US3515543A - Hafnium alloys - Google Patents

Description

United States Patent US. Cl. 75-134 7 Claims ABSTRACT OF THE DISCLOSURE Hafnium-base alloys suitable for nuclear reactor control rods consist of, by weight, at least one of the betastabilising elements zirconium 30% and niobium 15- 30% up to 2% in total amount of at least one of the betastabilising elements chromium, molybdenum, nickel and iron, up to 0.5% silicon, and up to 2% in total amount of one or both of the alpha stabilising elements tin and aluminum, the total content of alloying elements being not greater than 30%, balance hafnium and impurities.

This invention relates to hafnium alloys.

Hafnium is used for control rods in nuclear reactors because of its high neutron absorption coefficient. However, there are occasions when a material with a lower coefficient but having a comparable or better strength and corrosion resistance is required.

According to the present invention, a hafnium-base alloy consists of, by weight, one or both of the beta stabilising elements, zirconium 530% and niobium 15 30%, 02% in total of one or more of the beta stabilising elements, chromium, molybdenum, nickel and iron, 0-0.5% silicon, 0-2% in total of one or both of the alpha stabilising elements, tin and aluminium, the total content of alloying elements being not greater than 30%, balance hafnium and impurities.

The term impurities as used herein excludes zirconium which is frequently present in hafnium up to about 4%.

The preferred ranges of composition are 5-25% zirconium, 15-25% niobium, 0.1-2% in total of one or Patented June 2, 1970 ice 0.5% silicon, 0.1-2% in total of one or both of tin and aluminium. Preferably, the total content of alloying elements should be between 10 and 30%.

The effect of the beta stabilising elements is to strengthen hafnium; the alpha stabilising elements, whilst having little effect on strength when added alone, generally improve the properties -of the beta stabilised alloy.

The addition of niobium and zirconium reduces the neutron absorption coefficient and improves tensile strength. Niobium is considerably more effective in this latter respect than zirconium when only one of these elements is present. When both are present, the tensile strength is more than double that of commercially pure hafnium whilst the ductility is reduced by only a small amount. Additions of small amounts of the other beta stabilising elements, chromium, molybdenum, nickel, iron and silicon and of the alpha stabilising elements, tin and aluminium, further improve the tensile properties.

Corrosion resistance depends upon the alloying ele ments added and the amounts present. Zirconium has little effect on the excellent corrosion resistance of hafnium but niobium tends to increase the amount of surface film formed on exposure to high temperature pressurised steam. Below 15% niobium there is an appreciable loss of corrosion resistance.

The corrosion resistance of the alloys of the invention was determined by exposing cleaned bright samples to steam at a pressure of 100 atmospheres and at a temperature of 400 C. for 72 hours. The corrosion behaviour was compared by the appearance of the surface of the samples in accordance with the following classification.

(A) Glossy, bright, thin, adherent film (including interference films).

(B) Glossy, bright, thick, adherent film.

(C) Mottled appearance with dull patches or streaks which are usually powdery.

(D) Dull but adherent film.

It is important that the film should be adherent with no tendency to powder or to flake off. In the above classification, class (A) has the best corrosion resistance.

The tensile properties and corrosion resistance of a number of alloys in accordance with the invention are shown by way of example in the table in comparison with pure hafnium. In the table, the abbreviation 4 /A, used in connection with the elongation values, refers to the gauge length of test pieces, that is, a gauge length equal to four times the square root of the area of the crosssection of the test piece. The corrosion resistance was demore of chromium, molybdenum, nickel and iron, 0.01- termined in accordance with the above classification.

TABLE Elongation,

Composition Corrosion 0.2% P.S.. U.T.S., percent Alloy (analysed) resistance t.s.i. t.s.i. on 4 v2 Unalloyed T3 25Zr-0.5Mo A 15.7 30.6 23

We claim:

1. A hafnium-base alloy consisting of, by Weight, at least one beta stabilising element selected from the group consisting of zirconium 5-30% and niobium 15-30%; up to 2% in total of at least one beta stabilising element selected from the group consisting of chromium, molybdenum, nickel and iron; up to 0.5% silicon; up to 2% in total of at least one alpha stabilising element selected from the group consisting of tin and aluminium, the total content of alloying elements being not greater than 30%; balance hafnium and impurities.

2. A hafnium-base alloy according to claim 1 in which the total content of alloying elements is between 10 and 30%.

3. A hafnium-base alloy according to claim 1 containing 525% zirconium.

4. A hafnium-base alloy according to claim 1 containing 15-25% niobium.

5. A hafnium-base alloy according to claim 1 containing (ll-2% in total of one or more of chromium, molybdenum, nickel and iron.

6. A hafnium-base alloy according to claim 1 containing 0.12% in total of one or both of tin and aluminium.

7. A hafnium-base alloy according to claim 1 containing 0.010.5% silicon.

References Cited UNITED STATES PATENTS 2,234,969 3/1941 Hensel et al. 75134 2,810,640 10/1957 Bolkcom et a1 75l34 RICHARD O. DEAN, Primary Examiner