Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys by Use of Melt Quenching-Induced Metastable Phase

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1 Materials Transactions, Vol. 43, No. 8 (2002) pp to 2016 Special Issue on Bulk Amorphous, Nano-Crystalline and Nano-Quasicrystalline Alloys IV c 2002 The Japan Institute of Metals OVERVIEW Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys by Use of Melt Quenching-Induced Metastable Phase Akihisa Inoue, Hisamichi Kimura and Kenji Amiya Institute for Materials Research, Tohoku University, Sendai , Japan The stabilization of supercooled metallic liquid occurs for a number of alloys where the constituent elements have the following three rules, i.e., multi-component of more than three elements, significant mismatches of atomic size above 12% among the main three elements, and negative heats of mixing among their main elements. The use of the stabilized supercooled liquid has enabled us to synthesize a number of novel advanced metallic materials. In this review, we present the alloy components, fabrication processings, structures, mechanical properties and applications of high-strength Al- and Mg-based alloys with glassy, nanocrystalline or nanoquasicrystalline phase which were developed on the basis of the concept of the utilization of stabilized supercooled liquid for the last one decade. (Received February 21, 2002; Accepted June 4, 2002) Keywords: aluminum base alloy, magnesium base alloy, nanocrystalline phase, nanoquasicrystalline phase, new long-periodic hexagonal phase, rapid solidification, powder metallurgy, mechanical property 1. Introduction In recent years, there have been strong demands of saving energy and natural resources which are essential to keep a good environment on earth through the decrease in carbon dioxide gas concentration. One of the contribution ways against these problems for materials scientists is to develop high specific strength materials by using raw materials which are abundant on earth and have high recycle ratios. As appropriate metallic materials leading to high specific strength, it is reasonable to select Al- and Mg-based alloys because of their light weight and abundant resources on earth. It is well known that ordinary high-strength Al- and Mg-based alloys have been developed on the basis of various strengthening mechanisms such as solid solution strengthening, precipitation strengthening, grain size refinement strengthening, dispersion strengthening, work strengthening and fiber strengthening etc. 1, 2) However, the use of these conventional strengthening mechanisms gives rise to an upper limit of tensile strength of about 600 MPa for Al-based alloys 1) and about 300 MPa for Mg-based alloys. 2) When one requires the development of a new type of Al- and Mg-based alloys with much higher tensile strength, a completely different strengthening mechanism must be taken into consideration. For the last one decade, we have paid attention to non-periodic structure alloys such as amorphous and quasicrystalline phases. 3) This is attributed to our discoveries on Al- and Mg-based amorphous alloys with high tensile strength of 900 to 1200 MPa 4) and 600 to 800 MPa, 5) respectively, in 1987 and Subsequently, we have also found that the homogeneous dispersion of fcc- Al or hcp-mg nanoparticles with grain size of 5 to 10 nm into Al- and Mg-based amorphous phases causes a further significant increase in tensile strength to 1560 MPa for Al-based alloy 6) and 1000 MPa for Mg-based alloy. 7) This paper intends to review our recent results on the developments of Al- and Mg-based nonequilibrium bulk alloys with good mechanical properties produced by the warm extrusion process of rapidly Inoue Superliquid Glass Project, ERATO, Japan Scienticic and Technology Corporation. solidified alloy powders. 2. Trials of Forming Al-Based Bulk Amorphous Alloys Containing Nanoscale fcc-al Phase Figure 1 shows the changes in tensile yield strength (σ 0.2 ), ultimate tensile strength (σ B ) and fracture elongation (ε f ) with testing temperature for Al 88 Ni 9 Ce 2 Fe 1 amorphous alloy containing fcc-al phase of 20% volume fraction (V f ). 8) The σ B value is 1450 MPa at room temperature and keeps high values above 1000 MPa in the temperature range up to 573 K. It is noticed that the elevated-temperature strength of 1000 MPa at 573 K is about 20 times higher than those 1) for conventional Al-based alloys. However, these strength values were obtained from melt-spun ribbon samples with a cross section Fig. 1 Testing temperature dependence of nominal tensile yield strength (σ y ), ultimate tensile strength (σ B ), fracture elongation (ε f ) and bending ductility for the melt-spun Al 88 Ni 9 Ce 2 Fe 1 amorphous alloy ribbon containing nanoscale fcc-al phase of 20% in volume fraction. The tensile tests were performed after the holding for 180 s at each test temperature.

2 Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys Fig. 3 Bright-field transmission electron micrograph of the Al 88.5 Ni 8 Mm 3.5 bulk alloy produced by warm extrusion at 723 K. Fig. 2 X-ray diffraction patterns of the bulk Al 85 Ni 5 Y 8 Co 2 alloys prepared by uniaxial hot pressing of atomized amorphous alloy powders at different press temperatures. of mm 2. We have tried to produce such a high strength alloy with a mixed structure of amorphous and Al phases in a bulk form by the warm extrusion or pressing technique of rapidly solidified alloy powders. When we use an Al 85 Ni 5 Y 8 Co 2 alloy with the largest supercooled liquid region of 34 K 9) before crystallization and the primary precipitation mode of fcc-al phase, bulk alloys with a mixed structure of amorphous and Al phases as shown in Fig. 2 10) have been produced by the hot-pressing technique. The compressive fracture strength of the bulk alloy pressed at the temperature (T p ) of 523 K reaches 1150 MPa, but high strength above 1000 MPa in a tensile deformation mode has not been obtained. Subsequent trials have been made with the aim of obtaining an Al-based bulk alloy with high tensile strength above 1000 MPa by use of the mixed structure of amorphous and Al phases. 3. Nanocrystalline Al-Based Alloys When we choose much higher extrusion temperatures between 623 and 723 K around crystallization temperature for the Al-based alloys with a mixed structure of amorphous and Al phases, nanocrystalline bulk alloys consisting of fine compound particles embedded in nanogranular Al matrix can be easily produced. As an example, the bright-field TEM image of the Al 88 5 Ni 8 Mm 35 (Mm: misch metal) bulk alloy produced by extrusion at 723 K is shown in Fig ) The structure consists of fine Al 3 Ni and Al 11 Mm 3 compounds embedded in Al matrix with grain sizes of 150 to 200 nm. It is seen that the compound particles disperse rather homogeneously within the grains and along the grain boundaries. The nanocrystalline Al-based alloys exhibit good mechanical properties of high tensile strength of about 900 MPa combined with high fatigue strength of 300 to 350 MPa after 10 7 cycles under a rotating beam bending stress condition. 11) On the other hand, the plastic elongation is in the range of 3 to 8% which are smaller than those 1) for the conventional Albased alloys. It is noticed that these strength values are much superior to those 1) for conventional Al-based alloys and previously developed powder metallurgy (P/M) Al-based alloys. The present high-strength Al-based alloys produced by the rapid solidification powder metallurgy (RS P/M) process have been used as a commercial name of GIGAS, 12) though the commercial alloys include more multi-component. 4. Nanoquasicrystalline Al-Based Alloys With the aim of improving the smaller elongation for the above-described high-strength type Al-based alloys in Al Ni Mm and Al Ni Mm Zr base systems, we searched for an appropriate Al-based alloy exhibiting much larger elongation without detriment to high strength with much higher Al concentrations. It was found that the use of Al M TM Ln (M = V, Cr, Mn, TM = Ti, Fe, Co, Ni, Cu, Ln = lanthanide metal) 3, 13 15) alloys with high Al concentrations of 92 to 95 at% produces a unique nanostructure consisting of fine spherical icosahedral quasicrystal particles surrounded by Al single phase, as exemplified for a single Al 93 Mn 5 Co 2 alloy powder prepared by the high-pressure gas atomization in Fig ) The icosahedral phase has a particle size ranging from 50 to 200 nm and its volume fraction is estimated to be as large as about 60%. The mixed structure is formed by the unique solidification mode in which the icosahedral phase precipitates as the primary phase from liquid, followed by the

3 2008 A. Inoue, H. Kimura and K. Amiya Fig. 4 Bright-field electron micrograph and selected-area electron diffraction pattern of a single Al 93 Mn 5 Co 2 alloy powder prepared by high-pressure gas atomization. precipitation of Al phase from the remaining liquid. These alloys were formed using atomized alloy powders, and can be easily consolidated by warm extrusion in the temperature range from 623 to 723 K at an extrusion ratio of 10. Figure 5 shows the relation between tensile strength and elongation for the extruded bulk icosahedral-base alloys, 3) together with the data of conventional Al-based alloys obtained from the JIS handbook and extruded bulk Al-based alloys from the atomized amorphous or amorphous + Al phase powders. 11) It is seen that the icosahedral base alloys exhibit high strength of 500 to 850 MPa as well as large elongation of 5 to 25% which are much superior to those for the conventional Albased alloys. It has further been recognized that the icosahedral base alloys exhibit high fatigue endurance limits of 240 MPa at room temperature and 210 MPa at 423 K after 10 7 cycles under the rotating-beam bending stress condition. 16) In addition, the icosahedral base Al 93 Cr 2 Ti 2 Fe 3 alloy also exhibits high elevated temperature strength above 350 MPa in the wide temperature range up to 573 K. 17) The elevated temperature strength level is superior to the goal level of the U.S.A. air force for combat planes in the temperature range below 573 K. As described above, these icosahedral base alloys can be divided into the following three types, as summarized in Table 1. That is, high strength type in Al M Ln TM Fig. 5 Relation between tensile strength and plastic elongation for the extruded bulk icosahedral (I) base alloys. The data of conventional Al-based alloys are also shown for comparison. system, high ductility type in Al M Cu TM system and high elevated temperature strength type in Al Cr Ti Fe system. Table 1 Distinction of alloy system, microstructure and mechanical properties for the quasicrystalline base bulk Al alloys. Type Alloy system Structure Mechanical properties High-strength alloy Al Cr Ce M Al+ Q σ f MPa Al Mn Ce ε p 5 10 MPa High-ducitility alloy Al Mn Cu M Al+ Q σ f MPa Al Cr Cu M ε p 12 30% High-elevated temperature strength alloy Al Fe Cr Ti Al + Q + Al 23 Ti 9 σ f 350 MPa at 573 K

4 Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys 2009 Table 2 Application fields of Al-based alloys with nanocrystalline or nanoquasicrystalline structure produced by warm extrusion or warm forging of atomized powders. 1 Robot parts: arm, finger, foot etc. 2 Machine parts 3 Die cast molds 4 Sporting goods materials: softball bat, tennis racket, golf club etc. 5 Light weight tools 6 Fishing reel 7 Gears in bicycle 8 Wheelechair parts 6. Applications of Nanocrystalline Al-Based Alloys Fig. 6 Scanning electron micrograph of atomized Al 95 Zr 1 Ni 1 Mm 3 alloy powder. Fig. 7 Ultimate tensile strength, yield strength, elongation and Charpy impact fracture energy as a function of volume fraction of compounds for the P/M Al 95 Zr 1 Ni 1 Mm 3 alloy. 5. Nanocompound Al-Based Alloys A similar fine mixed structure as that for the icosahedral base alloys was also formed in Al 95 Zr 1 Ni 1 Mm 3 alloy. 18) As exemplified in Fig. 6, the atomized powders with particle sizes below 32 µm consist of primary precipitates of Al 3 Zr surrounded by the Al + Al 11 Mm 3 phase. These powders can be consolidated into a bulk form by the warm die-forging technique at 673 K. Figure 7 summarizes the relation between tensile strength and Charpy impact fracture energy for the die-forged Al Zr Ni Mm alloys. 18) It is noticed that the powder-forged alloys exhibit a good combination of high strength above 600 MPa and high impact fracture energy of 15 to 25 J/cm 2. These alloys also exhibit large plastic elongation of 8 to 15% even in the powder-forged state. The good combination of strength and ductility is due to the unique mixed structure in which the fine compound particles are dispersed homogeneously as primary precipitates from the supercooled liquid even at the Al-rich composition. It is important to point out that the Al 3 Zr compound is a metastable phase which can be obtained only from the direct precipitation from the supercooled liquid. 19) The metastability seems to enable the fine precipitation state. Although no actual data are presented in the present review, the above-described nanostructure Al-based alloys exhibit high strain-rate superplasticity, i.e., large elongation of 500 to 700% in a high strain rate range of 0.1 to 1.0 s 1 and at temperatures between 840 and 890 K. Owing to the simultaneous achievements of high strength, high elastic modulus, high ductility, high fatigue strength, high elevated temperature strength, good corrosion resistance and high strain-rate superplasticity, the newly developed Al-based nanocrystalline alloys have been used in a number of application fields summarized in Table 2. All these application fields require high strength combined with light mass as well as high reliability. As examples, some actual parts of sporting goods materials, fishing reel and gears in bicycle are shown in Fig. 8. Although these Al-based alloys have been produced by the RS P/M process which is rather expensive, the mass production system in the RS P/M process has been developed, resulting in a significant reduction of materials cost. From these combined effects, the application fields of the nanocrystalline Al-based alloys have increased steadily for the last six years and the importance of these new nanostructure alloys as engineering materials is expected to become more and more significant in the near future. 7. Mg-Based Bulk Glassy Alloys In 1988, Mg-based glassy alloys with a large supercooled liquid region before crystallization were found in Mg Ni Ln 20) and Mg Cu Ln 21) systems. As exemplified for Mg Cu Ln system in Fig. 9, these ternary Mg-based alloys have extremely wide glass-formation ranges. When we choose appropriate alloy compositions in Mg Cu Ln and Mg Ni Ln base systems, bulk glassy alloys can be formed in the diameter range up to 12 mm by various casting processes. 22) Figure 10 shows outer shape of Mg Cu Y Pd Ag bulk glassy alloy rods with diameters of 8, 10 and 12 mm prepared by water quenching of the molten alloys inside the steel tubes. These bulk glassy alloys exhibit rather high tensile fracture strength of 600 to 700 MPa at room temperature, 23) though no distinct plastic elongation is seen. However, we have noticed a significant degradation of the high strength after room-temperature aging for several months presumably because of the progress of structural relaxation caused by room temperature aging. 24) Considering that the relaxation-induced embrittlement is es-

5 2010 A. Inoue, H. Kimura and K. Amiya Fig. 8 Application examples of nanocrystalline Al-based alloys: sporting goods (a, b), fishing reel (c) and gears in bicycle (d). These data were taken from YKK and Daiwa Corporations. Fig. 10 Outer shape of bulk glassy Mg 60 Cu 20 Y 10 Ag 5 Pd 5 alloy rods with diameters of 8, 10 and 12 mm. Fig. 9 Composition range in which an amorphous phase is formed in Mg Cu Y system by melt spinning. The numbers represent the crystallization temperature (T x ) and the glass transition temperature (T g ). in conjunction with good bending ductility after annealing at the extrusion temperatures of 573 and 623 K was judged as an appropriate alloy composition. Subsequently, the appropriate alloy was atomized to produce rapidly solidified alloy powders with particle size fraction below 32 µm. A successive powder metallurgy process (RS P/M) consisting of gas atomization, followed by collection, sieving, pre-compaction, evacuation, sealing and then warm consolidation was made in a well controlled atmosphere with a moisture concentration below 2 ppm. Figure 11 shows a schematic illustration of the sequential RS P/M process developed by our group. 25) A perfect protection system is necessary for the RS P/M treatment of Mg-based alloys against the dangerous explosion. We have reported that high tensile yield strength values of 500 to 600 MPa are obtained for the RS P/M Mg 95.5 Mm 2 Y 2.5 and Mg 95 Ca 2.5 Zn 2.5 alloys. 26) The tensile strength is very high, but the plastic elongation is usually below 2%. The poor ductility prevents the use of the RS P/M Mg-based alsential for Mg-based bulk glassy alloys with low melting temperature and low glass transition temperature, a subsequent development of a high strength Mg-based alloy was focused on nanocrystalline Mg-based alloys with high Mg concentrations above 85 at%. 8. Mg-Based Nanocrystalline Alloys We have searched for an appropriate alloy composition for the RS P/M process by measuring the composition dependence of Vickers hardness and bending ductility in as meltspun and annealed states. The alloy with the highest hardness

6 Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys 2011 Fig. 11 Schematic illustration showing the sequential closed processing system in which the production of atomized Mg-based amorphous powders and their consolidation into a bulk form can be carried out in a well-controlled atmosphere. loys. We investigated the mechanism for the high strength of the RS P/M Mg Mm Y and Mg Ca Zn alloys. Figure 12 shows the tensile yield strength values in the Hall-Petch relation, 27, 28) together with the data of the conventional Mgbased alloys. One can see a rather good linear relation between σ 0.2 and dg 1/2, though some scattering are recognized. The linear relation indicates that the present high strength is due partly to the grain size refinement strengthening. These Mg-based alloys have a mixed structure of fine compounds embedded in hcp Mg matrix and the existence of the second phase causes the scattering of the data. Consequently, the present data were re-plotted by using the Fisher-Hart-Pry relation 29) including the volume fraction and particle size of the dispersed phase. The use of the relation yields a much better linear relation, indicating that the high strength of the present alloys is due to the combination of grain size refinement strengthening and dispersion strengthening. Based on the information that the strength of the Mg-based alloys obeys approximately the Hall-Petch relation, it is concluded that a new Mg-based alloy with much higher tensile strength above 600 MPa may be obtained for the alloy with fine grain sizes of 100 to 200 nm. Fig. 12 Relation between tensile yield strength (σ 0.2 ) and grain size (dg 1/2 ) for various RS P/M Mg-based alloys. 9. Nanogranular Mg-Based Alloys with Novel Longperiod Hexagonal Structure 9.1 Alloy components Although the development method of a new Mg-based alloy with high strength above 600 MPa was presented, it was very difficult to obtain such a fine grain structure with grain sizes of 100 to 200 nm in Mg-rich alloys because of easy grain growth of Mg phase with low melting temperature during warm extrusion of atomized powders. This difficulty is supported from a number of experimental data reported up to

7 2012 A. Inoue, H. Kimura and K. Amiya date. That is, the grain size of the RS P/M alloys has been reported to be about 250 nm for Mg 95.5 Mm 2 Y 2.5, 26) 100 nm for Mg 95 Ca 2.5 Zn 26) 2.5 and 450 nm for Mg 85 Ca 5 Al ) These results also indicate that the increase in solute contents is not effective for the decrease in grain size and a completely different new mechanism must be introduced for the grain size refinement of the Mg phase. Here, we have paid attention to special alloy systems in which the constituent elements have the following three rules, 31 33) i.e., (1) multi-component consisting of more than three elements, (2) significantly different atomic size mismatches above 12% among the three main constituent elements, and (3) suitable negative heats of mixing among their main elements. This is because the alloys with the three component rules can have highly stabilized supercooled liquid against crystallization and lead to the formation of bulk glassy alloys by various casting processes. It is expected that the stabilized supercooled liquid also produces a novel metastable phase even in a crystalline state. We examined the compositional dependence of Vickers hardness and bending ductility in as-spun and annealed states for several Mg-based alloy systems which satisfy the above-described component rules. As a result, we noticed that two alloy systems of Mg Ca Al and Mg Zn Y can keep high hardness combined with good bending ductility even in the annealed state. Furthermore, it has been recognized that only Mg Zn Y alloys can have nanogranular structure with fine grain sizes of 100 to 200 nm by the formation of a novel phase with new atomic configurations. 34) The microstructure and mechanical properties of the RS P/M Mg Zn Y alloys are explained in the following. 9.2 Mechanical properties 34) The X-ray diffraction pattern of the atomized Mg 97 Zn 1 Y 2 powders indicates that the alloy powders with a particle size fraction below 32 µm consist mainly of hcp Mg phase including a small amount of second compound phase. The atomized powders can be easily consolidated into fully dense bulk forms by the warm extrusion technique of the atomized powders and the density of 1.84 Mg/m 3 measured by the Archimedian method remains almost unchanged over the whole extrusion temperature range. The extruded bulk alloys exhibit good mechanical properties as shown in Fig. 13. The tensile yield strength ranges from 610 to 420 MPa in conjunction with large elongation of 5 to 16% and these properties change systematically with extrusion temperature. The extruded alloy also exhibits high elevated temperature strength, i.e., above 500 MPa in the temperature range up to 423 K and above 400 MPa at the temperatures below 473 K. These strength values are much superior to those (200 to 300 MPa at 423 K) 35) for the heat-resistant type WE54-T6 (Mg 5Y 3Mm 0.4Zr alloy) and TMT-WE43 (Mg 4Y 3Mm 0.4Zr alloy) alloys. Figure 14 shows the comparison of the strength and elongation of the Mg Zn Y alloys with those for conventional Mg-based alloys. It is clearly seen that the strength of the present alloy is about three times higher than those 35) for the AZ91 (Mg 9Al 1Zn alloy) and ZK60(Mg 6Zn 0.4Zr alloy) alloys prepared by ingot metallurgy including heat treatment and about 1.5 times higher than those for the RS P/M AZ91 and ZK61 alloys. It is thus concluded that the RS P/M Mg Zn Y alloys have the best combination of tensile yield Fig. 13 Tensile yield strength (σ 0.2 ) and elongation (δ) as a function of extrusion temperature for the RS P/M Mg 97 Zn 1 Y 2 alloy and Mg 96 Zn 1 Y 3 alloys. Fig. 14 Relation between tensile yield strength (σ 0.2 ) and elongation (δ) for the RS P/M Mg Zn Y alloys. The data of the RS P/M AZ91 and ZK61 alloys, and conventional IM-aging Mg-based alloys are also shown for comparison. strength and plastic elongation among all kinds of Mg-based alloys. The specific strength of the RSP/S Mg Zn Y alloys was also compared with those for conventional metallic alloys. As summarized in Fig. 15, the specific tensile strength and specific Young s modulus of the Mg Zn Y alloys are 330 MPa/(Mg/m 3 ) and 23 GPa/(Mg/m 3 ), respectively. It is noticed that the specific tensile strength is much higher than

8 Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys 2013 Fig. 16 Strain-rate dependence of the flow stress and elongation to failure at 623 K for the RS P/M Mg 97 Zn 1 Y 2 alloy extruded at 573 K. Fig. 15 Specific stiffness (E/ρ) and specific tensile yield strength (σ 0.2 /ρ) for the RS P/M Mg Zn Y and Mg Ca Al alloys. The data of conventional Mg-, Al- and Ti-based alloys are also shown for comparison. those 36) for all kinds of metallic alloys. Although the RS P/M Mg Zn Y alloys exhibit high elevated temperature strength in the temperature range below 473 K, the further increase in temperature causes a significant decrease in flow stress. When the alloy is deformed at 623 K and at a high strain rate of 0.1 s 1, the alloy exhibits large elongations exceeding 650%, as shown in Fig. 16. One can also see a good linear relation between true flow stress and strain rate and the slope of the linear relation corresponding to the strain rate sensitivity exponent (m-value) also shows a rather large value of 0.4. The m-value also shows that the M Zn Y alloy has superplasticity. Figure 17 shows the change in the sample shape before and after testing at 623 K under various strain rates ranging from 0.02 to 0.8 s 1. When the strain rate is adjusted to the range from 0.06 to 0.1 s 1, the sample is significantly deformed in a homogeneous deformation mode. The large deformability in the high strain rate range indicates that the Mg-based alloy also has a good formability into various shapes at the high temperature of 623 K. 9.3 Novel structure of the RS P/M Mg 97 Zn 1 Y 2 alloy 34) With the aim of clarifying the reason for the extremely high tensile yield strength and large elongation for the RS P/M Mg Zn Y alloys, the microstructure of the alloy with the highest tensile strength extruded at 523 K was examined by high-resolution TEM. The RS P/M alloy consists of very fine equiaxed Mg grains with grain sizes of 100 to 200 nm, as 9.4 Mechanism for the formation of the novel longperiod hexagonal structure In the equilibrium phase diagram of Mg Zn Y ternary system, there is no appreciable solid solubility limit of Zn and Y elements into Mg phase at room temperature. 37) It is therefore interpreted that the long-period hexagonal phase can be regarded as a reinforced solid solution saturated with Zn and Y elements. Considering that the atomic size ratio is 1.14 for Mg/Zn and 1.13 for Y/Mg, 38) the reinforced solid solushown in Fig. 18. However, we cannot observe a high density of dislocations inside the grains. Instead of dislocations, one can see a high density of planar faults in almost all the grains. There have been no data on the formation of planar faults in the ordinary hexagonal closed packed (hcp) Mg phase. Consequently, we paid attention to the planar faults. When the sample is tilted, we can see long-period structure in all the Mg grains, as exemplified in Fig. 19. The selected-area electron diffraction pattern taken from the region A in the highresolution TEM image reveals the existence of extra spots at the 1/3 and 2/3 positions to the 0001 reflection spot, in addition to the fundamental reflection spots of the hcp Mg phase. These results indicate that the structure in the faulted region changes to a novel long-periodic hexagonal structure in which the periodicity is three times longer than that for the ordinary hcp Mg phase. In order to clarify the atomic configuration in the long-periodic hexagonal structure, the high-resolution TEM image is shown in Fig. 20. It is seen that the stacking of atomic configurations is three times longer than the ABABAB type packing for the ordinary hcp-mg phase. This atomic configuration mode obtained from the high-resolution TEM image is consistent with the interpretation derived from the selected-area electron diffraction pattern.

9 2014 A. Inoue, H. Kimura and K. Amiya Fig. 17 Change in the sample morphology of the RS P/M Mg 97 Zn 1 Y 2 alloy before and after tensile testing at 623 K and various strain rates. (a) before deformation, (b) s 1, (c) s 1 (d) s 1 (e) s 1, (f) s 1, (g) s 1. Fig. 18 Bight-field TEM image of the RS P/M Mg 97 Zn 1 Y 2 alloy obtained by extrusion at 573 K. Fig. 19 Bright-field TEM image and selected-area electron diffraction pattern of the RS P/M Mg 97 Zn 1 Y 2 alloy obtained by extrusion at 573 K. tion must include a higher degree of strains and hence the long-period structure is presumed to be introduced for the relaxation of significant strains generated by the reinforced solution. It is thus said that the selection of the alloy systems with the three component rules produces the novel structure through the formation of the reinforced solid solution resulting from the increase in the stability of the supercooled liquid. 9.5 Mechanism for high tensile strength and large elongation The long-period hexagonal structure requires a more longer scale diffusion for grain growth as compared with the ordinary hcp-mg phase. This also implies that the grain growth of the long-period structure is much difficult in dynamic recrystallization and dynamic recovery reactions during warm extrusion of the atomized powders. Therefore, we can obtain anomalous fine grains of 100 to 200 nm even in the Mg-rich

10 Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys 2015 alloy and the fine grain structure is the main reason for the achievement of high tensile strength for the RS P/M Mg Zn Y alloys. In addition to the high tensile strength, it is characterized that the present alloy also has large elongation. The RS P/M alloy includes very fine Mg 24 Y 5 particles with a particle size of about 10 nm in the Mg matrix, as exemplified in Fig. 21. However, no appreciable precipitation of Mg 24 Y 5 particles along the grain boundary of the Mg matrix is seen. The absence of any precipitates along the grain boundary suggests that the fine Mg 24 Y 5 particles precipitate in the dynamic re- Fig. 20 High-resolution TEM image of the RS P/M Mg 97 Zn 1 Y 2 alloy. crystallization reaction during warm extrusion. Hence, the nanograin structure without gain boundary precipitates is presumed to result in the large elongation as well as the high tensile strength. 10. Effectiveness of Stabilized Supercooled Liquid for the Development of Advanced Materials In this review, we have pointed out that the universal feature in all the alloys used in the present development is the use of special multi-component alloys with the three component rules. Figure 22 summarizes the changes in the resulting structures with solute concentration for the special alloys with significant atomic size mismatches above 12% and suitable negative heats of mixing. The special alloys can have a stabilized supercooled liquid state leading to the suppression of crystallization reaction during cooling from liquid. That is, the nucleation and growth reactions can occur at lower temperatures in the highly supercooled liquid. The transformation mode is favorable for the formation of various nonequilibrium phases even in bulk alloys obtained at slow cooling rates. Finally it is important to point out that the other alloys such as nanocrystaline Al-based alloys, nanocrystalline Febased soft magnetic alloys and nanoquasicrystalline Al-based alloys except some bulk glassy alloys and novel long-period hexagonal Mg Zn Y alloys were developed before the concept on the alloy components of stabilized supercooled liquid was found. That is, the other alloys have incidentally satisfied the three component rules. It is thus concluded that all the alloys developed in our groups for the last 15 years have the three component rules. This agreement allows us to expect that the further development of a new advanced alloy along the similar alloy designs leads to a novel metallic material with useful properties. Fig. 21 High resolution TEM image and nanobeam electron diffraction pattern of the RS P/M Mg 97 Zn 1 Y 2 alloy obtained by extrusion at 573 K.

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