Tech Notes Superplasticity Superplasticity has several different variations in terms 3000 of microstructural Temperature, mechanisms and Kelvins deformation conditions,. 2500 503 These include 473 • Micrograin superplasticity 0 423 • Transformation L/L superplasticity 2000 • Internal stress superplasticity 1500 At this time, only micrograin superplasticity is of importance in the fabrication of parts. For 1000 micrograin superplasticity, high ductilities are observed Tensile fracture strain, fracture Tensile only under certain conditions, and the basic 500 requirements are: • Very fine grain size material (on the order of 10 m, or 400 in., or finer). 10-6 10-5 10-4 10-3 0.01 0.1 1 10 • A controlled strain rate, Strain rate, 1/s usually 0.0001 to 0.1/sec. • Relatively high Tensile fracture strain versus initial strain rate for a Zn-22Al alloy having a grain size of temperature, greater than 2.5 m (100 in.) tested at temperatures ranging from 423 to 503K. approximately one-half the absolute melting point. ductility is seen to vary and ’ is the strain rate. components can substantially with strain The characteristics of be fabricated in a A current research goal is rate, as shown in the graph superplastic alloys indicate single step. development of for a zinc-aluminum that unusual forming finer-grain materials that eutectoid alloy. As shown, capability should be exhibit superplasticity at ductility reaches a possible, but control of This information is from lower forming temperatures. maximum at a specific forming process parameters ASM HANDBOOKS strain rate, with significant is important to reach the full ONLINE, Volume 14B, Characteristics of losses in ductility as the potential of this class of Metalworking: Sheet superplastic metals strain rate is increased or material. Such process Forming —> Superplastic Only a limited number of decreased relative to this controls are more Sheet Forming —> commercial alloys are maximum. demanding than Requirements for superplastic, and these are corresponding requirements Superplasticity formed via methods and It is well known that the for conventional forming conditions that are different primary factor related to this processes, and the from those for conventional behavior is the rate of superplastic forming of metals. However, more and change of flow stress with sheet metals is a more alloys have been strain rate, usually technology that is different grain-refined to induce measured and reported as from the conventional superplasticity. m, the strain-rate sensitivity processes. exponent: For a superplastic metal However, superplastic that is tensile tested under m = ln / ln ’ forming offers advantages proper conditions of over other fabrication temperature, the observed where is the flow stress, methods in that complex 96 ADVANCED MATERIALS & PROCESSES/MAY 2008.