Trace element effects on ductility and fracture of Ni-Cr-Ce alloys
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KANDRA
The effect of trace additions of Ce, ranging from 0 to 180 at. ppm, on the tensile behavior of a Ni-20Cr alloy is presented. For alloys without Ce a transition from ductile transgranular to brittle intergranular fracture mode is observed at high temperatures and for low strain-rate tests. Additions of Ce suppress this transition with a resulting increase in ductility. Maximum effects are observed for temperature and strain rate values where fracture in Ce-free alloys occurs via grain boundary cavitation. The reduced cavitation rate of Ce-containing alloys is suggested to be the result of an increase in both interfacial energy and grain boundary mobility.
I.
INTRODUCTION
this paper, the effect of temperature, strain-rate, and composition on the tensile strength, ductility, and fracture mode are presented.
Tr E phenomena of high temperature intergranular fracture is controlled primarily by the process of cavitation which includes cavity nucleation, growth, crack formation, and propagation leading to final fracture. There is now a large body of evidence 1'2 which indicates that minor trace elements, especially those with a high interfacial activity, greatly influence this process. The presence of these trace elements promotes drastic reductions in ductility in the temperature range of 0.5 to 0.8 TM. In addition it is now apparent that this high temperature intergranular fracture mode is not necessarily an intrinsic mode of fracture present for all alloys but to the contrary, could be minimized or be totally suppressed in ultra-pure materials or for alloys with specific alloying additions such as with Zr, 3 Ce, 4 or B. 5 In nickel alloys, O and S are probably two of the most harmful elements which promote severe intergranular cracking.: The elaboration of ultra-high purity alloys is a difficult and very complex task which is seldom achieved on a commercial basis. Another very effective way of reducing the O and S content to a very low level is through the addition of refining aids such as Ca, Mg, Mn, La, or Ce. 2 It has been shown recently 6 that Ce is extremely effective in controlling the free O and S due to the low free energy of formation of various oxides, sulfides, and oxisulfides. The alloys thus produced contain a free O and S content of about 5 ppm or less and can therefore be considered as "high purity" alloys. By varying the amount of Ce additions a range of alloys containing various trace concentrations of O and S has been produced. 6,7 In order to improve our understanding of the mechanism of intergranular fracture and to elucidate further the role played by trace elements, an investigation was conducted to study the mechanical behavior of these Ni-Cr-Ce alloys. In Table I.
II.
EXPERIMENTAL
PROCEDURES
A. Alloy Preparation The alloys used in this investigation were solid solution Ni-20Cr (wt pct) with Ce concentrations ranging from 0 to 180 atomic ppm. They were prepared by vacuum induction melting (VIM) at Special Metals Corp. followed by casting and hot rolling. Ce was added to the melt in
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