Change in creep rate and microstructure in an aged, low-copper amalgam.

Low-copper amalgams aged at 37 degrees C, 50 degrees C, 60 degrees C, 70 degrees C, and 80 degrees C for periods of 7 and 30 days were examined using a scanning electron microscope and an x-ray diffractometer. Gamma-one Ag-Hg grain size and gamma 2 Sn-Hg volume fraction, and surface area were determined by quantitative metallography. At 60 degrees C and below, gamma 1 was the predominant matrix phase. Little beta 1 Ag-Hg was found. In this temperature range, both gamma 1 grain size and ADA creep were found to obey an Arrhenius law. A linear relationship was found between the log of the mean ADA creep and the mean gamma 1 grain size. Above 60 degrees C, the gamma 1 phases and the majority of the gamma Ag-Sn particles disappeared. In their place, beta 1 appeared. Gamma-two volume fraction increased during aging at temperatures over 60 degrees C, but a coarsening of gamma 2, reflected in a decrease in gamma 2 surface area per unit volume, also took place. Aging for 30 days or less resulted in decreases in ADA creep. At 60 degrees C and below, the decrease was caused by increases in gamma 1 grain size. Above 60 degrees C, the decrease in ADA creep was caused by several factors, one of which was the appearance of large beta 1 grains.

Corrosion of the eta'(Cu-Sn) phase in dental amalgam.

Previous studies have shown preferential corrosion of the eta'(Cu-Sn) phase in high-copper dental amalgam both in vitro and in vivo, while samples of pure eta' have shown high corrosion resistance. To clarify this contradiction, samples of pure eta' crystals mixed with other phases were prepared and tested. Evaluation of the corrosion resistance was based on the results of coulometry at constant potential and potentiodynamic polarization. The corrosion susceptibility of eta' in the matrix of gamma 1(Ag-Hg) was considerably higher than the susceptibility of isolated eta'. The susceptibility of pure eta' also could be increased by plating it will small amounts of Hg. It was concluded that in dental amalgam, the presence of mercury in the phases surrounding eta' reduces its resistance to corrosion. Although eta' is more resistant to corrosion than gamma 2(Sn-Hg) which appears in low-copper amalgams, it is the least corrosion resistant major phase in high-copper amalgams and can suffer deterioration.

A study of high copper amalgams. I. A comparison of amalgamation on high copper alloy tablets.

Two types of high copper alloy powder have been amalgamated by plating tablets of compacted powder with Hg. Gamma1 Ag-Hg crystals form on both types of tablet. On one type, zeta Cu-Sn crystals are also formed. An amalgamation mechanism for this latter type of high copper amalgam is discussed.

New information on high copper amalgam corrosion.

Anodic polarization of "pure" Cu6Sn5 was investigated. Cu6Sn5 is found to be passive in nearly neutral synthetic saliva. Anodic corrosion activity (characteristic for tin) previously reported for a high copper amalgam must result from the presence of the gamma2 phase or tin in the matrix phase.

Analysis of Asgar-Mahler reaction zone in Dispersalloy amalgam by electron diffraction.

Selected area reflection electron diffraction has been used to determine the phases present in the reaction zones surrounding the Ag-Cu eutectic dispersant in Dispersalloy amalgam. The experiments were carried out on two kinds of specimens, (1) Dispersalloy amalgam and (2) Dispersalloy amalgam using 30, 50, 75, and 100% more mercury than found in commercially available capsules. The results show that the reaction zones in the specimens of the present investigation consist of Cu6Sn5 and Ag2Hg3. These diffraction data confirm the presence of the phases in the reaction zone which are suggested by the elemental analysis obtained by Mahler and his associates using an electron probe. Moreover, the strong intensity of the eta-Cu6Sn5 diffraction rings suggests a composition which is predominantly Cu6Sn5, again, in agreement with the electron probe data.