Influence of processing technology on phase transformations in a.pdf

Materials Science and Engineering A 462 2007 334 338 Infl uence of processing technology on phase transformations in a rare earth containing Mg Zn Zr alloy Jitka Pelcov a Bohumil Smola Ivana Stul kov a Charles University Faculty of Mathematics and Physics Ke Karlovu 5 CZ 121 16 Prague 2 Czech Republic Received 30 August 2005 received in revised form 13 December 2005 accepted 15 December 2005 Abstract An investigation was carried out into the effect of annealing on the precipitation processes and stability of the microstructure in the magnesium alloy Mg 3wt Zn 1wt Nd 0 5wt Zr produced by various solidifi cation conditions The alloy was produced by squeeze casting and spray forming with and without subsequent extrusion The phase transformations were studied by means of relative electrical resistivity changes during isochronal annealing in the temperature range from 293 to 783K The microstructure of selected states was analyzed by transmission electron microscopy 2006 Elsevier B V All rights reserved Keywords Mg alloys Precipitation Electrical resistivity Microstructure Spray forming 1 Introduction Magnesium alloys are attractive for space aeronautical automobile or leisure and tool applications owing to their specifi c properties such as low density high specifi c strength good machinability and availability The use of low cost Mg base alloys is limited owing to their moderate mechanical and creep properties at elevated temperatures These could be improved by the use of modern processing technologies composites rapidly solidifi ed alloys nano particle reinforced alloys etc or by using non traditional alloying elements such as rare earths RE Spray forming as an example of the rapid solidifi cation processing technique is one possibility of reducing the cost of manufactured components by reducing of the number of pro ductionstepsnecessarytogetherwiththeadvantageofenhanced properties associated with microstructural refi nement and sup pressed macro segregation 1 This technique was used in the study of the alloy Mg 3wt Zn 1wt Nd 0 5wt Zr Zinc is often used as an alloying element in commercial Mg alloys Duringdecompositionofsupersaturatedsolidsolutionofbinary Mg Zn alloys Guinier Preston GP zones and metastable MgZn MgZn2 Mg2Zn3precipitates are observed Zirconium Corresponding author Tel 420 2 21911372 fax 420 2 21911618 E mail address pelcova j email cz J Pelcov a is added to refi ne the grain size and to participate in the devel opment of phases leading to a higher ratio of proof stress to tensile strength and increase the creep resistance Most Mg Zr alloys contain RE elements such as cerium neodymium and lanthanum which form eutectic systems with magnesium and improvethecastabilityowingtotheformationofgrainboundary networks of relatively low melting point eutectics Continuous alloy development has led to major improvements in room temperature mechanical properties of Mg Zr based alloys The signifi cant improvement in high temperature properties enables themostrecentalloystobeusedupto573Kcomparedto423K for earlier Mg Zr alloys 2 Mg Zn Zr alloys known as ZK alloys are widely used commercially for their high strength good plasticity and corrosion resistance 3 4 A strengthening effect of RE on wrought Mg Zn Zr RE due to the formation of RE containing particles which suppresses dynamic recrys tallisation during extrusion was found by Luo et al 5 The addition of 3wt Nd to the Mg Zn Zr alloy can effectively improve the yield strength and ultimate tensile strength of the alloy at higher temperatures as a result of grain refi nement and the formation of the Mg12Nd phase 6 In Mg Zn RE Zr alloys if the cooling rates are high enough quasicrystals can be formed The Zn Mg Y icosahedral quasicrystal phase with a fi ve fold symmetry was fi rst 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 have synthesized a family of icosahedral quasicrystals with RE Y 0921 5093 see front matter 2006 Elsevier B V All rights reserved doi 10 1016 j msea 2005 12 110 J Pelcov a et al Materials Science and Engineering A 462 2007 334 338335 Table 1 Composition of the alloy Mg 3wt Zn 1wt Nd 0 5wt Zr nominal composition AlloyZn wt REa wt Zr wt Mg wt Squeeze cast and extruded material4 190 980 5894 25 Spray formed material3 411 190 3895 02 Spray formed and extruded material3 080 910 3295 69 a RE consists of Nd with a small amount of Y Nd Sm Dy Gd Tb Ho and Er The presence of quasicrystals brings an improvement in mechanical properties such as higher hardness higher thermal stability due to stabilization of grain boundaries higher corrosion resistance and ductility etc In the work presented in this paper the infl uence of pro cessing technology on the phase transformations in Mg 3wt Zn 1wt Nd 0 5wt Zr alloy was studied by means of rela tiveresistivitychangesduringisochronalannealingupto783K The microstructure of selected states was studied by transmis sion electron microscopy TEM 2 Experimental details A detailed investigation of the microstructural development and phase transformations in the course of isochronally increas ing temperature 273 783K was carried out using electrical resistometry The results of electrical resistometry were corre lated with those of microhardness measurement carried out in the same way as the resistometry measurements An analysis of themicrostructurewasundertakenusingTEMonselectedstates of the material The alloy Mg 3wt Zn 1wt Nd 0 5wt Zr was pre pared by squeeze casting or the spray forming technique with and without subsequent extrusion The temperature of the melt insprayformingwas1013K theprocessgasbeingAr 1vol O2 Squeeze casting was undertaken in a protective atmosphere ofAr 1 SF6 Extrusionwascarriedoutat623Kwithareduc tionof50 1afteraone hourpreheatat573K Thecompositions of the alloys studied are listed in Table 1 Thechangesinrelativeresistivityduetoisochronalannealing weredeterminedintherange293 783Kinstepsof30K 30min Each annealing step was followed by quenching in liquid nitro gen for annealing temperatures up to 513K and in water at roomtemperatureforhigherannealingtemperatures Heattreat ment was carried out in a stirred oil bath up to 513K and in a furnace with an argon protective atmosphere at higher temper atures The four contact specimens in the shape of a letter H were used in the resistivity measurements at 77K after each heating step Relative resistivity changes were obtained to an accuracy of 10 4using the dc four point method with a dummy specimen in series The effect of parasitic thermo Table 2 Measured and calculated density of the alloys studied AlloyMeasured density kg m3 Calculated density kg m3 Squeeze cast and extruded material1828 51819 Spray formed material1598 31813 Spray formed and extruded material1813 51803 electromotive forces was suppressed by current reversal The value of the electrical resistivity was measured also at 293K in selected states of the material to obtain the residual resis tivity ratio RRR 293K 77K which does not depend on thespecimenform factorandincreaseswithincreasingmaterial purity ChangesinthemicrohardnessHV0 1 Vickershardnesswith 0 1kg load were measured following the same treatment as resistivity measurements to reveal the thermal stability of the mechanical properties related to the microstructural develop ment Transmission electron microscopy electron diffraction ED and X ray microanalysis EDX were used to determine the structure and morphological characteristics of the phases pre cipitated using a JEOL JEM 2000FX electron microscope and aLinkAN10000microanalyzer ThespecimensforTEMwere prepared by the same isochronal annealing procedure as those for electrical resistivity and hardness measurements 3 Results and discussion The grain size was about 1 m for spray formed alloys The values of density are listed in Table 2 and compared to the cal culated values The high discrepancy between the measured and calculated density of the spray formed material indicates a large porosity content about 12vol which induces extreme brit tleness Table 3 summarizes the values of the RRR parameter in the denoted states of heat treatment and Vickers microhardness HV0 1 in the as prepared state The lowest values of micro hardness and the RRR parameter in the spray formed alloy also support the presence of voids The microhardness value is con siderably higher and comparable for both extruded alloys the Table 3 Values of the RRR parameter and microhardness HV0 1 AlloyRRR as prepared state RRR minimum of resistivity RRR after annealing up to 783K HV0 1 as prepared Squeeze cast and extruded material3 529 3 823 633K2 48795 3 Spray formed material2 665 3 08452 4 Spray formed and extruded material3 120 3 593 693K2 67286 3 336J Pelcov a et al Materials Science and Engineering A 462 2007 334 338 Fig 1 Response of relative resistivity changes to isochronal annealing up to 783K in Mg Zn RE Zr alloy with various preparation conditions spray formed and extruded spray formed non extruded squeeze cast and extruded squeeze cast and extruded 2nd run and spray formed and extruded 2nd run squeeze cast as well as the spray formed alloy in the initial state Relative resistivity changes 0owing to isochronal annealing annealing curves in the alloys investigated are compared in Fig 1 The resistivity annealing curve of the spray formed alloy shows a negligible increase during isochronal annealing up to 603K followed by a continuous decrease in resistivity to 783K 14 This is most probably caused by a precipitation process leading to the purifi cation of the matrix which is verifi ed by the increasing RRR value Table 3 The spray formed and extruded material responds to anneal ing by a decrease in resistivity in two temperature ranges 423 543K decrease of 8 and 603 693K decrease of 18 Conglomerates of rectangular particles size l m of a complex phase containing Zn and Nd were observed in the as preparedstateofthesprayformedandextrudedalloy seeFig 2a ED patterns could not be indexed on the basis of any known phase of Mg Zn Nd and Zr containing alloy The C base centred orthorhombic cbco reciprocal lattice was constructed Table 4 Measured angles of goniometer position and those between the poles in deg Position123 Gonio angle 11 015 50 0 Gonio angle 1 0 7 38 0 1 27 313 0 227 3 21 3 313 021 3 Table 5 Angles between poles of the cbco phase in deg Position123 Pole 102 001 216 102 27 113 4 001 27 1 21 5 216 13 421 50 using four ED patterns obtained from different particles Lattice parametersofthecbcophasewereestimatedtobea 0 997nm b 1 149nm and c 0 974nm This interpretation was verifi ed by a tilting experiment on a single coarsened particle of this phase in the spray formed and extruded alloy after isochronal annealing up to 543K Three ED patterns were indexed unam biguously as 102 001 and 216 pole patterns of the cbco phase Fig 3 The measured and calculated angles between the poles agree very well with each other within the experimental error 0 5 see Tables 4 and 5 A relatively large dislocation density was observed in some grains of the spray formed and extruded material in the as pre pared state Fine particles of a complex phase containing Y and Zn in grain interiors up to 30nm as well as at the grain bound aries up to 50nm were present in this material see Fig 2b Coarsening of the cbco phase and recovery of the dislocation substructure was observed after annealing up to 543K which leads to a slight decrease in HV0 1 7 The develop ment of fi ne dense dispersed precipitates of Zn Y and a Nd Fig 2 Structure of spray formed and extruded Mg Zn RE Zr alloy in as prepared state a dark conglomerates of rectangular particles cbco Zn Nd Mg phase b fi ne particles of Y Nd containing phase Bright fi eld TEM J Pelcov a et al Materials Science and Engineering A 462 2007 334 338337 Fig 3 Diffraction patterns of a cbco particle in spray formed and extruded Mg Zn RE Zr alloy isochronally annealed up to 543K a pole 102 b pole 001 c pole 216 containing phase Fig 4 was detected after annealing to 693K which resulted in a resistivity decrease in the range 603 693K A slight increase in microhardness was associated with this pro cess 7 The increase in the RRR value after annealing to 693K Table 3 confi rms the higher effective purity of the matrix due to the precipitation processes Unlike annealing of non extruded material annealing of spray formed and extruded material to higher temperatures leads to an increase of more than 25 above the initial resistivity value An increase above the initial resistivity value was not observed in spray formed material supporting the assumption of a higher concentration of solutes in the as prepared state but not in the extruded material where precipitation could occur during the thermal treatment at 573 623K The increase above the as prepared value in spray formed and extruded alloys is caused most probably by the dissolution of precipitates containing Nd and simultaneous pre cipitationofthephasecontainingZn Zr needlesandellipsoids observed in TEM specimens after annealing up to 753K see Fig 4 Fine precipitates of Zn Y phase in spray formed and extruded Mg Zn RE Zralloyafterisochronalannealingupto693K Bright fi eldTEM Fig 5 Experimental data show that a contribution of 1at Nd in the Mg matrix to the residual resistivity is relatively high 77n m at 10 79 4n m at 11 95n m at 12 at 77K It cannot only compensate the decrease caused by the depletion of Zn and Zr solute in the matrix due to precipitation of Zr Zn phase but can also lead to the pronounced resistivity increase observed The response of relative resistivity changes to isochronal annealing of the squeeze cast and extruded specimen can be described by a continuous slight decrease of resistivity values up to 603K minimum 13 At higher annealing temper atures above 633K a very pronounced increase is observed more than 60 above the value of as prepared alloy similar to that in the spray formed and extruded material It indicates an increasingconcentrationofsolutesinthematrixafterdissolution of precipitated phases as confi rmed by the decrease in the RRR value Table3 Thisresultinvokedthemeasurementofso called second runs in which the stepwise isochronally annealed spec imens of extruded materials spray formed as well as squeeze cast were again isochronally annealed from 293 to 783K Fig 5 FineneedlesofZn Zrcontainingphaseinthesprayformedandextruded Mg Zn RE Zr alloy isochronally annealed up to 753K Bright fi eld TEM 338J Pelcov a et al Materials Science and Engineering A 462 2007 334 338 Relative resistivity changes during the second run Fig 1 dashed lines are substantially larger than those in the fi rst run and the shape of resistivity annealing curves is similar This result clearly proves that the isochronal heat treatment up to higher temperatures signifi cantly suppress the infl uence of pro cessing technology The supersaturation provided by the fi rst isochronal annealing up to 783K enables the development of metastablephasesduringthesecondrunofisochronalannealing from room temperature to 523K The shape of the main resis tivity decrease 513 603K indicates that several precipitation processes take place simultaneously Depending on the resistiv ity response the temperature range of precipitation processes differs substantially in spray formed specimen after extrusion and the same specimen exposed to a second run with the same isochronal heat treatment 4 Conclusion The alloy Mg 3wt Zn 1wt Nd 0 5wt Zr alloy p