Studies on embrittlement of ordered Ni4Mo

Author

Mahesh Sanganeria

Date of Award

5-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

C. R. Brooks

Committee Members

A. J. Pedraza, J. E. S

Abstract

The alloy Ni₄Mo in the ordered condition is strong and has potential for use at moderately high temperature (approx. 600°C), but it is extremely brittle in the ordered condition. Ni₄Mo based commercial alloy Hastelloy B-2 is used in the disordered condition for its good strength and corrosion resistance. However, the possibility of ordering, and thereby embrittlement, during fabrication by welding is of concern. The present work involves an attempt to understand the phenomenon of embrittlement in ordered Ni₄Mo with the aim of exploiting its potential use.

The alloy Ni₄Mo undergoes an ordering transformation below 868°C from a short-range order (SRO), FCC structure (α) to a tetragonal structure (β) , accompanied by a significant increase in hardness, yield strength and a drastic reduction in ductility. The α phase can be retained by rapid cooling from 868°C, and then the rate of ordering controlled by choice of aging temperature and time. On aging in the temperature range 700-800°C, an independent reaction commences, which involves the movement of high angle, former α-boundaries. Fracture in the embrittled ordered alloy occurs, clearly, along the high angle, former α-boundaries.

However at increased aging times, the fracture morphology becomes finer (without any significant change in ductility) which does not correlate with the former α-grain size. It has been established in this work that this finer fracture surface morphology is associated with the fracture along the migrated, high angle boundaries.

Attempts were made to improve the ductility of ordered Ni₄Mo by microalloying. Arc melted, microalloyed buttons were processed to obtain a fine grain, homogeneous a structure and then ordered by aging at 725°C for four hours. Thin slices of the ordered samples were bent to test for ductility. Elements Hf, Zr, Ce, Nb, Mg, La, and Mn were selected to act as a scavenger of impurities from the grain boundary but these elements did not seem to improve the ductility. Among the elements which are likely to segregate to the grain boundary and improve the strength of intrinsically weak boundaries, boron and beryllium were tested. Boron produced no observable change in ductility, however, beryllium appeared promising as the sample showed a little bending before fracturing.

The aging (up to 20 hours) behavior of cold worked, disordered structure a has been determined for 700°C, 775°C and 850°C (T_c = 868°C) in terms of microstructural observation and microhardness measurement. Three degrees of cold work, 43%, 68% and 80% reduction in area, were investigated. Aging at 850°C resulted in recrystallization of cold worked a into strain-free equiaxed α (with a marked hardness decrease), followed by short-range ordering and then long-range ordering (with a marked hardness increase). Then the long-range ordered structure on continued aging showed softening which appeared to be associated with domain growth. Aging at 700°C and 775°C did not result in recrystallization of cold worked a into strain-free a. Instead, it appears that long range ordering of cold worked disordered α (accompanied by a marked increase in hardness) followed by recrystallization into low dislocation density ordered β structure occurred. There is also a contribution of short-range ordering (which has been reported to exist at initial stages of aging cold worked a structure at 700°C) in increasing hardness in the initial stage of aging. Recrystallization of β was optically resolved on aging 43% cold worked sample for 150 minutes at 775°C or 300 minutes at 700°C.

The possibility of ordering and embrittlement, during fabrication by welding, in some Hastelloy B-2 components, in use at Tennessee Eastman Co., Kingsport, Tennessee was examined. Two samples were investigated to study the effect of welding. In the as-received condition these samples showed cracks on the surface which was exposed to chemicals in application. Metallographic observation Vlll showed that these cracks were intergranular. However, bending samples with the cracks did not propagate them intergranularly. These samples showed an extensive bend ductility. The cause of the cracks is intergranular corrosion.

Recommended Citation

Sanganeria, Mahesh, "Studies on embrittlement of ordered Ni4Mo. " Master's Thesis, University of Tennessee, 1989.
https://trace.tennessee.edu/utk_gradthes/13068