Influence of processing technology on phase transformations in a.pdf

Materials Science and Engineering A 462 (2007) 334338 Infl uence of processing technology on phase transformations in a rare-earth-containing MgZnZr 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 MgZn alloys GuinierPreston (GP) zones and metastable MgZn, MgZn2, Mg2Zn3precipitates are observed. Zirconium Corresponding author. Tel.: +420 2 21911372; fax: +420 2 21911618. E-mail address: pelcova.jemail.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 MgZr 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 MgZr based alloys. The signifi cant improvement in high-temperature properties enables themostrecentalloystobeusedupto573Kcomparedto423K for earlier MgZr alloys 2. MgZnZr 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 MgZnZrRE, 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 MgZnZr 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 MgZnRE(Zr) alloys, if the cooling rates are high enough, quasicrystals can be formed. The ZnMgY 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) 334338335 Table 1 Composition of the alloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr (nominal composition) AlloyZn wt.%REawt.%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 (273783K) 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 weredeterminedintherange293783Kinstepsof30K/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 104using 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 material2.6653.08452 4 Spray formed and extruded material3.120(3.593)693K2.67286 3 336J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338 Fig. 1. Response of relative resistivity changes to isochronal annealing up to 783K in MgZnREZr 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 (423543K decrease of 8% and 603693K decrease of 18%). Conglomerates of rectangular particles (sizel?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 MgZnNd- 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 ?1.07.38.0 127.313.0 227.321.3 313.021.3 Table 5 Angles between poles of the cbco phase (in deg) Position123 Pole102001216 10227.113.4 00127.121.5 21613.421.50 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 MgZnREZr alloy in as prepared state, (a) dark conglomerates of rectangular particles cbco ZnNd(Mg) phase, (b) fi ne particles of YNd containing phase. (Bright-fi eld TEM). J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338337 Fig. 3. Diffraction patterns of a cbco particle in spray formed and extruded MgZnREZr 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 603693K. 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 573623K. 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- cipitationofthephasecontainingZnZr(needlesandellipsoids) observed in TEM specimens after annealing up to 753K, see Fig. 4. Fine precipitates of ZnY phase in spray formed and extruded MgZnREZralloyafterisochronalannealingupto693K.(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 ZrZn 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. FineneedlesofZnZrcontainingphaseinthesprayformedandextruded MgZnREZr alloy isochronally annealed up to 753K. (Bright-fi eld TEM). 338J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338 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 (513603K) 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 pre- pared by spray forming shows a high porosity (about 12%) that causes extreme brittleness