Precipitation effects during hot deformation of a copper alloy
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INTRODUCTION
S T A T I C precipitation studies of a number of hardenable alloys have been undertaken in an effort to improve their mechanical properties and to identify optimum thermomechanical treatment conditions. Alloy composition and precipitation-time-temperature (PTT) characteristics usually are examined. Thermomechanical treatments produce strain hardening and static precipitation in a solution-treated alloy that is deformed and aged at low temperatures. The controlled roiling of steels and other alloys, where the temperature of the rolling passes is lowered over the range of aging temperatures, has been extensively studied. Such experiments showed the effects of static precipitation in the hardening and softening processes. On the other hand, dynamic precipitation, defined as precipitation that occurs during the hot deformation of materials, has been largely ignored--probably because of the difficulty of distinguishing static from dynamic effects. Since investigations on dynamic precipitation are based primarily on low-strain-rate tests, the applicability of the results is restricted in practice to low strain rates and relatively high temperatures of deformation. Dynamic precipitation of carbonitrides in highstrength, low-alloy (HSLA) steels has been investigated by Jonas and Sakai and co-workers. I~ 8r The delay of dynamic recrystallization by carbonitride precipitation has been studied on the basis of mechanical test results. When dynamic precipitation or other structural processes affect dynamic recrystallization, the flow-stress peak becomes displaced. Compression tests performed over a wide range of strain rates allowed identification of the start and finish times for dynamic precipitation. One can see from PTT curves of dynamic and static precipitation L. BLAZ, Associate Professor, is with the Department of Structure and Mechanics of Solids, Academy of Mining and Metallurgy, Cracow 30-059, Poland. E. EVANGELISTA, Professor, is with the Department of Mechanics, University o f Ancona, Ancona 1-60131, Italy. M. NIEWCZAS, formerly Graduate Student, Academy of Mining and Metallurgy, Cracow, Poland. is Research Associated, D e p a r t m e n t of Materials Science and E n g i n e e r i n g , M c M a s t e r University, Hamilton, Ont., Canada. Manuscript submitted June 17, 1991. METALLURGICAL AND MATERIALS TRANSACTIONS A
that dynamic precipitation is notably accelerated by deformation and proceeds faster than the static aging that occurs in a material with a preexisting dislocation substructure. [~ In torsion tests of solution-treated aluminum alloy AA6015, Evangelista e t a l . Igt found that the flow curves exhibited a sharp peak and a rapid decline to failure at 570 and 623 K, which coincided with the precipitation of semicoherent particles contributing to hardening. At higher temperatures, as precipitates grew, the pinning effect progressively decreased, and softening attained a steady state due to a very fast recovery. Hot deformation activation energy at a temperature range of 570 to 620 K was ver
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