Microstructures And Properties Of The Processed Mg-Y-Zn Alloys By Rolling And Rapid-solidification

Global effort in magnesium materials development has diversified in recent years, targeting new automotive,aerospace,and biomedical applications.The 1990's saw a concerted effort in the development of creep-resistant magnesium casting alloys for automotive power trains.These alloys are based on the addition of alkaline earth or rare earth alloying elements to Mg-Al system and provide adequate elevated temperature performance up to 175℃.The R&D efforts in this area resulted in the application of the AJ62(Mg-6Al-2Sr) alloy in BMW engine blocks and the AE44(Mg-4Al-4RE) in the GM engine cradle.Other new creep-resistant alloys are based on Mg-Sn-Ca,Mg-Sc-Ce and Mg-Gd-Nd.Scandium- and gadolinium-containing alloys,which offer high creep resistance,are candidates for aerospace engine applications.Mg-Zn-RE(where RE = rare earth elements) alloy systems offer a base for further development too.Early in 1970's,the Mg-Zn-Y-Zr system of casting alloys had been studied in China and it was found that a close relation between the microstructure and property of the alloy and the ratio of Y/Zn existed in the alloy system.Our nation had nine casting magnesium alloys early in 1980's,among them,six rare-earth element containing magnesium alloys had been developed by the Institute of Aerospace Materials for nearly 30 years since the P.R.C. had been founded.The rare-earth containing magnesium alloys are applied broadly in the aerospace industry.The ZM3 alloy was brought into application as early as in 1967;the ZM4 alloy,in which the Zn content is up to 2～3%,was developed.An Mg-Zn-Y-Zr alloy was acquired in 1980 and the Y element is it's main alloying element,at the same time,the ZM6 alloy with Nd as the main alloying element was developed too.Our nation kept ahead of other countries in the world in the R-D of the creep-resistant magnesium alloys then.Padezhnova E M et al.of the USSR found three ternary equilibrium phases in the 1980's,i.e.W-Mg₃Zn₃Y₂,Z-Mg₃Zn₆Y and X-Mg₁₂ZnY phases.They reported that the W-Mg₃Zn₃Y₂ phase belongs to an isostructure of AlMnCu₂.In recent years,Z.P.Luo et al. identified the Z- Mg₃Zn-6Y as a stable icosahedral quasicrystalline phase,and determined the X-Mg₁₂ZnY phase as an 18R type of long-period stacking(LPS) structure by an electron diffraction technique.Inoue A.et al.fabricated an Mg₉₇Zn₁Y₂(at.%,i.e.WZ73) alloy with excellent tensile yield strength above 600 MPa by rapid solidification(RS) techniques of powder metallurgy in 2001.Since then,the Mg-Zn(Cu,Ni)-Re(-Zr) alloys,which have a LPS phase as a secondary phase,has been studying as an elevated temperature creep resistant Mg-based alloy.The Mg₉₇Zn₁Y₂(at.%) is marked as WZ73 according to the standard of ASTM.The study by Inoue A.et al.shows that the property of RS/PM AZ91 alloy is lower than one of the WZ73 processed by same technology.The proceeded work about the analysis of microstruetures and strengthening mechanisms of the RS/PM and RS/melt-spun WZ73 alloys shows that the strengthening effect is mainly from the existing LPS phase.While,the examined hardness of the WZ73 alloy with LPS structure is lower than that of the commercial AZ91 alloy at room temperature according to our experimental data.The microstructure characteristics of the WZ73 alloy,processed by ingot metallurgy (IM)/rolling and annealing upon the conventionally cast alloy and by rapid solidification (RS)/melt-spinning and aging upon the ribbon,which have been not introduced by other papers systematically,will be generally illustrated by a series of schemed experiments in this paper.Among them,after examining the as-cast alloy through X-ray diffraction(XRD), general optical microscopy(OM),scanning electron microscopy(SEM) and differential thermal analysis(DTA) complexly,the evolutionary direction of microstructure prescribed by thermodynamics in the WZ73 alloy is reflected from the experimental data of the as-cast alloy;after analysizing the data including phase constitutions and textures from XRD and images from LOM,SEM and TEM,the actual evolution paths selected by kinetics are depicted in detail in the as-spun alloy and rolled alloy;by testing hardess on micro-hardness machine and macro-hardness machine and tensile property,the mechanical properties on an Instron-type machine are displayed and the analysis of fractures is attached.At the same time the microstructure of a conventional AZ91 alloy processed by melt-spinning technique and its evolution during aging and micro-hardness were obtained for comparison with those of the RS/melt-spun WZ73 alloy;the mechanical properties and fracture morphologies of the as-cast pure magnesium were examined too to analyze the deformation mechanism of the alloy with LPS phase during rolling and tensile test.The strong influences of thermodynamic non-equilibrium mechanism,which entails the factual complexity of microstructures typically during IM/rolling and annealing, RS/melt-spinning and ageing for strengthening the creep resistant magnesium alloy,are presented in this work.Thus a general characteristics of WZ73 with originality and important fundamental data is concluded in this thesis,they are,(1) The ternary compound LPS phase,as the secondary phase of the cast WZ73 alloy, mainly resulted from the eutectic transformation and precipitating out of the supersaturated a-Mg during cooling.(2) The yield tensile strength,ultimate tensile strength and percent elongation of the rolled Mg₉₇Zn₁Y₂ alloy is 282 and 298 MPa and 5.2%,which is higher than 85 and 154 MPa and 5.4%of the as-cast alloy respectively.In the rolled WZ73 alloy,a mixed fracture mode including brittle and ductile fracture patterns is different from the single brittle fracture in the as-cast alloy;the evolution of deformation mechanism during tensile test is from mainly dislocation induced strain with strengthening effect in the as-cast WZ73 alloy to dislocations sliding produced plastic strain in which the strengthening effect hardly existed in the rolled WZ73 alloy.(3) With increasing rolling reduction upon the cast alloy,a rolling texture of {0001} developed gradually and the LPS structure of X-Mg₁₂ZnY phase in the rolled alloy shrank and distributed along the rolling direction.A lot of fine particles precipitated in the alloy when the annealing temperature was up to 400℃.With the temperature rising,the amount of precipitates was gradually reduced.The second recrystallized grains ofα-Mg were obtained during annealing the rolled alloy at 500℃,and the grain size also is similar to the size of the dendritic grain in the as-cast alloy.With increasing annealing temperature from 350℃to 500℃,an annealing texture evolution proceeded before and aider second re-crystallization process finished and a microstructure evolution about ternary Mg-Y-Zn compound from a LPS phase to an L-type approximant of icosahedral quasicrystal to an icosahedral quasicrystal during rolling and annealing.(4) There was a cellular/dendritic transition during melt-spinning process of WZ73 alloy,and a mild age strengthening with the hardness of 113HV/0.05 at 300℃;more importantly,there was a different evolution of morphologies of the matrix and inter-metallic compounds from general secondary phase strengthened alloys when age temperatures up to the 500℃had been introduced to the as-spun alloy.Dendritic and cellular arrays evolved into grains; cellular grains,which structure is an L-type approximant of icosahedral quasicrystal different from the LPS structure,were transformed at 500℃at new grain boundaries.(5) The commercial AZ91 alloy undergoes a cellular/dendritic transition during melt-spinning too.A strengthening effect due to aging was observed:a maximum hardness of 110 HV/0.05 and an age-harden-ability of 50%were obtained when the ribbon was aged at 200℃.Theβ-Mg₁₇Al₁₂ phase exhibits net and dispersion types of distribution during precipitation.The dispersion of precipitates in dendritic grains or cells is the main source of strengthening.The cooling rate of the rapid solidification/melt-spinning upon the AZ91 alloy in this experiment is theoretically calculated to be 10⁵ to 10⁶℃/s.Resulting from the experiments and comparison and analysis in this paper,a prospect which is the applications of WZ73 alloy in structure parts processed by plastic deformation, sheet and extrusion materials,light-weight structure parts with elevated temperature performance,is expected;the research about Mg-Zn(Cu,Ni)-Re(-Zr) alloy with LPS phase and the develop about processing technology upon them are encouraged to proceed.