047Effect of misch metal on elevated temperature tensile ductility of the Cu-Zn-Bi alloy

047Effect of misch metal on elevated temperature tensile ductility of the Cu-Zn-Bi alloy

ability of Cu. The Bi in Cu-Zn-Bi tertiary alloys, however,embrittles Cu because of its tendency to form a Bi film along phase boundaries, prohibiting the hot rolling over 271.4°C,which is the melting temperature of Bi. Misch (Ms) metal,which is a mixture of Ce, La, Nd, etc., can form a variety of stoichiometric compounds with Bi, and may reduce the Bi film formation along ␣and ␤phase boundaries. In the present study, the effect of Ms metal on tensile behavior of Cu-Zn-Bi alloys was examined at room temperature and 300°C to quantify the hot rollability. It was found that elevated temperature tensile elongation of Cu-Zn-Bi alloy was significantly improved, while the Bi film along phase boundaries was greatly inhibited, with the addition of 0.1wt pct Ms metal. It was also noted that the Bi film along phase boundaries was greatly inhibited probably due to the for-mation of intermetallic compound between Ms metal and Bi. The addition of 0.3 wt pct Ms metal was, however, not beneficial to the tensile ductility of Cu-Zn-Bi alloy at 300°C.Since the solubility of Ms metal in Cu is very low, the Ms metal, added over 0.1 wt. pct, may therefore remain as impu-rities. These impurities would eventually reduce the tensile elongation of Cu-Zn-Bi alloy at room and particularly high temperatures.

Free-cutting leaded brass (Cu-Zn-Pb), which is a widely used material for potable-water plumbing systems, contains over 0.5 wt pct Pb for the machinability of Cu.[1]In leaded brass, tiny globules of essentially pure Pb are produced by eutectic reaction both at grain boundaries and throughout the matrix during solidification. These Pb globules act as chip breakers enhancing the machining characteristics of Cu significantly.[1,2]Due to the health hazard of Pb, however,alternative alloying elements have been sought.[1]The Pb-free brass with alternative free-machining additive must have similar machinability compared to that of its leaded coun-terpart. The Bi has a significant potential as a nontoxic alter-native to Pb for enhancing the machinability of Cu.[1,3]Used alone, however, Bi embrittles Cu because of its tendency to form a Bi film along phase boundaries.[1,4,5]This characteristic is ascribed to the large difference in surface tension between Cu and Bi.[1,6]Due to this film formation of low melting temperature element, hot rolling of Cu-Zn-Bi alloys is considered to be practically impossible. At present,the embrittlement of Cu-Zn-Bi alloys can be reduced by adding another element that has a low surface tension and is soluble in Cu but not in Bi. The most promising elements are P, I, and Sn.[1]The addition of any one of them in an amount larger than a critical level causes Bi “dewetting” at grain boundaries. Misch (Ms) metal, which is a mixture of Ce, La, Nd, etc., can form a variety of stoichiometric com-pounds with Bi. Also, Ms metal tends to be present in the boundary area. The ease of forming intermetallic compounds at grain boundaries suggests that the addition of Ms metal in Cu-Zn-Bi alloys may reduce the formation of Bi film at phase boundaries. The objective of the present study was therefore to examine the effect of Ms metal on room and elevated temperature tensile properties of Cu-Zn-Bi alloys to quantify the hot rollability. The dewetting effect of Ms metal was discussed based on the micrographic and fracto-graphic examinations.

1060—VOLUME 36A, APRIL 2005

METALLURGICAL AND MATERIALS TRANSACTIONS A

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Effect of Misch Metal on Elevated Temperature Tensile Ductility of the Cu-Zn-Bi Alloy

YOUNGHWAN JANG, SANGSHIK KIM,and SEUNGZEON HAN

Cu-Zn-Bi alloy has been regarded as the most promising substitute for free-cutting leaded brass (Cu-Zn-Pb), which

contains over 0.5 wt pct of hazardous Pb for the machin-

Fig. 4—Changes of high-angle grain-boundary spacing in austenite accord-ing to the deformation compared with the changes of ferrite grain size.

YOUNGHWAN JANG, Graduate Student, and SANGSHIK K IM,Professor, are with the Division of Materials Science and Engineering,Engineering Research Institute, Gyeongsang National University, Chinju,660-701, K orea. Contact e-mail: sang@gsnu.ac.kr SEUNGZEON HAN,Senior Researcher, is with the Materials Engineering Department, Korea Institute of Machinery and Materials, Changwon, 641-010, Korea.Manuscript submitted September 24, 2004.

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