A Mechanistic Study of the Microalloying Effect in NiAl Base Alloys
- PDF / 448,556 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 14 Downloads / 157 Views
A MECHANISTIC STUDY OF THE MICROALLOYING EFFECT IN NiAl BASE ALLOYS R.D. Field, D.F. Lahrman, R. Darolia Engineering Materials Technology Laboratories, GE Aircraft Engines, 1 Neumann Way, Cincinnati, OH 45215. ABSTRACT Alloys based on the B2 compound NiAl have significant potential for applications in hot sections of aircraft engines due to their low density, high melting point, and high thermal conductivity. A major disadvantage of this class of materials is low ductility at ambient temperatures. Recently, it was discovered that small levels of certain elements (eg. Fe, Ga, Mo) result in dramatic improvements in room temperature ductility. In this paper, results are presented from a mechanistic investigation of the "microalloying" effect. Tensile and compression testing as a function of temperature and orientation has been performed on both the binary compound and microalloyed material. Data on ductile to brittle transition temperatures, critical resolved shear stress values as a function of temperature on the different slip systems, and dislocation structures from TEM analysis of the tested specimens are presented. These data are discussed in terms of possible mechanisms for the microalloying effect in NiAl alloys. INTRODUCTION A major limitation to the application of NiAl based alloys in turbine engines is low room temperature ductility. For single crystal NiAl, no RT plastic elongation is seen in oriented tensile specimens, whereas for and oriented NiAl (so called "soft" orientations), plastic elongation up to 2% can be obtained [1]. The plastic behavior is highly sensitive to composition and impurity content. The stoichiometric dependence of strength and ductility has been explained in terms of vacancies and anti-site defects within the structure [2]; however, the effects of impurities are not well understood. Alloying studies have shown beneficial effects of microalloying on the room temperature tensile ductility of "soft" oriented NiAl single crystals [3]. In room temperature tensile tests, up to 6% plastic elongation to failure has been obtained in oriented NiAl alloys containing 0.1 and 0.25 at% Fe. Significant improvements in ductility have also been made by alloying with Ga or Mo. For the Fe additions, the ductility improvements are sometimes accompanied by a reduction in yield strength. While ductility enhancements are obtained at microalloying levels, the beneficial effects are either reduced or disappear at higher levels. Low temperature heat treatments have also been found to increase RT ductility in NiAl. This was first observed in polycrystalline specimens [4], and more recent work has confirmed a ductility enhancement in "soft" oriented single crystal specimens [5]. The increased ductility is accompanied by a decrease in yield strength. In this paper, some early results are presented from a study to determine the mechanisms for these ductility enhancements in NiAl alloys. EXPERIMENTAL PROCEDURE Two alloys were studied: stoichiometric binary NiAl (D5) and an alloy containing 0.1at% Fe (D183). All o
Data Loading...
