Brittle fracture and grain boundary chemistry of microalloyed NiAl
- PDF / 1,074,943 Bytes
- 9 Pages / 593.28 x 841.68 pts Page_size
- 31 Downloads / 237 Views
I. INTRODUCTION The recent confirmation1 of an earlier observation2 that limited room-temperature tensile ductility can be obtained in stoichiometric NiAl has renewed interest in this potentially useful high-temperature structural material. NiAl has the ordered B2 crystal structure and is stable over a relatively wide composition range: —4559 at. % Ni at 400 °C.3 However, relatively small deviations from stoichiometry can result in significant defect-induced hardening in NiAl.4"6 Therefore, several recent efforts to improve its ductility have concentrated on the stoichiometric alloy. The room-temperature fracture mode of stoichiometric NiAl is predominantly intergranular.1 In the past, intergranular fracture has often been linked to the segregation of harmful impurities to grain boundaries: for example, many of the elements in groups IVA-VIA of the periodic table are well-known embrittlers of ironbase alloys (see, e.g., Refs. 7 and 8); likewise Sb9 and Bi10 are known to embrittle Cu, and S embrittles Ni3Al.n However, there is now growing evidence that 754
http://journals.cambridge.org
intermetallics like Ni3Al and Ni3Si,12 15 and even some elemental metals like fee iridium,16'17 have intrinsically brittle grain boundaries; i.e., these materials are brittle even in the absence of harmful grain boundary impurities. One of our objectives here is to determine whether the same is true for NiAl. Another objective is to determine whether it is possible to suppress grain boundary fracture in NiAl by the addition of suitable microalloying elements, and to correlate the fracture mode and ductility with the grain boundary segregation behavior of the added element. Boron, carbon, and beryllium were chosen as potentially beneficial additions: boron because it suppresses intergranular fracture and dramatically improves ductility in Ni3Al,12'18 carbon because of its chemical similarity to boron and also because first-principles quantum mechanical calculations19 have shown that carbon is better than even boron in improving the cohesive strength of Ni6 clusters, and beryllium because of its mildly beneficial effect in Ni3Al.20 We will also evaluate in this paper the room-temperature mechanical properties of NiAl
J. Mater. Res., Vol. 5, No. 4, Apr 1990
Downloaded: 12 Mar 2015
IP address: 139.78.24.113
E. P. George and C.T. Liu: Brittle fracture and grain boundary chemistry of microalloyed NiAl
and its alloys, and discuss critically the possible factors (yield strength, grain size, grain boundary chemistry, microalloy additions, etc.) that influence ductility and fracture mode. II. EXPERIMENTAL PROCEDURE
Four alloys with chemical compositions corresponding to pure, stoichiometric NiAl, NiAl + 300 wt. ppm each of B and C, and NiAl + 500 wt. ppm Be were arc melted using high-purity elemental starting materials and a Ni-4 wt.% B master alloy, and drop cast into cylindrical copper chill molds measuring 25.4 mm in diameter and 102 mm in length. After homogenizing in vacuum for 24 h at 1100 °C, the ingots were canned in mild steel
Data Loading...
