摘要
对热挤压+峰时效后的Mg-8.78Gd-2.82Y-0.47Zr(%,质量分数)合金进行表面机械研磨处理(SMAT),并分析其显微组织、力学性能和断裂方式等。结果表明,经SMAT后合金表面形成了梯度结构,可分为剧烈变形层、弱变形层和未受应变影响的基体层。SMAT使得表面亚结构数量增加,细晶粒呈现随机取向。SMAT后合金抗拉强度进一步上升,达449 MPa,延伸率为3.04%。强度的提升主要归因于表面变形层的晶粒细化和位错密度的增加。断裂形貌分析表明,SMAT后剧烈变形层的断口形貌以脆性特征为主,而靠近中心的弱变形层存在韧性和准解理断裂的混合特征。SMAT态合金较低的延伸率可能与剧烈变形层的脆性开裂有关。
Magnesium alloys had wide application prospects because of their advantages of low density and high specific strength.However,the weak deformability and poor mechanical properties of magnesium alloys limited their further development of Mg alloys.The addition of rare elements such as Gd and Y could improve the mechanical properties of the alloys through solid solution strengthening and precipitation strengthening;and the addition of Zr could refine the grains of magnesium alloys and improve the tensile properties at room temperature.Mg-Gd-Y-Zr alloy showed high tensile strength and elongation attributed to the solid solution strengthening and precipitation strengthening.The alloy after hot extrusion usually exhibited a uniform and fine grain structure;the mechanical properties could be further improved with subsequent peak-aging treatment.Surface mechanical attrition treatment(SMAT)was carried out for alloys to further improve and the mechanical properties.The surfaces of alloy samples were impacted by steel balls at room temperature,the severely deformation of the surface could produce ultra-fine grains varied from nanometer to submicrometer.A gradient structure was obtained from the surface to the center of the sample after SMAT.Gradient structures showed synergistic strengthening effect,and usually had a good combination of strength and ductility superior to that of traditional homogeneous materials.SMAT was performed on the Mg-8.78Gd-2.82Y-0.47Zr(%,mass fraction)alloy after hot extrusion and peak aging(PA).Microstructures were analyzed by optical microscope(OM)and transmission electron microscope(TEM),and the mechanical properties were tested by nanoindentation tests and tensile tests at room temperature.The contribution of yield strength from each layer of the gradient structure was estimated according to different strengthening mechanisms.The fractography after tension was analyzed by scanning electron microscope(SEM).The results showed that a gradient structure was formed on the surface after SMAT,which can be divided into severely deformed layer of 0~300μm depth from surface,less deformed layer of 300~600μm,and the non-deformed matrix layer deeper than 600μm.The nanoindentation hardness of the SMATed alloy also distributed gradiently,which decreased with the increase of the depth from the surface.The highest nanoindentation hardness was tested at 30μm depth from the surface,which was 2.31 GPa.The average hardness of severely deformed layer and less deformed layer were about 2.13 and 1.94 GPa,respectively.After PA+SMAT,coarse grains were divided into fine grain structure with different orientations in the severely deformed layer,with an average size of 233.8 nm.The dislocation density near surface increased after SMAT,and lattice distortion and dislocation arrays were observed.The dislocation density of severely deformed layer was estimated through high resolution TEM(HRTEM)and inverse fast Fourier transform(IFFT)image,which was about 7.09×10^(14) m^(-2).The secondary phases in the alloy didn't change after SMAT,and the precipitates wereβ'phases.The ultimate tensile strength,yield strength and engineering strain of the hot extruded+peak aged alloy was 414.5 MPa,315.2 MPa and 4.58%.After SMAT,the ultimate tensile strength and yield strength of the alloy increased to 449.0 and 388.9 MPa,while the elongation was 3.04%.The increment of yield strength chouldcaculated as 73.7 MPa.The increment of yield strength provided by the grain boundary strengthening and dislocation strengthening in the severely deformed layer was estimated respectively.The strength increment was mainly attributed to the grain refinement and increasing of dislocation density of the severely deformed layer.Fractography showed that the fracture morphology of the severely deformed layer was mainly brittle fracture,while the less deformed layer near the center showed mixed characteristics of ductile and quasi-cleavage fracture.The relatively low elongation of the SMATed sample might be related to the brittle fracture of the severely deformed layer.The gradient structure formed by SMAT could effectively strengthen the magnesium alloy,but it was difficult to improve the plasticity at the same time.This might be related to the lack of dislocation slip systems and the poor deformation ability of the hexagonal close-packed lattice of magnesium alloy.In the future,further research should be done to solve the problem of strength-plasticity balance of magnesium alloys.
作者
苑潇逸
宁江利
陶世洁
徐博
Yuan Xiaoyi;Ning Jiangli;Tao Shijie;Xu Bo(Department of Metal Materials and Processing Engineering,College of Metallurgy and Energy,North China University of Science and Technology,Tangshan 063210,China;Key Laboratory of Modern Metallurgy of the Ministry of Education,Tangshan 063210,China)
出处
《稀有金属》
EI
CAS
CSCD
北大核心
2022年第12期1546-1555,共10页
Chinese Journal of Rare Metals
基金
国家国防科技工业局国防基础科研项目(JCKY2018407C008)
华北理工大学杰出青年基金项目(JQ201702)资助
作者简介
苑潇逸(2000-),男,河北衡水人,硕士研究生,研究方向:金属材料的强韧化,E-mail:2327795303@qq.com;通信作者:宁江利,教授,电话:0315-8805420,E-mail:ningjiangli@ncst.edu.cn。
