Mechanical Behaviors of Nanostructured FCC Metals with Low Stacking Fault Energy

Author:Li Jian Guo

Supervisor:li yu long suo tao


Degree Year:2018





The properties of metals produced with the traditional methods have gradually not met the requirements of the quickly developing engineerings.However,the nanocrystalline(NC)metals present some excellent mechanical properties,such as high strength,improved good ductility,severe plastic deformation(SPD)methods are used to fabricate the bulk nanostructured(NS)metals or alloys,but the NS materials always show little or even no ability to deform plastically due to the introduction of high density of dislocations during the SPD process.In order to produce the high-performance NS methals with strength-plasticity matching,scholars propose a number of improving methods,one of them is tailoring the stacking fault energy(SFE)of metals to creat numerous deformation twins in the nanocrystalline grains by SPD.The enhancement of dislocation storage ability can effectively increase the plasticity of NS metals.Nevertheless,the mechanical properties of NS alloys with low SFE were seldom systematically investigated except for the tensile properties under quasi-static loading.The results show that the uniform elongation rate definitely increases with the decreasing SFE of metals,but due to the lack of experimental exploration the deformation and fracture mechanisms still is unknown,especially no reports on the mechanical behaviors under different loading conditions,for example,the extreme conditions like high strain rate and low temperature systematically investigate the mechanical properties and deformation behavior not only helps us interpret the deformation and failure mechanisms in depth but also guide us to further improve the comprehensive performance of NS metals。In the present work,the solid solution copper aluminum(Cu-Al)alloy with face-centered cubic(FCC)structure is used.The addition of Al atoms can effectively reduce the SFE of alloys.The equal channel angular pressing(ECAP)method is employed to produce the NS Cu-Al alloy,the minimum average grain size can reach to about 90nm.Then mechanical behaviors of the different SFE NS Cu-Al alloys are comprehensively studied at a wide range of strain rates and temperatures by using electric universal materials tester and split Hopkinson pressure bar.Transmitted electron microscope(TEM)is adopted to observe the microstructure feature before and after deformation and make a comparison to thoroughly understand the deformation mechanisms.On the basic of compressive experiment results the deformation kinematics factors including strain rate sensitivity(SRS)and temperature sensitivity are analysed and the strain rate jump tests are also conducted under quasi-static loading to calculate the SRS fator.Meanwhile,based on the thermal activity theory the corresponding thermal active volume is measured.Because different deformation mechanisms correspond to a different thermal active volume we can use the calculation results to qualitatively discuss the controlling mechanisms during the deformation.It is worthy noting that under high strain rate loading a transition from uniform deformation to localized shearing deformation occurs in the NS Cu-Al alloys when the SFE becomes lower.The microscopy observations verify the formation of adiabatic shear bands(ASB).The thermal plastic instability results in the abrupt stress collapse on the stress-strain curves.This is the first report on the ASB in nanocrystalline metals with FCC structure under dynamic uni-axial compression.The micro morphology of the ASB is observed by using optical microscope(OM)and scanning electron microscope(SEM).After that,the microstructure characteristics inside and outside the ASB are compared through TEM observations and nano-indentation tester is operated to explore the different mechanical properties caused by the microstructural differences.In order to better understand the formation process of the ASBs in the NS Cu-Al with the lower SFE,this work firstly explains the formation reasons of the ASBs from the mechanical side,and then in the perspective of materials science the underline mechanisms are discussed.The experimental results indicate that in the crystalline materials after SPD texture should be the main factor which contributes to the localized shearing deformation.Therefore,based on the crystalline placticity finite element method(CPFEM)and the adiabatic temperature rise during the high strain rate loading is considered at the same time,a themo-mechanics coupling CPFEM polycrystalline model is built to verify the influence of texture components on the formation of ASBs in the NS Cu-Al alloys with the low SFE.Finally,with regard to the problem of low ductility in the NS metals or alloys after SPD,the tension fracture behaviors of the NS Cu-Al alloy with the lowest SFE are studied in different loading directions to consider the effects of texture.The mechanical curves and microstructural observations tell us the controlling mechanisms which can lead to the early softening behavior and even failure.Though the plastic performance still is not satisfied now,this work has already studied the effects of texture and micro-voids on the fracture under tension tests.Weakening the intensity of texture and avoiding the emergence of micro-voids during materials processing may be effective to improve the plastic ductility of low SFE NS alloys.To sum up,the conclusions can be shown as follows:(1)The refinement of grain size can significantly enhance the strength of NS Cu-Al alloy.Under quasi-static loading,the yield strength of the NS Cu-Al alloy with the lowest SFE can arrive at higher than 750MPa.At a high strain rate the flow strength is over 1.1GPa.It is interesting to note that after SPD the medium/high SFE metals always present nearly perfectly elastic-plastic deformation behavior,but in this work,a large number of plain defects like deformation twins and satcking faults appear in the grains of NS Cu-Al alloys when the SFE value becomes lower.The microstructural evolution makes the NS alloys show different behaviors observed from the stress-strain curves:under quasi-static laoding,NS Cu-6.87at.%Al(?SFE=21mJ/m2)strats to display an apparent hardening behavior at the early stage of plastic deformation and then it disappears when the true strain is larger than a certain strain.For NS Cu-11.14 at.%Al alloy,a continuous strain hardening behavior appears even at a large deformation.Compare the before-and-after microstructural features,it can be observed that in medium/high SFE alloys the dislocations emission and interactions at or near the grain boundaries(GBs)should be the main controlling deformation mechanisms.However,when the SFE decreases the appearance of deformation twins and stacking faults can contribute to accomadate the plastic deformation.It is clear that the interfaces between the matrix and deformation twins or stacking faults can inhibit the movement of dislocations to enhance the storage ability in the grains.As such it improves the strain hardening ability during the plasticity.But TEM observations and active volume calculations reveal that the emission and movement of partial dislocations at the GBs accomadate the plastic deformation.(2)Under high strain rate loading,the decrease of SFE causes the occurrence of a transition from uniform plastic deformation to localized shearing instability.The microstructural evolution makes the mechanical properties change apparently.From the point of mechanis,at a high strain rate the enhanced strength and temperature sensitivity of NS Cu-Al alloys,the increased but still relatively low SRS together lead to the formation of ASBs in NS FCC alloys with low SFE.In the perspectives of materials science,the anisotropic adiabatic shearing behaviors under dynamic uni-axial compression indicate that micro-texture plays an important role during the formation of ASBs.X-ray diffraction measurement shows that after SPD the texture components in the NS Cu-Al alloys mainly includes Goss texture({110}<001>),Brass texture({110}<112>)and the rotated Goss texture results from the transition from{110}<112>to{110}<001>through the connection of fiber({110}<uvw>).The pre-existence of strong textures is favor for the beginning of localized plastic deformation.(3)The CPFEM models verify the contribution of Goss texture which dominates in the NS Cu-Al alloy with low SFE to the formation of ASBs.The simulation stress-strain curves show good agreement with the experimental results.Based on the varation of macroscopic mechanical responses the formation of ASBs can be described in several stages.Moreover,in terms of effects of several typical textures on the formation of ASBs for FCC metals,the Cubic texture({100}<001>)is effective to restrict the occurrence of ASBs,in the other textures,the appearance of ASBs is related to the number of slip systems that can be actived easily,the less can more easily lead to the initiation of localized deformation.(4)The strong texture after SPD in the NS Cu-Al alloy can significantly affect the tension fracture behaviors,including shearing failure and typical tension fracture.Meanwhile,the fracture mechanisms are strongly dependent on the loading rate.Under quasi-static tension,the initiation and growth of micro-voids are the main reason for the premature softening and failure in the NS Cu-Al alloy.However,at a high strain rate the adiabatic shearing deformation which is caused by the heat generation inside a localized shearing zone is responsible for the sudden failure.