High-strain-rate superplasticity in an AZ91 magnesium alloy processed byingot metallurgy rout

Materials Transactions,V ol.43,No.1(2002)pp.78to80c 2002The Japan Institute of MetalsRAPID PUBLICATIONHigh-Strain-Rate Superplasticity in an AZ91Magnesium Alloy Processed by Ingot Metallurgy RouteHiroyuki Watanabe1,Toshiji Mukai1,Koichi Ishikawa1and Kenji Higashi21Osaka Municipal Technical Research Institute,Osaka536-8553,Japan2Department of Metallurgy and Materials Science,Osaka Prefecture University,Sakai599-8531,Japan High-strain-rate superplastic magnesium alloy,AZ91,was processed through the ingot metallurgy route.The relationship between working temperature and resulting grain size indicated that the grain size tends to decrease with decreasing working temperature in AZ91. Based on this preliminary result,the ingot was hot extruded at a relatively low temperature of523K with a reduction ratio of44.A very fine grain size of1.7µm was attained only by hot extrusion.Thefine-grained structure was stable blow573K.Owing to thefine grain size, high-strain-rate superplasticity was observed at temperatures of∼548K.(Received September21,2001;Accepted November14,2001)Keywords:magnesium alloy,grain refinement,hot extrusion,high-strain-rate superplasticity1.IntroductionRecent research activities of magnesium become higher in order to reduce the weight of components such as motor vehi-cles especially from the ecological point of view.This is not only a result of magnesium’s relatively low density,which can directly and substantially reduce weight,but is also a re-sult of its good damping characteristics,dimensional stability, machinability,and low casting cost.These attributes enable magnesium to economically replace many zinc and aluminum die castings,as well as cast iron and steel components.An AZ91alloy is the most widely used cast magnesium alloy. This is because this alloy shows a good combination of high strength at room temperature,good castability and excellent corrosion resistance.Die casting has been a principal technique for the fabri-cation of magnesium components because of its high pro-ductivity,suitable strength,quality and dimensional control. The fabrication is also performed by semi-solid process-ing of thixotropic molding.However,fabrication by plas-tic forming also has considerable potential because the al-loys have higher ductility and strength than the castings,1) and because plastic forming enables high productivity.It is expected that processes utilizing plastic forming will be developed.In wrought magnesium alloys,grain refine-ment is essential for attaining superior mechanical prop-erties such as higher strength2)and higher ductility3–6)at room temperature,and high-strain-rate superplasticity at el-evated temperatures.7)Especially,high-strain-rate superplas-ticity,defined as superplasticity occurring at strain rates at or above10−2s−1,8)is of great interest because it is ex-pected to result in economically-viable,near-net-shape form-ing techniques under the commercial hot working rates. High-strain-rate superplasticity has also been demonstrated in magnesium-based materials,especially in powder metallurgy (PM)processed materials.7)To date,however,superplastic forming of magnesium-based materials has not been adopted industrially.One of the reason is that the simple and inexpen-sive process for attaining veryfine grained materials has not been established in magnesium-based materials.Although se-vere plastic deformation,such as equal-channel-angular ex-trusion(ECAE),and PM route using rapidly solidified pow-der(RSP)are the promising route for attainingfine-grained magnesium-based materials,it is expected from the industrial point of view that the high-strain-rate superplastic materials produced by conventional working technique,such as extru-sion and rolling are to be developed.It has been pointed out that hot deformation accompanied by DRX is a promising route for refining the microstruc-ture of magnesium alloys.9)To date,grain refinement dur-ing hot deformation has also been demonstrated in ingot met-allurgy(IM)and PM AZ91alloy.10–17)The relationship be-tween working temperature and resulting grain size is shown in Fig.1for AZ91prepared by IM(extrusion,ECAE and rolling)and PM(extrusion of RSP and machined chip)routes. The grain size tends to decrease with decreasing working temperature.This suggests the strategies for grain refine-ment in magnesium:decrease the temperature in the process. The present paper was motivated by this preliminary result. The purpose of the present investigation is two-fold.First, to refine the microstructure of IM AZ91alloy by hot extru-sion at a relatively low temperature.Second,to examine the high-strain-rate superplastic behavior of the extruded mate-rial.2.ExperimentalThe material used in the present study was a commercial magnesium alloy,AZ91.The alloy was initially produced by ingot casting.The material was solution treated at686K for 48h,followed by hot extrusion at523K with a reduction ratio of44.Microstructures were observed by a color laser3D pro-file microscope using a light screen mode and a transmission electron microscope.Tensile specimens,machined directly from the extruded bars,had tensile axes parallel to the extruded direction.The specimens had a gauge length of5mm and a gauge diame-ter of2.5mm.To investigate the high-strain-rate superplasticHigh-Strain-Rate Superplasticity in an AZ91Magnesium Alloy Processed by Ingot Metallurgy Route79Fig.1The relationship between working temperature and the resulting grain size in AZ91magnesium alloy.behavior of the extruded material,constant strain rate tensile tests were carried out at1×10−2s−1at temperatures ranging from523to623K in air.In addition,a strain rate change test was carried out at548K to obtain the strain rate sensitivity exponent,m,over the wide range of strain rates.3.Results and Discussion3.1Microstructures of extruded materialThe microstructure of as-extruded AZ91was composed of equiaxed matrix grains and uniformly dispersedfine particles. The particles were observed to reside in the grain boundaries. The particle seems to be Mg17Al12.18)The particle diameter was estimated to be about0.3µm from the transmission elec-tron micrographs.The matrix grain size,d(d=1.74L;L is the linear intercept size),of the as-extruded material was mea-sured to be1.7µm.It is noted thatfine-grained AZ91was at-tained only by hot extrusion.The grain refinement is probably attributed to the dynamic recrystallization during hot extru-sion.10)The grain size of the present material is also included in Fig.1.It is evident that the small grain size of the present material is also owing to the low extrusion temperature.The typical microstructure of extruded material is shown in Fig.2after the specimen was annealed at548K for1.8ks. The grains were stillfine at this temperature.The grain sizes at given temperatures are shown in Fig.3.The grain size was remained almost unchanged at temperatures below573K,but rapidly increased at higher temperatures.It is suggested that the Mg17Al12precipitates can not contribute to pin thegrain Fig.2Typical microstructure of extruded AZ91followed by static anneal-ing at548K for1.8ks.Fig.3The variation in grain size as a function of annealing temperature in AZ91processed by various routes.boundaries no longer at high temperatures.In Fig.3,the grain sizes of AZ91alloy processed by various routes are also in-cluded.It is obvious that PM alloy processed from RSP shows the high grain size stability even at673K.This is probably associated with the existence of stable oxide particles,which are introduced during atomization process.3.2Superplastic propertiesThe variation in(a)flow stress and(b)elongation-to-failure as a function of temperature is shown in Fig.4.Theflow stress was determined at afixed strain of0.1.Theflow stress decreased monotonically with temperature below573K,but increased rapidly above this temperature and again decreased with temperature.The increase inflow stress above575K is probably associated with the rapid grain growth as has been shown in Fig.3,since theflow stress offine-grained material often depends on the grain size.The maximum elongation of 354%was obtained at a temperature of548K.The elongation value decreased at temperatures above598K in connect with the increase inflow stress and grain growth.Iwasaki et al.12) have already examined the superplastic behavior of IM AZ91 alloy at a temperature of573K.The data of IM AZ91by Iwasaki et al.12)is included in Fig.4.Their extruded material has similar grain size to the present result,although there is no description on the precipitates.It is evident that the present material exhibited lowerflow stress and larger elongation at a80H.Watanabe,T.Mukai,K.Ishikawa and K.HigashiFig.4The variation in (a)flow stress and (b)elongation-to-failure as a function of temperature at a strain rate of 1×10−2s −1of as-extruded AZ91.The data for documented IM AZ91alloy 12)is also included in thefigure.Fig.5The variation in flow stress as a function of strain rate of as-extruded AZ91at a temperature of 548K obtained by strain rate change test.temperature of 573K in spite of the similar initial grain size.The lower extrusion temperature of the present material may resulted in different distribution and size of precipitates,and thus difference in grain size stability at elevated temperature.To characterize the effect of strain rate on plastic flow be-havior,strain rate change test was carried out at 548K.The variation in flow stress as a function of strain rate is plotted in Fig.5.The flow stress for the present alloy increased with strain rate.The m -value,which was estimated from the slope of the line,exhibited 0.5in the strain rate range investigated.This high m -value of 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