Flow and Fracture of Dental Alloys Determined by a Microbend Tester
The microstructure of an alloy is the fundamental variable on which the mechanical properties of an alloy depend. For testing the mechanical properties, however, common tests, such as tensile strength or hardness, are generally used. This may be due to the fact that the microstructure study is more complicated and time-consuming. The common method of microstructure study is to perform a series of tensile tests and then obtain the microstructure of the specimens after the tests are completed so as to correlate the structure with the performance data. The actual contribution of each observable microstructure to flow and fracture is rather difficult to detect. When the structure is examined after the fracture has taken place, some of the evidence is lost that would have been available if continuous observation of the structure had been possible. In 1955 Flinn and Trojan' developed an apparatus by means of which the microstructure of an alloy under stress can be studied. The stress can be increased while the microstructure is under observation. Thus any changes that may take place can be detected in any of the phases. When an alloy with a complicated microstructure is studied, the microbend test shows its main advantage, since it is rather difficult to predict how the different phases of a complicated structure will behave under different stresses. With this test the effect of such stress on various phases may be continuously watched under the microscope. The purpose of this study was to observe the behavior of different phases of cobalt-base and gold alloys used in dentistry for partial denture framework when they were subjected to bending stresses.
Effect of Casting Conditions on Some Mechanical Properties of Cobalt-Base Alloys
The casting of cobalt-base alloys in dentistry is comparatively new in relation to the casting of gold alloys and other restorative procedures. It has been less than thirty years since these alloys were introduced to dentistry, but each year they have become more popular. A survey of the choice of dental alloys for removable partial dentures conducted by Lane' in 1949 showed that five times as many removable partial dentures were cast of the cobalt-base alloys as of gold alloys. Today this ratio may be even higher, since cobalt-base alloys are the choice of many dentists and laboratory technicians. Unfortunately, even with such a high demand for this type of alloy in dentistry, little is known by the profession about the physical and mechanical properties of the alloy or about its basic nature. Similar alloys of cobalt are widely used in industry at high temperatures, and consequently more work is done and reported with this type of alloy used at high temperatures than at room temperature. Although acceptable dental techniques for casting these alloys are available, a completely dependable, accurate, and easy-to-apply technique has not yet been developed. Such a technique probably cannot be developed until more is known about the basic nature of these alloys and about the effect of variations in casting procedures upon their properties. The purpose of this study was to determine the physical properties of cobalt-base alloys and to relate these properties to the microstructure, as well as to observe the influence of casting variables on both microstructure and physical properties. In this way the most acceptable casting practice can be selected which is revealed by both physical tests and microstructure.
A survey of dental alloys.
No other material is so universally and successfully employed in restoring carious teeth to a state of usefulness as is dental amalgam. Besides forming such a large proportion of the fillings, the number of amalgam restorations made annually is tremendous. The office of the Surgeon General stated that 68,170,326 permanent fillings were inserted by Army dentists from December 7, 1941, to September 1, 1945, and most of them were of amal­ gam.1 Historically, amalgams were first intro­ duced in Europe in 1826 (under the name of “ silver pastes” ) and in America in 1833. Partly because of their inferior and undependable performance, but largely because they were first used by charlatans, the adoption of amalgams by the dental profession was slow. What has been called an “ amalgam war” raged, beginning in 1845. The American Society of Dental Surgeons passed a resolution in 1845 which reads: Resolved, That any member of the Society who shall hereafter refuse to sign a certificate pledging himself not to use any amalgam, and, moreover, protesting against its use, under any circumstance, in dental practice, shall be ex­ pelled from the Society.