Thermal microstructural stability of AZ31 magnesium after severe plastic deformation

    Thermal Microstructural Stability of AZ31 Magnesium after Severe Plastic Deformation J.P. Young, H. Askari, Y. Hovanski, M.J. Heiden, D.P. Field PII: DOI: Reference:

S1044-5803(14)00407-0 doi: 10.1016/j.matchar.2014.12.026 MTL 7782

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

22 August 2014 26 November 2014 29 December 2014

Please cite this article as: Young JP, Askari H, Hovanski Y, Heiden MJ, Field DP, Thermal Microstructural Stability of AZ31 Magnesium after Severe Plastic Deformation, Materials Characterization (2014), doi: 10.1016/j.matchar.2014.12.026

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ACCEPTED MANUSCRIPT Thermal Microstructural Stability of AZ31 Magnesium after Severe Plastic Deformation

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J P Younga1, H Askaria, Y Hovanskib, M J Heidena and D P Fielda School of Mech. and Matls. Eng., Washington State University, 405 Spokane St, Pullman, WA 99163-

Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland WA 99354, USA

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2920 USA

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Corresponding author. Email: [email protected]. Tel: +1 208 409 3465

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Abstract

Both equal channel angular pressing and friction stir processing have the ability to refine the

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grain size of twin roll cast AZ31 magnesium and potentially improve its superplastic properties. This

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work used isochronal and isothermal heat treatments to investigate the microstructural stability of twin

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roll cast, equal channel angular pressed and friction stir processed AZ31 magnesium. For both heat treatment conditions, it was found that the twin roll casted and equal channel angular pressed materials were more stable than the friction stir processed material. Calculations of the grain growth kinetics showed that severe plastic deformation processing decreased the activation energy for grain boundary motion with the equal channel angular pressed material having the greatest Q value of the severely plastically deformed materials and that increasing the tool travel speed of the friction stir processed material improved microstructural stability. The Hollomon-Jaffe parameter was found to be an accurate means of identifying the annealing conditions that will result in substantial grain growth and loss of potential superplastic properties in the severely plastically deformed materials. In addition, Humphreys’s model of cellular microstructural stability accurately predicted the relative microstructural stability of the severely plastically deformed materials and with some modification, closely predicted the maximum grain size ratio achieved by the severely plastically deformed materials.

ACCEPTED MANUSCRIPT Keywords: Electron Backscatter Diffraction; AZ31 magnesium; Equal Channel Angular Pressing; Friction Stir Processing; Microstructural Stability; Superplasticity

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1. Introduction

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The push for ever lighter components in the transportation industry has driven recent interest in

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light weight magnesium alloys [1-3]. However, with their hexagonal close packed (HCP) structure, magnesium and its alloys possess limited formability at room temperature [4-6]. This has motivated

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interest in alternative forming mechanism such as superplasticity [7-10]. Superplastic deformation is

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defined as a state in which a polycrystalline material is deformed well past its typical failure strain without forming a necked area. This deformation is achieved through grain boundary sliding (GBS)

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rather than dislocation motion and therefore requires a fine grain size typically less than 10 µm and

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moderate strain rates [11,12]. To achieve the strain rates necessary for superplasticity to be utilized in a manufacturing process, the material most often must be heated to temperatures near 0.5Tm where Tm

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is the absolute melting temperature. Therefore, the ideal microstructure for superplastic forming is not only a material with a fine grain size but also a material with significant microstructural stability at the deformation temperature.

Severe plastic deformation (SPD) processing has emerged as one of the most common means of achieving the fine grain size necessary for superplastic forming. SPD processes are defined as methods of metal forming that induce very high strains on a bulk work piece without significant net shape change to the specimen [13]. Equal channel angular pressing (ECAP) has become one of the most highly developed SPD processes. Originally developed by Segal, this process involves the pressing of a billet through an angled die with channels of equal cross section [14,15]. Simple shear strain is induced at the plane where the two die channels meet and repeated pressing can result in significant strain accumulation. The effects of ECAP on the grain refinement [16,17], texture [18-20] and superplastic

ACCEPTED MANUSCRIPT properties [21-23] of magnesium alloys has been well studied. In general, it has been found that pressing temperature, route, number of passes and die angle will have an effect on grain refinement.

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The texture of ECAP billets has been shown to be dependent on die geometry with the basal plane generally aligning itself with the shear plane. Finally, multiple studies have shown that the fine grain size

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produced by ECAP significantly improves the superplasticity of magnesium alloys. Despite these

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conclusions, much less work has been performed on the stability of microstructures produced by ECAP.

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One recent study by Straska et al. [24] used isochronal heat treatments between 170°C and 500°C to assess the microstructural stability of extruded ECAP (EX-ECAP) AZ31. It was found that the activation

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energy for grain boundary motion (Q) varied with annealing temperature. At low temperatures Q was equal to 115 kJ/mol, at intermediate temperatures Q decreased to 33 kJ/mol and at high temperatures

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Q increased to 164 kJ/mol. Very similar results were found by Kim et al. for purely ECAP AZ31 [25]. Both

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Staska et al. and Kim et al. attributed this variance in Q to a decrease in dislocation density during the

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intermediate temperatures due to an accelerated recovery process. These results suggest that ECAP can have a significant effect on the grain growth kinetics of AZ31 magnesium, however as both studies used isochronal heat treatments and assumed an ideal grain growth exponent of 2, there is more to be investigated in the stability of ECAP AZ31. The other SPD process considered in this study was friction stir processing (FSP). Originally developed as a welding process (friction stir welding (FSW)) by The Welding Institute (TWI) in the early 1990s, this solid state joining processing uses a non-consumable, cylindrical welding tool consisting of a shoulder and centered pin. The tool pin drills into the work piece and the frictional heat generated by the pin and shoulder plasticizes the material resulting in a mixing action that joins the work piece [26, 27]. Like all welding processes, FSW produces distinct microstructural zones, typically designated as the heat affected zone (HAZ), the thermo-mechanically affected zone (TMAZ) and the nugget or fusion zone (FZ). Under the proper processing conditions, FSP has been shown to refine the grain size of the FZ of

ACCEPTED MANUSCRIPT many metals, including magnesium [28-30]. This grain refinement has motivated investigations in to the effects of FSP on the superplastic forming of magnesium. Mohan et al. compared the superplastic

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deformation properties of FSP AZ91 and AZ31 magnesium and found that the increased Al content of the AZ91 alloy improved the microstructural stability but that deformation temperatures above 330°C

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saw a rapid drop in strain to failure due to accelerated grain growth [31]. Chai et al. also showed that

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the main failure mechanism during superplastic testing of FSP AZ91 was grain growth and cavity

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coalescence [32]. Finally, Jain et al. showed that although the kinetics of superplastic deformation in a fine grain Mg-1.2 Zn-1.7 Y-0.53 Al-0.27 Mn alloy were slower than that of the AZ31, Mg-10.6 Zn-2.3 Y

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and Mg-4.3 Zn-0.7 Y alloys tested, the thermal stability of the first alloy improved strain to failure during superplastic deformation [33]. Investigations into the static grain growth behavior of FSP magnesium

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alloys are fairly limited, but abnormal grain growth in the FZ of FSP/W ZK60 magnesium alloy [34] and Al

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alloys [35, 36] has been reported. The results from these studies suggest the FSP parameters will have a

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significant effect on the properties and stability of the produced microstructures. The results from the ECAP and FSP studies mentioned above suggest that to fully utilize these SPD processes to enhance the superplastic properties of magnesium alloys, we must develop a better understanding of the thermal stability of the produced microstructures. Utilizing both isochronal and isothermal heat treatments, this work explores the microstructural stability of AZ31 magnesium in the twin roll cast (TRC), equal channel angular pressed (ECAP) and friction stir processed (FSP) conditions and develops an approach for calculating the superplastic forming limit for the SPD microstructures. 2. Experimental Procedure Twin roll cast AZ31 (Mg- 3 wt% Al- 1wt% Zn) magnesium sheet (supplied by POSCO©) of an average thickness of 4 mm was used in this study. Throughout this work this material will be referred to as the TRC condition. Specimens for microstructural analysis were cut with a water jet from the sheet

ACCEPTED MANUSCRIPT and cold mounted in epoxy for observation of the normal direction (ND) transverse direction (TD) plane. Specimens were mechanically ground with SiC paper to a grit of 1200 and then polished using first 1µm

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and then 0.25µm diamond paste on synthetic velvet cloths. Specimens were washed in anhydrous alcohol and rotated 90° between each polishing step, with each polishing step continuing until the

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scratches from the previous step were eliminated. The final polishing step used 0.05 µm colloidal silica

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on a low nap cloth. The final step continued until Kikuchi patterns could be observed. Microstructural

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analysis was performed using electron backscatter diffraction (EBSD) with a FEI field emission scanning electron microscope (FESEM). Scans were run with an accelerating voltage of 20KeV and a probe current

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of ~10 nA. The orientation data generated from the scans was analyzed using TSL Orientation Imaging Microscopy (OIM) Analysis EBSD software where a filter was used to eliminate all data with a confidence

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index (CI) less than 0.1. CI standardization was the only clean up method used on the orientation data.

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The texture, represented in pole figures, and the average grain diameter of all microstructures analyzed

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in this work were calculated from the orientation data from a minimum of 10,000 grains. The friction stir processed (FSP) materials were produced as bead-on-plate welds. The welds were run with the processing direction parallel to the rolling direction (RD) of the TRC plate. A tool with a 25.4 mm diameter threaded shoulder and a 4.5 mm diameter and 2.26 mm tall pin was used for all processing conditions. Two processing conditions were investigated in this study, both with a tool rotation rate of 700 RPM and tool feedrates of 300 mm/min and 1000 mm/min. These processed specimens will be referred to as FSP 3-7 and FSP 10-7 throughout this work, respectively. Specimens of the FSP material were cut from the cross section of each of the welds, cold mounted in epoxy for observation along the processing direction and prepared for microstructural analysis in the same manner as the TRC material. Microstructural analysis of the FZ of each of the processed conditions was performed using EBSD in the same manner as the TRC material.

ACCEPTED MANUSCRIPT Equal channel angular pressing (ECAP) was performed on strips cut from the as-received material. Strips measuring 12 mm in width and 101.6mm in length were cut with their major axis parallel

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to the RD of the TRC sheet. The strips were stacked to create billets of 12 by 12 mm2 and pressed at 200°C through a die with an angle of 90° and 0° internal arc of curvature. Each billet was pressed for a

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total of 4 passes where the billet was rotated around its long axis by 90° between passes. This route is

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referred to as route Bc in the literature. After the fourth pass, specimens for microstructural analysis

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were cut from the center of the billet, cold mounted in epoxy for observation along the pressing direction and prepared for microstructural analysis in the same manner as the TRC material.

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Microstructural analysis of the ECAP material was performed using EBSD in the same manner as the TRC material.

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Analysis of the microstructural stability of the TRC, ECAP and FSP materials was performed by both isochronal and isothermal heat treatments. The isochronal heat treatments were designed to allow

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for observation of the reactions of individual grains at different temperatures. Specimens were cut from the TRC, ECAP and 3-7 FSP materials and prepared for microstructural analysis. A Vickers microindenter was then used to create fiduciary marks on the surface of the prepared specimens. The specimens were annealed at temperatures between 100°C and 400°C in an argon furnace for ten minutes and then allowed to cool in the argon environment. A more rapid quench would have been more effective at preserving the microstructure but would have resulted in degeneration of the polished surface. Microstructural analysis was performed following each heat treatment where the fiduciary marks were used to observe the same area on each specimen. Isothermal heat treatments of the TRC, ECAP and FSP materials were designed to allow for calculation of the grain growth kinetics of each of the materials. Specimens of the same volume were cut from the TRC, ECAP, FSP 3-7 and FSP 10-7 materials and annealed at temperatures between 200°C

ACCEPTED MANUSCRIPT and 500°C for 10, 30, 60 and 120 minutes. Specimens were water quenched, cold mounted in epoxy and then prepared for microstructural analysis in the same manner as the TRC material.

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3. Results

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3.1. Twin Roll Cast and SPD microstructures

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Figure 1 shows the microstructure and texture of the TRC material represented in an orientation

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map and pole figure, respectively. The microstructure of the TRC material appears mostly heterogeneous with coarse, fine and twinned grains. The average grain diameter (including twin grains)

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was found to be 15.3±1.2 µm. The pole figure in Fig. 1(b) shows a weak basal texture in line with ND and

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a weak (10-10) peak parallel to the RD of the TRC material.

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Orientation maps and pole figures representing the microstructures and textures of the FZ of the two FSP conditions are shown in Figures 2 and 3. Figs. 2(a) and 3(a) show that both friction stir

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processing conditions resulted in significant grain refinement in the FZ of the processed material and produced a mostly homogenous microstructure of fine, equiaxed grains. The average grain diameters of both processing conditions were found to be 1.41±0.54 µm and 1.67±0.75 µm for the FSP 3-7 and FSP 10-7 materials, respectively. The two processing conditions produced similar textures showing a basal peak elevated approximately 45° above the processing direction, as shown in Figures 2(b) and 3(b). These pole figures show that the FSP 3-7 processing condition produced a stronger texture than the FSP 10-7 condition. The maximum texture peak intensity (measured in units of MRD - multiples of a random distribution), the average grain diameter and the average grain boundary misorientation angle for the TRC, FSP and ECAP materials are summarized in Table 1. The microstructure of the ECAP billet is shown in Figure 4. The orientation map, Fig. 4(a), shows that the process significantly refined the microstructure, producing mostly fine, equiaxed grains. At 3.82

ACCEPTED MANUSCRIPT ± 1.07 µm, the average grain diameter was found to be larger than that of the FSP materials. The texture of the ECAP material shown in Fig. 4(b), shows a basal peak with a similar orientation as the FSP

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materials but a much weaker maximum peak intensity, as summarized in Table 1.

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3.2. Isochronal heat treatments

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The average grain diameter as a function of heat treatment temperature for the TRC, FSP 3-7 and ECAP material is shown in Figure 5. All three materials show no significant change in average grain

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diameter until 250°C. At a temperature of 250°C and above the ECPA and TRC materials show grain

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growth that increases with temperature where the grain growth rate of the TRC material was greater than that of the ECAP material. At 250°C the FSP 3-7 material shows a sudden increase in grain size. This

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rapid growth rate continues with increasing temperature. This rapid increase in grain size is illustrated

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by the orientation maps shown in Figure 6. These orientation maps, generated from scans taken of the same area of the specimen’s surface, show a sudden increase in grain size following the heat treatment

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at 250°C, where several large grains consume all surrounding grains. Note that the scan used to generate the orientation map at 400°C was taken at a much lower magnification than the previous scans. For reference, the fiduciary marks used for locating the previous scans can be observed as the four marks forming a rectangle in the center of the 400°C orientation map. For comparison, Figures 7 and 8 show the orientation maps of the ECAP and TRC following the same heat treatments, respectively. The ECAP material shows an increase in grain size with increasing temperature but no sudden jump as was observed in the FSP 3-7 material. The orientation maps of the TRC material show increasing grain growth with increasing heat treatment temperature. 3.3. Isothermal heat treatments Figure 9 shows the grain size as a function of time and temperature for the TRC, FSP 3-7, FSP 107 and ECAP materials. Note that 200°C heat treatments of the ECAP were not performed as no

ACCEPTED MANUSCRIPT previously tested material had shown any significant increase in grain size at 200°C. The TRC material shows normal but minor grain growth for all temperatures above 200°C. For all three SPD materials it

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was found that there is a temperature dependent dwell time (td), below which only minor grain growth would occur. This is most clearly illustrated by the 300°C and 350°C curves of the FSP 3-7 material. At

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heat treatment temperatures of 300°C and 350°C only minor grain growth occurred up until a heat

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treatment time of 60 minutes. At 60 minutes, there is a sudden increase in the average grain diameter

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when annealed at 300°C and 350°C. The FSP 10-7 material shows a similar trend with a significant increase in grain size after 60 minutes at 300°C and 350°C. The effects of td were less pronounced in the

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ECAP material, with only the 350°C heat treatment showing a minor jump between the 10 and 30

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3.3.1. Grain growth kinetics

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minute heat treatments.

The average grain size (D) of most polycrystalline materials following a heat treatment of time

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(t) at temperature (T) can be estimated by:

where Do is the initial grain size, n is the grain growth exponent and k is the result of the Arrhenius equation:

where ko is a material constant, R is the gas constant and Q is the activation energy for grain boundary motion [37]. The grain growth exponents can be calculated by rearranging equation (1) to yield:

ACCEPTED MANUSCRIPT and plotting ln(ΔD) as a function of ln(t). The resulting linear curve will have a slope equal to 1/n. These curves for the TRC material are shown in Figure 10. Plotting Dn-Don as a function of t will produce linear

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curves with a slope of k, as shown in for the 250°C, 300°C and 350°C heat treatments of the TRC in

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Figure 11. With the values of k calculated, the natural log form of equation 2:

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can be used to plot ln(k) as a function of 1000/T in Kelvin to produce a curve with a slope of –Q/R, thus allowing for calculation of the activation energy of grain boundary motion. This curve for the TRC

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material is shown in Figure 12.

This approach was used to calculate the n, k and Q values of the TRC material using the average

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grain diameter data for all heat treatment temperatures above 200°C. The n, k and Q values of the SPD

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materials were calculated using the average grain diameter data for heat treatments temperatures at

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which td was greater than 10 minutes. These results are summarized in Table 2. 3.3.2. Annealing Parameter

As td was found to vary with temperature, it follows that the annealing limit for microstructural stability would be both a function of annealing time and temperature. The Hollomon-Jaffe parameter has been shown to correctly describe the effects of annealing parameters on the hardness of steel alloys [38]. Its equation is as follows: (5) where T is the annealing temperature in kiloKelvin, C is a material constant and t is the annealing time in hours [39]. For steels, C has been shown to vary with carbon content with values typically between 15 and 21 [38, 40]. As this parameter has not been previously applied to magnesium alloys, the following

ACCEPTED MANUSCRIPT method was developed for the calculation of C. Equation 4, the natural log form of the Arrhenius

(6)

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equation for k, can be rearranged to:

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As both equation 5 and 6 are phenomenological, we can let Q/R = Hp and ln(ko)= C, and as k is a

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rate and thus inversely proportional to t, we can rewrite equation 6 as the equation 5 where the

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Hollomon-Jaffe constant (C) is equal to ln(ko). As ko is a material constant, this approach gives Hp values that are dependent on the microstructural conditions. A similar approach was used by Limarga et al. for

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the calculation of C in the Larson Miller parameter [41]. The values of C were calculated for the TRC and the SPD materials using the k and Q values calculated in section 3.3.1. The C values are summarized in

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Table 3.

Figure 13 shows plots of the change in average grain diameter (ΔD) as a function of the

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Hollomon-Jaffe parameter (Hp) for the TRC, FSP 3-7, FSP 10-7 and ECAP materials. The TRC material, Figure 13(a), shows a mostly linear relationship between ΔD and Hp. All three SPD materials show a significant change in the slope of the ΔD vs. Hp curve when the annealing conditions resulted in significant grain growth. In Figures 13 (b-d), this transition is indicated by a change in marker shape and color. While the FSP 10-7 and the ECAP materials show a significant change in the slope of the ΔD vs. Hp curves, the FSP 3-7 shows a less substantial change in slope but a significant jump in ΔD. The Hp value that resulted in this change in slope was designated Hpcritical, where Hpcritical can serve as the limit below which the annealing conditions will not result in significant grain growth and thus designated as the annealing limit for superplastic forming for these SPD microstructures. The Hpcritical values of the SPD material are summarized in Table 3. Discussion

ACCEPTED MANUSCRIPT Results from the isochronal and isothermal heat treatments of the TRC material show that it was relatively stable under both heat treatment conditions. The activation energy for grain boundary motion

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of the TRC material was found to be 167 kJ/mol. This is 75 kJ/mol higher than that of as-cast pure magnesium (92 kJ/mol) [42]. Wang et al. have reported a Q value for AZ31 magnesium of 110 kJ/mol,

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where they attributed the increase in activation energy to be due to the presence of the intermetallic

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phases [43]. In this study it appears that the TRC process, which involves both casting and rolling in a

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single step, contributed to the microstructural stability because of the high processing temperature that results in a low energy microstructure.

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SPD processing led to a decrease in the activation energy for grain boundary motion in all three processing conditions. The activation energy for the ECAP material was found to be 154 kJ/mol, still

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significantly greater than previously reported Q values for AZ31 (110 kJ/mol) [43] and for pure magnesium (92 kJ/mol) [42]. This result closely matches the Q values reported by Straska et al. and Kim

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et al. [24, 25] for EX-ECAP and ECAP AZ31 in the high temperature regimes of their studies. The increase over pure magnesium can be explained by the presence of the intermetallic phases, however, the increase over the reported Q value for AZ31 is not expected. Miao et al. [45] has argued that a fine grain size and strong basal texture resulted in a decrease in the activation energy of their hot rolled AZ31. The Q values from the FSP materials support this conclusion, where their fine grain size and texture likely resulted in a decrease in the activation energy. A possible explanation for the variance in Q values of the SPD material is discussed below, but it is important to note that the Hpcritical values of the three SPD materials follows the trend of the activation energies, with the ECAP material having the greatest Hpcritical value and the FSP 3-7 material having the lowest Hpcritical value. The results from the isochronal heat treatments, the calculated Q values, and the Hpcritical values all suggest that the microstructures produced by FSP for this study are less thermally stable than the ECAP

ACCEPTED MANUSCRIPT and TRC microstructures The grain growth behavior during annealing of a FSW ZK60 magnesium alloy has been described as abnormal grain growth (AGG) where the grain growth behavior was dependent

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upon the heterogeneous distribution of second phase particles [34]. Similar studies on grain growth of FSW AA2095 and 7010 aluminum alloys have shown that welds produced at higher heat inputs resulted

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in larger grains and greater microstructural stability in the FZ [35, 36]. Unlike these results, increasing

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the FSP feedrate from 300 mm/min to 1000 mm/min in this study resulted in no significant decrease in

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the average grain diameter in the FZ. In a study of FSW AZ31 magnesium, Commin et al. observed that although increasing the feedrate did result in a decrease in grain size for low tool rotational speeds, at

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high tool RPMs the effects of feedrate on grain size were negligible [46]. Our results suggest that at 700 RPMs the difference in heat input between the two feedrates did not have a significant influence on the

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average grain diameter.

The results from the isothermal heat treatments suggest that the different feed rates had an effect

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on the microstructural stability of the FSP materials. The FSP 10-7 material was found to have greater Q and Hpcritical values than the FSP 3-7 material. Several studies [35, 36, 47] have used the theory of cellular microstructures, purposed by Humphreys [48], to explain the grain growth behavior in FSW and ECAP Al alloys. While Attallah et al. [35] and Hassan et al. [36] used Humphreys model to explain how the fine grain size and distribution of second phase particles had an influence on AGG in FSW Al, Yu et al. used Hunphreys’ model to argue that the annealing behavior of the ECAP Al is better described as a continuous process of grain coarsening or continuous recrystallization rather than AGG. Yu et al. argued that this was because there was little to distinguish the “recrystallized” from the “unrecrystallized” grains and that there was little change in the texture following grain growth [47]. Figure 14 shows orientation maps of the (a) FSP 3-7, (b) FSP 10-7 and (c) ECAP material following heat treatments of 350°C for 10 minutes, 300°C for 60 minutes and 300°C for 120 minutes, respectively. Note that these annealing conditions correspond to Hp values one annealing step below the Hpcritical value for each of the

ACCEPTED MANUSCRIPT three materials. Fig. 14 (a) shows that several large grains have grown to consume the surrounding smaller grains in what appears to be AGG. The orientation maps of the FSP 10-7 and ECAP materials,

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Figs. 14 (b) and (c), show that certain grains have begun to grow, but there are many growing grains and these grains are more randomly distributed. This may explain why the FSP 3-7 material showed much

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greater grain growth than the FSP 10-7 and ECAP materials. As the annealing conditions approach

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Hpcritical, only a few grains in the FSP 3-7 material begin to grow abnormally whereas a significantly

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greater number of grains experience growth in the FSP 10-7 and ECAP materials. This results in the grains in the FSP 10-7 and ECAP materials encountering other growing grains and reaching a state of

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equilibrium at a much lower grain size than the FSP 3-7 material. These conclusions match previous descriptions of grain coarsening in ECAP Al [47] but also suggest that under the proper processing

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conditions, FSP AZ31 magnesium will experience grain coarsening rather than AGG. Humphreys’ model of cellular microstructure stability considers the ratio between the

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misorientation of a single (sub)grain and the mean misorientation of (sub)grains making up a microstructure. According to this model, a lower mean grain misorientation ( ) and a stronger texture make it more likely that a particular grain will grow abnormally. This will lead to greater microstructural instability and therefor a lower Q value for a particular microstructure. [48]. Figures 1-4 and Table 1 show that the materials in this study followed this trend, with the FSP 3-7 material having the greatest maximum texture peak intensity and the lowest

thus resulting in the lowest Q and Hpcritical values. In

Humphreys’ analysis it is stated that microstructures with a

greater than 15° will be stable and

unlikely to grow abnormally. Evidence of this is seen in Fig. 14(a) where the FSP 3-7 material, the only SPD microstructure with a

< 15°, showed evidence of AGG. Humphreys’ analysis considered two sets

of microstructural components; a set of mean properties

corresponding to the average

grain diameter, misorientation, boundary energy and mobility, and the properties of a “particular” (sub)grain corresponding to

. The condition for microstructural instability was found to be

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and

are the mean and “particular” (sub)grain radii, respectively. The relationship between

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where

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the boundary energy and the misorientation followed the Read-Shockley equation where:

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and the boundary mobility related to the misorientation by:

where γm, Θm and Mm are the boundary energy, misorientation and mobility of high angle boundaries

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(HABs). In Humphreys’ analysis it was shown that there is not only a minimum

ratio for instability to

occur but also a maximum size ratio to which the abnormal grain would grow. To calculate this ,

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maximum size ratio, we take

, and

and set Eqn. 7 equal to zero:

Eqn. 10 represents a state at which the (sub)grains will cease to grow abnormally. The roots of Eqn. 10 will give the maximum X to which the (sub)grains will grow:

In Humphreys’ analysis, Θm was assumed to equal 15°. Yu et al. showed that using the average misorientation of the ECAP Al for Θ resulted in agreement between the experimental results and the calculated maximum X value. In this study, rather than assuming Θm to be 15°, the average misorientation of all HABs, that is all boundaries with a misorientation greater than 15°, was calculated for each of the three SPD materials. Θ was then assumed to equal the total average misorientation of

ACCEPTED MANUSCRIPT each SPD microstructure. Humphreys’ development of Eqn. 8 was based upon the boundary mobilities for aluminum where B and n were stated to equal 5 and 4, respectively. Applying these constants to the

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SPD materials in this study it was found that Humphreys’ model significantly overestimated the ratio of the FSP materials and underestimated the ratio of the ECAP material when compared to the

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experimental values. These results are summarized in Table 4. In an attempt to match Humphreys’

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model to the experimental results, different values of n and B were applied to Eqn. 9. It was found that

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decreasing n to 3 and B to 4 results in a more accurate estimates of the maximum grain size ratio. As Humphreys’ approach was originally developed for aluminum, the improved accuracy of the model with

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lower constants may be explained by the difference in boundary mobility between aluminum and magnesium alloys though the physical implications of a change in these constants has not been

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investigated in this work. If this is the case, these results support the accuracy of Humphreys’ model and

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suggest that the stability of the SPD material can be explained by his approach. The values of Q, Hpcritical,

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the experimental maximum grain size ratios, and the calculated maximum grain size ratios all follow the same trend for the SPD materials. Conclusions

This work investigated the microstructural stability of four AZ31 magnesium microstructural conditions. One in the as-twin roll cast state and three modified by severe plastic deformation processes, including equal channel angular pressing and two friction stir processing conditions. The following general conclusions were found: 1. All three SPD processes significantly refined the grain size and produced similar textures that varied in intensity. 2. The results from the isochronal heat treatments show that the TRC, ECAP and FSP material processed at 300 mm/min and 700 RPM were stable up to approximately 250°C. Above 250°C,

ACCEPTED MANUSCRIPT the TRC and ECAP materials showed minor grain growth while the FSP 3-7 material showed a significant increase in grain size and rapid grain growth.

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3. Calculations of the activation energy for grain boundary motion from isothermal heat treatments showed that all three SPD processing conditions decreased the value of Q when

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compared to the TRC material and suggest that the FSP material was less stable than the ECAP

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material.

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4. The Hollomon-Jaffe parameter was found to be a good annealing parameter by which the limit for superplastic forming can be determined.

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5. Microstructural evidence of abnormal grain growth was found in the FSP 3-7 material while the grain growth behavior of the FSP 10-7 and ECAP material is better described as continuous

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recrystallization or grain coarsening.

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6. Humphreys’s model of cellular microstructural stability accurately describes the annealing

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Acknowledgments:

This work was made possible by a National Priorities Research Program grant from the Qatar National Research Fund under grant number NPRP 09-611-2-236 and by a Graduate Assistance in Areas of National Need (GAANN) grant through the U.S. Department of Education. CFDA number: 84.200

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35. Attallah, M.M. and Salem, H.G. “ Friction stir welding parameters: a tool for controlling abnormal grain growth during subsequent heat treatment.” Materials Science and Engineering A. 391 (2005):51-59.

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42. Frost H J, Ashby M F (Eds.), Deformation-Mechanism Maps, the plasticity and creep of metals and ceramics, Pergamon Press, New York, 1982, pp. 43–45.

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43. Wang X, Hu L, Liu K, and Zhang Y. “Grain growth kinetics of bulk AZ31 magnesium alloy by hot

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pressing.” Journal of Alloys and Compounds, 527 (2012): 193-96

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44. Lambri O A, Riehemann W, and Trojanova Z. “Mechanical spectroscopy of commercial AZ91 magnesium alloy.” Scripta Materialia, 45 (2001): 1365-71 45. Miao Q, Hu L, Wang X, and Wang E. “Grain growth kinetics of a fine-grained AZ31 magnesium alloy produced by hot rolling.” Journal of Alloys and Compounds, 493 (2010): 87-90. 46. Commin, L., Dumont, M., Masse, J.E. and Barrallier, L. “Friction stir welding of AZ31 magnesium alloy rolled sheets: Influence of processing parameters.” Acta Materialia, 57 (2009): 326-34. 47. Yu, C.Y., Sun, P.L., Kao, P.W. and Chang, C.P. “Evolution of microstructure during annealing of a severely deformed aluminum.” Materials Science and Engineering A. 366 (2004):310-317.

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Table 1: Summary of the average grain diameter, texture and average grain boundary misorientation of the TRC, FSP and ECAP materials

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Table 2: Grain growth kinetics of the TRC and SPD materials arranged in descending order of activation energy for grain boundary motion. Table 3. Calculated C and Hpcritical values of the TRC and SPD materials.

values for the SPD materials

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Table 4. Experimentl and calculated

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Figure Captions:

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Figure 1: Microstructure and texture of the TRC material represented in an orientation map (a) and pole figures (b). Figure 2: Microstructure and texture of the FSP 300 mm/min 700 RPM material represented in an orientation map (a) and pole figures (b) where the processing direction is normal to the page.

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Figure 3: Microstructure and texture of the FSP 1000 mm/min 700 RPM material represented in an orientation map (a) and pole figures (b) where the processing direction is normal to the page.

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Figure 4: Microstructure and texture of the ECAP material represented in an orientation map (a) and pole figures (b) where the pressing direction is normal to the page. Figure 5: Results from the isochronal heat treatments shown as the average grain diameter as a function of heat treatment temperature and material. Figure 6: Microstructure of the FSP 3-7 material as a function of heat treatment temperature. Figure 7: Microstructure of the ECAP material as a function of heat treatment temperature. Figure 8: Microstructure of the TRC material as a function of heat treatment temperature. Figure 9. Average grain diameter as a function of time and temperature for the (a) TRC, (b) FSP 3-7, (c) FSP 10-7 and (d) ECAP materials. Figure 10: ln(ΔD) as a function of ln(time) and temperature for the TRC material. The slopes of the linear curve fits were used to calculate the grain growth exponents. Figure 11: Dn-Don as a function of time and temperature for the 250°C, 300°C and 350°C heat treatments of the TRC material. The slopes of the linear curve fits were used to calculate k. Figure 12. ln(k) as a function of temperature for the TRC material. The slope of the linear curve fit was used to calculate the activation energy of grain boundary motion.

ACCEPTED MANUSCRIPT Figure 13.Change in average grain diameter (ΔD) as a function of Hp for the (a) TRC, (b) FSP 3-7, (c) FSP 10-7 and (d) ECAP materials.

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Figure 14. Orientaion maps of the (a) FSP 3-7 material following an anneal at 350°C for 10 minutes, (b) FSP 10-7 material following an anneal at 300° for 60 minutes and (c) ECAP material following an anneal at 300°C for 120 minutes.

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15.32±1.2

1.41±0.54

1.67±0.75

3.82±1.07

5.96

54.39

25.26

14.41

26.23±2.29

11.58±1.88

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Max. Texture Peak Intensity (MRD) Ave. Grain Boundary Misorientation (°)

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Ave. Grain Diameter (µm)

Twin Roll Cast

35.36±2.9

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500°C

k n k n k n k n

2.47E+03 4.83

1.89E+04 5.18

4.44E+04 5.4

1.04E+05 5.55 1.30E+03 5.7 1.83E+03 3.8 3.28E+05 4.6

8.81E+05 6.2 1.35E+04 5 3.67E+03 4.8 4.73E+05 5.6

2.44E+10 9.1 4.49E+04 6 8.78E+05 4.7 1.53E+04 5

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Q(kJ/mol) 167 154 89 42

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ECAP

FSP 10-7

FSP 3-7

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23.95

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FSP 3-7 33.16 92.21 35.53

FSP 10-7 16.09 33.36 16.82

ECAP 4.39 2.39 3.36

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Table 4 . Experimental n=4, B=5 n=3, B=4

ACCEPTED MANUSCRIPT Ref. No.: MTL-13495 Title: Thermal Microstructural Stability of AZ31 Magnesium after Severe Plastic Deformation Highlights ECAP and FSP both decreased the thermal microstructural stability of TRC AZ31 Mg.

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An annealing parameter was used to find the thermal limits of superplastic forming.

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The recrystallization mechanism was effected by the FSP conditions.

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Humphreys’s model accurately described the annealing behavior of the SPD materials.

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