The effect of 0-4 weight percent chromium on transformations of alpha phase Ni-Mo-Cr alloys with nickel : molybdenum atom ratio equal to 4 : 1

Author

Ke Vāsudēvan

Date of Award

8-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

E. E. Stansbury

Abstract

The effect of 0-4 wt.% Cr on transformations of the alpha phase in Ni-Mo-Cr alloys with nickel:molybdenum atom ratio equal to 4:1 is reported using resistivity and hardness measurements, optical and scanning electron microscopy, and x-ray diffraction methods. The resistivity was measured during continuous heating, cooling, or isothermally for periods of more than a day on alloys in various initial states including cold worked. The isothermal transformation behavior of the short range ordered alpha phase was also studied at 600, 700, 800 and 850°C for times upto 4500 hours.

The SRO "K-State" effect exists in all Ni₄Mo + (0-4) wt.% Cr alloys in the retained alpha phase as shown by a negative temperature coefficient of resistivity and a decrease in resistivity with cold work. The resistivity decreases with an increase in cold work to 68% RA and then increases. This indicates that decrease in SRO compensates for the increase in resistivity caused by increased defect concentration to 68% RA. Above 68% RA the converse is true. The SRO resistivity decreases linearly with an increase in chromium content. The lattice parameter of the alpha phase Ni₄Mo increases with an increase in chromium content. The critical temperature of the order-disorder transformation decreases with an increase in chromium content, which is 868±2, 865, 850, 845 and 825°C for the Ni₄Mo, 0.41, 0.54, 0.74 and 2.08 wt.% Cr alloys respectively. About 3 wt.% Cr is sufficient to suppress long range ordering of Ni₄Mo in alloys cooled at rates faster than 1°C/min from above the critical temperature.

Based on isothermal resistivity measurements at 700°C, time of initiation of ordering of Ni₄Mo is not influenced by prior cold work. However, at later stages of aging the ordering rate is higher the greater the degree of prior cold work. Cold work more than 68% RA substantially increases the ordering rate at all times beyond 20 minutes. Ordering in cold worked Ni₄Mo at 700°C, can be said to occur by a process of heterogeneous nucleation which appears optically as a homogeneous reaction.

Based on resistivity measurements and microstructure, ordering in Ni₄Mo is complete in 24 hours at 850°C, 10 hours at 800°C, 96 hours at 700°C and 2000 hours at 600°C. The kinetics of ordering of Ni₄Mo are only slightly affected by additions of chromium to 0.74 wt.%. At 2.08 wt.% Cr and above, the rate decreased substantially at all temperatures but 700°C.

Distinct {111} f.c.c. striations are observed in the beta phase microstructure on isothermal aging. Since the striations usually form before the maximum hardness, it is suggested that they are mechanical twins introduced by stresses from the small lattice parameter changes associated with the ordering. Also, tapered {110} f.c.c. striations are observed in the beta phase, often coexisting with the {111} f.c.c striations. In the higher chromium-containing alloys, short Widmanstatten plates, tentatively identified as beta phase, form parallel to {111} f.c.c. planes.

The long range ordered beta domains possess six distinct orientations in correspondence to a {110} f.c.c. habit plane. Time at temperature results in growth of domains to large amoeba-shaped, irregularily bounded regions. Domain growth is inhibited by the addition of chromium and often domains appear as doe-eye or flame-shaped formations.

Based on hardness, resistivity and optical microstructure on alloys aged for times to 4500 hours, the solubility of chromium in the beta phase is about 4 vit.% at 600°C and about 0.4 wt.% at 700, 800 and 850°C. Widmanstatten gamma phase is observed in chromium containing alloys. In the higher chromium alloys, the gamma phase is present in two morphologies, namely, thin Widmanstatten gamma striations/plates parallel to {111} f.c.c. planes and massive Widmanstatten plates. The formation of the Widmanstatten gamma plates is associated with a high hardness. In alloys located in the three phase α + γ + δ region (higher temperatures), a discontinuous precipitation of δ and γ phases is observed.

Recommended Citation

Vāsudēvan, Ke, "The effect of 0-4 weight percent chromium on transformations of alpha phase Ni-Mo-Cr alloys with nickel : molybdenum atom ratio equal to 4 : 1. " Master's Thesis, University of Tennessee, 1982.
https://trace.tennessee.edu/utk_gradthes/15108