原位SEM研究LDMD Ti-6Al-4V合金β晶界对拉伸断裂行为的影响
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摘要
目的研究β晶界对激光直接熔化沉积(LDMD)Ti-6Al-4V合金裂纹形核或传播行为的影响,以澄清合金的断裂机制,为合金性能的改善提供理论依据。方法采用LDMD Ti-6Al-4V合金粉末,在Ti-6Al-4V基板上逐层堆积形成沉积层。沿沉积层扫描方向截取试样,在室温下观察样品的微观组织形貌,并对原位拉伸过程中的微观组织演化进行实时研究。同时研究β晶界对微裂纹萌生、扩展和断裂的影响行为,总结断裂机理。结果 LDMD Ti-6Al-4V合金组织宏观呈现出沿构造方向生长的粗大柱状β晶,β晶内由板条状α晶和整齐排列的具有相同生长取向的α簇组织组成,并有少量孔洞缺陷。采用原位扫描电镜拉伸样品时发现,在横向拉力作用下,样品最初在孔洞周围发生变形,之后裂纹的萌生扩展主要沿β晶界进行,β晶界对拉力起阻碍作用,造成样品的伸长率较低。拉伸过程中,微观组织主要沿着β晶界周围的α相变形,并且孔洞缺陷引起的应力集中使得缺陷周围变形最严重,变形方向与拉力方向呈45°。结论孔洞缺陷决定了样品的初始变形位置,而β晶界则决定了裂纹传播的方向,且由于拉伸试样的截取方向与β晶界相垂直,导致样品的伸长率较低,所以β晶界对样品的力学性能及断裂机理起决定作用。
The work aims to clarify the fracture mechanism of laser direct melting deposition(LDMD) Ti-6Al-4V alloy by studying the effect of β grain boundary on the nucleation or propagation behavior of cracks, and provide a theoretical basis for improving the mechanical properties of the alloy. The build was formed on Ti-6Al-4V substrate with LDMD Ti-6Al-4V alloy powder. Samples were taken along the scanning direction of the build. The microstructures of the sample and the real-time changes of the microstructure during in-situ tensile test were studied at room temperature. The effect of β grain boundary on crack initiation, propagation and fracture in tensile test was also investigated, and the fracture mechanism was summarized. The microstructure of the LDMD Ti-6Al-4V sample was composed of coarse columnar β grain that grew parallel to the direction of the build macroscopically. Within the β grain, plenty of lamellar α and α colonies with the same orientation in α plates were observed. Hole defects in a small amount were also identified. The tensile sample by in-situ SEM showed that under the transverse tensile load, the initial deformation of the sample was formed around the hole due to the stress concentration and the crack propagated along the β grain boundary. The β grain boundary hindered the tensile force and resulted in the lower elongation of the sample. During the tensile, the microstructure mainly deformed along phase α surrounding β grain boundary and the hole defects caused more serious deformation due to concentration of stress. There was an angle of 45° between the deformation direction and the tensile direction. The hole defect determines the initial deformation position of the sample, and the β grain boundary determines the direction of crack propagation. Since the direction of the tensile specimen taken is perpendicular to theβ grain boundary and the sample has a lower elongation, so β grain boundary plays an important role in mechanical properties and fracture mechanism of sample.
The work aims to clarify the fracture mechanism of laser direct melting deposition(LDMD) Ti-6Al-4V alloy by studying the effect of β grain boundary on the nucleation or propagation behavior of cracks, and provide a theoretical basis for improving the mechanical properties of the alloy. The build was formed on Ti-6Al-4V substrate with LDMD Ti-6Al-4V alloy powder. Samples were taken along the scanning direction of the build. The microstructures of the sample and the real-time changes of the microstructure during in-situ tensile test were studied at room temperature. The effect of β grain boundary on crack initiation, propagation and fracture in tensile test was also investigated, and the fracture mechanism was summarized. The microstructure of the LDMD Ti-6Al-4V sample was composed of coarse columnar β grain that grew parallel to the direction of the build macroscopically. Within the β grain, plenty of lamellar α and α colonies with the same orientation in α plates were observed. Hole defects in a small amount were also identified. The tensile sample by in-situ SEM showed that under the transverse tensile load, the initial deformation of the sample was formed around the hole due to the stress concentration and the crack propagated along the β grain boundary. The β grain boundary hindered the tensile force and resulted in the lower elongation of the sample. During the tensile, the microstructure mainly deformed along phase α surrounding β grain boundary and the hole defects caused more serious deformation due to concentration of stress. There was an angle of 45° between the deformation direction and the tensile direction. The hole defect determines the initial deformation position of the sample, and the β grain boundary determines the direction of crack propagation. Since the direction of the tensile specimen taken is perpendicular to theβ grain boundary and the sample has a lower elongation, so β grain boundary plays an important role in mechanical properties and fracture mechanism of sample.
引文
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[16]高晓龙,李海军,刘晶,等.低温对TC4脉冲激光焊接接头组织和性能的影响[J].材料科学与工程学报,2017,35(5):836-840.GAO Xiao-long,LI Hai-jun,LIU Jing,et al.Effect of low temperature on the microstructure and mechanical properties of pulsed Nd:YAG laser welding for TC4 titanium sheet[J].Journal of materials science&engineering,2017,35(5):836-840.
[17]小威廉·卡丽斯特,大卫?来斯威什.国外名校名著材料科学与工程基础[M].第四版.北京:化学工业出版社,2015.CALLISTER W D,RETHWISCH D G.Fundamentals of materials science and engineering[M].4th Edition.Beijing:Chemical Industry Press,2015.
[2]李永涛.钛合金激光增材制造缺陷研究[D].大连:大连理工大学,2017.LI Yong-tao.The study on defect formation in laser additive manufacturing titanium alloy[D].Dalian:Dalian University of Technology,2017.
[3]CARROLL B E,PALMER T A,BEESE A M.Anisotropic tensile behavior of Ti-6Al-4V components fabricated with directed energy deposition additive manufacturing[J].Acta materialia,2015,87:309-320.
[4]王琦.TC4钛合金的低温拉伸性能[J].材料工程,2009(3):54-55.WANG Qi.Tensile mechanical properties of TC4 alloy at243 K[J].Journal of materials engineering,2009(3):54-55.
[5]?KERFELDT P.Additive manufacturing of Ti-6Al-4V:Relationship between microstructure,defects and mechanical properties[D].Sweden:Lule?Tekniska Universitet,2016.
[6]?KERFELDT P,ANTTI M L,PEDERSON R.Influence of microstructure on mechanical properties of laser metal wire-deposited Ti-6Al-4V[J].Materials science and engineering:A,2016,674:428-437.
[7]章敏.送粉式和送丝式的钛合金激光增材制造特性研究[D].哈尔滨:哈尔滨工业大学,2013.ZHANG Min.Research on laser additive manufacturing characteristics of titanium alloy with powder and wire[D].Harbin:Harbin Institute of Technology,2013.
[8]王华明,张述泉,王向明.大型钛合金结构件激光直接制造的进展与挑战(邀请论文)[J].中国激光,2009,36(12):3204-3209.WANG Hua-ming,ZHANG Shu-quan,WANG Xiangming.Progress and challenges of laser direct manufacturing of large titanium structural components(invited paper)[J].Chinese journal of lasers,2009,36(12):3204-3209.
[9]KIM Y,SONG Y B,LEE S H.Microstructure and intermediate-temperature mechanical properties of powder metallurgy Ti-6Al-4V alloy prepared by the prealloyed approach[J].Journal of alloys and compounds,2015,637:234-241.
[10]夏麒帆,梁益龙,杨春林,等.TC4钛合金拉伸变形行为的研究[J/OL].稀有金属,2018[2019-04-19].http://kns.cnki.net/kcms/detail/11.2111.TF.20180521.1404.005.html.XIA Qi-fan,LIANG Yi-long,YANG Chun-lin,et al.Study on tensile deformation behavior of TC4 titanium alloy[J/OL].Chinese journal of rare metals,2018[2019-04-19].http://kns.cnki.net/kcms/detail/11.2111.TF.20180521.1404.005.html.
[11]LI P H,GUO W G,YUAN K B,et al.Effects of processing defects on the dynamic tensile mechanical behavior of laser-solid-formed Ti-6Al-4V[J].Materials characterization,2018,140:15-29.
[12]LU J X,CHANG L,WANG J,et al.In-situ investigation of the anisotropic mechanical properties of laser direct metal deposition Ti6Al4V alloy[J].Materials science and engineering:A,2018,712:199-205.
[13]REN Y M,LIN X,FU X,et al.Microstructure and deformation behavior of Ti-6Al-4V alloy by high-power laser solid forming[J].Acta materialia,2017,132:82-95.
[14]AL-BERMANI S S,BLACKMORE M L,ZHANG W,et al.The origin of microstructural diversity,texture,and mechanical properties in electron beam melted Ti-6Al-4V[J].Metallurgical and materials transactions A,2010,41(13):3422-3434.
[15]陈静,张霜银,薛蕾,等.激光快速成形Ti-6Al-4V合金力学性能[J].稀有金属材料与工程,2007,36(3):475-479.CHEN Jing,ZHANG Shuang-yin,XUE Lei,et al.Mechanical properties of Ti-6Al-4V alloy by laser rapid forming[J].Rare metal materials and engineering,2007,36(3):475-479.
[16]高晓龙,李海军,刘晶,等.低温对TC4脉冲激光焊接接头组织和性能的影响[J].材料科学与工程学报,2017,35(5):836-840.GAO Xiao-long,LI Hai-jun,LIU Jing,et al.Effect of low temperature on the microstructure and mechanical properties of pulsed Nd:YAG laser welding for TC4 titanium sheet[J].Journal of materials science&engineering,2017,35(5):836-840.
[17]小威廉·卡丽斯特,大卫?来斯威什.国外名校名著材料科学与工程基础[M].第四版.北京:化学工业出版社,2015.CALLISTER W D,RETHWISCH D G.Fundamentals of materials science and engineering[M].4th Edition.Beijing:Chemical Industry Press,2015.