Heat treatment of single crystals

Concepts relating to the heat treatment of single crystal superalloy articles are disclosed. A first concept is the use of a heat treatment sequence which includes incipient melting of the article being heat treated followed by one or more steps which essentially heal the incipient melting damage. A second concept relates to the treatment of previously heat treated single crystal articles which have incipient melting. Methods are disclosed for healing this damage and for recovering the mechanical properties of such articles. A third concept disclosed is a particular cycle which permits effective heat treatment of a specific single crystal alloy composition.

DESCRIPTION 
1. Technical Field 
This invention relates to the heat treatment of single crystal superalloy 
articles. 
2. Background Art 
Superalloys are metallic materials, usually based on nickel or cobalt, 
which have useful properties at temperatures on the order of 1400.degree. 
F. and above. Most commonly used superalloys are based on nickel and 
derive much of their strength from the formation of a precipitate phase 
based on Ni.sub.3 M where M is Al or Ti. The mechanical properties are 
strongly affected by the gamma prime morphology. Such alloys contain 
various other constituents, some of which melt at relatively low 
temperatures. The localized melting is termed incipient melting and is 
often a limiting factor in the use of superalloy articles. 
It is well known to heat treat cast superalloy articles for the purpose of 
improving their mechanical properties. Typically such heat treatments 
include a solutionizing step in which the article is heated to a 
temperature above the temperature at which the gamma prime phase is taken 
into solid solution. The article is then cooled, and reheated at a lower 
temperature for controlled reprecipitation of the gamma prime phase. The 
solutionizing step cannot exceed the incipient melting temperature. The 
ability to control the size and distribution of the gamma prime phase 
permits substantial control over mechanical properties. The application 
heat treatment of single crystal articles is described in U.S. Pat. No. 
4,116,723. It is well known that the incipient melting temperature of a 
cast superalloy may under certain conditions be increased by heat 
treatments at temperatures below but near the incipient melting 
temperature for periods of time sufficient to permit partial 
homogenization of low melting regions in the article. This is disclosed in 
U.S. Pat. Nos. 2,798,827; 3,753,790; and 3,783,032. None of these patents 
refer to single crystal articles and all patents have as a major objective 
the avoidance of incipient melting. 
U.S. Pat. No. 4,209,348 discloses a specific single crystal alloy 
composition and heat treatment therefor. 
DISCLOSURE OF INVENTION 
The invention relates to the heat treatment of single crystal superalloy 
articles. According to one embodiment of the invention, such single 
crystal articles are heat treated using a heat treatment cycle during the 
initial stages of which incipient melting occurs within the articles being 
heat treated. During a subsequent step in the heat treatment process 
substantial diffusion occurs in the article with the result that the 
detrimental effects of the previous incipient melting are essentially 
completely removed. A related embodiment is a heat treatment process for 
the repair of single crystal articles which have previously undergone 
incipient melting during a heat treatment process. The use of a heat 
treatment cycle similar to that previously described permits the 
substantial elimination of the detrimental effects and evidence of prior 
incipient melting thereby permitting the use of parts which would 
otherwise be scrapped. The third embodiment of the invention is a heat 
treatment cycle for a particular widely used single crystal article 
composition whose nominal composition is 10% Cr,5% Al, 1.5% Ti, 4% W, 12% 
Ta, 5% Co, balance essentially nickel. Articles of this composition may 
successfully be heat treated with essentially zero rejection rate, using 
an initial heating step at about 2300.degree. F. followed by controlled 
heating to about 2340.degree. F. followed by controlled heating at a 
lesser rate to about 2350.degree. F. with a holding time at 2350.degree. 
F. of about two hours followed by a controlled heating to a temperature of 
about 2375.degree. F. with a hold at 2375.degree. F. for about 30 minutes 
followed by controlled cooling at a rate in excess of about 100.degree. F. 
per min to a temperature below that of 2000.degree. F. Use of this cycle 
reduces the rejection rate from a figure of about 40% encountered with 
prior heat treatment cycles to less than 2% and permits compositional 
modifications which result in increased mechanical properties. 
Other features and advantages will be apparent from the specification and 
claims and from the accompanying drawings which illustrate an embodiment 
of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 illustrates the variation of the incipient melting point during the 
course of heat treatments as suggested in the prior art namely U.S. Pat. 
No. 2,798,827; U.S. Pat. No. 3,753,790 and U.S. Pat. No. 3,783,032. As 
shown in FIG. 1 with time at temperature and increasing temperature, the 
incipient melting temperature (I.M.T.) increases, but at all times the 
heat treatment temperature remains below the incipient melting 
temperature. The increase in incipient melting temperature is the result 
of the diffusion and homogenization of the low melting point alloy phases. 
We have found that in the case of single crystals that the incipient 
melting temperature may be exceeded during the course of the heat 
treatment without permanent detrimental effects, provided that the time 
and temperature of the remainder of the heat treatment is sufficient to 
heal the damage caused by the incipient melting which has occurred. It 
should be noted that this possibility is limited to single crystals. In 
polycrystalline material, either equiaxed material or directionally 
solidified material, any incipient melting which occurs will occur largely 
at the grain boundaries and when incipient melting occurs at grain 
boundaries the detrimental effects of the incipient melting cannot be 
recovered by subsequent heat treatment. Thus the observation and the 
benefits of the invention are limited to the case of single crystals which 
of course have no internal grain boundaries. 
One aspect of the invention is illustrated in FIG. 2. FIG. 2 displays a 
heat treatment cycle similar to that shown in FIG. 1 except that during 
the course of the heat treatment cycle the temperature of the articles 
being heat treated exceeds the incipient melting temperature. At the point 
in the cycle where the article temperature exceeds the incipient melting 
temperature, some incipient melting will occur. Provided the article is a 
single crystal and provided that the portion of the heat treatment cycle 
subsequent to the incipient melting temperature is conducted at the proper 
time and temperature, we have found that the detrimental effects of the 
incipient melting can be completely eradicated from the article. The 
healing effect of a proper solution heat treatment on a superalloy single 
crystal article can be seen in FIGS. 3 and 4. FIG. 3 is a photomicrograph 
of a nickel base superalloy single crystal article after exposure at a 
temperature of 2375.degree. F. for two hours. Since the incipient melting 
temperature of this alloy in the as cast condition is approximately 
2360.degree.-2365.degree. F., incipient melting has occurred as can 
clearly be seen in the photomicrograph. FIG. 4 is a photomicrograph at the 
same magnification of the same alloy which had undergone incipient melting 
at 2375.degree. F. and subsequently been given at a healing heat 
treatment. FIG. 4 clearly illustrates that evidence of incipient melting 
has been completely eliminated. Measurements of mechanical properties 
confirm that the prior incipient melting has not caused any permanent 
detriment on these properties after it has been properly heat treated. 
Thus there are at least two major aspects to this invention. The first 
aspect is the use of a solution heat treatment which intentionally causes 
incipient melting coupled with subsequent repair of the resultant damage. 
This permits the use of shorter heat treatments at higher temperatures 
without concern for incipient melting. As will be discussed below the use 
of higher temperatures can increase mechanical properties and the 
elimination of concern about incipient melting can permit compositional 
changes for yet higher strengths. The second aspect of the invention is 
the repair of single crystal articles which have previously undergone 
incipient melting. 
With respect to the first aspect, the use of a stepped heat treatment cycle 
which deliberately produces some degree of incipient melting, the required 
steps in the heat treatment cycle are that the alloy be to a temperature 
below but within about 25.degree. F. of its incipient melting temperature 
and held below its incipient melting temperature for a period of time 
sufficient to achieve substantial amount of alloy homogenization. The 
temperature is then increased to a temperature above its incipient melting 
temperature thereby causing incipient melting of the alloy article, 
preferably less than 5% by volume and holding the article at a temperature 
above the incipient melting temperature for a period of time sufficient to 
cause the incipient melting temperature of the alloy to increase to above 
that temperature at which the alloy article is being held for a period of 
time sufficient to cause substantial healing of the previously incurred 
incipient melting. 
The second aspect of the invention is the use of a stepped heat treatment 
to repair single crystal damage caused by previous heat treatment above 
the incipient melting temperature. In this situation the incipient melted 
article is held at a temperature below but within 25.degree. F. of its 
incipient melting temperature for a period of time to achieve a 
significant (e.g. 50% by vol.) amount of homogenization and partial 
healing of prior incipient melting. The temperature is increased to above 
its actual incipient melting temperature and held at this temperature for 
a time sufficient to cause an increase in the incipient melting 
temperature to a temperature above the holding temperature. The article is 
held at this temperature for a time sufficient to cause further 
homogenization and healing of prior incipient melting. This heat treatment 
is depicted in FIG. 5. 
U.S. Pat. No. 4,209,348 describes a nickel base single crystal superalloy 
composition and article having highly desirable combination of mechanical 
properties and resistance to oxidation and corrosion at elevated 
temperatures. The article composition is from about 8 to about 12% 
chromium, from about 4.5 to about 5.5% aluminum, from about 1 to about 2% 
titanium, from about 3 to about 5% tungsten, from about 10 to about 14% 
tantalum, from about 3 to about 7% cobalt, the balance essentially nickel 
where all percentages are weight percentages. This is the composition of 
the samples shown in FIGS. 3 and 4. FIG. 6 illustrates a heat treatment 
cycle which has been developed to produce maximum mechanical properties in 
this alloy. The alloy in its as cast condition has an incipient melting 
temperature of 2360.degree.-2365.degree. F., although it will be 
appreciated that differences in solidification procedures may cause slight 
differences in incipient melting temperature and that even within the same 
part differences in section size can result in minor differences in 
incipient melting temperature. As shown in FIG. 6 the article is initially 
heated to a temperature of 2300.degree..+-.25.degree. F. and held at this 
temperature for at least 30 minutes. This initial treatment helps insure 
that the parts are at a uniform temperature. The parts are next heated to 
a temperature of 2340.degree..+-.10.degree. F. at a rate of about 
0.5.degree.-2.degree. F. per minute and from 2340.degree. F. are heated to 
a temperature of 2350.degree..+-.5.degree. F. at a slower rate of about 
10.degree..+-.5.degree. F. per hr. The parts are held at 2350.degree. F. 
for a time of 2.+-.1 hour and are then heated to a temperature of 
2370.degree..+-.10.degree. F. at a rate of about 0.1.degree.-1.degree. F. 
per minute. The parts are held at 2375.degree. F. for a period of about 
15-60 minutes and are then cooled to a temperature below 1000.degree. F. 
at a rate in excess of 100.degree. F. per minute. This final cooling rate 
plays an important role in producing an optimum gamma prime morphology. 
When parts of this composition were initially evaluated, a single step 
solution heat treatment at 2340.degree. F. was employed in order to avoid 
melting of the parts due to temperature fluctuations and variations in 
incipient melting temperature in the parts. The minimum figure used for 
design purposes for the intended application was a minimum of 10 hours to 
1% creep and 36 hours rupture life when tested at 1800.degree. F. with a 
36 ksi applied load. When the parts were given a single step 2340.degree. 
F. solution heat treatment, the time to 1% creep under these testing 
conditions was typically less than 9 hours and the typical rupture life 
was 40 hours. In addition rejection rates of up to 40% were encountered. 
These results were unsatisfactory and led to the present invention. When 
the present invention was employed with a specific cycle shown in FIG. 6, 
with a 2375.degree. F. maximum temperature, the time to 1% creep was 
increased from typically less than 9 hours to about 23 hours and the 
rupture life was increased from about 40 hours to about 74 hours under the 
1800.degree. F./36 ksi testing conditions. 
The composition range for the alloy permits the sum of the aluminum and 
titanium to range from 5.5 to 7.5 weight percent. The initial production 
specification added the requirement that the sum of the aluminum and 
titanium lie between 6 and 7 percent. However, the problems with incipient 
melting made it necessary to restrict the aluminum and titanium to lie 
between about 6 and 6.2% since higher levels of aluminum and titanium 
increase the problems of incipient melting. Since aluminum and titanium 
form the gamma prime phase which is in large part responsible for the 
strength of the alloy, the restriction of the aluminum and titanium to the 
lower end of the range reduce the potential mechanical properties of the 
material and substantially diminished the benefits attributed to the 
composition and single crystal form. Through the use of the present 
invention it was found that the aluminum and titanium content could 
readily be increased to more than 6.6%. The significance of this will be 
appreciated by noting that an alloy composition having (Al+Ti) equal to 
6.03% had a yield strength at 1100.degree. F. of about 135 ksi whereas an 
alloy having (Al+Ti) equal to 6.6 wt% had an 1100.degree. F. yield 
strength of about 158 ksi when both alloys were given the same solution 
heat treatment according to the present invention. Thus not only does the 
heat treatment of the present invention completely avoid detrimental 
results due to incipient melting but it also permits the use of higher 
strength variations on the original alloy composition which produce yield 
strengths almost 20% stronger than the compositions which were formerly 
used due to problems with incipient melting. 
It should be understood that the invention is not limited to the particular 
embodiments shown and described herein, but that various changes and 
modifications may be made without departing from the spirit and scope of 
this novel concept as defined by the following claims.