Production of Zr 55 Cu 30 Ni 5 Al 10 Glassy Alloy Rod of 30 mm in Diameter by a Cap - Cast Technique

Authors stressed an importance of low oxygen content of slowly cooled ingots for the homogeneous amorphous half top layers of arc-melted button ingots. The quenching procedure was also not favored to produce amorphous top layers. However authors made a liquid quenching machine to enhance the cooling speed. Photographs Figs.4(b) and 5 are troublesome. Originality as well as scientific significance of the title paper is in question. Summary of the title paper This paper represents a new casting technique and required oxygen level for successful production of Zr55Cu30Ni5Al10 glassy alloy rod up to the size of 30mm in diameter. This casting technique is named as “cap-cast method”, namely, the arc-melting of this particular alloy followed by quick casting into water cooled copper mold. A copper “cap”is utilized to compress the alloy melt at 1kN from the top surface just after the casting. This cap is needed to prevent the shrinkage of the alloy and to enhance the cooling speed of the alloy melt. Low oxygen level, being less than 45 mass ppm in the starting zirconium metal, has essential importance to obtain homogeneous Zr55Cu30Ni5Al10 glassy alloy rod. Otherwise, crystalline inclusions are solidified in the glass matrix. It is also stated that “the formation of a single glassy phase in the half topside of master alloy” was confirmed for the as-arc melted button ingots, being cooled naturally on the copper hearth. It is concluded that (1) the purity level of alloy component is an important factor to produce glassy alloy rods with large size and (2) the Zr55Cu30Ni5Al10 glassy alloy rods of up to 30mm in diameter could be produced by means of the cap-cast technique, though the critical sizes of glassy alloy rods was 20mm by the conventional cast technique using copper molds. Critical comments for the title paper 1. Enough evidences ,e.g. DSC or DTA curves showing the glass transition temperature, Tg, and crystallization temperature, Tx, were not exhibited for the amorphous layers covering the half topside of the arc-melted alloy buttons. 2. In comparison with the cooling speed, oxygen content, being less than 45 mass ppm for the starting zirconium metal in the button ingots was claimed to be the essential factor for the stability of amorphous layer. Since the topside of the arc-melted buttons was slowly cooled rather than the bottom which was directly contacting the copper hearth, the crystalline state is quite likely realized there. The authors of this paper say “cooling speed in the melt-quenching procedures was the most important factor” and the cap-cast method was designed to realize “rapid cooling” on the one hand. If authors claim that slow cooling of the low-oxygen samples is a key factor for obtaining the amorphous state on the other hand, the cap-cast method may be no longer needed for the production of glassy alloy rods. There are contradictory statements in one paper. 3. The cap-cast method was not explained clearly to readers. For production of glassy alloy rods, e.g. φ30mm x 30mm in size, the amount of about 144g master alloy is required due to its density. Within the best knowledge of the present author, however, such large amount of master alloy can not be melted by a conventional arc-melting machine. At least four times high power machine is needed for the production. Maximum power could exceed 40kW. Homogenization of the 144g sample should be very troublesome. No explanation was described with respect to the homogenization process for the ingots as well as the sizes of the arc-melting machine for the cap-cast method. Cooling water is also a trouble for the arc-melting. In the conventional case, cooled 20-litter water is needed per minute to prevent melting of the copper hearth on which samples are arc-melted. A lot of water is needed for the cap-cast machine. There is no such essential information in this paper. 4. The arc-melting method is very useful to make small metal ingots, being typically 25g in weight and consisting of refractory metals and/or transition metals. Shortcoming of this method is the in-homogeneity of the products. Due to large temperature gradient from the top to the bottom of the melt during the melting procedure exceeds 10 4 degree per cm, since the top layers are heated by the argon plasma but the bottom layers touch water-cooled copper hearth. The melt-quenching of arc-melted ingots into the water-cooled mold may easily result in less-homogeneous products. It is hardly conceivable that homogenous glassy alloy rods may be directly casted from the arc-melted ingots. 5. Figure 3(b) and Figure 4(b) exhibit cross sections of amorphous rods. The former and the latter correspond to the cases to the φ20mm andφ30mm rods, respectively. It is not understandable why Fig.4(b) is a kind of patch-work photograph which consists of four sections. Straight boundaries can be clearly seen both horizontally and vertically. Again any explanation about this point is not given in this paper. 6. Figure 5 shows a very homogeneous and smooth HREM photograph observed by the JEOL-4000 FX machine. Unfortunately, JEOL-4000 FX machine has not existed since 1990 in the Institute of Materials Research, Tohoku University within the best knowledge of the present author. 7. A HREM photograph, Fig.5, is not understandable since the picture seems too smooth relative the other HREM photographs of glassy alloys previously published by the present authors[1], Matsuura et al. [2] and Hirata et al.[3]. As easily seen in the attached photograph, enlarged picture of Fig.5, shows rather coarse “pixels” relative to the photograph of Fig.4. It is our afraid that the coarse pixels were the indications of a computer-processed image. 8. Originality of this paper is not well-deserved since production of Zr55Al10Ni5Cu30 glassy alloy rod with diameter of 30 mm using the suction casting method with copper mold [4] has been already reported by Inoue and Zhang in 1996[5]. For this reason, the superiority of the cap-cast method is not fully recognized in comparison with the suction casting method. Considering all these points, there are paradoxical indications for the successful production of glassy alloy rods with large sizes. Authors stressed an importance of low oxygen content of slowly cooled ingots for the homogeneous amorphous half top layers. The quenching procedure was also not favored to produce amorphous top layers. However authors made a liquid quenching machine to enhance the cooling speed. Photographs Figs.4(b) and 5 are troublesome. Originality as well as scientific significance of the title paper is in question. Authors are definitely suggested to withdraw their paper for rewriting reasonably. References[1] Y.Yokoyama ,P.K.Liaw, M.Nishijima,K.Hiraga, R.A.Buchanan and A.Inoue; Mat.Trans. 47(2006) 1286-1293.[2] M.Matsuura,K.Konno, M.Yoshida, M.Nishijima and K.Hiraga; ibid 47(2006) 1264-1267.[3] A.Hirata , T.Morino, Y.Hirotsu, K.Itoh and T.Fukunaga; ibid.48(2007) 1299-1303.[4] A.Inoue and T.Zhang; Mat.Trans.,JIM 36(1995) 1184-1187.[5] A.Inoue and T.Zhang; ibid 37(1996) 185-187.