Component made of an intermetallic compound with an aluminum diffusion coating

A component made of an intermetallic compound of titanium and aluminum, or of alloys of such intermetallic compounds with alloying additions forming the base material, and with an aluminum diffusion coating on the base material, is provided. The component has, between the base material and the aluminum diffusion coating, a closed zone which is close to the surface and has a recrystallization structure. For this purpose, the component is cold-formed or slightly melted in a zone which is close to the surface, is then annealed at the recrystallization temperature, and finally has an aluminum diffusion coating applied to the recrystallized zone. The process is used for components in engines and, particularly, for components in the hot-gas duct of an engine.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to a component made of an intermetallic compound 
consisting of titanium and aluminum or made of alloys of such 
intermetallic compounds with alloying additions so as to form the base 
material and with an aluminum diffusion coating on the base material. 
This base material has interesting characteristics for the construction of 
engines. It has mechanical characteristics which are comparable to those 
of conventional titanium alloys while the specific weight is low, but can 
be used at significantly higher operating temperatures. However, the 
ductility of this base material at room temperature is lower and must 
therefore be improved by the use of alloying elements and heat treatment 
processes, as they are known from German Patent document DE 30 24 645. 
While, in the case of conventional titanium alloys, an oxygen embrittlement 
in an oxidizing atmosphere begins at temperatures starting at 550.degree. 
C., in the case of intermetallic compounds made of titanium and aluminum, 
this temperature is at 700.degree. C. The oxygen embrittlement is 
disadvantageous because the already low ductility further deteriorates at 
room temperature and results in a brittleness which is known with respect 
to ceramic components. 
In order to use this base material for components which are subjected to 
operating temperatures of 700.degree. C., as occur preferably in the case 
of components in the compressor and turbine range of engines, a closed and 
no-defect aluminum diffusion coating is required on the 
high-temperature-stressed component surfaces. 
When conventional aluminum diffusion coatings are used on components made 
of the base material, no closed aluminum diffusion coating is achieved. 
Disadvantageously, coating defects occur. These coating defects include 
areas of extremely non-uniform coating thicknesses such as trough-shaped 
coating structures which have no coating on the bottom of the trough. When 
the coating is extremely thick, these troughs and defects can be covered 
with aluminum. However, when the component is stressed, these areas will 
disadvantageously break open and the aluminum covering will chip off. 
It is an object of the present invention to provide a component of the 
above-mentioned type, and a process for its manufacture, in which no 
coating defects occur and which can be used at operating temperatures of 
700.degree. C. 
According to the present invention, this object is achieved in that, 
between the base material and the aluminum diffusion coating, the 
component has a closed zone which is close to the surface and has a 
recrystallization structure. 
As determined in comprehensive development work, a closed aluminum 
diffusion coating grows in an undisturbed and uniform manner only on such 
a recrystallization structure of an intermetallic compound base material 
consisting of titanium and aluminum, or of alloys of such intermetallic 
compounds with or without alloying additions. The advantages of the 
invention are that the application range of such base materials is 
significantly expanded, and conventional technologies and processes which 
are suitable for mass production can be used for producing such 
components. 
In the case of a preferred embodiment of the invention, the intermetallic 
compound is TiAl. In the case of this base material, it could be 
determined that crystallites with a high stacking fault density occur in 
the form of crystallographic twin planes in the crystallite. These 
crystallites exhibit a plate structure, as has not been observed in the 
case of conventional titanium alloys. In the case of conventional aluminum 
diffusion coatings, the twin planes remain uncoated. It is only after a 
zone is formed which is close to the surface and has a crystallization 
structure that components made from the base material could be represented 
with a closed aluminum diffusion coating. 
A particularly high density of crystalline plate structures is exhibited by 
base materials made of alloys from intermetallic compounds with a 
constituent of TiAl of between 50 and 95% by volume and with a Ti.sub.3 Al 
constituent of between 5 and 50% by volume. In the case of components made 
of critical base materials, which have a higher proportion of titanium 
than TiAl and, therefore, tend to have more oxygen embrittlement, 
uniformly thick aluminum diffusion coatings can be implemented in an 
advantageous manner through the use of the closed zone according to the 
invention. The closed zone is close to the surface and consists of a 
recrystallization structure. 
For improving the ductility of the components made of intermetallic 
compounds, preferably up to 4% alloying additions made of niobium, 
tantalum, tungsten, vanadium, or mixtures thereof are contained in the 
component material. 
The depth of the closed zone which is close to the surface and has a 
recrystallization structure amounts to at least 0.1 .mu.m. A 
recrystallization structure depth between 1 and 10 .mu.m was found to be 
practical because it can be prepared in a low-cost manner, preferably by 
using a cold forming which is close to the surface. Recrystallization 
structure depths between 0.1 and 1 .mu.m are preferably implemented by 
laser melting and recrystallizing close to the surface. In the case of 
recrystallization structure depths of above 100 .mu.m, the risk increases 
that large-volume crystallites with a plate structure are formed during 
the recrystallization which would prevent a closed aluminum diffusion 
coating. 
A process according to the present invention for producing the components 
of the above-mentioned type is achieved by the following process steps. 
The component is cold-formed or slightly melted in a zone which is close 
to the surface. The component is then annealed at the recrystallization 
temperature, and finally an aluminum diffusion coating is applied to the 
recrystallized zone. This process has the advantage that low-cost process 
steps are provided which are suitable for mass production so that 
components can be used in engine construction which are improved in a 
low-cost manner. 
For the surface cold forming, a shot peening or machining of the surface 
areas of the component to be recrystallized is preferably carried out. 
During shot peening, the component is blasted by ceramic balls made of 
Al.sub.2 O.sub.3, by glass beads, or by steel balls. In this case, the 
crystalline structure of the base material is disturbed and internal 
stress enters into the surface of the base material. During the subsequent 
recrystallization annealing below the melting temperature of the material, 
a finely crystalline recrystallization structure is formed on which an 
aluminum diffusion layer can grow in an undisturbed manner. For surface 
areas which are not to be coated, protective measures must be taken during 
the shot peening such as using covers or screens. 
For the machining or cold forming close to the surface, pressure rollers, 
presses, rollers, striking tools or pressure grinding tools may be used. 
Preferably, the recrystallization structure may also be formed by the fact 
that, in the areas which finally are to be coated with aluminum, the 
surface of the component is first rastered by using a laser beam. In the 
process, the surface is slightly melted. This has the advantage that 
particularly low depths of the recrystallization structure between 0.1 and 
1 .mu.m can be implemented and the surface areas can be rastered, melted 
and recrystallized in a geometrically exact manner without using any 
additional protective measures. 
In the case of a preferred implementation of the process, a recrystallizing 
and an aluminum diffusion coating is carried out using a heat cycle in 
that first the component, which is cold-formed on the surface or slightly 
melted on the surface and solidified, is heated to the recrystallization 
temperature in a system for aluminum diffusion coating. After the 
recrystallization has taken place, the temperature is set for the aluminum 
diffusion coating and the transmitted aluminum-containing gas is supplied 
at the same time. 
This implementation of the process fully utilizes the technical conditions 
of a system for aluminum diffusion coating because, in such systems, the 
component can be heated independently of the coating process. In addition, 
contamination danger is reduced because there is no removal or 
modification between the recrystallization annealing and the coating. This 
also reduces the cost of the process. 
Preferably, the component is subjected to a reduced pressure or to a 
protective atmosphere during recrystallization so that the heat cycle to 
the feeding of the aluminum-containing donor gas takes place under a 
protective gas or at a reduced pressure. This has the advantage that the 
component surfaces continue to be protected from impurities and oxidation 
processes. 
The powder pack process is known for the aluminum diffusion coating of 
structural members made of an iron base alloy, a nickel base alloy or a 
cobalt base alloy. In addition, many different aluminum donors are used 
for generating aluminum donor gases. The preferred process for the 
aluminum diffusion coating is the powder pack process, and an aluminum 
donor of the ternary alloy Ti/Al/C is used for generating a donor gas. In 
this case, the carbon constituent has the effect that the residual oxygen 
concentrations remaining in the powder pack are bound or neutralized by 
use of carbon monoxide formations or carbon dioxide formations, whereas Ti 
and Al correspond to the base material and therefore promote the growth 
process of an aluminum diffusion coating on the base material. 
The figures illustrate embodiments for an aluminum diffusion coating of 
components made of intermetallic compounds of titanium and aluminum. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates an aluminum diffusion coating 1 of components made of 
intermetallic compounds of titanium and aluminum without a zone which is 
close to the surface and which has a recrystallization structure, the base 
material 2 being solidified in large-volume crystallites 3 to 8. One of 
the crystallites 3 exhibits a pronounced plate structure with stacking 
faults in the form of twin planes 9. On the lines 10 of these fault points 
intersecting along the surface, the aluminum diffusion coating has 
trough-shaped faults. A faultless coating is found only on the 
crystallites 4, 5 and 8 which have no plate structure. The outlined cutout 
A was examined by means of a metallographic section. The result is 
illustrated in FIG. 2. 
FIG. 2 is the photo of a metallurgical micrograph of a material according 
to FIG. 1 in the area of the cutout A. For this purpose, a moving blade of 
an engine made of TiAl was coated in a powder pack system with a ternary 
alloy made of Ti/Al/C as an aluminum donor on its blade surface. The 
aluminum diffusion coating 1 shows considerable defects in the area of the 
crystallite 3 with a pronounced plate structure. 
FIG. 3 illustrates an aluminum diffusion coating 1 of components made of 
intermetallic compounds of titanium and aluminum with a zone 11 which is 
close to the surface and which has a recrystallization structure. The base 
material 2 exhibits large-volume crystallites 12 to 14. Crystallite 12 has 
a plate structure and crystallites 13 to 15 do not have a plate structure. 
In the proximity of the surface, the base material has a closed zone 11 
with a recrystallization structure which is uniformly covered without 
fault points by a closed layer of aluminum. The outlined cutout B was 
examined by means of a metallographic section. 
FIG. 4 is a photo of a metallurgical micrograph through a material 
according to FIG. 3 in the area of the cutout B. For this purpose, a guide 
blade of an engine made of 60% by volume TiAl and 40% by volume Ti.sub.3 
Al was first cold-formed on the surface to a depth of 5 .mu.m by means of 
shot blasts, then recrystallization-annealed in an aluminum powder pack 
system, and finally provided with an aluminum diffusion coating 1 having a 
thickness of 5 .mu.m. As illustrated in the metallurgical micrograph, a 
completely uniform aluminum coating 1 has grown evenly over the 
crystallite 12 with an originally extremely pronounced plate structure 
during the aluminum diffusion process in the aluminum powder pack system 
on the base material 2. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.