Weakest Amalgam Phase — MFDS Part 1 MCQ

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ModerateDental MaterialsWeakest Amalgam PhaseMFDS Part 1

In a set conventional low-copper dental amalgam, which phase has the lowest strength and greatest susceptibility to corrosion?

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Correct answer: D — Gamma-2 phase (Sn7–8Hg)

The gamma-2 tin-mercury phase is the weakest and most corrosion-susceptible phase of set conventional low-copper amalgam. Its degradation reduces tensile strength and contributes to creep and marginal breakdown. The gamma phase is residual unreacted Ag3Sn alloy and is relatively strong, while gamma-1 is the principal silver-mercury matrix and is stronger than gamma-2. Epsilon is a copper-tin alloy phase, and the silver-copper eutectic is associated with admixed high-copper alloys rather than being the weak matrix phase of conventional amalgam. In high-copper amalgams, gamma-2 is reduced or eliminated because tin preferentially forms the eta phase, Cu6Sn5, improving corrosion resistance and marginal integrity.

Reference: Beech DR. High copper alloys for dental amalgam. International Dental Journal. 1982;32(3):240–251. https://pubmed.ncbi.nlm.nih.gov/1068242/