Precipitation hardening in nickel-copper base alloy monel K 500
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I.
MONEL*
INTRODUCTION
K 500 is a nickel-base age-hardenable
*MONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
alloy containing a substantial amount of copper together with small amounts of iron, manganese, aluminum, and titanium. Age hardening in this alloy is caused by the formation of coherent, ordered (Ni3(A1, Ti)) precipitates. [IJ This alloy not only possesses excellent corrosion resistance like other monel alloys but also has a high room-temperature strength, comparable to those of some Ni-Cr-base alloys and low-alloy precipitation hardening steels. Although this alloy has been in use for some time, its age-hardening behavior does not appear to have been studied in detail previously. This article describes the age-hardening response of MONEL K 500 and attempts a systematic analysis of the relationship between the microstructure of the alloy and its strength. The microstructure has been studied with special reference to the dislocation arrangements and the dislocation-precipitate interactions in specimens containing different volume fractions and sizes of precipitates. An effort has been made to compare the observed increase in strength due to precipitation with that predicted theoretically on the basis of the available models. MONEL K 500, like some other commercial, nickel-base, precipitation-hardenable alloys (e.g., NIMONIC* PE16 and NIMONIC 105), is *NIMONIC is a trademark of Inco Alloys International, Inc., Huntington, WV.
well suited for a comparative study of this kind, since the ordered phase forms as coherent, nearly stress-free (zero misfit), spherical precipitates. These particles, after isothermal aging treatments, cover a wide range of precipitate sizes. In the present work, the theoretical increase in strength in the underaged condition has been estimated by a combination of the B r o w n - H a m t21 and G.K. DEY, R. TEWARI, S.L. WADEKAR, and P. MUKHOPADHYAY, Scientific Officers, are with the Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India. P. RAO, Graduate Student, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Manuscript submitted January 12, 1993. METALLURGICAL TRANSACTIONS A
the Haasen-Labusch models, t3j In the peak-aged and slightly overaged conditions, a theoretical estimate of the increase in strength has been obtained on the basis of the formulation proposed by Nembach and Neite. t41 For significantly overaged conditions, the relationship suggested by Kocks tSl and Hirsch and Humphrey [61 has been used. While analyzing the experimental data, the anisotropy of the matrix has been taken into account. The dislocation line tension has been evaluated with respect to its dependence on the size and volume fraction of the particles. Loss of solid solution strengthening of the solute-depleted matrix in the aged specimens has been taken into account while determining the strengthening contribution of the precipitates. The effect of precipitates on fracture morphology h
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