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Effects of annealing at high pressure on structure and mechanical properties of Al87Ni7Gd6 metallic glass
Intermetallics 10 (2002) 1099–1103 www.elsevier.com/locate/intermet
Effects of annealing at high pressure on structure and mechanical properties of Al87Ni7Gd6 metallic glass B.C. Koa, P. Wesselinga, O.L. Vatamanua, G.J. Shifletb, J.J. Lewandowskia,* a
Department of Materials Science and Engineering, Case Western Reserve University, Case School of Engineering, Cleveland, OH 44106, USA b Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903, USA Received 8 July 2002; accepted 29 July 2002
Abstract The effects of annealing and annealing with a superimposed pressure of 940 MPa on the primary crystallization behaviour of a-Al and the resulting micro-hardness have been studied for as-quenched Al87Ni7Gd6 metallic glass. Isothermal annealing experiments were conducted for 30 min at 188 C, 191 C, and 205 C in silicone oil maintained either at atmospheric pressure (i.e. 0.1 MPa) or at 940 MPa. XRD analyses detected the evolution of structure with annealing at 0.1 MPa, while specimens annealed with 940 MPa pressure exhibited sharper diffraction peaks than those annealed at 0.1 MPa. DSC measurements were conducted on the as-received amorphous ribbons as well as ribbons annealed at different temperatures at either 0.1. MPa or with 940 MPa superimposed pressure. Specimens annealed with 940 MPa pressure exhibited higher onset temperatures (i.e. Tx1) and temperatures for the first exothermic peak (i.e. Tp1) for primary crystallization. TEM measurements revealed an increase in the volume fraction of aAl with increases in annealing temperature, while micro-hardness measurements revealed an increase in hardness with increasing amounts of a-Al. Specimens annealed with 940 MPa pressure exhibited further increases in both the volume fraction of a-Al and resulting micro-hardness. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: B. Glasses, metallic; Mechanical properties at ambient temperatures; Precipitates; C. Extrusion; Hot isostatic pressing
1. Introduction Al-based metallic glasses and devitrified versions of these alloys are receiving increasing attention because of their extremely high specific strength [1–3]. While fully amorphous Al-based glasses exhibit strengths in the 1 GPa range, partially crystallized metallic glasses containing a homogeneous distribution of Al nano-crystals (i.e. diameter < 10 nm) have been reported to exhibit tensile fracture strengths of 1560 MPa [1, 2, 4]. Although there have been recent breakthroughs in the processing of various compositions of bulk (e.g. > 10 mm thick) metallic glasses, amorphous metals based on the Al system have only been possible to this point with melt spinning, thereby limiting thicknesses to the range < 250 um. The production of amorphous powders is also possible via a number of different techniques, but the slower cooling rates obtained in such processing * Corresponding author. Fax: +1-413-431-9519. E-mail address: [email protected] (J.J. Lewandowski).
further limits the size of powders which can be obtained as fully amorphous. In this regard, consolidation or forming processes at warm/high temperatures are needed in order to produce bulk metallic glasses or nanocomposites [4, 5]. The ability to produce high strength, partially devitrified Al metallic glasses via heat treatment/annealing [1–4] has prompted a number of processing studies designed to consolidate amorphous powders into fully dense material [4, 5]. As such, one of the key features in these processes is to understand and document the effects of various processing variables (e.g. T, pressure, time, etc.) on the resulting structure and properties of the amorphous material. While it is clear that annealing at sufficiently high temperatures can produce devitrification, much less work has investigated the effects of changes in stress state (e.g. pressure) on devitrification. As most industrial consolidation and metal working processes include pressure in some form, as reviewed recently [6], it is also important to evaluate the effects of confining pressure on devitrification as well as on flow/fracture.
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00164-4
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B.C. Ko et al. / Intermetallics 10 (2002) 1099–1103
Only a few studies have directly investigated the effects of hydrostatic pressure on the flow and fracture of amorphous metals and these are summarized in this conference [11] while the individual studies are summarized elsewhere [6–12]. The effects of annealing with confining pressure on structure evolution has been directly investigated in a number of amorphous metal systems [13–18], although the results are somewhat conflicting depending on the test techniques utilized and systems under investigation. Some of these previous investigations have indicated that crystallization can be promoted by annealing with high pressure [13–15], while others have shown that crystallization can be inhibited by annealing with high pressure [16–18], depending on the alloy system, pressure, temperature, and other experimental variables. The work reported in this paper is part of a larger study investigating the effects of processing conditions on the resulting structure and properties of amorphous metals. The aim of this study is to report on preliminary experimental results on the effects of annealing at different temperatures with 0.1 MPa and 940 MPa superimposed hydrostatic pressure. The effects of such exposures on structure evolution and resulting hardness are reported in this work for Al87Ni7Gd6 metallic glass.
2. Experimental procedures The materials used in these experiments were supplied from the University of Virginia. Melt-spun ribbons with approximately 30 mm thickness and 3 mm width were provided and held at 0 C until any experiments were conducted. The ribbons had the composition of Al87Ni7Gd6 in at.%. The as-received ribbons were characterized by a combination of XRD (Scintag X1 apparatus) using Cu-Ka radiation; thermal analysis via differential scanning calorimetry (MDSC, TA 2910) using a heating rate of 20 K/min; Transmission electron microscopy (TEM) analyses on a Philips CM20 using foils electro-polished at approximately 30 C in a 30% Nitric acid+70% Methanol solution. The current and voltage settings were 0.1 A and 10 V, respectively. Micro-hardness was measured with a BUEHLER hardness machine at a load of 50 g. At least eight measurements were made for each condition. Isothermal annealing experiments were all conducted for 30 min in silicone oil (GE, USEP SF 1147) maintained at 188, 191, and 205 C. In addition, identical annealing experiments were conducted with 940 GPa superimposed pressure via the use of an Innovare Hydrostatic Extrusion Rig, the details of which are described elsewhere [6]. In these cases, the oil was first heated to the desired stable annealing temperature. The as-received ribbon was then immersed in the oil and the oil compressed to produce a hydrostatic pressure of 940
MPa. The hydrostatic extrusion apparatus is instrumented with pressure sensors and the pressure variation was less than 55 MPa during the annealing with pressure experiments. Annealing with 940 MPa pressure was also conducted for 30 min, enabling a direct comparison to the specimens simply annealed at 0.1 MPa. All specimens were water quenched after the various annealing experiments and maintained at 0 C while awaiting subsequent analyses. The analyses described above for the as-received foils were similarly conducted on the specimens annealed at 0.1 MPa and 940 MPa.
3. Results and discussion Fig. 1 summarizes the XRD results obtained on the as-received ribbon as well as from the ribbons isothermally annealed for 30 min at 188, 191, and 205 C under 0.1 and 940 MPa, respectively. The as-quenched state exhibited a relatively broad peak centered at 38 , consistent with the lack of long range order exhibited by amorphous Al. The specimens annealed at 0.1 MPa exhibited a sharpening of the broad peak as well as the evolution of other peaks corresponding to (200) Al. Annealing with 940 MPa produced more distinct sharpening of the broad peak as well as more distinct secondary (200) Al peak. The small distinct diffraction peaks at 38.58, 44.84, 65.35, and 78.35 correspond to crystalline a-Al with lattice constant of 4.038 A˚, 4.039 A˚, 4.035 A˚, 4.044A˚, respectively. In the case of annealing with 940 MPa pressure, the Al (111) peak at 38.53 becomes sharper while other Al (200) peaks start to appear at 191 C. This was not observed for identical ribbons simply annealed at 0.1 MPa and 191 C. Fig. 2 provides TEM dark-field images of the asreceived ribbon as well as ribbons annealed at 188 and 205 C with 0.1 and 940 MPa pressure. The as-received
Fig. 1. XRD results of the Al87Ni7Gd6 alloy annealed at different temperatures with either 0.1 or 940 MPa pressure.
B.C. Ko et al. / Intermetallics 10 (2002) 1099–1103
ribbons did not exhibit any evidence of a-Al in any of the five as-received ribbons examined. Annealing for 30 min at 188 C and 205 C with 0.1 MPa pressure clearly produced increasing amounts of a-Al. Annealing with 940 MPa at these temperatures further increased the number density and volume fraction of a-Al beyond those exhibited by the specimens annealed at 0.1 MPa, as quantified elsewhere [19]. In addition, quantitative results on a-Al nanocrystal density evolution during continuous heating have shown that particle density increases continuously between 200 and 290 C from 1.91022 to 4.31022 nanocrystals per cubic meter [20]. Continuous DSC was performed at 20 K/minute on the as-received ribbon as well as after isothermal annealing at different temperatures under 0.1 and 940 MPa in order to compare the thermal stability of the resulting materials after such exposures. As shown in Fig. 3, the onset (Tx1) temperature of the as-quenched material is 189 C and the peak temperature for the first peak (Tp1) is 223 C. Fig. 3 reveals effects of annealing at 0.1 MPa on both Tx1 and Tp1, while annealing also clearly reduces the enthalpy change associated with the first peak. Annealing with 940 MPa pressure further reduces the enthalpy change associated with the first peak, while additionally shifting both Tx1 and Tp1 to
1101
temperatures higher than those exhibited by specimens simply annealed at 0.1 MPa. These effects are summarized in Fig. 4, which illustrates the effects of annealing at 0.1 MPa and 940 MPa on both Tx1 and Tp1. The observation of the increase in both Tx1 and Tp1 for specimens annealed at 0.1 MPa is consistent with an increase in the thermal stability of the partially crystallized metallic glass. The thermal stability of the remaining structure is enhanced due to the higher volume fraction of a-Al, which is accompanied by solute enrichment of the remaining amorphous [14] matrix. The further increase in both Tx1 and Tp1 with annealing at 940 MPa is consistent with the higher volume fraction of a-Al present in specimens annealed with these higher pressures, as shown in Fig. 2. The continuous DSC scans following heat treatment at the indicated conditions shown in Fig. 3 reveal that either more Al nanocrystals form by nucleation, further growth of existing particles occurs, or a combination of both processes occur. Both cases (i.e. additional nucleation,
Fig. 3. DSC traces of the Al87Ni7Gd6 alloy annealed at different temperatures with either 0.1 MPa or 940 MPa pressure.
Fig. 2. TEM dark-field images of the Al87Ni7Gd6 alloy (a) as-quenched, annealed at (b) 188 C, 0.1 MPa (c) 188 C, 940 MPa (d) 205 C, 0.1 MPa and (e) 205 C, 940 MPa. The light spots are a-Al.
Fig. 4. Effects of annealing and annealing with either 0.1 or 940 MPa pressure on the primary onset (Tx1) and peak (Tp1) temperatures for the Al87Ni7Gd6 alloy. As-quenched values are also provided for comparison.
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further growth) should produce additional solute enrichment of the remaining amorphous matrix, consistent with the observations shown in Figs. 3 and 4. Ongoing TEM quantitative analyses of particle densities, sizes, etc. are underway in order to establish the source(s) of these observations. The mechanical properties of the annealed ribbons were significantly different than that of the as-received ribbons, which exhibited Hv=3.33 0.08 GPa. Fig. 5 summarizes the effects of annealing at 0.1 MPa and 940 MPa on the resulting micro-hardness. Annealing at 0.1 MPa produced increases in Hv from 3.33 ( 0.08) GPa in the as-received ribbons to values of 4.6 ( 0.05) GPa in the ribbon annealed for 30 mins at 205 C. The increase in Hv with annealing is entirely consistent with the increases in the amount of a-Al present in specimens annealed under these conditions and shown in Fig. 2. The higher Hv exhibited by the specimens annealed with 940 MPa is also consistent with the greater volume fraction of a-Al present in specimens annealed with 940 MPa in comparison to those simply annealed at 0.1 MPa. The strengthening observed in these devitrified amorphous Al ribbons may arise from a number of sources [2, 3, 21, 22]. It has been reported that the higher hardness, which accompanies devitrification in such systems is due to solid solution hardening of the remaining amorphous matrix via solute rejection [21, 22]. Dispersion hardening approaches have also been suggested where the presence of the a-Al particles in the amorphous matrix produces hardening via processes analogous to dispersion hardening of crystalline metals [2]. The sources of the strengthening in the present studies are under continuing investigation.
4. Conclusions The effects of annealing and annealing with a superimposed pressure of 940 MPa on the primary crystallization behaviour of a-Al and the resulting microhardness have been studied for as-quenched Al87Ni7Gd6 metallic glass. Isothermal annealing experiments were conducted for 30 min at 188, 191 and 205 C in silicone oil maintained either at atmospheric pressure (i.e. 0.1 MPa) or at 940 MPa. XRD analyses detected the evolution of structure with annealing at 0.1 MPa, while specimens annealed with 940 MPa pressure exhibited sharper diffraction peaks than those annealed at 0.1 MPa. DSC measurements were conducted on the asreceived amorphous ribbons as well as ribbons annealed at different temperatures at either 0.1 MPa or with 940 MPa superimposed pressure. Specimens annealed with 940 MPa pressure exhibited higher onset temperatures (i.e. Tx1) and temperatures for the first exothermic peak (i.e. Tp1) for primary crystallization. TEM measurements revealed an increase in the volume fraction of aAl with increases in annealing temperature, while microhardness measurements revealed an increase in hardness with increasing amounts of a-Al. Specimens annealed with 940 MPa pressure exhibited further increases in both the volume fraction of a-Al and resulting microhardness. The results suggest that annealing with 940 MPa at these temperatures promotes the evolution of aAl.
Acknowledgements Financial support provided by Boeing Company via a subcontract from the DARPA SAM Program is gratefully acknowledged. Approved for public release. Distribution unlimited. Experimental assistance from Joshua Caris and Awlah Awadalla are also acknowledged.
References
Fig. 5. Effects of annealing and annealing with either 0.1 or 940 MPa pressure on the micro-hardness using 50 g indentation load. Values for the as-quenched ribbons also provided for comparison.
[1] Kim YH, Inoue A, Masumoto T. Mater Trans JIM 1991; 32:331. [2] Inoue A, Horio Y, Kim YH, Masumoto T. Mater Trans JIM 1992;33:669. [3] Chen H, He Y, Shiflet GJ, Poon SJ. Nature 1994;367(10):541. [4] Inoue A, Kimura H. J Light Metals, 2000;1:31. [5] Eckert J, Seidel ML, Xing Q, Borner I, Weib B. Nanostructured Materials 1999;12:439. [6] Lewandowski JJ, Lowhaphandu P. International Materials Reviews 1998;43(4):145. [7] Lowhaphandu P, Montgomery SL, Lewandowski JJ. Scripta Metall et Materialia 1999;41:19. [8] Lowhaphandu P, Ludrosky LA, Montgomery SL, Lewandowski JJ. Intermetallics 2000;8:487. [9] Lewandowski JJ. Mater Trans JIM 2001;42(4):633.
B.C. Ko et al. / Intermetallics 10 (2002) 1099–1103 [10] Lewandowski JJ, Lowhaphandu P. Philosophical Magazine A 2002;82(17)(in press). [11] Lewandowski JJ, this conference proceedings, 2002. [12] Davis LA, Kavesh S. J Mater Sci 1975;10:453. [13] Yao B, Guo HC, Wang J, Ding BZ, Li H, Wang AM, Hu ZQ. Physica B 1996;228:379. [14] Gu XJ, Ye F, Zhou F, Lu K. Mater Sci Eng 2000; A278:61. [15] Wang WH, He DW, Zhao DQ, Yao YS. Appl Phys Lett 1999; 75:2770. [16] Ye F, Lu K. Acta Mater 1998;46:5965.
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[17] Imura T, Suwa M, Fuji K. Mater Sci Eng 1988;97:247. [18] Jiang JZ, Zhou TJ, Rasmussen H, Kuhn U, Eckert J, Lathe C. Appl Phys Lett 2000;77(22):3553. [19] Ko BC, Lewandowski JJ. Unpublished research, Case Western Reserve University, 2002. [20] Gao M and Shiflet GJ. Unpublished research, University of Virginia, 2002. [21] Hono K, Zhang Y, Tsai AP, Inoue AI, Sakurai T. Scripta Metall Mater 1995;32(2):191. [22] Zhong ZC, Jiang XY, Greer AL. Mater Sci Eng 1997;A226– 228:531.
Effects of annealing at high pressure on structure and mechanical properties of Al87Ni7Gd6 metallic glass B.C. Koa, P. Wesselinga, O.L. Vatamanua, G.J. Shifletb, J.J. Lewandowskia,* a
Department of Materials Science and Engineering, Case Western Reserve University, Case School of Engineering, Cleveland, OH 44106, USA b Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903, USA Received 8 July 2002; accepted 29 July 2002
Abstract The effects of annealing and annealing with a superimposed pressure of 940 MPa on the primary crystallization behaviour of a-Al and the resulting micro-hardness have been studied for as-quenched Al87Ni7Gd6 metallic glass. Isothermal annealing experiments were conducted for 30 min at 188 C, 191 C, and 205 C in silicone oil maintained either at atmospheric pressure (i.e. 0.1 MPa) or at 940 MPa. XRD analyses detected the evolution of structure with annealing at 0.1 MPa, while specimens annealed with 940 MPa pressure exhibited sharper diffraction peaks than those annealed at 0.1 MPa. DSC measurements were conducted on the as-received amorphous ribbons as well as ribbons annealed at different temperatures at either 0.1. MPa or with 940 MPa superimposed pressure. Specimens annealed with 940 MPa pressure exhibited higher onset temperatures (i.e. Tx1) and temperatures for the first exothermic peak (i.e. Tp1) for primary crystallization. TEM measurements revealed an increase in the volume fraction of aAl with increases in annealing temperature, while micro-hardness measurements revealed an increase in hardness with increasing amounts of a-Al. Specimens annealed with 940 MPa pressure exhibited further increases in both the volume fraction of a-Al and resulting micro-hardness. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: B. Glasses, metallic; Mechanical properties at ambient temperatures; Precipitates; C. Extrusion; Hot isostatic pressing
1. Introduction Al-based metallic glasses and devitrified versions of these alloys are receiving increasing attention because of their extremely high specific strength [1–3]. While fully amorphous Al-based glasses exhibit strengths in the 1 GPa range, partially crystallized metallic glasses containing a homogeneous distribution of Al nano-crystals (i.e. diameter < 10 nm) have been reported to exhibit tensile fracture strengths of 1560 MPa [1, 2, 4]. Although there have been recent breakthroughs in the processing of various compositions of bulk (e.g. > 10 mm thick) metallic glasses, amorphous metals based on the Al system have only been possible to this point with melt spinning, thereby limiting thicknesses to the range < 250 um. The production of amorphous powders is also possible via a number of different techniques, but the slower cooling rates obtained in such processing * Corresponding author. Fax: +1-413-431-9519. E-mail address: [email protected] (J.J. Lewandowski).
further limits the size of powders which can be obtained as fully amorphous. In this regard, consolidation or forming processes at warm/high temperatures are needed in order to produce bulk metallic glasses or nanocomposites [4, 5]. The ability to produce high strength, partially devitrified Al metallic glasses via heat treatment/annealing [1–4] has prompted a number of processing studies designed to consolidate amorphous powders into fully dense material [4, 5]. As such, one of the key features in these processes is to understand and document the effects of various processing variables (e.g. T, pressure, time, etc.) on the resulting structure and properties of the amorphous material. While it is clear that annealing at sufficiently high temperatures can produce devitrification, much less work has investigated the effects of changes in stress state (e.g. pressure) on devitrification. As most industrial consolidation and metal working processes include pressure in some form, as reviewed recently [6], it is also important to evaluate the effects of confining pressure on devitrification as well as on flow/fracture.
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00164-4
1100
B.C. Ko et al. / Intermetallics 10 (2002) 1099–1103
Only a few studies have directly investigated the effects of hydrostatic pressure on the flow and fracture of amorphous metals and these are summarized in this conference [11] while the individual studies are summarized elsewhere [6–12]. The effects of annealing with confining pressure on structure evolution has been directly investigated in a number of amorphous metal systems [13–18], although the results are somewhat conflicting depending on the test techniques utilized and systems under investigation. Some of these previous investigations have indicated that crystallization can be promoted by annealing with high pressure [13–15], while others have shown that crystallization can be inhibited by annealing with high pressure [16–18], depending on the alloy system, pressure, temperature, and other experimental variables. The work reported in this paper is part of a larger study investigating the effects of processing conditions on the resulting structure and properties of amorphous metals. The aim of this study is to report on preliminary experimental results on the effects of annealing at different temperatures with 0.1 MPa and 940 MPa superimposed hydrostatic pressure. The effects of such exposures on structure evolution and resulting hardness are reported in this work for Al87Ni7Gd6 metallic glass.
2. Experimental procedures The materials used in these experiments were supplied from the University of Virginia. Melt-spun ribbons with approximately 30 mm thickness and 3 mm width were provided and held at 0 C until any experiments were conducted. The ribbons had the composition of Al87Ni7Gd6 in at.%. The as-received ribbons were characterized by a combination of XRD (Scintag X1 apparatus) using Cu-Ka radiation; thermal analysis via differential scanning calorimetry (MDSC, TA 2910) using a heating rate of 20 K/min; Transmission electron microscopy (TEM) analyses on a Philips CM20 using foils electro-polished at approximately 30 C in a 30% Nitric acid+70% Methanol solution. The current and voltage settings were 0.1 A and 10 V, respectively. Micro-hardness was measured with a BUEHLER hardness machine at a load of 50 g. At least eight measurements were made for each condition. Isothermal annealing experiments were all conducted for 30 min in silicone oil (GE, USEP SF 1147) maintained at 188, 191, and 205 C. In addition, identical annealing experiments were conducted with 940 GPa superimposed pressure via the use of an Innovare Hydrostatic Extrusion Rig, the details of which are described elsewhere [6]. In these cases, the oil was first heated to the desired stable annealing temperature. The as-received ribbon was then immersed in the oil and the oil compressed to produce a hydrostatic pressure of 940
MPa. The hydrostatic extrusion apparatus is instrumented with pressure sensors and the pressure variation was less than 55 MPa during the annealing with pressure experiments. Annealing with 940 MPa pressure was also conducted for 30 min, enabling a direct comparison to the specimens simply annealed at 0.1 MPa. All specimens were water quenched after the various annealing experiments and maintained at 0 C while awaiting subsequent analyses. The analyses described above for the as-received foils were similarly conducted on the specimens annealed at 0.1 MPa and 940 MPa.
3. Results and discussion Fig. 1 summarizes the XRD results obtained on the as-received ribbon as well as from the ribbons isothermally annealed for 30 min at 188, 191, and 205 C under 0.1 and 940 MPa, respectively. The as-quenched state exhibited a relatively broad peak centered at 38 , consistent with the lack of long range order exhibited by amorphous Al. The specimens annealed at 0.1 MPa exhibited a sharpening of the broad peak as well as the evolution of other peaks corresponding to (200) Al. Annealing with 940 MPa produced more distinct sharpening of the broad peak as well as more distinct secondary (200) Al peak. The small distinct diffraction peaks at 38.58, 44.84, 65.35, and 78.35 correspond to crystalline a-Al with lattice constant of 4.038 A˚, 4.039 A˚, 4.035 A˚, 4.044A˚, respectively. In the case of annealing with 940 MPa pressure, the Al (111) peak at 38.53 becomes sharper while other Al (200) peaks start to appear at 191 C. This was not observed for identical ribbons simply annealed at 0.1 MPa and 191 C. Fig. 2 provides TEM dark-field images of the asreceived ribbon as well as ribbons annealed at 188 and 205 C with 0.1 and 940 MPa pressure. The as-received
Fig. 1. XRD results of the Al87Ni7Gd6 alloy annealed at different temperatures with either 0.1 or 940 MPa pressure.
B.C. Ko et al. / Intermetallics 10 (2002) 1099–1103
ribbons did not exhibit any evidence of a-Al in any of the five as-received ribbons examined. Annealing for 30 min at 188 C and 205 C with 0.1 MPa pressure clearly produced increasing amounts of a-Al. Annealing with 940 MPa at these temperatures further increased the number density and volume fraction of a-Al beyond those exhibited by the specimens annealed at 0.1 MPa, as quantified elsewhere [19]. In addition, quantitative results on a-Al nanocrystal density evolution during continuous heating have shown that particle density increases continuously between 200 and 290 C from 1.91022 to 4.31022 nanocrystals per cubic meter [20]. Continuous DSC was performed at 20 K/minute on the as-received ribbon as well as after isothermal annealing at different temperatures under 0.1 and 940 MPa in order to compare the thermal stability of the resulting materials after such exposures. As shown in Fig. 3, the onset (Tx1) temperature of the as-quenched material is 189 C and the peak temperature for the first peak (Tp1) is 223 C. Fig. 3 reveals effects of annealing at 0.1 MPa on both Tx1 and Tp1, while annealing also clearly reduces the enthalpy change associated with the first peak. Annealing with 940 MPa pressure further reduces the enthalpy change associated with the first peak, while additionally shifting both Tx1 and Tp1 to
1101
temperatures higher than those exhibited by specimens simply annealed at 0.1 MPa. These effects are summarized in Fig. 4, which illustrates the effects of annealing at 0.1 MPa and 940 MPa on both Tx1 and Tp1. The observation of the increase in both Tx1 and Tp1 for specimens annealed at 0.1 MPa is consistent with an increase in the thermal stability of the partially crystallized metallic glass. The thermal stability of the remaining structure is enhanced due to the higher volume fraction of a-Al, which is accompanied by solute enrichment of the remaining amorphous [14] matrix. The further increase in both Tx1 and Tp1 with annealing at 940 MPa is consistent with the higher volume fraction of a-Al present in specimens annealed with these higher pressures, as shown in Fig. 2. The continuous DSC scans following heat treatment at the indicated conditions shown in Fig. 3 reveal that either more Al nanocrystals form by nucleation, further growth of existing particles occurs, or a combination of both processes occur. Both cases (i.e. additional nucleation,
Fig. 3. DSC traces of the Al87Ni7Gd6 alloy annealed at different temperatures with either 0.1 MPa or 940 MPa pressure.
Fig. 2. TEM dark-field images of the Al87Ni7Gd6 alloy (a) as-quenched, annealed at (b) 188 C, 0.1 MPa (c) 188 C, 940 MPa (d) 205 C, 0.1 MPa and (e) 205 C, 940 MPa. The light spots are a-Al.
Fig. 4. Effects of annealing and annealing with either 0.1 or 940 MPa pressure on the primary onset (Tx1) and peak (Tp1) temperatures for the Al87Ni7Gd6 alloy. As-quenched values are also provided for comparison.
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further growth) should produce additional solute enrichment of the remaining amorphous matrix, consistent with the observations shown in Figs. 3 and 4. Ongoing TEM quantitative analyses of particle densities, sizes, etc. are underway in order to establish the source(s) of these observations. The mechanical properties of the annealed ribbons were significantly different than that of the as-received ribbons, which exhibited Hv=3.33 0.08 GPa. Fig. 5 summarizes the effects of annealing at 0.1 MPa and 940 MPa on the resulting micro-hardness. Annealing at 0.1 MPa produced increases in Hv from 3.33 ( 0.08) GPa in the as-received ribbons to values of 4.6 ( 0.05) GPa in the ribbon annealed for 30 mins at 205 C. The increase in Hv with annealing is entirely consistent with the increases in the amount of a-Al present in specimens annealed under these conditions and shown in Fig. 2. The higher Hv exhibited by the specimens annealed with 940 MPa is also consistent with the greater volume fraction of a-Al present in specimens annealed with 940 MPa in comparison to those simply annealed at 0.1 MPa. The strengthening observed in these devitrified amorphous Al ribbons may arise from a number of sources [2, 3, 21, 22]. It has been reported that the higher hardness, which accompanies devitrification in such systems is due to solid solution hardening of the remaining amorphous matrix via solute rejection [21, 22]. Dispersion hardening approaches have also been suggested where the presence of the a-Al particles in the amorphous matrix produces hardening via processes analogous to dispersion hardening of crystalline metals [2]. The sources of the strengthening in the present studies are under continuing investigation.
4. Conclusions The effects of annealing and annealing with a superimposed pressure of 940 MPa on the primary crystallization behaviour of a-Al and the resulting microhardness have been studied for as-quenched Al87Ni7Gd6 metallic glass. Isothermal annealing experiments were conducted for 30 min at 188, 191 and 205 C in silicone oil maintained either at atmospheric pressure (i.e. 0.1 MPa) or at 940 MPa. XRD analyses detected the evolution of structure with annealing at 0.1 MPa, while specimens annealed with 940 MPa pressure exhibited sharper diffraction peaks than those annealed at 0.1 MPa. DSC measurements were conducted on the asreceived amorphous ribbons as well as ribbons annealed at different temperatures at either 0.1 MPa or with 940 MPa superimposed pressure. Specimens annealed with 940 MPa pressure exhibited higher onset temperatures (i.e. Tx1) and temperatures for the first exothermic peak (i.e. Tp1) for primary crystallization. TEM measurements revealed an increase in the volume fraction of aAl with increases in annealing temperature, while microhardness measurements revealed an increase in hardness with increasing amounts of a-Al. Specimens annealed with 940 MPa pressure exhibited further increases in both the volume fraction of a-Al and resulting microhardness. The results suggest that annealing with 940 MPa at these temperatures promotes the evolution of aAl.
Acknowledgements Financial support provided by Boeing Company via a subcontract from the DARPA SAM Program is gratefully acknowledged. Approved for public release. Distribution unlimited. Experimental assistance from Joshua Caris and Awlah Awadalla are also acknowledged.
References
Fig. 5. Effects of annealing and annealing with either 0.1 or 940 MPa pressure on the micro-hardness using 50 g indentation load. Values for the as-quenched ribbons also provided for comparison.
[1] Kim YH, Inoue A, Masumoto T. Mater Trans JIM 1991; 32:331. [2] Inoue A, Horio Y, Kim YH, Masumoto T. Mater Trans JIM 1992;33:669. [3] Chen H, He Y, Shiflet GJ, Poon SJ. Nature 1994;367(10):541. [4] Inoue A, Kimura H. J Light Metals, 2000;1:31. [5] Eckert J, Seidel ML, Xing Q, Borner I, Weib B. Nanostructured Materials 1999;12:439. [6] Lewandowski JJ, Lowhaphandu P. International Materials Reviews 1998;43(4):145. [7] Lowhaphandu P, Montgomery SL, Lewandowski JJ. Scripta Metall et Materialia 1999;41:19. [8] Lowhaphandu P, Ludrosky LA, Montgomery SL, Lewandowski JJ. Intermetallics 2000;8:487. [9] Lewandowski JJ. Mater Trans JIM 2001;42(4):633.
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