crystal nanotwinned samples were then cut through a femtosecond laser micromachining platform,and tested on micro-scale mechanical testing apparatus to study the mechanical behavior of nanotwins in different twin boundary orientations.Furthermore,scanning electron microscope(SEM)and backscattered electron diffraction(EBSD)were employed to study the underlying deformation mechanisms.The main research conclusions are as follows:(1)After the Cu-Al alloy was treated with LNT-DPD,a high-density twinned structure was obtained due to the low stacking fault energy.The size of a single crystal grain is 1.5mm and enough to meet the requirements of micro-scale tensile testing.(2)When the tensile direction is parallel to the twin boundaries,the deformation mechanism of single-crystal nano twins is the slipping across the twin boundaries of threading dislocations.The threading dislocations first nucleate and move forward in the wide twin layer or matrix,and then the screw dislocations deposited on the twin boundary cross slip to the twin boundary and decompose into two partial dislocations.The partial dislocations can further constrained to a full screw dislocation and cross slip again into the slip plane in the neighboring twin or matrix lamellae.Repeating this process,the threading dislocations eventually pass through all the twin layers and contribute to plastic deformation.(3)When the tensile direction is parallel to the twin boundary,the strength of the material has a linear relationship with the reciprocal of the twin spacing.As the twin spacing continues to decrease,the sample shows higher and higher strength.Because the threading dislocations bulge out and slip in the form of Orowan dislocations under the constraints of adjacent twin boundaries,the external force required for slipping is inversely proportional to the twin spacing.(4)When the tensile direction is inclined to the twin boundary,the plastic deformation mechanism changes to dislocation slip along the twin boundary.The yield strength of the single crystal nanotwinned sample is higher than that of the single crystal sample without twins,and at the same time it shows more obvious work hardening and tensile plasticity,compared with those of tensile tests parallel to twin TBs.(5)When the tensile direction is parallel to the twin boundary,along with the increase of material strength,the tensile ductiity is reduced to a certain extent.Due to the absence of grain boundaries,single-crystal nano twins no longer fracture through the plastic dimple nucleationgrowth mechanism.Compared with polycrystalline samples,single crystal nano twin samples have higher uniform plastic deformation ability.