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A process for the fabrication of Al2O3­YAG eutectic composites using an undercooled melt formed by the melting of an Al2O3­YAP eutectic specimen has been developed. An Al2O3­YAG eutectic structure with fine lamellar spacing was formed. However, the Al2O3­YAG casting has porosities with diameters of 4­9μm. The composition is limited to the Al2O3­YAP eutectic composition. In this paper, a new method for the fabrication of Al2O3­YAG eutectic composites through the transformation from fine Al2O3 and YAP powder mixtures is proposed. The enhancement of YAG formation due to the fine Al2O3 and YAP powders and the 11% volume expansion in the transformation from the Al2O3­ YAP system to the Al2O3­YAG system contributed to the formation of dense Al2O3­YAG eutectic compact. The Al2O3­YAG compact featured a uniform microstructure consisting of spherical-like Al2O3 and YAG grains. In addition, the forming temperature was reduced to 1873K, which is approximately 100K lower than that in the previous process using the undercooled melt. The proposed process provides a dense Al2O3­YAG eutectic compact with a fine and uniform microstructure over a wide composition range of Al2O3­xmol%Y2O3 (10 < x < 30). [doi:10.2320/matertrans.M2012047]

In Sn-Ag-Cu solder balls, unusual microstructures consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic stratures are sometimes observed. However, the formation mechanism of these unusual microstructures is still unclear. Therefore, in this study, the solidification process has been investigated to clarify the nucleation and growth of binary and ternary eutectic structures in Sn-1.0Ag-0.5Cu alloy by using thermal analysis and interruption tests.Cu-enriched zone was observed around β-Sn in the liquid before the nucleation of Sn-Cu6Sn5 binary eutectic structure. Sn-Cu6Sn5 binary eutectic structure formed in these regions. Moreover, some Ag-enriched zones were observed around the Sn-Cu6Sn5 binary eutectic structure in the liquid before the initiation of the ternary eutectic solidification. The unusual Sn-Ag3Sn binary eutectic structure formed in these regions just after the temperature reached the ternary eutectic point. The ternary eutectic structures were classified into three types depending upon the fluctuation of the remaining liquid composition at the ternary eutectic temperature.

To study the supercooling of many liquid metals with high melting temperatures, the investigation was performed for the cooling curves of electrostatic levitation (ESL) experiments, which had been originally obtained for the measurements of many physical properties. The largest supercooling over the literature values was found for liquid Ru (428 K), Ta (721 K), W (601 K), and Ir (438 K), where temperatures in the parentheses mean the supercooling of respective liquid metals. This indicates the validity of ESL for the supercooling experiments of liquid metals and alloys because of being rather free from the heterogeneous nucleation. This ESL was applied to the study of supercooling of homogeneous liquid phase in the composition range from 29 at% Nb to 71 at% Nb of eutectic Ni-Nb system, whose eutectic point is present at 40.5 at% Nb and 1448 K. The experimental result shows a poor supercooling tendency of homogeneous liquid phase around the eutectic composition in spite of large supercooling far apart from this eutectic composition. This characteristic feature was discussed based on the classical nucleation theory coupled with the knowledge recently found, the existence of concentration fluctuations in the homogeneous liquid phase near the eutectic point.

A new liquid-phase bonding method was developed to establish a strong bonding between Al bearing Mg alloys and steels by intentionally forming a thin and uniform reaction layer at the bonding interface. By adopting Ag as an interlayer between the Mg alloys and steel, Mg-Ag eutectic melt is produced at 773 K and nano-scale Fe-Al reaction layer is uniformly formed at the melt-steel interface during the isothermal solidification of the melt which is driven by the diffusion of Ag into the Mg alloy. At the completion of the solidification, exceptionally strong bonding, exceeding the yield strength of the base Mg alloys, is achieved.

Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β-Sn, Ag3Sn and Cu6Sn5. In order to reveal the solidification process of the alloy, the technique for quenching samples during freezing was used to evaluate the freezing characteristics of near eutectic ternary Ag-Cu-Sn alloys. In the present study, Sn-3.5Ag-0.5Cu and Sn-3.5Ag-1.5Cu (mass%) samples were investigated. These samples were subjected to the thermal program to give a cooling rate of about 0.05°C/s. In the case of Sn-3.5Ag-0.5Cu alloy, the solidification structure almost corresponded with the estimation from equilibrium phase diagram, though the solidification process was different. In this study the Sn-Ag3Sn eutectic nucleated immediately after primary β-Sn solidified. On the other hand, eutectic Cu6Sn5 did not nucleate easily, though the temperature of the specimen indicated at 217°C (ternary eutectic temperature). In the case of Sn-3.5Ag-1.5Cu alloy, the solidification structure as well as solidification process were quite distinct from the estimation of equilibrium phase diagram. Primary Cu6Sn5 was the first phase formed in the liquid. This was followed by the nucleation and growth of psud-primary Ag3Sn. The recalescence was observed when β-Sn nucleated and grew as a halo. The ternary eutectic structure emerged after Sn-Ag3Sn binary eutectic solidification.

Accepted: 2010-07-06, 資料番号: SA1002744000

Recently, lead-free solders have been used in electronic equipments. Sn-Ag-Cu system is an important alloy for lead-free solders. There have been so many reports for mechanical properties of bonding. It is important to understand the evolution of solidified structure of these alloys, though, such reports are few. In order to understand the solidification process of Sn-Ag-Cu alloys, Sn-Ag alloys have been used in this study. Three alloys, hypo-eutectic, eutectic, and hyper-eutectic alloys have been prepared. The specimen was quenched during solidification and the solidified structure was interrupted and compared with thermal history. In the case of hypo-eutectic alloy, the specimen was composed by primary Sn and eutectic. It was found that a small undercooling was necessary for nucleation of eutectic. Such undercooling was not observed in eutectic and hyper-eutectic alloys. This may be interpreted by the difference in the liquid composition when the eutectic solidification starts. In the case of eutectic and hyper-eutectic alloys, the eutectic as well as β-Sn may form at almost the same time of recalescence. In the case of hyper-eutectic alloy, it was found that a large undercooling was not necessary for nucleation of Ag3Sn, even though it was facetted phase. Since the primary Ag3Sn was surrounded by the halo of β-Sn and eutectic, the Ag3Sn phase may be a site for nucleation of β-Sn and eutectic.

When the Al2O3-YAP metastable eutectic system was heated above the metastable eutectic temperature but below the equilibrium eutectic temperature, undercooled melt was formed. Solidification in the equilibrium path accompanied the melting of the metastable eutectic system, resulting in a fine and uniform eutectic structure. A novel solidification process using undercooled melt formation was developed. In this paper, the influences of the initial Al2O3-YAP eutectic particle size, forming pressure and holding time at the maximum temperature on the microstructure and/or the mechanical property of the Al2O3-YAG compact were examined to optimize the forming conditions. The Al2O3-YAG compact produced from the Al2O3-YAP eutectic particles with diameters less than 20 mm exhibited a higher flexural strength and the smaller porosity as compared to that produced from Al2O3-YAP eutectic particles with diameters ranging from 20 mm to 45 mm. The volume fraction of the porosity was approximately 4% under a forming pressure of 10 MPa and a holding time of 60 s. The flexural strength of the Al2O3-YAG compact decreased with the increase in the holding time due to the increase in the lamellar spacing. For Al2O3-YAP eutectic particles with diameters less than 20 mm, a low forming pressure of 10 MPa and a short holding time of 60 s were sufficient to synthesize a dense Al2O3-YAG compact. Thus, the solidification technique that uses undercooled melt produced by melting the Al2O3-YAP metastable eutectic system has good advantages for shaping. [doi:10.2320/matertrans.MB200705]

Structure and tensile properties of a high-purity Al-Si hypo-eutectic alloy, which was H-Alloy of 99.98 mass% purity, have been investigated comparing with a conventional one which was L-Alloy of 99.89 mass% purity. Although the base structure in both of the alloys was composed of proeutectic α-phase and eutectic structure, the eutectic structure of H-Alloy was finer than that of L-Alloy. Coarse crystals of plate-like silicon were observed in L-Alloy, while was not observed in H-Alloy. Based on the results of Brinell Hardness Test on the solidification structure, it was found that there was little difference of the hardness between the both alloys. Tensile tests were also performed in an atmosphere at room temperature. The elongation of H-Alloy was twice as large as that of L-Alloy, though the tensile strength of L-Alloy and H-Alloy were almost the same. Al-Fe-Si system compounds were observed at the interface between α-phase and eutectic silicon in L-Alloy.

We present a simple thermodynamic calculation for a strongly partitioning eutectic system, to examine how the critical nucleus energy changes, depending upon assumptions of the chemical diffusion. The calculations show that for strongly partitioning systems, the maximum undercooling may occur at a composition significantly different than the eutectic composition, particularly if the rate of diffusion is slow in the undercooled state. These simple calculations emphasize the role that partitioning and composition may play in determining optimal compositions in metallic glass systems, which typically occur near (but not at) deep eutectic compositions.

The electrical resistivity of compound forming Bi-Te system in the liquid state was studied by the dc-four probe method. Anomalous behavior around the intermetallic compound Bi2Te3, which was reported in the previous study, was not found. In the present study, anomalous behaviors of the temperature coefficient of the electrical resistivity were found at both two eutectic compositions present in this system. These behaviors were discussed based on the effective medium theory of electrical conduction. It is concluded that the concentration fluctuation at the eutectic composition(90 at%Te) grows in the homogeneous liquid phase with the approach to the eutectic temperature.

The electrical resistivity, ρ, of compound forming Bi-Te system in the liquid state was studied by the dc-four probe method. Anomalous behavior of p around the intermetallic compound Bi 2 Te 3 in the previous study was notfound. Anomalous behaviors of the temperature dependence of p, dp/dT, were found both at two eutectic compositions in this system. These behaviors were discussed based on the effective medium theory of electrical conduction. The concentration fluctuation at the eutectic composition (90 at %Te) grows in the homogeneous liquid phase with the approach to the eutectic temperature.

We investigated the relation between hydrogenation property and eutectic structure of a Mg-Mg2Ni two-phase alloy that is made by melting Mg and Ni in a sealed steel crucible. Both the hydrogen content and the plateau pressure increase with the rise of the measurement temperature of PCT (Pressure-Composition-Temperature) curve for both the furnace cooled and water cooled specimens. In these specimens, the maximum hydrogen contents increase up to 5.6 mass% as the increase of the cycle number of PCT measurement at 400°C. On the other hand, the maximum hydrogen content of the heat treated specimen does not depend on the PCT cycle number, and maintains a higher value of about 5.6 mass%. The eutectic structures are fine for both the furnace cooled and water cooled specimens, and coarse for the heat-treated specimen. The hydrogenation behavior depends on the morphology of the eutectic structure, that is, the coarsening of the eutectic structure causes the acceleration of hydrogenation.

Sn-Ag 3 Sn eutectic alloy is known as an attractive candidate to replace Sn-Pb soldering alloy. Since the solidified structure directly relates to the mechanical properties of joints, understanding of solidified structure of this alloy system is very important. Therefore, in order to understand the revolution of solidified structure, unidirectional solidification experiments have been carried out using eutectic Sn-Ag 3 Sn alloy. When the growth velocity is larger than the critical value, Sn-dendrites solidifies as a primary phase. This is attributed to the skewed coupled zone in this alloy system. In this case. the volume fraction of the primary Sn increases with increasing growth velocity and approaches a constant value. Interdendritic eutectic is composed with fibrous Ag 3 Sn and Sn matrix. On the other hand, when the growth velocity becomes lower, a eutectic structure appears without any primary phase. In this case, a piece of eutectic Ag 3 Sn becomes large and changes from fibrous to plate-like with decreasing growth velocity. This morphological change may be due to the anisotropy of surface tension of eutectic Ag 3 Sn.

The effects of Ga content on the microstructure, thermal behavior and mechanical properties of Sn-Zn eutectic alloy were examined in this study. Results show that Ga was dissolved in both Sn and Zn phases. This gave rise to irregular eutectic structure with misaligned, less distributed massive Zn-rich phase, relatively low melting point, and solid solution strengthening effect. Due to the inhomogeneous dissolution feature of Ga in Sn matrix, Sn-Zn-Ga alloys exhibit a broad melting range and an alternate normal-irregular eutectic structure. Notably, the addition of Ga into the Sn-Zn alloy will improve the tensile strength without reducing the ductility when the Ga content ranges from 0.05 to 1 mass%.

Glass formation is basically to avoid nucleation upon quenching. However, the growth of each nucleus is also important as certain undercooling is needed. Thus the subsequent competition between the growth of crystalline phases and the formation of amorphous phase should be considered. In this paper, we summarized our recent studies in Pd-Ni-Cu-P system and La-Cu-Ni-Al alloy systems. It is concluded that the glass forming ability of a eutectic alloy system depends on the type of the eutectics, i.e. symmetric or asymmetric eutectic coupled zone. For the alloy systems with symmetric eutectic coupled zone, the best glass forming alloys should be at or very close to the eutectic composition. For the alloys with asymmetric eutectic coupled zone, the best glass forming alloys should be at off-eutectic compositions.

Aluminium–silicon anomalous eutectic growth has been studied numerically using a phase-field model developed to predict multiple phases. The Si phase grew very slowly in a melt of eutectic composition, but then grew substantially faster when in contact with the solidifying Al phase. The Al-liquid interface was irregular while the Si-liquid interface was flat as a consequence of the low interfacial energy between liquid and Al phases compared with that between liquid and Si. By developing a mathematical nucleation criterion, eutectic growth patterns were predicted and are in good agreement with experimental observations. Two mechanisms were shown to stop locally the growth of the Si phase during solidification. The first is competitive growth between the Al and Si phases, and arises when faster growing Al surrounds the tip of the solidifying Si. The second is due to nucleation and growth of Al ahead of the solidifying Si. It also has been shown that the melt is more likely to be trapped within the Si phase than in the Al phase.

This paper presents a selection of the eutectic systems in the Al 2 O 3 -Y 2 O 3 -ZrO 2 pseudo-ternary system and the production of the undercooled melt due to melting of the metastable eutectic structure at the metastable eutectic temperature. In the pseudo-ternary system, there are two eutectic systems; one is the Al 2 O 3 -YAG(Y 3 Al 5 O 12 )-ZrO 2 equilibrium system and the other is the Al 2 O 3 -YAP(YAlO 3 )-ZrO 2 metastable system. The eutectic system was selected by maximum melt temperature before solidification. YAG nucleation was inhibited in the melt when heated above 2250 K and consequently the metastable eutectic solidification path was selected. The selection controlled by the melt temperature before solidification was similar to that of the Al 2 O 3 -Y 2 O 3 system. Furthermore, the undercooled melt formation was found when the Al 2 O 3 -YAP-ZrO 2 metastable eutectic structure was heated up to temperatures above the metastable eutectic temperature. The addition of ZrO 2 to the Al 2 O 3 -Y 2 O 3 system inhibited the YAG nucleation remarkably and enhanced the undercooled melt formation in comparison with the Al 2 O 3 -Y 2 O 3 pseudo-binary system. Ease of the undercooled melt formation in the pseudo-ternary system will be beneficial for the melt shaping processes.

There are two eutectic reactions in the Al2O3-rich portion of the Al2O3–Y2O3 pseudo-binary system; one is the Al2O3-YAG (yttrium-aluminum-garnet) equilibrium eutectic reaction, the other is the Al2O3-YAP (yttrium-aluminum-perovskite) metastable reaction. The selection of eutectic systems was examined, in terms of maximum melt temperatures before solidification and of cooling rates, for specimens in a composition range of 13.5 to 28.5 mol%Y2O3. YAG nucleation did not occur at a cooling rate of more than 5 K/s when eutectic composition specimens were cooled from 2173 K . Heating of the melt to temperatures above 2273 K also inhibited YAG nucleation. The inhibition of YAG nucleation led to the production of undercooling melt below the metastable eutectic temperature, and consequently resulted in a metastable solidification path. Selection of the eutectic system was controlled by YAG nucleation. At around 2273 K no heat release was detected by optical DTA measurement, although the effect of the maximum melt temperature on YAG nucleation suggested some change in the melt at about 2273 K.

The effect of adding Ag up to 1 mass% on the microstructural and mechanical properties of Sn–Cu eutectic solder alloy was examined. Without Ag, primary β–Sn grains are surrounded by the eutectic network band of Cu6Sn5 needle precipitates/β–Sn. With increasing Ag content, the primary β–Sn grain size and the eutectic network size become finer. In the eutectic band fine Ag3Sn particles appear in addition to Cu6Sn5 precipitates. The DSC experiment revealed the presence of four endothermic reactions on heating for Sn–Cu–Ag alloys; the two peaks near 217°C correspond to the Sn–Cu–Ag ternary eutectic melting reaction and those at 223–225°C/224–226°C are for Sn–Cu binary melting. The 0.2% proof stress and tensile strength decrease with the addition of 0.1 mass%Ag and then gradually increase up to 1 mass%Ag. Even with 1% Ag, they are less than the values for a Sn–0.7Cu binary alloy. In contrast, elongation increases with increasing Ag content up to 1%. Thus, the addition of Ag to Sn–0.7Cu alloy can effectively improve its ductility. The strain rate dependence of 0.2% proof stress of Sn–0.7Cu–0.5Ag is similar to that of Sn–Ag eutectic alloy but is different from that of Sn–Cu eutectic alloy. A small amount of added Ag results in a change of the deformation mechanism of Sn–Cu alloy.

This paper demonstrates the undercooled melt formation from the metastable eutectic structure by heating procedures in the Al 2 O 3 -YAG system. Phase transformation during heating/cooling procedures is detected by optical DTA equipment. Firstly, the Al 2 O 3 -YAP metastable eutectic structure is obtained by solidification after heating the melt above 2273 K. The undercooled melt formation is confirmed when the Al 2 O 3 -YAP metastable eutectic structure composed of crystalline phases is heated up to temperatures above the metastable eutectic temperature for the equilibrium eutectic composition (18.5 mol% Y 2 O 3 ) and the metastable eutectic composition (23.5 mol% Y 2 O 3 ). The undercooled melt formation is immediately followed by the solidification in the Al 2 O 3 -YAG equilibrium path for the 18.5 mol% Y 2 O 3 specimens. For the 23.5 mol% Y 2 O 3 specimens, the obtained undercooled melt is kept even for as long as 3600 s without solidification. Furthermore, the fine and uniform Al 2 O 3 -YAG eutectic structure is obtained when the melting and the solidification simultaneously occur. The solidification with the exothermic heat beside the melting with the endothermic heat has a great advantage for heat release of the latent heat. Consequently, the high growth rate leads to the fine eutectic structure in the off-eutectic composition.

The influence of sodium (Na) on nucleation and growth of the Al-Si eutectic in a commercial hypoeutectic Al-Si-Cu-Mg foundry alloy has been investigated. The microstructural evolution during eutectic solidification was studied by a quenching technique. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites by EBSD, the eutectic solidification mode could be determined. The results show that the eutectic solidification starts near the mould wall and evolves with front growth opposite the thermal gradient on a macro-scale, and on a micro-scale with independent heterogeneous nucleation of eutectic grains in interdendritic spaces. Na-modified alloys therefore behave significantly differently from those modified by other elemental additions.

The effect of eutectic modification by strontium on nucleation and growth of the eutectic in hypoeutectic Al-Si foundry alloys has been investigated by electron back-scattering diffraction (EBSD) mapping. Specimens were prepared from three hypoeutectic AlSi base alloys with 5, 7 and 10 mass%Si and with different strontium contents up to 740 ppm for modification of eutectic silicon. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites? the growth mode of the eutectic could be determined. The mapping results indicate that the eutectic grew from the primary phase in unmodified alloys. When the eutectic was modified by strontium, eutectic grains nucleated separately from the primary dendrites. However, in alloys with high strontium levels, the eutectic again grew from the primary phase. These observed effects of strontium additions on the eutectic solidification mode are independent of silicon content in the range between 5 and 10 mass%Si.

Pure boron (B) has been applied to the liquid phase diffusion bonding of Ni-base superalloy, Rene80, as an intermediate layer, considering that B has very high diffusivity in Ni and Ni-B system forms eutectic point much lower than their melting points. Rene80 could be bonded well through the liquid phase diffusion bonding with the intermediate layer of pure B. During isothermal solidification, the width of eutectic layer, i.e. liquid layer at bonding temperature, was given by a linear function of square root of holding time. Bonding time of the Rene80 /B/ Rene80 joint was much short, and it took 4.8 ks for isothermal solidification and homogenization. But, when bonding time was too short or intermediate material was thick, boride of (Cr, Mo, W, M) X B y . was produced along the bonded line, where M means minor elements Ni, Ti and Co. Bonded joint was slightly hardened after bonding due to increase of B. Tensile strength of the Rene80/B/Rene80 joint at 1144 K (1600F) was obtained up to 567 MPa, and it was 90% of base metal.

We have experimentally found a Sn-Zn-Mg2Sn eutectic alloy with the eutectic composition of 82Sn-10Zn-8Mg (mol%) and with nearly the same eutectic temperature of 454 K as that of the Pb-Sn eutectic alloy. The eutectic alloy was made in air by covering Sn, Zn and Mg with the 59KCl-41LiCl (mol%) composition eutectic melt in an Al2O3 crucible, melting the metals at 1073 K and washing away the alkali-chlorides with distilled water after cooling to room temperature. The as-cast alloy easily oxidized at temperatures above 503 K in air after melting. It was found that the Sn-Zn-(Mg, Al)2Sn eutectic alloys that were made by replacing 0.8 to 4.0 mol% of Mg with Al in the ternary eutectic composition with the same alloying method did not oxidize in air after melting.

Ti/Ti 5 Si 3 functionally graded material was fabricated by a eutectic bonding method. A metal Ti and an intermetallic compound Ti 5 Si 3 were bonded by utilizing a eutectic reaction, where Ti 5 Si 3 was chosen because of its high heat resistance and high oxidation one. In the case of bonding the stoichiometric Ti 5 Si 3 with the pure Ti, cracks were formed within the Ti 5 Si 3 part. Since the cracking was suspected to occur due to the low ductility of the stoichiometric Ti 5 Si 3 , a Ti-rich Ti 5 Si 3 which is ductiler than the stoichiometric Ti 5 Si 3 was applied in place of the stoichiometric Ti 5 Si 3 . As a result, a sound FGM without any detectable defects was realized.

The fabrication of bulk ceramic composites with homogeneous structure in the nano-scale is a difficult challenge. Multi-step methods based on synthesis and sintering of ceramic nanopowders, or methods using deposition from gas phases are complex and require high energy input with very low energetic efficiency, so they are not practical. However, bulk ceramic composites with a naturally self-assembled structure may be formed by solidification from eutectic melts. Smaller eutectic spacing may be obtained by using rapid solidification. We report here the rapid solidification in an arc-image furnace of eutectic melts in the system alumina-Y 3 Al 5 O 12 -zirconia. Ceramic eutectic composites grown from melt have homogeneous bulk structure and lamella sizes in the submicron to the nanoscale range.

Adding 1_??_6% of Fe to the purest Al (99.999%), the effect of cooling velocities on the microstructure of cast Al-Fe alloys has been studied. As the result of retardation of the crystallization of FeAl3, the eutectic point is supercooled if the alloys are cooled rapidly, and the apparent eutectic point shifts to the Fe-side. On the contrary, if the alloys are cooled slowly, the primary crystals of FeAl3 segregate in the mother-liquor and sink to the bottom of the ingot. Utilising the phenomena of segregation of the primary crystals, the composition for the eutectic point of the Al-Fe system has been studied and assigned to be 1.7% Fe. Mechanical properties of the Al-Fe alloys, cooled at various rates, have also been measured. Adding 1_??_10% Si to Al-Fe alloys, containing 5% of Fe, the molten alloys are cooled very slowly (10°/1 hr.) from the liquid in a Tammann-tube. Then structure of the ingot is examined as to the state of segregation of the primary crystals, the sorts of the ternary compounds and also the change of the phases during cooling. Further the composition of the mono-variant eutectic line, existing in the Al-Fe-Si system, has been determined by analysing the segregated portion of the ingots.