Conventional spark plug electrodes may include a nickel-based alloy in which elements are selectively included to improve properties of the electrode. For example, a known nickel-based alloy, identified as Champion Alloy 522, includes nickel (Ni), titanium (Ti), silicon (Si), manganese (Mn), chromium (Cr), iron (Fe), zirconium (Zr), carbon (C), and a small quantity of unavoidable impurities. The proportion of each element contributes to the properties of the alloy. Specifically, the known alloy includes 94.5-95.5% Ni, 0.20-0.30% Ti, 0.35-0.55% Si, 1.80- 2.10% Mn, 1.65-1.90% Cr, 0.10-0.20% Zr, and a maximum of 0.50% Fe, 0.05% C and 0.25% other impurities.

SUMMARY In one general aspect, a nickel, aluminum-based electrode alloy includes aluminum, silicon, chromium, manganese, iron, carbon, titanium, zirconium, and impurities, and the remainder nickel.

Aluminum is present in the NiAl alloy in a preferred range of 0.75 to 2.2 weight percent. A more preferred range is 1.5 to 2.1 weight percent. An even more preferred amount of aluminum in the alloy is about 1.8 weight percent.

Silicon is present in the alloy in a preferred range of 1.1 to 1.75 weight percent. A more preferred amount of silicon in the alloy is in the range of 1.1 to 1.3 weight percent. An even more preferred amount of silicon in the alloy is about 1.2 weight percent.

Chromium is present in the alloy in a preferred range of 1.45 to 1.9 weight percent. A more preferred range is 1.45 to 1.8 weight percent. An even more preferred amount of chromium in the alloy is about 1.6 weight percent.

Manganese is present in the alloy in a preferred range of 0.3 to 0.6 weight percent.

A more preferred range is 0.3 to 0.5 weight percent. An even more preferred amount of manganese in the alloy is about 0.4 weight percent.

Iron occurs naturally in nickel and is present in a preferred amount less than 0.5 weight percent. A more preferred amount is less than 0.2 weight percent.

An even more preferred amount of iron in the alloy is less than 0.1 weight percent.

Titanium, zirconium and impurities, such as carbon, are present in the NiAl alloy in an amount preferably less than 0.2 weight percent. A more preferred amount is less than 0.1 weight percent zirconium and less than 0.02 weight percent titanium. An even more preferred amount is less than 0.05 weight percent zirconium and less than 0.01 weight percent titanium. An even more preferred amount is less than 0.005 weight percent titanium. Referring specifically to carbon as an impurity, it should be present at 0.05 weight percent or less. Nickel comprises the remainder of the composition of the alloy.

When used as the electrode material in a spark plug, the NiAl alloy has the considerable advantage of forming a durable oxide film or scale on the electrode that restricts harmful oxidation and sulfidation to reduce corrosion of the electrode. The alloy also improves the performance of the electrode by reducing gap growth.

Other features and advantages will be apparent from the following description, including the drawings, and from the claims.

DESCRIPTION OF THE DRAWING Fig. 1 is a front view of a spark plug.

DESCRIPTION Referring to Fig. 1, a spark plug 100 includes an insulator 105, an outer shell 110, a ground electrode 115 attached to outer shell 110, and a firing electrode 120 adjacent to ground electrode 115. A spark gap 125 is defined between ground electrode 115 and firing electrode 120. Ground electrode 115 and firing electrode 120 are made of a nickel, aluminum alloy (NiAl alloy) that includes nickel (Ni), aluminum (Al), titanium (Ti), silicon (Si), manganese (Mn), chromium (Cr), iron (Fe), and zirconium (Zr). Each element contributes to the properties of the NiAl alloy.

Aluminum is present in the NiAl alloy in a preferred range of 0.75 to 2.2 weight percent. A more preferred range is 1.5 to 2.1 weight percent. An even more preferred amount of aluminum in the alloy is about 1.8 weight percent.

Aluminum is added to the NiAl alloy primarily because it forms a durable and stable oxide film in the form of aluminum oxide (A1203). On an atomic scale, the alloy is made of grains ordered in a grain arrangement. The oxide film is formed in the grain boundaries, where the alloy is likely to be attacked in the processes of oxidation and sulfidation during combustion. These processes corrode the electrodes by causing grains to drop out of the grain boundary. The formation of a durable aluminum oxide film restricts harmful oxidation and sulfidation and, thus, reduces corrosion.

Aluminum also contributes to faster work hardening than would occur during the normal plastic deformation associated with cold working of the alloy.

During cold working, larger grains are broken into smaller grains. Because of energy level differences, the aluminum atoms migrate to the grain boundaries. The aluminum atoms are smaller than the nickel atoms they replace, which makes the material harder to work further and thereby improves work hardening. After cold

working, an alloy of nickel and aluminum generally is harder than the alloy without aluminum.

Silicon is present in the alloy in a preferred range of 1.1 to 1.75 weight percent. A more preferred amount of silicon in the alloy is in the range of 1.1 to 1.3 weight percent. An even more preferred amount of silicon in the alloy is about 1.2 weight percent. Silicon forms a durable oxide film similar to the film formed by aluminum. In particular, the silicon undergoes a reaction with oxygen attacking the alloy in the grain boundaries. The reaction produces the oxide, silicon oxide (Si02), which, in turn, creates a compressive force on the surface to form a protective oxide scale.

Chromium is present in the alloy in a preferred range of 1.45 to 1.9 weight percent. A more preferred range is 1.45 to 1.8 weight percent. An even more preferred amount of chromium in the alloy is about 1.6 weight percent.

Chromium is added because it reacts with oxygen to form a durable chromium oxide (Cr203) scale with properties similar to the aluminum and silicon oxides.

However, the weight percentage of chromium is restricted because its presence reduces thermal and electrical conductivity, both of which must be maintained to ensure performance of the spark plug.

Manganese is present in the alloy in a preferred range of 0.3 to 0.6 weight percent. A more preferred range is 0.3 to 0.5 weight percent. An even more preferred amount of manganese in the alloy is about 0.4 weight percent.

Manganese reacts with sulfur present in combustion gases in the combustion chamber more than it reacts with oxygen due to sulfur's higher level of energy.

The reaction produces manganese sulfide (MnS) as a film on the surface of the electrode. This film generates a compressive force on the surface similar to the compressive force produced by silicon oxide. The film and compressive force block additional sulfur and oxygen from passing through to the alloy below.

Iron occurs naturally in nickel and is present in a preferred amount less than 0.5 weight percent. A more preferred amount is less than 0.2 weight percent.

An even more preferred amount of iron in the alloy is less than 0.1 weight percent.

Like chromium, the weight percentage of iron is limited because it affects thermal and electrical conductivity.

Titanium and zirconium are present in the alloy because they occur naturally in nickel as trace elements. Together with impurities, such as carbon, they are present in the NiAl alloy in an amount preferably less than 0.2 weight- percent. A more preferred amount is less than 0.1 weight percent zirconium and less than 0.02 weight percent titanium. An even more preferred amount is less than 0.05 weight percent zirconium and less than 0.01 weight percent titanium. An even more preferred amount is less than 0.005 weight percent titanium. Referring specifically to carbon as an impurity, it should be present at a maximum of 0.05 weight percent. Titanium forms a durable oxide film that reduces corrosion.

However, titanium is more expensive and more difficult to process than aluminum.

Zirconium also forms a durable oxide film that reduces corrosion.

Nickel comprises the remainder of the composition of the alloy. Nickel has the beneficial property of high temperature integrity.

The percentages of the elements in the alloy, although reported here as weight percentages, are based on atomic percentages. Table 1 illustrates the weight percentages and atomic percentage of one composition of the alloy. The composition was developed based on the atomic percentages of aluminum, silicon, and chromium necessary to form a durable oxide scale with similar proportions of each oxide. For instance, to have similar proportions of silicon oxide, aluminum oxide, and chromium oxide, the atomic percentages of the elements are one silicon atom for every two aluminum atoms and chromium atoms. From this ratio, the atomic percentages and weight percentages can be calculated. Constraints on the compositions generated by this method are the availability of alloys designed according to this method and manufacturability of the alloy into electrodes. Al i si Mn Cr Fe Zr Ni Density 2.70 4.50 2.33 7.43 7.19 7.86 6.49 8.90 At. Wt. 26.98 47.88 28.09 54.94 51.996 55.85 94.22 58.69 Total Percent Wt. % 1.8 0.004 1.2 0. 4 1.6 0.09 0.01 balance 100.000 At. % 3.713 0.005 2. 378 0.405 1. 713 0. 090 0.006 balance 100. 000 Table 1 To test the NiAl alloy, ground and firing electrodes were fabricated from the alloy using a composition specified to be within the weight percentage ranges described above. Spark plugs then were fabricated using the electrodes and the spark plugs mounted in combustion engines. Ground and firing electrodes also were made using Champion Alloy 522, and finished spark plugs were installed in internal combustion engines. The engines were operated using leaded fuel.

Periodically, the spark gaps were examined for spark gap growth (i. e., the increase in gap distance between the ground and firing electrodes caused by erosion of the electrodes). After 180 hours of operation, the ground electrode fabricated from Champion Alloy 522 had eroded almost completely. After 200 hours of operation, the ground electrode fabricated from the NiAl alloy showed little erosion of either the ground or firing electrode. An analysis of the results showed a 30-40% increase in performance of the electrode material as measured by reduced gap growth. Results also indicate that the NiAl alloy electrodes provide increased performance when used to combust unleaded fuel.

Other embodiments are within the scope of the following claims.