Study on the Microstructure and Mechanical Behaviors of Mg-RE Alloys Subjected to Friction Stir Processing

Author:Han Jing Yu

Supervisor:peng li ming chen juan


Degree Year:2018





Magnesium alloys with rare-earth elements addition,so called Mg-RE alloys in short,could achieve considerable high-temperature mechanical properties and corrosion-resistance,while the conventional advantages such as high specific strength are still reserved.Consequently,Mg-RE alloys are receiving a growing attention from both researchers and industry.As a newly developed severe plastic deformation(SPD)process,Friction Stir Processing(FSP)exerts high strain and strain rate to refine grains,to break and distribute particles,and to promote microstructure homogenization,without changing the dimensions of the workpieces.Although researchers have begun to apply FSP to Mg-RE alloys to optimize microstructure and improve mechanical properties,the evolution mechanism of microstructure and texture,as well as the effect of species and content of RE elements on which during FSP remain unclear.The efforts on the approach provides significant knowledge in choosing processing parameters and prolonging the service life of Mg-RE alloys.In this paper,we have developed six kinds of Mg-RE alloys of different composition,including light RE element containing Mg-Nd-Zn alloys,heavy RE element containing Mg-Gd and Mg-Gd-Zn alloys for friction stir processing.The grain structure,phase constitution and distribution,together with the texture of the parent material before and after FSP were systematically characterized.The mechanical properties of the stirring zone were also tested.In combination with the temperature field distribution,the effects of processing parameters,alloy composition and phase constitution on the discipline of microstructure evolution,as well as the corresponding strengthening and toughening mechanism within the stirring zone were carefully discussed.In addition,the influence of subsequent heat treatment is also considered.The aim is to accurately modify and control the microstructure on needs.The specific research work and main conclusions are presented as follows.The peak temperatures for the top layers of the stirring zone(SZ)were all close to600℃during FSP of Mg-3Nd-0.2Zn-0.4Zr(wt.%)alloys(NZ30)with various processing parameters.At the same time,the peak temperatures within central SZ were all about 520℃for the three processing groups,while the depth of central SZ changes with heat input.The increase of heat input expands the range of the stirring zone.The microstructure of the SZ consists of mainly singleαphase with the dissolution of Mg12Nd phase previously located at the grain boundaries in the parent materials(PM).Along the vertical direction on the cross section,the texture of the stirring zone changes from<0001>Mg||ND direction to<0001>Mg||PD direction.The basal planes tend to parallel with the outer surface of the stirring tool.FSP has eliminated the casting defects and significantly enhanced the plasticity of sand casting NZ30 alloy.The heavy rare earth element containing Mg-6 wt.%Gd(G6),Mg-10 wt.%Gd(G10)and Mg-14 wt.%Gd(G14)alloys were selected for FSP.No defects were found in either as cast or solution-treated conditions.When the processing parameters are the same,the temperature distribution of the middle layer,along with the bottom layer of the SZ is similar for both conditions.However,the top layer difference is quite obvious.Compared with the solution-treated parent material,the AC-FSP is featured for smaller grain size,higher HAB fraction and weaker average texture.The difference is attributed to the sub-micron residualβphase present at the grain boundaries.With the increase of Gd content,the average strength of the texture in SZ decreases significantly.In dilute Mg-Gd alloys,soft texture S transmits to hard texture P with the increase of heat input.The study found that the recrystallization mode is affected by the content of Gd during the FSP procedure,and the grain growth and reorientation occurs during the subsequent migration of the primary crystal formed by dynamic recrystallization.Segregation of Gd elements at grain boundaries causes abnormal strong texture in T4 condition.After low heat input FSP of the parent materials of Mg-14Gd-2.5Zn-0.4Zr(wt.%)alloy GZ142 in different heat treatment conditions,the grains are refined and the second phase particles are broken and evenly dispersed.Theβphase dendrite in the parent materials become refinedβ-(Mg,Zn)3Gd phase,and the X phase is broken into finer X phase near the grain boundaries.The 14H-LPSO structure in the crystal has been retained in the DRXed grains.In summary,the types of the second phase will not change with low heat input.In contrast,high heat input FSP can change the X phase intoβphase.During FSP procedure,the grain refinement and the second phase breaking occur even before the stirring tool arrives.The deformed materials will be deposited to their eventual positions after being stirred within the"transport zone".GZ142 alloy in different conditions generally develop weak textures after FSP,Considering that the texture strength of GZ102.5 alloy with relatively lower Gd content is also weak,the existence of a large number of second phases has changed the way of dynamic recrystallization of alloys,promoting the weakening of texture during deformation.The as-cast and LT4 treated GZ142 alloys are mainly refined by CDRX,while the PM HT4 condition alloy is mainly refined by CDRX and GDRX.The YS and UTS of the SZ in GZ142 alloys have been greatly improved after FSP.The strengthening effect is mainly provided by refined grains in combination with the broken particles.The refined grain boundary LPSO structure has lost the coherent relationship with the Mg lattice,consequently leads to the weakening of the ability to coordinate the deformation.The yield point phenomenon during the tensile test of the bottom sample is ascribed to the seducement of strain hardening effect resulted from grain refinement.The contribution of twinning to yield stress in Mg-Gd-Zn alloys could be negligible.A certain type of microstructure featured by small grains filled with LPSO lamellas has been successfully prepared and characterized.The results indicate that the short solution heat treatment after FSP could lead to the formation of LPSO lamellae only within the interior of the grains.The lamellae has a 14H type with a relatively weak texture.The structure has demonstrated excellent ductility.