Dynamic Recrystallization in Tial Alloys

  • PDF / 3,316,018 Bytes
  • 5 Pages / 420.48 x 639 pts Page_size
  • 51 Downloads / 315 Views

DOWNLOAD

REPORT


DYNAMIC RECRYSTALLIZATION IN TIAL ALLOYS D.S. LEE*, D.M. DIMIDUK*, S. KRISHNAMURTHYt *Materials Directorate, WPAFB, OH 45433 rUES, Inc, Materials Research Division, Dayton, OH 45432 ABSTRACT TiAl alloys with Cr, V, and Nb additions show promise as high temperature materials due to their high temperature strength and modulus. Dynamic recrystallization has been shown to be important for the processing and superplastic forming of these materials. Earlier studies have indicated that dynamic recrystallization may also occur during tensile straining at or near the proposed use temperature of these alloys. A systematic study has been conducted to determine the effects of various parameters - temperature, strain, strain rate, and microstructure, on the occurrence of dynamic recrystallization near expected use conditions. The conditions near the ductile to brittle transition temperature which bound the onset of dynamic recrystallization in tension and compression were investigated. The findings will be presented and correlations will be drawn between the contributions of dynamic recrystallization and high temperature to the observed ductility. INTRODUCTION Titanium aluminides are currently being studied for a variety of military and commercial applications. The alloys of most interest combine some combination of gamma and alpha-2 phases, in either a duplex or fully lamellar microstructure, in order to obtain a balance of properties. While recent advances in processing and alloy development have made these alloys contenders to applications for 1500'F, many basic questions remain which may ultimately affect their performance in use. One of the most basic problems with this class of alloys is its lack of room temperature ductility. In fact, the tensile ductility does not rise appreciably above 2-4% until the material reaches its ductile to brittle transition temperature. This is typically around 700800*C, depending upon the alloy composition and prior thermomechanical history. This phenomenon, although widely known, is not well understood. The mechanisms which might account for this change in behavior include increased dislocation activity [9], increased twinning, dynamic recrystallization, or some combination these mechanisms. A study by Krishnamurthy et al. [1] of a near gamma alloy, showed that dynamic recrystallization could be found in tensile specimens near the necked region for specimens tested above 900°C. In specimens tested at 1000'C the dynamic recrystallization consumed the entire necked region in a duplex alloy. The fully lamellar alloy studied was also fully recrystallized in the necked region, except for lamellar grains, which were oriented in the direction of the tensile axis. The true strains calculated in the affected region were all above 1.0. Given that gamma/alpha-2 alloys are expected to be used at or near their ductile to brittle transition temperature, it is of interest to determine whether dynamic recrystallization is occurring at these temperatures, and if so, under what conditions. Additionally, if d

Data Loading...