I LOW TEMPERATURE METALS Brookhaven （我低温金属布鲁克黑文）.pdfVIP

LOW TEMPERATURE METALS A . Hurlich General DynamicslAstronautics San Diego, California Before we consider how low temperatures affect the engineering properties ofspe- cific metals and alloys, we should review the general effects of low temperature upon the properties of metals. As the temperature is lowered, the hardness, yield strength, tensile strength, modulus of elasticity, and fatigue resistance of almost all metals and alloys increase. Unfortunately, many engineering metals and alloys become embrittled at reduced. temperatures so that structures fabricated from them fracture or shatter unexpectedly at low temperatures when loaded to stress levels at which performance would be satis- factory at normal temperatures. Brittle fracture is generally characterized by little or no deformation of the metal in the vicinity of the fracture, or by a distinctive fracture having bright crystalline appearing facets. Considerable confusion has, unfortunately, existed about the nature of the behav- ior that is the opposite of brittle. Some refer to this as ductile, while others usethe term tough. Although both terms are correct, they do not describe the same charac.teristic of a materials behavior. Ductility is commonly expressed in terms of percentage elongation in gauge length, and reduction in area, of a tensile specimen that is tested to fracture. Ductility is thus a measure of the deformation undergone by a smooth test specimen of uniform cross. section that is slowly and uniaxially stressed until fracture occurs. Toughness, on the other hand, is a characteristic that permits a metal to absorb energy by under- going plastic deformation rather than fracturing in the presence of high stress con- centrations (cracks or notches) and multiaxial stress distributions, underh