ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 28 ARCHAEOMETALLURGICAL FRACTURE ANALYSIS Case studies demonstrate the benefits of employing fractographic analysis to study cracking and fracture mechanisms in heritage alloys. Russell Wanhill, Emmeloord, the Netherlands Omid Oudbashi,* Art University of Isfahan, Iran; The Metropolitan Museum of Art, New York *Member of ASM International TABLE 1 — CRACKING AND FRACTURE IN HERITAGE GOLD, SILVER, BRONZE, AND IRON ALLOYS Microscopic features Interpretations Heritage alloys Significances Microvoid coalescence: transgranular ‘dimpled’ rupture Ductile overload at this location All, if locally or generally unembrittled Too high stresses and consequent fractures Faceted transgranular cracking Brittle cleavage Stress corrosion cracking (SCC) Wrought iron Silver, bronze Impact fractures Potential and actual fractures Intergranular cracking Microstructural embrittlement Stress corrosion cracking (SCC) Gold, silver, wrought iron Gold, silver, bronze Impact fractures Potential and actual fractures Intergranular and faceted transgranular cracking Brittle fracture Stress corrosion cracking (SCC) SCC and microstructural embrittlement Wrought iron Silver, bronze Silver Impact fractures Potential and actual fractures Frangibility and actual fractures This article provides an overview of cracking and fracture mechanisms in heritage gold, silver, low-tin bronze, and wrought iron alloys. Understanding these mechanisms can be important for restorers, and possibly for conservators and curators as well. Metallography is widely used (when sampling is permitted) for studying archaeometallurgical artifacts in detail. However, in our opinion fuller comprehension often requires fracture surface examinations and analysis (i.e., fractography). This can be problematic because fracture surfaces must have been well-preserved during long-term burial, and fractographic knowledge is seldom available for these materials. To alleviate this situation, we have collated information and data from our case studies that include fractographic analysis and illustrate its potential usefulness. HERITAGE ALLOYS FRACTOGRAPHY OVERVIEW With information from several colleagues, we have gradually accumulated a limited-sample database for cracking and fracture of heritage alloys (Table 1). Although suitable artifacts and samples are uncommon, the database is robust with respect to different artifacts, their provenances and time periods; and it already shows a variety of cracking and fracture types. Examples are given in Figs. 1‒4 and concisely discussed in the subsequent paragraphs. Microvoid coalescence is a local or general ductile overload failure. The Fig. 1 — Ductile overload failure areas: (a) Luristan (Iran) sheet artifact, circa 1000 B.C. The arrow points to a microvoid- nucleating particle. (b) Luristan Iron Age vessel, 1300-650 B.C. (c) Roman pile-shoe, 340-400 A.D. (a) (b) (c)
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