Improving the Low Temperature Ductility of NiAl
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IMPROVING THE LOW TEMPERATURE DUCTILITY OF NiAl Sumit Guha, Paul R. Munroe and Ian Baker Thayer School of Engineering, Dartmouth College, Hanover, NH 03755 ABSTRACT As part of a study aimed at developing a ductile NiAl-based alloy, ingots of Ni-Fe-Al alloys were cast and hot extruded to rods. The purpose of the iron additions was two-fold viz; to produce a change in the slip vector from to and, in one alloy, to add a L12-structured ductile second phase. Extruded Ni-20A1-3OFe was two-phase, containing a pro-eutectic B2 phase in a fine lamellar structure (< 0.5 gm spacing) of B2+L1 2 phases. Room temperature tensile testing of both single extruded and double extruded alloys resulted in 8% and 22% plastic elongation and yield stresses of 850 and 760 MPa respectively. Fracture in both cases occurred by ductile tearing of the eutectic and transgranular cleavage of the proeutectic phase at 1350 MPa. The ductility in double extruded condition is higher than that reported earlier in rapidly solidified wires by Inoue et al., J. Mat. Sci., 19, (1984), 3097. By comparison, extruded single-phase B2-structured Ni30AI-2OFe exhibited a fracture strength of 780 MPa, no plasticity and a mixture of intergranular fracture and transgranular cleavage. This is in contrast to earlier work by Inoue et al. (ibid) where a yield stress of 400 MPa, 5% plastic strain and a mixture of dimple and intergranular fracture was reported.
INTRODUCTION NiAl is a B2-structured (ordered bcc) intermetallic which usually deforms by (hk0) slip at temperatures below -500'C(1 ). For ductility slip presents a fundamental problem since it provides only three independent slip systems( 2) and, thus, does not satisfy Von Mises
criterion for general plastic flow(3). As part of a study attempting to develop a ductile NiAl-based alloy, ingots of Ni-30A1-2OFe and Ni-20A1-3OFe (in at. %) were cast. Iron was chosen for two reasons. Firstly, at low temperatures, polycrystalline FeAl deforms by slip( 4 ,5), therefore iron additions may change the slip vector from to . Furthermore, FeAl exhibits
ductility when iron-rich(5 ,6). Secondly, in Ni-20A1-3OFe second phase formation is based upon the ductile L12 structured (ordered fcc) intermetallic Ni3Fe(7). Both of the alloys examined here, Ni-30A1-2OFe and Ni-20A1-30Fe, have been previously studied by Inoue et al.(8 ) in a wire form made by an 'in-rotating-water' rapid solidification technique. Ni-20A1-3OFe wires contained a duplex L12 (Ni,Fe) 3(A1,Fe) plus B2 (Ni,Fe)(Al,Fe) structure of 0.2tm grain size and on room temperature tensile testing exhibited a yield strength of -600 MPa, 17% plastic strain and dimple type fracture. By comparison, Ni-30A1-2OFe wires were single phase, of 4pm grain size, and exhibited a yield strength of -400 MPa, 5% plastic strain and a mixture of dimple and intergranular fracture. Three reasons were cited for the low temperature ductility: grain size refinement (although unalloyed NiAl with 4Rtm grains is not ductile at room temperature( 9)), suppression of ordering and suppression of grain boun
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