CA1067256A

CA1067256A - Skin melted articles

Info

Publication number: CA1067256A
Application number: CA271147A
Authority: CA
Inventors: Bernard H. Kear; Anthony F. Giamei
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1976-02-17
Filing date: 1977-02-07
Publication date: 1979-12-04
Also published as: GB1530776A; DE2706844A1; JPS5299929A; FR2341665A1; BE851514A

Abstract

SKIN MELTED ARTICLES

Abstract of the Disclosure A class of articles is described having a bulk crystalline substrate and a surface layer having a refined structure.
In one embodiment, the surface layer may be amorphous. The articles are the result of rapid surface melting, by a laser or other high energy source, and rapid cooling. The surface composition is restricted to a limited class of materials based upon eutectic systems.

Description

BACKGROUND OF THE LNVENTION

Field of Invention - The invention relates to the field of metallic articles having surface properties and microstructures which differ from the properties and microstructures of the underlying substrate.
Description of the Prior Art - Although most prior art dealing with composite microstructure materials is not predicated on a surface melting technique, there is some prior art which shows articles with remelted surface layers.
U.S, Patent 3,388,618 shows a steel cutting tool having a surface portion with a refined structure. The structure is relatively deep, about 1/4 inch.
U.S. Patent 3,838,288 speaks generally of a two step surface melting process, without giving details, except to say that the formation of pores is minimized.
U.S. Patent 3,505,126 discloses a method for refining the grain size of a metal plate by surface melting to half the thickness of the sheet~ reversing the sheet and remelting the other half. Some of the example language indicates that thick plate (1/2") is contemplated.
U.S. Patent 3,773,565 speaks of a surface refinement technique for steel bearing races in which the surface is melted to a depth of about lmm and allowed to resolidify.

-2-~06~'~S6 A description of surface refinement in aluminum alloys, by surface melting, is given in APP1. Phys. Letters 21 (1972) 23-5.
SUMMARY OF THE INVENTION
The present invention relates to articles having surface layers with microstructures which differ significantly from the microstructure of the underlying substrate. The surface layer has a refined microstructure which results from surface melting followed by extremely rapid solidification. To obtain the desired microstructures,cooling rates of 104F/sec and up are necessary and this requires that the surface layer be restricted to very thin layers, less than about 50 mils in thickness.
If the surface layer is of a suitable deep eutectic, and the cooling rates during solidification are great enough, crystallization during solidification may be eliminated and the surface layer may be amorphous.
If the surface layer is based on an alloy of transition metals and metalloids, unique precipitate morphologies may be produced.
Other features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

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In accordance with an embodiment of the invention, a metallic article having a composite microstructure comprises:
aO a crystalline substrate, bo a resolidified surface layer : having an ultra fine microstructure with at least one of the surface layer grain dimensions being less than about 1,000 A
with the total thickness of the surface layer being from about .1 mil to about 50 mils, c. an epitaxial layer separating the substrate and the surface layer, with the thickness of the substrate being at least four times the thickness of the surface layer.

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DESCRIPTION OF THE DRAWINGS
, Fig. 1 shows a portion of the nickel-boron phase diagram, and illustrates a typical deep eutectic system.
Fig. 2 shows a transverse optical micrograph of a skin melted Pd-Cu-Si article.
Fig. 3 is an optical micrograph of Vickers micro-hardness indentations in the article of Fig. 3.
Fig. 4 shows an electron micrograph of a fracture surface in a Pd-Cu-Si article.
Fig. 5 shows a microcrystalline surface layer in a Ni-Co-Cr-Mo B alloy.
Fig. 6 shows an optical micrograph of overlapping skin melted layers.

DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention concerns metallic articles having surfaces layers with metallurgical structures and properties which differ from those of the underlying substrate. The articles have a surface layer with a much finer structure, different chemical and mechanical properties, and, optionally a different chemical composition than the underlying substrate.
This invention involves eutectic systems which are systems having specific compositions with melting points that are less than the melting points of the alloy constituents. Eutectics may form between two or more elements or compounds. Fig. 1 shows a portion of the nickel-boron phase diagram showing a eutectic at 18.4 atomic percent boron with the eutectic trough extending from 0 to 25 percent boron. For the purposes of this application deep eutectics are those in which the eutectic melting point is substantially less than the melting point of the major e~tectic constituent. In the Ni-B system shown in Fig. 1, the major component of the eutectic is Ni with a melting point of 1453C while the eutectic temperature is 1080C.
Briefly, the articlesare produced as follows: a metallic intermediate article is provided having a shape which closely corresponds to the desired final configuration. The surface layer (at least) of the intermediate article is of substantially eutectic composition over some fraction of the total surface layer area.

~0~;~256 A portion of the surface area generally corresponding to that part of the area which is of substantially eutectic composition, is rapidly melted to a depth on the order of about .1-50 mils. Melting is performed by heating the surface of the article with a form of energy which will be substantially absorbed at the surface. Since melting occurs under conditions which heat the surface without significantly heating the substrate, after melting is complete, cooling of the surface layer by heat flow into the substrate is extemely rapid, usually at least about 105~C/sec. By varying the heating parameters, the cooling rate may be controlled.
This high cooling rate results in an ultra fine microstructure, which in combination with the selected surface chemistry is responsbile for the novel surface properties of the article.
This invention may conveniently be described in terms of two major embodiments, the first involves a surface layer which is at least partially amorphous. The second embodiment involves a surface layer which is at least partially micro-crystalline.
The two embodiments are related in that they both comprise articles which have then resolidified surface layers whose microstructure is extreme~y fine, with at least one of the grain dimensions being less than about l,OOOA. In stating this relationship the amorphous state is regarded as comprising material whose grain size is on the order of atomic size, about 5A For convenience these articles are described as 1067'~56 having a composite microstructure i.e. a fine surface microstructure and a coarse substrate microstructure.

Embodiment 1 Partially or Wholly Amorphous Surface Layer Briefly it can be said that amorphous materials lack the regular~long range order which is characteristic of crystalline materials and have structures similar to super-cooled liquids such as glasses.
Amorphous metals, which have heretofore usually been produced only by complete melting followed by extremely rapid solidification in shapes of restricted geometry (with at least one dimension being restricted to less than about 5 mils), have been found to possess properties which often include high strengths, high hardness, good fatigue resistance, and resistance to oxidation and corrosion. The rapid solidification is commonly obtained by cooling the liquid metal on a cold solid chill,and this is the reason that at least one dimension must be small. The present invention provides these advantages of amorphous materials in a surface layer, on a~d integral with a massive crystalline substrate.
So far as is currently known, compositions which have been made amorphous have invariably been of approximately eutectic composition. The closer to exact eutective composition, the more readily a composition can usually be made amorphous by rapid solidification. Likewise, the deeper the eutectic (measured in terms of percent depression of the absolute $~f~7'~

melting temperature of the eutectic point from the absolute melting temperature of the major phase), the more readily a eutectic can usually be made amorphous. The process of the invention is broadly applicable to any deep eutectic.
Classes of eutectics which have been made amorphous by rapid solidification include:
a. Eutectics between transition metals and metalloids which usually contain from about 15 to about 30 atomic percent of the metalloid. Examples include Ni + B, Ni + P, Ni + P + C, Fe + B and Ti-Be (as used herein the term metalloid includes elements chosen from the group consisting of C, B, P, Ge, Se, Te, Ga, As, Sb, Be and Si, and mixtures thereof).
Boron and phosphorous are the preferred metalloids while iron, cobalt, and nickel are the preferred transition elements.
b. Eutectics between nontransition metals and metalloids, for example, Ag + Ge, c. Eutectics between early transition metals and late transition metals, elements 21 to 28 are the transition elements; the low number elements are the early transition elements typified by Ti while the high number elements are the late transition metals, typified by Co. An appropriate eutectic would be that between Ti and Co.

106'^~256 d. Eutectics between transition metals and nontransition metals typified by Cu-Zr, e. Eutectics between nontransition metals typified by Au-Sn.
Eutectics of the first group, between transition metals and metalloids, and eutectics of the third group, between early and late transition metals are preferred for the purposes of this invention. Although most of the previously listed example systems listed are binary, the ternary and higher 0 eutectic compositions exhibiting deep eutectic troughs may of course be made amorphous, and in fact, indications are that the more complex eutectics may be made amorphous with comparatively greater ease than the simple systems. For this reason, the minimum and preferred minimum eutectic temperature depressions (from the major constituent melting point) for various multi component eutectics are listed in Ta~le I.

TABLE I
Number of Min. M.P. (% ) Pref.M.P. ~o/
Components Depression J Depression~ J

3 10 20

4 7 17 -~,4' 5 10 1o~7~56 In its simplest form, this embodiment of the present invention consists of a crystalline substrate with at least partially amorphous resolidified surface layer having a thickness of from about .1 to about 50 mils. Since cooling rates are inversely related to melt thickness, and since the formation of amorphous materials require high cooling rates, the surface layer thickness is preferably less than 20 mils and most preferably less than 5 mils. In order to obtain rapid cooling the substrate must be at least about four times as thick as the surface layer. Ihe composition of the substrate and the surface layer are substantially identical in this simple form. The surface layer and the substrate will be separated by an epitaxial layer, a layer which was melted but which solified in oriented crystalline form with the orientation of the individual crystals in the epitaxial layer being generally related to the orientation of the underlying crystals in the substrate. The thickness of the epitaxial layer will vary from about .001 to about 1 mil.
While the preceding form in which substrate and surface layer have identical compositions will undoubtedly be useful in certain applications, this identical composition form may not offer the precise combination of substrate and surface layer properties desired. For example in crystalline form the eutectics of transition metals and metalloids are usually extremely brittle, although they have significant ductility i72S6 in the amorphous state. Thus it might be desirable to have an amorphous surface layer on a ductile substrate. For this reason, the preferred form of this embodiment is one in which the chemistry of the surface layer differs significantly from the chemistry of the substrate.
A variety of means, well known to those skilled in the metallurgical art, may be used to produce a eutectic surface layer on a substrate of differing composition. Since these are more method related than product related, they will not be discussed here.
Once the surface layer is produced of substantially eutectic composition with a depth of from about .1 to about 50 mils, the surface may be partially melted as previously described to produce a surface layer which is at least partially amorphous.
Experimental work has been performed on alloys containing 90.7% Pd, 4.2% Cu and 5.1% Si. This system behaves essentially as the Pd-Si system with Cu substituting for Pd.
The melting point of pure Pd is 1552C (1825~K~ while the melting point of the Pd-5% Si eutectic is 800C (1073K).
Thus the additio~ of 5 weight percent, (about 15.5 atomic percent) Si to Pd depresses the absolute melting point of Pd about 41V/o~ A continuous carbon dioxide (infrared) laser was used to melt a portion of the surface. The laser operating conditions were: a power output of 3000 watts, a spot size of .020 inches and a spot travel rate of 50 feet 10~;72S6 per minute. The power density was about 4 x 107 watts per square inch, and the dwell time of the laser on a particular spot was about .00003 seconds. The maximum melt depth was about 7 mils and the average melt depth was 3-4 mils. Fig. 2 shows a transverse optical micrograph of a skin melted surface. The featureless region 1 is the area which has been skin melted. The unmelted remainder of the sample displays the features typical of the crystalline eutectic structure. The extremely thin epitaxial layer is not resolvable optically. Evidence of partial preferential melting of certain phases can be seen at the interface between the substrate and surface layer. X-ray analysis of the skin melted region gave diffuse patterns characteristic of liquids and glasses rather than the sharp peaks character-istic of crystalline materials. Transmission electron microscopy showed a complete lack of structure in the featureless region (within the limits of the electron microscope used), indicating that the material in the feature-less region is in fact amorphous. Previous experimenters had noted that amorphous Pd-Cu-Si was slightly softer and significantly more ductile than crystalline Pd-Cu-Si and these findings were confirmed. Further, the resistance of the amorphous layer to etchants commonly used to prepare metallographic samples gives some indication of its resistance to chemical corrosion. Fig. 3 is a micrograph of Vickers microhardness identationsin the amorphous region ~0~7Z56 of the alloy. The curved slip lines indicate that the sample has great ductility, and the symmetry and lack of any straight line pattern again indicates that the sample is not crystalline. Prior experimenters have observed that amorphous materials have a unique fracture surface morphology, which is terms vein like fracture. This phenomenon is shown in Fig. 4, a replica electron micrograph of a fracture surface across a skin melted region in Pd-Cu-Si. The boundaries of the skin melted region 1 are the free surface 2 of article and the melt depth limit 3. The fracture surface shows the vein like fracture 4 which is an area of interconnected, irregular raised portion on the fracture surface, while the crystalline substrate dispJays a more ordinary fracture surface morphology.
A particular characteristic of amorphous materials is that they are thermodynamically unstable, and if heated above a specific temperature (termed the glass transition temperature) will crystallize. If crystallization occurs near the glass transition temperature the resultant crystal structure will be extremely fine with at least some crystal dimensions on the order of less than about l,OOOAD. Such a fine grain structure may have advantages for specific applications since it is more thermodynamically stable than the amorphous structure and has reasonably good mechanical properties due to the fineness of its structure (the well known Hall-Petch relationship predicts improved properties in fine microstructures).

~0f~;7'~S6 This invention also contemplates the articles which result from the heat treatment of articles with amorphous surface layers at temperatures above the glass transition temperature.
Such articles have a crystalline substrate, an epitaxial layer, and a surface layer which is at least partially microcrystalline (grain size generally less than about l,OOOA).
The fraction of the surface layer which is microcrystalline will be substantially equal to that fraction of the surface layer which was amorphous before the heat treatment. This heat treatment may be applied to any article with an at least partially amorphous surface layer, regardless of whether the chemical composition of the surface layer is the same a~, or different than that of the substrate.

Embodiment 2 The second major embodiment is an article with a crystalline substrate and microcrystalline surface layer restricted in chemical composition.
The resolidified surface layer is microcrystalline and its composition is essentially that of a eutectic between a material chosen from the group consisting of transition metals and mixtures thereof and a material chosen from the group consisting of metalloids, and mixtures thereof.
C, B and P and mixtures thereof are preferred metalloid elements. The metalloid constituents is prefera~ly present in an-amount from about 15 to about 30 atomic percent.

~O'~Z:~6 Certain combinations of metalloid elements and transition metal elements as major alloy constituents are especially preferred. These include B and P and mixtures thereof in combination with Ni, Fe and cobalt and mixtures thereof, and C, B, and P and mixtures thereof in combination with Ni and Co and mixtures thereof. Of course the preceding are only guidelines wh;ch suggest the major alloy constituents and of course other minor alloy constituents, both metal and metalloid may be present in amounts up to those which do not seriously affect the formation and presence of the desired metalloid rich particles. The surface layer thickness in this embodiment is preferably from about .1 to about 50 mils. The microcrystalline grains in the surface layer are elongated, with a length to diameter ratio of at least 5:1, and a diameter of less than about 4,000A and preferably less than about l,OOOA~. The grains form with the axis of elongation parallel to the direction of heat flow, thus the microcrystalline grains in the surface layer are predominately oriented with their long axis perpendicular in the surface. Most importantly, at least one of the phases in the surface layer is supersaturated in the metalloid constituent. This supersaturation of the metalloid element contributes to the extremely high hardnesses displayed by surface layers of this type. Experimental work has been done on an alloy containing 15~/o CO~ 15% Cr, 5% Mo, 4/O B, balance Nickel (weight percent composition). The behavior 10tj7ZS6 of this system is predictable by reference to the Nickel-Boron phase diagram (Fig. 1), since cobalt, chromium, and molybdenum all have significant solid solubility in nickel. A eutectic composition exists at 4 weight percent boron (18.5 atomic percent) the melting point of pure nickel is 1453C (1726K), and the eutectic temperature is 1140C (1413K), thus the addition of 4 weight percent to nickel depresses the absolute melting point by about 18%. A 3 Kilowatt laser beam with a .020 inch diameter spot size was traversed across the sample at a rate of 50 feet per minute to produce the melt zone shown in Fig. 5, a transmission electron micrograph. The structure is a lamellar eutectic with a lamellar spacing of about 380 A. This alloy is a candidate for the production of amorphous coatings, however, the skin melting experiments did not produce a cooling rate sufficient to suppress crystallization and microcrystalline structure resulted.
Fig. 6 is an optical micrograph of a sample of the same material which was skin melted using overlapping passes and this figure shows that the skin melting technique can produce a wide surface layer and in fact the complete surface of an article might be so treated. The Vickers microhardness of the matrix in Fig. 4 is about 650 kg/mm2 while the Vickers hardness of the skin melted surface layer is about 1250 kg/mm2.
Table I gives approximate Vickers hardness numbers for some other "hard" materials.

TABLE II

Material Vickers Hardness (k~/mm2) Hardened Tool Steel 650-700 Matensite 800 Tungsten Carbide 1600-3000*
Cemented Tungsten Carbide 1300-1400 (with Cobalt) Quartz 1100 Ni-15Co-15Cr-5Mo-4B 650 substrate Ni-15Co-15Cr-5Mo-4B 1250 surface layer * varies with crystallographic orientation Thus it can be seen that the skin melted microcrystalline surface layer (supersaturated with B) compares very favorably with other 'lhard" materials. Equally as interesting as the hardness is the ductility of these coatings, which is evident in their resistance to cracking and spalling under the heavy localized loads which accompany hardness testing.
Fig. 6 shows a transmission electron micrograph of the surface layer, showing a lamellar structure with a spacing of 380A.
It is believed that in this case, the cooling rate was sufficient to suppress normal solidification at the equilibrium freezing point and that this fine structure resulted from the "solid state~l decomposition and crystallization of the supercooled liquid at a temperature below the normal 10t;7'~56 solidification temperature, but above the glass transition temperature for the alloy (about 500C).
This invention presents a novel composite type of article which possesses a unique combination of properties which heretofore have not been available in a single article.
Those coatings of the present invention which are at least partially amorphous will ~ave great utility in the prevention of corrosion and other forms of chemical attack. Such amorphous coatings will be useful in the chemical industry, in bearing surfaces which must operate with contaminated lubricants, and in other similar applications. Many of the amorphous materials also display high surface hardness in addition to chemical corrosion resistance and such coatings will be exceptionally useful for heavy duty bearing applications. Another use for such coatings is the protection of articles, such as gas turbine blades and vanes, from erosion and corrosion. Such erosion, caused by ingested dust and grit is a major factor which limits ~he effective life of gas turbines. The safe operating temperature of such coatings will necessaily be less than the glass transition temperature of the surface composition.
The supersaturated, microcrystalline coatings will have similar uses, except that the coatings will not be subject to crystallization and hence will not be temperature limited except to the extent that grain growth occurs. In addition, the extreme hardness of certain of these coatings indicates ~l~tj7'~5~

that cutting tools might be fabricated with microcrystalline cutting surface.
Both of the previously described embodiments of articles with composite microstructures, may be regarded as being the results of a continuously variable process. Consider a deep eutectic comprised of a transition metal and a metalloid, N + 4% B, for example. This material satisfies both the criteria set forth for embodiments 1 and 2 above.
If this material were melted in a fashion which resulted in extremely high cooling rates, perhaps about 108~F/sec, an amorphous structure might be produced. The resultant article would be that described above in Embodiment 1. If melting conditions were selected that produced lesser cooling rates, perhaps 106F/sec, a microstructure consisting of a nickel solid solution matrix containing discrete, approximately equiaxed borides might be produced. The borides would form precipitation in the material below the normal solidification temperature. If the cooling rate was reduced even more, to perhaps 104F/sec, a fibrous eutectic microstructure would result by a normal solidification mechanism. In all three of these situations, the solidified material would be supersaturated in boron because the cooling rates employed are so far removed from e~uilibrium conditions.

~0~7~:56 All three microstructures are unusual and are believed novel when considered as a composite microstructure, in combination with a bulk crystalline substrate. Even the third microstructure described, the fibrous eutectic structure is novel by virtue of its extreme fineness.
It will be appreciated that these three resultant surface microstructures which fo~ the previously de~cribed embodiments, are the result of a continuous process of skin melting. Because solidification rates vary as a function of position within the melt, surface layers with mixtures of these three microstructures are quite likely to form.
Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A metallic article having a composite microstructure comprising:
a. a crystalline substrate, b. a resolidified surface layer having an ultra fine microstructure with at least one of the surface layer grain dimensions being less than about 1,000 .ANG., with the total thickness of the surface layer being from about .1 mil to about 50 mils, c. an epitaxial layer seperating the substrate and the surface layer, with the thickness of the substrate being at least four times the thickness of the surface layer.

2. An article as in claim 1 wherein the composition of the surface layer is substantially the same as the composition of the substrate.

3. An article as in claim 2 wherein the surface layer is at least partially amorphous.

4. An article as in claim 2 wherein the surface layer is at least partially microcrystalline, with at least one dimension of the surface layer grains being less than about 1,000 .ANG..

5. An article as in claim 1 wherein the composition of the surface layer differs significantly from the composition of the substrate.

6. An article as in claim 5 wherein the surface layer is at least partially amorphous.

7. An article as in claim 5 wherein the surface layer is at least partially microcrystalline, with at least one dimension of the surface layer grains being less than about 1,000 .ANG..

8. An article as in claim 1 wherein the surface layer composition is of substantially eutectic composition.

9. An article as in claim 8 wherein the absolute eutectic temperature is at least about 15% less than the absolute melting point of the major eutectic constituent.

10. An article as in claim 9 wherein the surface layer is at least partially amorphous.

11. An article as in claim 10 wherein the surface layer composition differs from the substrate composition.

12. An article as in claim 1 wherein the surface layer is based on a eutectic based on a material selected from the group consisting of transition metals and mixtures thereof, and contains from about 15 to about 30 atomic percent of a material chosen from the group consisting of metalloids and mixtures thereof.

13. An article as in claim 12 wherein the transition metal is predominately one chosen from the group consisting of iron, nickel, and cobalt, and mixtures thereof.

14. An article as in claim 12 wherein the metalloid is predominately one chosen from the group consisting of C, B, and P and mixtures thereof.

15. An article as in claim 12 wherein the surface layer is made up of at least two phases and at least one of the phases is supersaturated with the metalloid element.

16. An article as in claim 12 wherein the surface layer is microcrystalline with at least one of the crystal dimensions being less than about 1,000 .ANG..