This invention relates to metallic articles having a substrate and a composite material applied thereto, where the composite is formed of unstabilized zirconium oxide particles embedded in a metal matrix, and to the preparation of such articles.
When some metals are cooled at a relatively high rate and in circumstances where their contraction is constrained, they have a tendency to crack and/or become porous. For example, weldments such as hard facings and thermally sprayed metallic coatings applied and bonded to a substrate are rapidly cooled during the application process. The metal is applied to a surface of the substrate in a partially or fully molten state and then cooled rapidly through the solidus temperature and to a lower temperature.
During the cooling, differential thermal strains arise because the contraction of the solidified surface of the applied metal is constrained by the neighboring substrate material to which it is adhered by the application process. The differential thermal strains produce thermal stresses. If the thermal stresses exceed the fracture strength of the applied metal or if the differential thermal strains exceed the fracture strain of the applied metal, the applied metal fractures and a crack and/or internal porosity results. The crack and/or internal porosity results in reduced performance of the article.
There are numerous techniques used to lessen the incidence of cracking and/or internal porosity in these circumstances. Stronger, more ductile metals are used, where such metals are available to meet the specific service requirements. The substrate may be heated prior to the application of the metal and maintained at elevated temperature during application of the metal, to reduce the initial temperature range over which the applied metal is cooled. The substrate and applied metal are then cooled more slowly to room temperature, so that the metal has the opportunity to deform plastically. Stress-relieving heat treatments and/or special cooling procedures are used in some cases.
Each of these techniques has been successful in some circumstances, but they all add costs and/or manufacturing complexity, and additionally may impair the basic functionality of the applied metal and the final article. There is a need for an approach which reduces the incidence of cracking and/or porosity yet does not add significant cost and complexity and does not adversely affect the performance of the applied metal and the final article. The present invention fulfills this need, and further provides related advantages.
This invention provides an article having a metallic alloy composite applied as a deposit to a substrate, and a method for its preparation. The metallic alloy composite may be applied in any operable form, with a metallic weldment and a thermally sprayed coating being of most interest. The metallic material has a reduced incidence of differential thermal cracking and/or porosity as compared with conventional metallic alloys, without changing the composition of the basic metallic material. Application techniques for the metallic composite material of the invention are essentially the same as those used for conventional metallic alloys.
In accordance with the invention, a method of preparing an article comprises the steps of furnishing a precursor of a metallic alloy having a solidus temperature of at least about 700xc2x0 C., and furnishing a mass of unstabilized zirconium oxide powder. The precursor of the metallic alloy and the mass of unstabilized zirconium oxide powder are combined to form a mixture. The mixture is applied as a heterogeneous composite material to a substrate to form the article. The composite material comprises a matrix consisting of the metallic alloy, and a plurality of unstabilized zirconium oxide particles distributed throughout the matrix. The step of applying occurs at an application temperature of greater than the solidus temperature of the metallic alloy. The step of applying occurs at a temperature which is no less than about 700xc2x0 C., is preferably greater than about 950xc2x0 C., and is most preferably greater than about 1200xc2x0 C.
The step of applying is preferably performed by either forming a weldment or thermally spraying a powder or wire. A suitable welding rod or wire for use in welding or spraying may be made by placing the powder mixture into a tube and then extruding, drawing, or swaging the tube to size. Upon melting, the net composition is that desired in the final material. All of the materials both within the tube and forming the tube sheath, other than the unstabilized zirconium oxide, together constitute the precursor of the metallic alloy.
A wide variety of metals may be used, but the metallic matrix is preferably an amorphous alloy such as a frictionally transforming amorphous alloy. In one embodiment, the present approach is beneficially utilized when the matrix alloy (amorphous or non-amorphous) has a maximum strain to failure in tension of less than about 10 percent, preferably less than about 5 percent, at room temperature. The unstabilized zirconium oxide powder is present in an amount such that the metal/unstabilized zirconium oxide composite material is from about 0.2 volume percent to about 8 volume percent, more preferably from about 0.2 volume percent to about 4.5 volume percent, of the unstabilized zirconium oxide particles. The unstabilized zirconium oxide particles preferably have an average size of from about 1 micrometer to about 20 micrometers.
An article according to the present approach comprises a substrate, and a composite material bonded to the substrate. The composite material comprises a metallic matrix and a plurality of unstabilized zirconium oxide particles distributed throughout the metallic matrix.
When a metallic alloy solidifies and cools, it contracts. Because of thermal gradients within the metal and surface constraints on the contraction of the metal, if any, the amount of contraction at any moment varies from location to location, typically being larger near the surface and smaller near the center of the metallic mass. This variation leads to the potential for cracking and/or porosity in the metallic mass. The potential for cracking and/or porosity is particularly great when the metallic mass is affixed to a substrate, because of the different thermal expansion coefficients of the metallic mass and the substrate.
Unstabilized zirconium oxide (also known as zirconia) exhibits a phase transformation upon cooling from a high-temperature tetragonal phase to a low-temperature monoclinic phase over a phase transformation range of from about 950xc2x0 C. down to about 700xc2x0 C. This phase transformation is accompanied by a specific volume increase of the zirconium oxide upon cooling through the phase transformation temperature range. When embedded in a metallic matrix which exhibits a volume decrease as it cools, the expansion of the zirconium oxide during its phase transformation counteracts at least some of the contraction of the metal, serving to lessen the effects of thermal strains and stresses. The composite material of a metallic alloy matrix with unstabilized zirconium oxide particles embedded therein is therefore less susceptible to cracking and development of porosity than is a comparable metallic alloy without the unstabilized zirconium oxide particles present, particularly when the composite material is applied to and bonded to the substrate.
The present approach also reduces the incidence of warping of the article due to internal stresses and the incidence of bond line failures between the deposited composite material and the substrate. When a metal is deposited on a substrate, the differential thermal expansion properties of the metal and the substrate result in a tendency of the substrate to warp as the metal deposit and substrate are cooled, causing the substrate and metal deposit to xe2x80x9ccurlxe2x80x9d. This warping applies a stress to the bond line between the metal deposit and the substrate, effectively reducing its strength. In the present approach, the internal stresses and strains resulting from the difference in the thermal expansions of the metal deposit and the substrate are reduced, reducing the tendency to curl and also reducing the degradation of the bond-line strength.
The present invention is therefore limited to the composite material deposit of zirconium oxide particles in a metallic alloy matrix, applied and bonded to a substrate. The problems associated with deposits applied and bonded to substrates are different from those of materials which are not applied and bonded to substrates. Specifically, the differential thermal expansion between the substrate and the applied and bonded deposit is not present for the case of materials that are not applied and bonded to substrates.
In some other applications of zirconium oxide, the ceramic is stabilized by the addition of other oxides (such as yttrium oxide) to suppress the effects of the tetragonal/monoclinic phase transformation. The present invention relies on beneficial results achieved through the operation of the phase transformation, and therefore unstabilized zirconium oxide must be used. Stabilized zirconium oxide is not operable in the present invention.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
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