Mechanical Properties, Fracture Behavior, and Grain-Boundary Chemistry of B-DOPED NiAI
- PDF / 1,246,595 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 40 Downloads / 170 Views
MECHANICAL PROPERTIES, FRACTURE BEHAVIOR, AND BOUNDARY CHEMISTRY OF B-DOPED NiAI
GRAIN-
E. P. GEORGE,' C. T. L|U,1 and J. J. LIAO2 'Mtasand Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 2MaeiasEngineeringDepartment, Auburn University, Auburn, AL 36849
ABSTRACT This paper summarizes the results of our work aimed at overcoming the intrinsic grainboundary weakness of NiAI by microalloying with boron. In previous work we have shown that 300 wppm boron is very effective in suppressing intergranular fracture in NiAI [1]. It does this by segregating strongly to the grain boundaries and strengthening them. Despite this dramatic effect on the fracture mode, however, boron is unable to improve ductility because it is a potent solid solution strengthener, more than doubling the yield strength relative to that of undoped NiA1. The present work attempts to decrease this deleterious hardening effect by lowering the bulk concentration of boron in NiA1. Our results show that if the boron concentration in the bulk is lowered to 30 wppm, the yield strength of boron-doped NiA1 is only about 30% higher than that of undoped NiAI. In addition, there is enough boron at the grain boundaries of this alloy to suppress intergranular fracture. Under these conditions, boron-doped NiAI has a tensile ductility of 2%, which is essentially identical to that of undoped NiA1. This result, namely that the strengthening of grain boundaries by boron does not by itself improve ductility, indicates that although grain boundaries might well be the weakest links in NiAI, cleavage planes are not much stronger. In other words, even though boron additions serve to strengthen the grain boundaries and suppress intergranular fracture, ductility is not improved, because the next brittle fracture mode, namely transgranular cleavage, takes over before significant plastic deformation can occur. INTRODUCTION After an early report in 1966 of limited (2%) room-temperature tensile ductility in polycrystalline NiAI [2], numerous later attempts to reproduce this ductility were unsuccessful, until Hahn and Vedula [3] recently showed that it was possible to obtain room-temperature plastic elongation of 2.5% in nearly stoichiometric, cast and extruded NiAI. Although it is not completely clear why the previous attempts were unsuccessful, it is now routinely possible to obtain plastic elongations of 2-3% in cast and extruded stoichiometric NiAI [e.g., Ref. 1]. Perhaps some of the earlier unsuccessful attempts, especially those using the powder-metallurgy approach [4,5], were plagued by interstitial element problems. For example, our recent work [1] has shown that as little as 300 wppm each of carbon or boron can embrittde NiAI, mainly by dramatically increasing the yield strength. Notwithstanding these recent demonstrations of tensile ductility in NiAI, the roomtemperature ductility of NiA1 remains relatively low (2-3%). Among the reasons commonly cited for the poor room-temperature ductility of NiAI are: (i) limited number of deformation modes (a co
Data Loading...
