US2848360A - Method of treating nickel-beryllium alloys - Google Patents

Description

lite METHOD OF TREATING NICKEL-BERYLLILHVI ALLOYS No Drawing. Application May 21, 1954 Serial No. 431,580

Claims priority, application France May 21, 1953 4 Claims. (Cl. 148-115) This invention relates to improved compositions of matter and particularly nickel base alloys. More especial ly the invention relates to improved metallic compositions particularly useful as secondary emissive substances.

it is an object of the invention to provide a composition of matter possessing a high secondary emission ratio when bombarded with primary particles.

Another object is to provide novel nickel-base alloys which have a high secondary emission ratio and at the same time possess other desirable mechanical and physical characteristics.

A further object is to provide novel alloys having high secondary emission ratios and satisfactory hardness yet readily workable.

A further object is to provide such alloys having strictly controllable, reproducible, characteristics.

A further object is to provide methods of controllably producing improved compositions of matter of the kind above specified.

Further objects and advantages of the invention, as well as the characteristic features thereof, will appear as the disclosure proceeds.

When alkali and alkali-earth metals are used as targets for primary particles such targets have a low secondary emission factor, in the neighborhood of unity. The oxides of alkali and alkali earth metal have a greater factor of secondary emission, which may be as high as five, as in the case of caesium and barium for example.

It is known moreover that certain alloys containing suitable oxides therein have a still higher emission factor. This is particularly true of alloys of nickel and metals in the second group of the periodical table when more or less oxidized. The secondary emission factor has then been found to attain a value of ten or more.

The use of such alloys however has not heretofore entered the field of industrial practice for various motives and especially owing to difliculty in the manufacture of such alloys and in the machining of articles made from the alloys.

We have now found that the proportion of the metal associated with the nickel is of major importance in alloys of the type just specified. Thus the nickel-beryllium alloys heretofore advocated for purposes of secondary emission contained less than 2.5% beryllium. In this condition the emission factor does not exceed a value of about 4.5. However, if the amount of beryllium be increased above 2.5%, say to a range of from 2.5% to 4.5% or more, it is found that the secondary emission ratio would reach exceedingly high values such as fifty or more.

The same is found to be true where magnesium is used instead of beryllium. In the case of magnesuim the most favorable range of concentration is from about 0.6% to about 2% magnesium by weight.

Moreover, the behavior of the new alloys when used as secondary emission targets is found to be frequently characterized by the phenomenon of persistent emission known a Malter effect. It will be recalled that Malter States Patent effect consists in the fact that the secondary emission continues as a decreasing rate for some time after the cessation of bombardment by the primary particles, and this efiect is turned to advantage in certain particular applications.

In alloys of the kind contemplated in the invention part of the beryllium or the magnesium present is in a state of solid solution in the nickel.

According to the invention, the alloys are heated in the presence of air to a temperature high enough to produce a migration of the beryllium or the maganesium towards the surface where it will oxidize in the presence of air or oxygen. In this way a manner of activation is produced which is believed to be responsible for the high secondary emissive capacity of the resulting alloys.

Crystallographically, X-ray investigation shows that in alloys containing more than 2.5% beryllium, there occurs a second phase or intermetallic compound with the formula NiBe, which may be called in accordance with the Hume-Rothery terminology, an electronic compound of nickel-beryllium to distinguish it from ionic or semiionic compositions. The electronic compound of nickelberyllium does not correspond to an interchange of valences.

The phenomenon of secondary emission depends on the surface migration of the beryllium atoms. This migration requires the presence of a saturated solid solution of beryllium in nickel. The Ni-Be phase then functions as a store or reservoir of beryllium with respect to the solid solution, inasmuch as the beryllium in that phase is more closely bound that it is in the solid solution, so that it will not flow out of said phase unless there is a drop in the beryllium concentration in the solid solution. Hence it is highly desirable to use alloys containing more than 2.5 beryllium in order to produce this beneficial regulating phase. The electronic NiBe compound possesses a centered cube structure, with a mesh lattice parameter this is, a unit-cell dimension, of a=2.6l4 A.

Diffraction patterns further disclose that in cases where the alloys are subjected to an insufiicient annealing treatment, the Ni-Be phase does not precipitate entirely. The addition of beryllium results in a contraction of the mesh. This general phenomenon corresponds with the formation of the solid solution, and it can be stated as an approximate rule that the mesh contraction increases with the beryllium concentration. For example, the mesh of nickel is equal to 3.517 A., and this is changed to 3.505 A., for a saturated solution of beryllium in nickel. When the effect is continued beyond the concentration of saturation, the solution becomes supersaturated. If one operates without annealing, a supersaturated unstable solution can be obtained and. the parameter of the cubic nickel mesh changes from 3.517 A. to 3.48 A ,for the supersaturated solution. The supersaturated condition is removed by subjecting the alloy to an annealing heat treatment, so that the unstable equilibrium is upset and the solution loses beryllium for return to its normal saturation having a mesh value of 3.505 A. The mesh parameter of the solid solution thus constitutes a criterion of the efiiciency of the annealing heat treatment, since the mesh is indicative of the state of stability of the alloy.

The annealing process is satisfactorily completed if the following conditions are found to be satisfied:

(1) Lines are present corresponding to the presence of the electronic compound NiBe (a=2.6l4 A.), and

(2) The mesh of the solid solution has attained a stable value corresponding to the just saturated condition of the solution (a=3.505 A.).

So long as the mesh parameters have not reached their final values, the alloy remains unstable and any attempt to oxidize the alloy in a controllable and repro- 3 ducible manner for the purposes of the invention in accordance with the above-given explanations of the part played by the Ni-Be phase, will fail.

Secondary emission tests with samples of alloys containing 4% beryllium have given the following results for short periods and immediately after the samples had been placed in operation, it being noted that these resuits were strictly reproducible. In the following table, the accelerating voltage is defined as the voltage across the anode and cathode and the extracting voltage" is the voltage between the anode and the collector electrode collecting the secondary electrons. The figures in the table indicate the secondary emission ratios observed for each set of values of the accelerating and extracting voltage:

It was observed that in the case of each sample tested, the secondary emission ratio increases in value during an initial stage. However, as the secondary output attains a relatively high value, c. g. for secondary output currents in excess of about 1 m A., under the circumstances of the tests the emissive coating was found to experience a kind of fatigue resulting in a gradual decay of the emissive factor.

The tested alloys were found to possess stability despite the variations in operating time and voltage to which they were subjected.

The secondary emission ratio may be considered as increasing with the proportion of beryllium content in the alloy. This rule is experimentally verifiable. Under practical condition the presence of the electronic NiBe compound first occurs after about 2.8% beryllium has been added. The presence of this compound causes a very considerable increase in the secondary emission factor by virtue of the reservoir or feeder effect of the Nil3e phase as previously disclosed. So long as the beryllium content in the alloy has not exceeded a moderate value, of say 5% by weight, the rate of diffusion of the Be constituent from the Ni-Be compound into the solid solution remains roughly proportional to the concentration of Ni-Be. The concentration of this compound in the alloy moreover exerts a further and important action. If the secondary emission effect is plotted as a function of the Ni-Be concentration, the following results are found to be approximately true:

(1) The emission is low for a concentration less than 2.5%;

(2) The emission increases as the concentration increases from 2.5% to 2.8%;

(3) Thereafter the emission increases further though at a gradually decreasing rate and asymptotically approaches a limiting value.

Similarly, crystallographical investigation of the nickelmaguesium alloys of the invention reveals that an alloy containing say 1% Mg by weight contains a small amount of the electronic Ni Mg compound which precipitates and likewise enhances secondary emission according to the teaching of this invention.

From another aspect, it has been found that the alloys specified herein pos ess remarkable mechanical characteristics. The Ni-Be alloys especially have a hardness, as measured by the Girschig sclerometer, of the approximate order of from 300 to 600 Vickers, in the areas thereof which contain the solid solution; while the NiBe compound may attain a Vickers hardness value as high as 700 or 800 or more.

Alloys according to the invention may be prepared by the following procedure. A carbon resistor, highfrequency furnace is used and a vacuum is created in it to a pressure not exceeding a value of the order of a few tenths of one torr a torr being the gas pressure equivalent to one millimeter of mercury. The furnace should be subjected to a special high-temperature degassing process and care must be taken to remove any traces of sulfur therefrom since this element is particularly objectionable for the purposes of the invention. This is because sulfur is very readily absorbed by beryllium. and magnesium. Traces of carbon up to a concentration of about one part in ten thousand are permissible.

The metals are melted in a crucible made from aluminous cement at a temperature higher than the melting point of nickel, i. e. higher than about 1452" C.

The following raw materials may be employed: 99.5% pure granular nickel, pure powdered beryllium or pure ingots of magnesium. The beryllium or magnesiumas the case may beis placed at the bottom of the crucible and is covered with the granular nickel compacted thereover, so as to protect the underlying lightweight metal and prevent its oxidation and/or evaporation. An alloy is obtained in this way containing from about 2.5% to 4.5% beryllium or from about 0.6% to 2% magnesium depending on the starting material.

Usually, the alloys are not produced at the first melt, but several successive charges are required. This is be cause the more fusible part of the metal will tend to be oxidized if the pressure reaches a value of l torr or more. When the pressure is less than 0.2 torr, the light metal is liable to evaporate, particularly where the metal is magnesium.

The process of preparing alloys according to the invention is preferably checked at various stages by micrographical tests and chemical analyses. The lumps of alloy produced are cleaned for removal of the surface layer of oxide. The beryllium alloy is greyish in colour and has a charactertistic surface appearance (reentrant pyramid crystallization). The magnesium alloy is bright and sometimes golden in appearance owing to oxidation.

The Ni-Be alloys can be softened by water-quenching from 1100 C.

In order to facilitate machining operations the alloys may be compressed at high temperature and pressure in a graphite mould, at a temperature slightly below the melting point, and a pressure of about 10 kg. per sq. cm. The resulting parts are highly compact while being sulficiently thin and soft to be machinable with carbide tools.

Emissive targets produced in accordance with the metallurgical process described are preferably polished first with emery paper then with metallographical alumina powder suspension and washed in alcohol. The parts are then heat treated twelve hours at about 450 C. This treatment is beneficial to homogeneity and structural precipitation and facilitates structural hardening through precipitation of the Ni-Be compound. Owing to such hardness, machining this alloy, as on a lathe, is extremely difiicult. The beryllium alloys are less brittle and tougher than the magnesium alloys. A final treatment at 800 C. for six hours in the presence of air is then applied for activating the electronic emissive properties of the product.

What we claim is:

1. A method of treating a nickel base alloy having included therein a metal selected from the group consisting of beryllium in an amount between 2.5% and 4.5%, by Weight, and magnesium, in an amount between 0.6% and 2.0%, by weight, in order to give to said alloy 2. high factor of real secondary emission, the relative concentration of nickel being sufiicient to provide a saturated solid solution of said metal in the nickel; said method comprising the steps of water-quenching the alloy from a temperature of about 1100 C., compressing the alloy at a pressure of about kg. per sq. cm. and at a temperature slightly below the melting point thereof, heating the alloy at a temperature of about 450 C., and finally heating the alloy at a temperature of about 800 C. in an oxidizing atmosphere until there is obtained the same parameter of mesh of the alloy as of said saturated solution of said metal in the nickel.

2. A method as in claim 1; wherein said final heating at a temperature of about 800 C. is effected in the presence of air for a period of about six hours.

3. A method as in claim 2; wherein said heating of the alloy at a temperature of about 450 C. is continued for about twelve hours.

4. A method of treating a nickel base alloy having beryllium included therein in an amount between 2.5% and 4.5%, by weight, in order to give to said alloy a high factor of real secondary emission, the relative concentration of nickel being sutficient to provide a saturated solid solution of beryllium in the nickel; said method comprising the steps of water-quenching the alloy from a temperature of about 1100 C., compressing the alloy at a pressure of about 10 kg. per sq. cm. and at a temperature slightly below the melting point thereof, heating the alloy at a temperature of about 450 C., and finally heating the 6 alloy at a temperature of about 800 C. in an oxidizing atmosphere until there is obtained the same parameter of mesh of the alloy as of said saturated solution of beryllium in the nickel.

References Cited in the file of this patent UNITED STATES PATENTS 1,986,585 Kroll Jan. 1, 1935 2,185,410 Lederer Jan. 2, 1940 2,223,862 Widell Dec. 3, 1940 FOREIGN PATENTS 382,573 Great Britain Oct. 27, 1932 404,012 Great Britain Jan. 8, 1934 OTHER REFERENCES The Journal of the Institute of Metals (Masing et al.), vol. 42, No. 2 1929), page 441 relied on.

Zeitschrift fiir technische Physick (Gille et al.), vol. 22 (1941), pages 228-236 relied on.

Williams: Heat Treatment and Properties of Some Be-Ni Alloys, Transactions of Am. Society for Metals, vol. (1948), pages 163 to 179, particularly page 168.

Claims (1)

1. A METHOD OF TREATING A NICKEL BASE ALLOY HAVING INCLUDED THEREIN A METAL SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM IN AN AMOUNT BETWEEN 2.5% AND 4.5%, BY WEIGHT, AND MAGNESIUM, IN AN AMOUNT BETWEEN 0.6% AND 2.0%, BY WEIGHT, IN ORDER TO GIVE TO SAID ALLOY A HIGH FACTOR OF REAL SECONDARY EMISSION, THE RELATIVE CONCENTRATION OF NICKEL BEING SUFFICIENT TO PROVIDE A SATURATED SOLID SOLUTION OF SAID METAL IN THE NICKEL; SAID METHOD COMPRISING THE STEPS OF WATER-QUENCHING THE ALLOY FROM A TEMPERATURE OF ABOUT 1100*C., COMPRESSING THE ALLOY AT A PRESSURE OF ABOUT 10 KG. PER SQ. CM. AND AT A TEMPERATURE SLIGHTLY BELOW THE MELTING POINT THEREOF, HEATING THE ALLOY AT A TEMPERATURE OF ABOUT 450*C., AND FINALLY HEATING THE ALLOY AT A TEMPERTURE OF ABOUT 800*C. IN AN OXIDIZING ATMOSPHERE UNTIL THERE IS OBTAINED THE SAME PARAMETER OF MESH OF THE ALLOY AS OF SAID SATURATED SOLUTION OF SAID METAL IN THE NICKEL.