[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】在含有稀有元素Mg–Zn–Zr的合金中，加工工艺对相变的影响-外文文献

Materials Science and Engineering A 462 (2007) 334338Influence of processing technology on phase transformations in arare-earth-containingJitka Pelcova, Bohumil Smola,K13Abstractalloformingisochronal microstructuremicroscopK forming1.automobilespecificgoodMg-baseandbe improved by the use of modern processing technologies(composites, rapidly solidified alloys, nano-particle reinforcedalloys, etc.) or by using non-traditional alloying elements, suchas rare-earths (RE).processingmanufductionpropertiespressedstudyisDuringMgZnMgZn,isopmenttensileallolanthanum,impronetwallotemperature mechanical properties of MgZr based alloys. Thesignificant improvement in high-temperature properties enablesthe most recent alloys to be used up to 573 K compared to 423 Kfor earlier MgZr alloys 2. MgZnZr alloys (known as ZK0921-5093/$doi:Spray forming, as an example of the rapid solidificationtechnique, is one possibility of reducing the cost ofactured components by reducing of the number of pro-steps necessary together with the advantage of enhancedassociated with microstructural refinement and sup-macro-segregation 1. This technique was used in theof the alloy Mg-3 wt.% Zn-1 wt.% Nd-0.5 wt.% Zr. Zincoften used as an alloying element in commercial Mg alloys.decomposition of supersaturated solid solution of binaryalloys GuinierPreston (GP) zones and metastableMgZn2,Mg2Zn3precipitates are observed. ZirconiumCorresponding author. Tel.: +420 2 21911372; fax: +420 2 21911618.E-mail address: pelcova.jemail.cz (J. Pelcova).alloys) are widely used commercially for their high-strength,good plasticity and corrosion-resistance 3,4. A strengtheningeffect of RE on wrought MgZnZrRE, due to the formationof RE-containing particles, which suppresses dynamic recrys-tallisation during extrusion was found by Luo et al. 5. Theaddition of 3 wt.% Nd to the MgZnZr alloy can effectivelyimprove the yield strength and ultimate tensile strength of thealloy at higher temperatures as a result of grain refinement andthe formation of the Mg12Nd phase 6. In MgZnRE(Zr)alloys, if the cooling rates are high enough, quasicrystals canbe formed. The ZnMgY icosahedral quasicrystal phasewith a five-fold symmetry was first reported by Luo et al.7 in a high-strength magnesium alloy containing Zn and Y.Following on from this, Niikura et al. 8 and Tsai et al. 9 havesynthesized a family of icosahedral quasicrystals with RE = Y, see front matter 2006 Elsevier B.V. All rights reserved.10.1016/j.msea.2005.12.110Charles University, Faculty of Mathematics and Physics,Received 30 August 2005; received in revised formAn investigation was carried out into the effect of annealing on the precipitationy Mg-3 wt.% Zn-1 wt.% Nd-0.5 wt.% Zr produced by various solidificationwith and without subsequent extrusion. The phase transformationsannealing in the temperature range from 293 to 783 K. They.2006 Elsevier B.V. All rights reserved.eywords: Mg alloys; Precipitation; Electrical resistivity; Microstructure; SprayIntroductionMagnesium alloys are attractive for space, aeronautical,or leisure and tool applications owing to theirproperties, such as low density, high specific strength,machinability and availability. The use of low-costalloys is limited owing to their moderate mechanicalcreep properties at elevated temperatures. These couldMgZnZr alloyIvana Stulkovae Karlovu 5, CZ-121 16, Prague 2, Czech RepublicDecember 2005; accepted 15 December 2005processes and stability of the microstructure in the magnesiumconditions. The alloy was produced by squeeze casting and spraywere studied by means of relative electrical resistivity changes duringof selected states was analyzed by transmission electronadded to refine the grain size and to participate in the devel-of phases leading to a higher ratio of proof stress tostrength and increase the creep resistance. Most MgZrys contain RE elements, such as cerium, neodymium andwhich form eutectic systems with magnesium andve the castability owing to the formation of grain boundaryorks of relatively low melting point eutectics. Continuousy development has led to major improvements in roomJ. Pelcova et al. / Materials Science and Engineering A 462 (2007) 334338 335Table 1Composition of the alloy Mg-3 wt.% Zn-1 wt.% Nd-0.5 wt.% Zr (nominal composition)Alloy Zn wt.% REawt.% Zr wt.% Mg wt.%Squeeze cast and extruded material 4.19 0.98 0.58 94.25Spray formed material 3.41 1.19 0.38 95.02Spray formed and extruded material 3.08 0.91 0.32 95.69aRE consists of Nd with a small amount of Y.Nd, Sm, Dy, Gd, Tb, Ho and Er. The presence of quasicrystalsbrings an improvement in mechanical properties, such as higherhardness, higher thermal stability due to stabilization of grainboundaries, higher corrosion-resistance and ductility, etc.In the work presented in this paper, the influence of pro-cessing technology on the phase transformations in Mg-3 wt.%Zn-1tiThesion2.andingresistometrylatedthetheofparedandinOoftionofwereEachgenroommentfurnaceatures.wereheatingtoaTable 2Measured and calculated density of the alloys studiedAlloy Measured densitykg/m3Calculateddensity kg/m3Squeeze cast and extruded material 1828 5 1819SpraySprayelectromotivintithepurity0.1resistimechanicalment.andstructurecipitatedapreparedfor3.vculatedcalculatedporositytleness.theHV0.1 in the as prepared state. The lowest values of micro-hardness and the RRR parameter in the spray formed alloy alsosupport the presence of voids. The microhardness value is con-siderably higher and comparable for both extruded alloys theTVAllo of resistivity) RRR (after annealing up to 783 K) HV0.1 (as prepared)Squeeze 2.487 95 3SpraySpraywt.% Nd-0.5 wt.% Zr alloy was studied by means of rela-ve resistivity changes during isochronal annealing up to 783 K.microstructure of selected states was studied by transmis-electron microscopy (TEM).Experimental detailsA detailed investigation of the microstructural developmentphase transformations in the course of isochronally increas-temperature (273783 K) was carried out using electrical. The results of electrical resistometry were corre-with those of microhardness measurement carried out insame way as the resistometry measurements. An analysis ofmicrostructure was undertaken using TEM on selected statesthe material.The alloy Mg-3 wt.% Zn-1 wt.% Nd-0.5 wt.% Zr was pre-by squeeze casting or the spray forming technique withwithout subsequent extrusion. The temperature of the meltspray forming was 1013 K, the process gas being Ar+1vol.%2. Squeeze casting was undertaken in a protective atmosphereAr + 1% SF6. Extrusion was carried out at 623 K with a reduc-of 50:1 after a one-hour preheat at 573 K. The compositionsthe alloys studied are listed in Table 1.The changes in relative resistivity due to isochronal annealingdetermined in the range 293783 K in steps of 30 K/30 min.annealing step was followed by quenching in liquid nitro-for annealing temperatures up to 513 K and in water attemperature for higher annealing temperatures. Heat treat-was carried out in a stirred oil bath up to 513 K and in awith an argon protective atmosphere at higher temper-The four contact specimens in the shape of a letter Hused in the resistivity measurements at 77 K after eachstep. Relative resistivity changes Delta1/ were obtainedan accuracy of 104using the dc four-point method withdummy specimen in series. The effect of parasitic thermo-able 3alues of the RRR parameter and microhardness HV0.1y RRR (as prepared state) RRR (minimumcast and extruded material 3.529 (3.823)633Kformed material 2.665 formed and extruded material 3.120 (3.593)693Kformed material 1598 3 1813formed and extruded material 1813 5 1803ve forces was suppressed by current reversal. Thealue of the electrical resistivity was measured also at 293 Kselected states of the material to obtain the residual resis-vity ratio, RRR =(293 K)/(77 K), which does not depend onspecimen form-factor and increases with increasing material.Changes in the microhardness HV0.1 (Vickers hardness withkg load) were measured following the same treatment asvity measurements to reveal the thermal stability of theproperties related to the microstructural develop-Transmission electron microscopy, electron diffraction (ED)X-ray microanalysis (EDX) were used to determine theand morphological characteristics of the phases pre-(using a JEOL JEM 2000FX electron microscope andLink AN 10000 microanalyzer). The specimens for TEM wereby the same isochronal annealing procedure as thoseelectrical resistivity and hardness measurements.Results and discussionThe grain size was about 1H9262m for spray formed alloys. Thealues of density are listed in Table 2 and compared to the cal-values. The high discrepancy between the measured anddensity of the spray formed material indicates a largecontent (about 12 vol.%), which induces extreme brit-Table 3 summarizes the values of the RRR parameter indenoted states of heat treatment and Vickers microhardness3.084 52 42.672 86 3336 J. Pelcova et al. / Materials Science and Engineering A 462 (2007) 334338Fig. 1. Response of relative resistivity changes to isochronal annealing up to783 K in MgZnREZr alloy with various preparation conditions (square, sprayformed and extruded; H17033, spray formed non-extruded; diamondsolid, squeeze cast andextruded; diamondsolid, squeeze cast and extruded 2nd run and; squaresolid-, spray formed andextruded 2nd run).squeeze cast as well as the spray formed alloy in the initialstate.Relative resistivity changes Delta1/0owing to isochronalannealing annealing curves in the alloys investigated arecompared in Fig. 1. The resistivity annealing curve of the sprayformed alloy shows a negligible increase during isochronalannealing up to 603 K followed by a continuous decrease inresistiawhiching(423543complepreparedEDphasecentredTable 4Measured angles of goniometer position and those between the poles (in deg)Position 1 2 3Gonio angle H9251 11.0 15.5 0.0Gonio angle H9252 1.0 7.3 8.01 27.3 13.02 27.3 21.33 13.0 21.3 Table 5Angles between poles of the cbco phase (in deg)Position 1 2 3Pole 1 0 2 0 0 1 2 1 61 0 2 27.1 13.40 0 1 27.1 21.52 1 6 13.4 21.5 0using four ED patterns obtained from different particles. Latticeparameters of the cbco phase were estimated to be a = 0.997 nm,b = 1.149 nm and c = 0.974 nm. This interpretation was verifiedby a tilting experiment on a single coarsened particle of thisphase in the spray formed and extruded alloy after isochronalannealing up to 543 K. Three ED patterns were indexed unam-biguously as 1 0 2, 0 0 1 and 2 1 6 pole patterns of the cbcophasepoleserrorgrainsparedZnariessubstructureleadsmentFig.(b)vity to 783 K (14%). This is, most probably, caused byprecipitation process leading to the purification of the matrix,is verified by the increasing RRR value (Table 3).The spray formed and extruded material responds to anneal-by a decrease in resistivity in two temperature rangesK decrease of 8% and 603693 K decrease of 18%).Conglomerates of rectangular particles (size lH9262m) of ax phase containing Zn and Nd were observed in the asstate of the spray formed and extruded alloy, see Fig. 2a.patterns could not be indexed on the basis of any knownof MgZnNd- and Zr-containing alloy. The C-base-orthorhombic (cbco) reciprocal lattice was constructed2. Structure of spray formed and extruded MgZnREZr alloy in as prepared state,fine particles of YNd containing phase. (Bright-field TEM).Fig. 3. The measured and calculated angles between theagree very well with each other within the experimental0.5, see Tables 4 and 5.A relatively large dislocation density was observed in someof the spray formed and extruded material in the as pre-state. Fine particles of a complex phase containing Y andin grain interiors (up to 30 nm) as well as at the grain bound-(up to 50 nm) were present in this material, see Fig. 2b.Coarsening of the cbco phase and recovery of the dislocationwas observed after annealing up to 543 K, whichto a slight decrease in HV0.1 (7%). The develop-of fine, dense dispersed precipitates of Zn-, Y- and a Nd-(a) dark conglomerates of rectangular particles cbco ZnNd(Mg) phase,J. Pelcova et al. / Materials Science and Engineering A 462 (2007) 334338 337Fig. 3. Diffraction patterns of a cbco particle in spray formed and extruded MgZnREZr alloy isochronally annealed up to 543 K. (a) pole 1 0 2, (b) pole 0 0 1,(c) pole 2 1 6.containing phase (Fig. 4) was detected after annealing to 693 K,which resulted in a resistivity decrease in the range 603693 K.A slight increase in microhardness was associated with this pro-cess ( +7%). The increase in the RRR value after annealingto 693 K, Table 3 confirms the higher effective purity of thematrix due to the precipitation processes. Unlike annealing ofnon-extruded material, annealing of spray formed and extrudedmaterial to higher temperatures leads to an increase of morethan 25% above the initial resistivity value. An increase abovethe initial resistivity value was not observed in spray formedmaterial supporting the assumption of a higher concentration ofsolutes in the as prepared state but not in the extruded material,where573623formeddissolutioncipitationobservFig.MgZnREZrFig. 5. Experimental data show that a contribution of 1 at.% Ndin the Mg matrix to the residual resistivity is relatively high(77 nOmega1m/at.% 10, 79.4 nOmega1m/at.% 11,95nOmega1m/at.% 12 at77 K). It cannot only compensate the decrease caused by thedepletion of Zn and Zr solute in the matrix due to precipitationof ZrZn phase but can also lead to the pronounced resistivityincrease observed.The response of relative resistivity changes to isochronalannealing of the squeeze cast and extruded specimen can bedescribedupatures(morethatincreasingofvsecondimenscast)precipitation could occur during the thermal treatment atK. The increase above the as prepared value in sprayand extruded alloys is caused, most probably, by theof precipitates containing Nd and simultaneous pre-of the phase containing ZnZr (needles and ellipsoids)ed in TEM specimens after annealing up to 753 K, see4. Fine precipitates of ZnY phase in spray formed and extrudedalloy after isochronal annealing up to 693 K. (Bright-field TEM).Fig.MgZnREZrby a continuous slight decrease of resistivity valuesto 603 K (minimum 13%). At higher annealing temper-(above 633 K), a very pronounced increase is observedthan 60% above the value of as prepared alloy) similar toin the spray formed and extruded material. It indicates anconcentration of solutes in the matrix after dissolutionprecipitated phases as confirmed by the decrease in the RRRalue (Table 3). This result invoked the measurement of so-calledruns, in which the stepwise isochronally annealed spec-of extruded materials (spray formed as well as squeezewere again isochronally annealed from 293 to 783 K.5. Fine needles of ZnZr containing phase in the spray formed and extrudedalloy isochronally annealed up to 753 K. (Bright-field TEM).338 J. Pelcova et al. / Materials Science and Engineering A 462 (2007) 334338Relative resistivity changes during the second run (Fig. 1,dashed lines) are substantially larger than those in the first runand the shape of resistivity annealing curves is similar. Thisresult clearly proves that the isochronal heat treatment up tohigher temperatures significantly suppress the influence of pro-cessing technology. The supersaturation provided by the firstisochronal annealing up to 783 K enables the development ofmetastable phases during the second run of isochronal annealingfrom room temperature to 523 K. The shape of the main resis-tivity decrease (513603 K) indicates that several precipitationprocesses take place simultaneously. Depending on the resistiv-ity response, the temperature range of precipitation processesdiffers substantially in spray formed specimen after extrusionand the same specimen exposed to a second run with the sameisochronal heat treatment.4. ConclusionThe alloy Mg-3 wt.% Zn-1 wt.% Nd-0.5 wt.% Zr alloy pre-pared by spray forming shows a high porosity (about 12%)that causes extreme brittleness. Ex