US3591353A - Bimetallic elements having heat-expansion characteristic - Google Patents

Abstract

A THERMOSENSITIVE ELEMENT COMPRISING A FIRST LAYER AND A SECOND LAYER OF A NON-CUBIC METAL HAVING A HIGH DEGREE OF CRYSTALLOGRAPHIC ORIENTATION IN WHICH THE DIRECTIONS OF CRYSTALLOGRAPHIC ORIENTATION ARE AT RIGHT ANGLES TO EACH OTHER.

Description

United States Patent ABSTRACT OF THE DISCLOSURE A thermosensitive element comprising a first layer and a second layer of a non-cubic metal having a high degree of crystallographic orientation in which the directions of crystallographic orientation are at right angles to each other.

The instant invention is directed to devices constructed of temperature sensitive materials and, more particularly to thermosensitive elements comprising two layers of material each having the same chemical composition. Such elements can be conveniently described as monometallic elements.

Thermosensitive elements and devices now available are generally of the bimetal type. A bimetallic thermosensitive device is a laminate comprising two or more layers of material, usually metals, having different linear thermal expansion coefficients. The laminate tends to change curvature with changes in temperature. Such thermosensitive elements are employed in thermometers, and temperature control equipment which are operative up to temperatures of approximately 1000 C. However, the bimetal thermosensiti-ve devices presently available are not satisfactory for use at temperatures in excess of about 800 F., particularly for prolonged periods of time.

One of the most serious problems encountered in the use of ordinary bimetallic elements at high temperatures is caused by diffusion across the interface between the layers. If a sufficiently long period of time were allowed, diffusion would produce a homogeneous strip having no thermal sensitivity. In shorter periods of time, particularly at elevated temperatures, diffusion does cause effective changes in the thermal activity of the bimetal element either by reducing the difference between the coefficients of linear expansion or by the gradual formation of an interfacial layer having a composition and thermal characteristics different from one or both of the original strips making up the bimetal.

It can therefore be appreciated that it would be very advantageous if temperature sensitive elements could be produced comprising two layers of strips of metal having the same chemical composition but having different thermal properties. As used herein, including the appended claims, the term metal is intended to include metals and alloys of metals.

It is therefore an object of the present invention to provide a two layer temperature sensitive element which is not subject to diffusional intercontamination.

It is an object of this invention to provide metallic temperature sensitive devices suitable for prolonged use at very high temperatures.

It is still a further object of the invention to provide a temperature sensitive element which is not subject to loss of sensitivity after prolonged use at high temperatures.

These and other related objects are achieved by providing a thermostat material, i.e., a temperature sensitive element, comprising two layers of metal having the same chemical composition and different thermal expansion properties bonded together to form a laminate of homogeneous composition. Such materials, i.e., those having the 3,591,353 Patented July 6, 1971 same or very similar chemical composition and differing thermal linear expansion properties can be conveniently referred to as crystallographically textured materials. The term crystallographically textured refers to materials having a substantial degree of preferred crystal orientation.

A metal sheet is simply a composite of an extremely large number of individual grains, each of which may be considered to be a crystal. The thermal expansion of a sheet cannot differ more with direction than the maximum difference in thermal expansion between two directions in a crystal of that material. For example, magnesium at approximately room temperature has thermal expansions of 26.5 and 25.1 10- per degree centigrade in the two crystal directions. This difference is too small to produce a workable deflection in a thermally active element. An appreciable difference, (on the order of at least about 10%) in thermal expansions between different crystallographic directions is necessary for most practical applications.

A cubic crystal expands equally in all directions and is therefore not applicable. Such common metals as iron, chromium, nickel, copper, gold, silver, tungsten, molybdenum, aluminum, platinum, tantalum and thorium are cubic metals and thus fail to quality in the present invention.

A further requirement is that there not be any change in crystal structure between room temperature and the service temperature. Cobalt and uranium undergo allotropic transformations when subjected to temperatures not substantially above 750 F. and 1200 F., respectively. Preferred crystallographic orientation would, to a large extent, be eliminated with the allotropic change and therefore these metals could not be used in applications where the maximum temperature exceeds the allotropic transformation temperature. Notably among the metals which might be used for bimetallic-type elements based on preferred crystallographic orientation and an ability to function after being subjected to a temperature of 1400 F. or below are beryllium, osmium, rhenium, ruthenium, titanium, yttrium, and zirconium.

Further exclusionary factors are high temperature strength, oxidation resistance, workability, weldability, or rate of loss preferred orientation as a function of time and temperature.

It is also necessary that the melting point be well above the required service temperature, For example, in a 1400 F. application, magnesium would be excluded from interest because of its relatively low melting point (1202 F.). Some of the other metals which would be similarly excluded are antimony, bismuth, cadmium, indium, tellurium, thallium, tin and zinc.

In preparing temperature sensitive devices of the type described above, an upper and lower layer of chemically identical or similar metal having different directions of crystal orientation, with respect to their length, are bonded together to form a laminate in which the directions of orientation are not parrallel. Preferably the directions of orientation should be at right angles to each other. The laminate is then incorporated into a temperature sensitive device in the usual manner.

In general metals and alloys which are crystallographically anisotropic, e.g., hexagonal metals such as titanium and zirconium, are characterized by measurable differences in thermal expansion along each crystallographic axis. Frequently the anisotropy, a characteristic generally considered to be detrimental, can be increased by suitable treatment of the metal, e.g., by suitably annealing the metal or by subjecting the metal to additional rolling. By way of example, a sheet of non-cubic alloy can be rolled to provide a sufficient degree of preferred crystal orientation, i.e., in the direction of rolling, and then pieces of corresponding size are cut from the rolled sheet, one in the longitudinal direction (the direction of rolling) and one in the transverse direction (at right angles to the direction of rolling). The two pieces are then laminated together, with their directions of orientation at right angles.

More particularly, it has been discovered that titanium alloys show a substantial difference between the coefficient of linear thermal expansion measured in the longitudinal direction and the coefiicient measured in the transverse direction.

Anisotropic thermal expansion in titanium alloys, e.g., an alloy containing about 8% aluminum, about 1% molybdenum, and about 1% vanadium; has been investigated and found to be of a magnitude suitable for use as a temperature sensitive material.

Table I, below, shows the results of dilatometric measurement of the thermal expansion in both the longitudinal and transverse direction on the above-mentioned aluminum-molybdenum-vanadium containing titanium alloy.

TAB LE I Mean expansion coefficient between room-temperature and the temperature shown in column 1 per degree 0.)

Temperature, 0. Longitudinal Transverse TABLE II Mean expansion coefficient between room temperature and the temperature shown in column 1 (10- per degree 0.)

Temperature) 0. Longitudinal Transverse Table III, below, shows the eifect of solution annealing for one hour at 1850 F.

TABLE III Mean expansion coetficient between room temperature and the temperature shown in column 1 (10- per degree 0.)

Transverse Temperature, 0. Longitudinal The temperature sensitive elements herein described can be easily prepared by cutting, at right angles to each other, thin fiat strips of metal from a sheet of suitably textured, i.e., crystallographically oriented metal. The thin strips are then bonded together, for example by riveting, welding, or diffusion bonding, to form a flat composite in which one of the strips has the direction of crystallographic orientation running at right angles to direction of orientation in the first strip, i.e., across the width of the strip. Due to the difference in thermal expansion between the two layers of the composite, the flat composite element bends into a curve when heated. Such a composite can be employed as the operative member of a thermostat, one end of the composite being fixed to a suitable support and the motion of the unfixed end being utilized to open and close an electrical circuit.

The temperature sensitive elements herein described can also be employed in thermometers, e.g., in the form of a helix which winds and unwinds in response to changes in temperature, the movement being indicated by a pointer which travels over a calibrated dial.

What is claimed is:

1. A thermosensitive element comprising a bonded composite of first and second layers of crystallographic textured non-cubic metals having a high degree of crystallographic orientation in which the directions of crystallographic orientation are at right angles to each other to produce a workable deflection in the element in response to temperature change.

2. A temperature sensitive element comprising a first layer of a crystallographically textured non-cubic metal and a second layer of the same metal, said first layer bonded to said second layer in different crystallographic directions to produce a workable deflection in the element in response to temperature change.

3. A temperature sensitive element comprising a first layer of a textured crystallographic titanium alloy and a second layer of the same textured titanium alloy, said first layer bonded to said second layer in diiferent crystallographic directions to produce a workable deflection in the element in response to temperature change.

4. The temperature sensitive element of claim 3 wherein the titanium alloy contains about 8 weight percent aluminum, about 1 weight percent molybdenum, and about 1 weight percent vanadium.

References Cited UNITED STATES PATENTS 1,985,181 12/1934 Matthews 29-195.S 2,234,748 3/ 1941 Dean et al. 29--195.5UX 2,940,163 6/1960 Davies 29198X 3,156,978 11/1964 Hanink et al 29 -198X FOREIGN PATENTS 209,931 8/1957 Australia -1755 ALLEN B. CURTIS, Primary Examiner US. Cl. X.R.