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تعداد صفحات این فایل: ۱۳ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Tripling hardness of gold by micro alloying coupled with cold processing~~en~~

Abstract

Gold has broad applications in jewelry, but suffers from low hardness. In the present work, we show that simultaneous addition of 0.1 wt.% Ce and Si can nearly double the hardness of fine gold. We further show that hardness can be tripled by subsequent cold processing. By systematic structural characterizations and thermal analysis, we find that Ce and Si tend to segregate to grain boundaries and impede grain growth. Our work is helpful for development of new gold micro alloys and for design of jewelry that is more wear and scratch resistant.

۱ Introduction

Gold has been utilized since ancient times in a wide variety of applications ranging from culture, decoration to finance, and remains one of the most important materials because of its excellent thermal conductivity, high electrical conductivity, and high corrosion resistance. The arsenic value of gold makes it one of the essential materials for jewelry making [1]. In modern jewelry industry, gold jewelry is fabricated by various techniques, such as press-forming, casting, and electro-forming, etc. [2e6]. Despite the fabrication routes, the value of gold jewelry is largely associated with the purity of gold [1,4].

Gold is one of the most ductile metals and exhibit low hardness, which are beneficial for jewelry fabrication [7]. However, for the same reason, fine gold jewelry can be easily deformed in use [7]. A common approach to circumvent this problem is to increase jewelry thickness to avoid deformation. Nonetheless, the usage of more gold adds more costs on the gold products and thus less profitability. Therefore, enhancing the hardness of fine gold is of great interest [7] and metallurgists have tried many methods [7e12], among which alloying with trace elements is a basic option. For example, addition of Ti can harden fine gold [9]. The underlying mechanism is attributed to precipitation strengthening [9]. Among the available additions, rare earth elements are of particular interest because not only can they increase the strength through solid solution strengthening, precipitation strengthening, grain refinement and dispersion strengthening, but also retard recovery and recrystallization induced softening [10,11]. It should be pointed out that the amount of addition cannot exceed 1 wt.% (wt.% weight percent hereafter), because the purity has to be maintained to be better than 99% to satisfy the customers’ traditional needs [1,7]. According to the customs in gold jewelry industry, such alloys are called gold-based micro-alloys [1,7,8,10,11,13]. Gold-based microalloys with addition less than 0.1 wt. % are special types of microalloys.

It has been found that rare earth elements and Si are effective in strengthening fine gold [11,14,15]. In the present work, we investigate the effect of simultaneous addition of Ce and Si on mechanical property of gold. We show that hardness of gold can be nearly doubled by simultaneous addition of small amount (less than 0.1% by weight) of rare earth element Ce and Si, and tripled by subsequent cold rolling. We further investigate the microstructure origin leading to the greatly enhanced hardness.

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