Hafnium containing high temperature Nb-Al alloy

An alloy is provided having exceptional strength at very high temperatures of 1200.degree. C. and higher. The alloy contains niobium, hafnium, and aluminum in the following atomic percentage ratios: ______________________________________ Concentration Range Ingredient From To ______________________________________ niobium balance essentially hafnium 5 18 aluminum 5 22 ______________________________________

CROSS REFERENCE TO RELATED APPLICATION 
The subject application relates to application Serial No. 202,357, filed 
June 6, 1988. It also relates to application Ser. No. 280,085 filed 
12/5/88; to application Ser. No. 279,639, filed 12/5/88; to application 
Ser. No. 290,399 filed 12/29/88; and to application Ser. No. 288,394 filed 
12/22/88. The text of the related application is incorporated herein by 
reference. 
BACKGROUND OF THE INVENTION 
The present invention relates generally to alloys and to shaped articles 
formed for structural use at high temperatures. More particularly, it 
relates to an alloy having a niobium base and which contains hafnium and 
aluminum additives. By a niobium base is meant that the principal 
ingredient of the alloy is niobium. 
There are a number of uses for metals which have high strength at high 
temperature. One particular attribute of the present invention is that it 
has, in addition to high strength at high temperature, a relatively lower 
density of the order of 7.8 to 8.8 grams per cubic centimeter (g/cc). 
In the field of high temperature alloys and particularly alloys displaying 
high strength at high temperature, there are a number of concerns which 
determine the field applications which can be made of the alloys. One such 
concern is the compatibility of an alloy in relation to the environment in 
which it must be used. Where the environment is the atmosphere, this 
concern amounts to a concern with the oxidation or resistance to oxidation 
of the alloy. 
Another such concern is the density of the alloy. One of the groups of 
alloys which is in common use in high temperature applications is the 
group of iron-base, nickel-base, and cobalt-base superalloys. The term 
"base", as used herein, indicates the primary ingredient of the alloy is 
iron, nickel, or cobalt, respectively. These superalloys have relatively 
high densities of the order of 8 to 9 g/cc. Efforts have been made to 
provide alloys having high strength at higher operating temperatures and 
significantly above those of the superalloys. 
It has been observed that the mature metal candidates for use in this field 
of high strength at high temperature can be grouped and such a grouping is 
graphically illustrated in FIG. 1. Referring now to FIG. 1, the ordinate 
of the plot shown there is the density of the alloy and the abscissa is 
the maximum temperature at which the alloy provides useful structural 
properties for aircraft engine applications. The prior art alloys in this 
plot are discussed in descending order of density and use temperatures. 
With reference to FIG. 1, the materials of highest density and highest use 
temperatures are those enclosed within an envelope marked as Nb-base and 
appearing in the upper right hand corner of the figure. Densities range 
from about 8.7 to about 9.7 grams per cubic centimeter and use 
temperatures range from less than 2200.degree. F. to about 2600.degree. F. 
Referring again to FIG. 1, the group of prior art iron, nickel, and cobalt 
based superalloys are seen to have the next highest density and also a 
range of temperatures at which they can be used extending from about 
500.degree. F. to about 2200.degree. F. 
A next lower density group of prior art alloys are the titanium-base 
alloys. As is evident from the figure, these alloys have a significantly 
lower density than the superalloys but also have a significantly lower set 
of use temperatures ranging from about 200.degree. F. to about 900.degree. 
F. 
The last and lowest density group of prior art alloys are the aluminum-base 
alloys. As is evident from the graph these alloys generally have 
significantly lower density. They also have relatively lower temperature 
range in which they can be used, because of their low melting points. 
The usefulness of the titanium-base alloys extends over a temperature range 
which is generally higher than that of the aluminum-base alloys but lower 
than that of the superalloys. Within this temperature range, alloy changes 
occur due to a phase transformation from hexagonal to cubic crystal 
structure. 
A novel additional set of alloys is illustrated in the figure as having 
densities about equal to those of the superalloys but with useful 
temperature ranges potentially extending beyond the superalloy temperature 
range. These ranges of temperature and density include those for the 
alloys such as are provided by the present invention and which are formed 
with a niobium base. 
BRIEF STATEMENT OF THE INVENTION 
It is, accordingly, one object of the present invention to provide an alloy 
system which has substantial strength at high temperature relative to its 
weight. 
Another object is to reduce the weight of the niobium alloys presently used 
in higher temperature application. 
Another object is to provide an alloy which can be employed where high 
strength is needed at high temperatures. 
Other objects will be in part apparent and in part pointed out in the 
description which follows. 
In one of its broader aspects, these and other objects of the present 
invention can be achieved by providing a niobium base alloy according to 
the following compositional ranges (in atomic percentages): 
______________________________________ 
Concentration Range 
Ingredient From To 
______________________________________ 
niobium balance essentially 
hafnium 5 18 
aluminum 5 22 
______________________________________ 
The phrase "balance essentially" as used herein is used to include, in 
addition to niobium in the balance of the alloy small amounts of 
impurities and incidental elements, which in character and/or amount do 
not adversely affect the advantageous aspects of the alloy.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1 and with particular reference to the envelope 
labelled "Nb-Hf base", it is evident that the density of alloy provided by 
the present invention ranges from about 7.8 to about 8.8 grams per cubic 
centimeter. This density range corresponds essentially to that of the 
iron, nickel, or cobalt base superalloys. However, as is also evident from 
the figure, the use temperature ranges from below 2000.degree. F. to above 
2500.degree. F. This use range is above that for which the iron, nickel, 
and cobalt superalloys are suitable. The upper useful range of the 
superalloys is about from 500.degree. F. to about 2200.degree. F. The 
alloys of the subject invention extend this range upward by more than 
300.degree. F. 
EXAMPLES 1 and 2 
Two alloy samples were prepared. One had a density of 8.8 grams per cubic 
centimeter (g/cc) and the other had a density of 7.9 g/cc. The alloy 
composition of these samples is set forth in Table I immediately below. 
TABLE I 
______________________________________ 
Ingredient Concentration 
in Atom % 
Ingredient Nb Hf Al Density in g/cm.sup.3 
______________________________________ 
Example 1 72 18 10 8.8 
Example 2 65 15 20 7.9 
______________________________________ 
The alloy samples were prepared by conventional ingot forming means and 
conventional tensile test bars were prepared from the alloy samples. 
Tensile tests were conducted on the samples at 900.degree. C. and at 
1200.degree. C. The results of these tests are tabulated in Table II, 
immediately below. 
TABLE II 
______________________________________ 
Yield Strength YS-900.degree. C. 
YS-1200.degree. C. 
______________________________________ 
Example 1 83 ksi 42 ksi 
Example 2 61 ksi 45 ksi 
______________________________________ 
From the data plotted in Table II, it is evident that the alloys of this 
invention have remarkable strength at elevated temperatures. The strength 
at 1200.degree. C. is two or three times greater, that is 200-300% greater 
than any other niobium alloy of such a low density. The specific strength 
(strength/density) is well above the value of any of the superalloys at 
the 1200.degree. C. temperature. This is shown in FIG. 2, where data for 
examples 1 and 2 are compared against data for commercial Ni, Co and 
Nb-base alloys.