2.5 SELECTION OF MATERIALS

The selection of the most suitable material for a given rocket engine part will be governed by- (1) The function, size, and shape of the part (2) Required mechanical properties; strength, stiffness or rigidity, hardness, and ductility, with particular consideration of the extreme temperature conditions in liquid rocket engines (3) Required physical and chemical properties; density, thermal conductivity, specific heat, coefficient of expansion, Poisson's ratio, strength-to-weight ratio, corrosion resistance, compatibility with propellants as a function of temperature (4) Considerations related to fabrication, such as forgeability, castability, weldability, machinability, and formability (5) Cost and availability (6) Existing industry and Government standards Extreme temperature and corrosion conditions combined with the need for very-high-strength-toweight ratios drastically narrow the choice of available materials. In particular, the extremely low temperatures encountered with cryogenic liquids have introduced serious materials problems. As a rule, tensile strength and yield strength increase with decreasing temperature. However, ductility is seriously affected. Apart from selecting the best alloys for extremely low temperatures, highest purity of the metals is mandatory.

The increased usage of liquid hydrogen has introduced additional problems which further narrow the selection of available materials. Specifically for the $\mathrm{LH}_{2}$ application, metals must exhibit:

Resistance to Low-Temperature Embrittlement

This can also be referred to as toughness or resistance to brittle fracture. Toughness, in general, describes resistance to fracture under shock-type loads and stresses. Most rocket engine parts are exposed to these loads; however, conditions are much more severe at the extremely low liquid-hydrogen temperatures. The tendency of various alloys to brittle failure is measured experimentally by the notched/unnotched tensile strength ratio. Typically, for 2014-T6 aluminum at $-423^{\circ} \mathrm{F}$, the ratio is 0.94 (longitudinal) and 0.83 (transverse).

Resistance to Thermal Shock

This is a measure of a material's ability to resist weakening or fracture as a result of sudden heating or cooling. The following properties appear to be requirements for high thermal shock resistance: High tensile strength ( $F_{\mathrm{tu}}$ ), high thermal conductivity ( $k$ ), low modulus of elasticity ( $E$ ), and a low coefficient of thermal expansion ( $\alpha$ ). The ratio $F_{\mathrm{tu}} k / E \alpha$ provides a relative measure of thermal shock resistance for comparison of different metals. Typical values are 5 to 8 for stainless steel and 40 to 48 for 2014-T6 aluminum.

Resistance to Hydrogen Embrittlement

Certain metals, such as steels and titanium alloys, have a tendency to embrittlement in a hydrogen atmosphere. This tendency is greatest in the intermediate-temperature range, but disappears at low and high temperatures. The effect is often delayed until a critical hydrogen concentration in the metal is reached when cracks start to appear, as a result of a marked decrease in ductility. Also, the embrittlement effect decreases with increasing strain in the metal. Heat-treated steels are more susceptible to it than annealed ones.

Resistance to Chemical Reactions

While low-temperature hydrogen is considered essentially noncorrosive, gaseous hydrogen forms hydrides with some metals, such as uranium and titanium.

For up-to-date detail on material properties, the reader is referred to material handbooks, industry (material supplier) information, and Government publications (Bureau of Standards).

The principal groups of materials used for liquid rocket engines are as follows:

Low-Alloy Steels

Uses for rocket engine components include pins, bolts, shafts, brackets, mounts, thrust chamber structure members, ducts, injector bodies, and certain pressure vessels. The standard grades AISI 4130, AISI 4140, AISI 4340, MAS 6434 are prominent in this group of steels.

The low-alloy steels are normally used in the temperature range from $-60^{\circ} \mathrm{F}$ to $600^{\circ} \mathrm{F}$. They are not suitable for corrosive environments. Elevated temperatures produce excessive creep, thus reducing the strength. Also, very low temperatures tend to induce brittleness in most of them. An exception is AMS (SAE 9310).

Austenitic Stainless Steels (300 Series)

Steels in this group possess the highest corrosion resistance in the family of stainless steels and are highly qualified for storable liquid propellant application. They are inherently tough and well adapted for fabrication by deep drawing and other similar means. They can be welded easily, can be soldered by proper technique, and are well suited for machining and forming under normal conditions. Ordinary sand castings, precision investment castings, and forgings can also be produced from these steels. They are widely employed in rocket engines using cryogenic and storable propellants. Parts such as regenerative-cooled thrust-chamber tubes and manifolds, injector bodies and domes, valve poppets and bodies, propellant ducts and tanks are made from these steels.

Martensitic-Type Stainless Steels ( 400 Series)

The steels in this group are hardenable, in which condition they exhibit their best mechanical as well as corrosion-resisting properties. The thermal conductivity of these steels is low but still the best of the stainless-steel family. They are specially suitable for hot working or forging. Their cold-forming characteristics are fair. They are well suited for most applications requiring high strength, hardness, and resistance to abrasion, wet and dry erosion, and moderate corrosion. They are not suitable for cryogenic applications, because of brittleness and shock sensitivity under these conditions. They are used for turbopump ball bearings and shafts, gears, valve actuators, and cams.

Semiaustenitic Stainless Steels

The steels in this group can be formed in the soft state and then precipitation hardened. They are intended for use in parts requiring corrosion resistance and high strength at operating temperatures up to $800^{\circ} \mathrm{F}$, and where such parts may require welding and soldering during fabrication. However, the corrosion resistance of this type of steel is not as good as that of the austenitic stainless steels. Rocket engine component parts, such as thrust chambers, pump shafts, levers, brackets, bellows, ducts, springs, clamp rings, valve poppets, housings, and pressure vessels, have been made from the steels of this group.

Aluminum Alloys

Pure metallic aluminum has a relatively low strength. However, the strength can be greatly increased by alloying aluminum with one or more metals or metalloids. This can be accomplished without affecting appreciably the other desirable properties of aluminum, such as low weight, corrosion resistance, ductility, good thermal and electric conductivity.

Wrought alloys of aluminum are generally of two types: one group that can be hardened by cold-working only (non-heat-treatable), such as $1100,3003,3004,5050$, and 5052 and a second group that will respond to both cold-working and heat-treatment, such as 2011, 2014, 2017, 2024, 6061, 6066, and 7075. Wrought-aluminum alloys are suitable for fabrication processes such as machining, shearing, drawing, stretch forming, spinning, stamping, and shape bending. Most of them are also adaptable for forging, welding, brazing, and soldering. Aluminum alloys can be cast by all three common casting methods: sand, permanent mold, and pressure die casting. Mechanical properties and workability of aluminum castings are excellent.

Aluminum alloys are the most widely used materials in rocket engine construction except where elevated temperatures are encountered. Typical applications are valve bodies and poppets, injector domes, propellant tanks and ducts, pump housings, impellers and inducers, and structure mounts.

Magnesium Alloys

Magnesium alloys have found many applications in rocket engines and vehicles because of their excellent strength-to-weight, fatigue and stiffness characteristics. These alloys are used to make pump housings, valve bodies, and structure mounts and are available in sheets, rods, and castings.

Magnesium sheet alloys can be formed at elevated temperatures. They are also suitable for various machining processes. They can be joined by fusion and resistance welding as well as by adhesive bonding. Magnesium alloys can be cast by all three common casting methods: sand, permanent mold, and pressure die casting. Certain cast alloys can also be welded and heat treated.

High-Temperature Nickel-Base Alloys

The metals included in this group are used primarily for their strength at temperatures up to $1700^{\circ} \mathrm{F}$. The majority of them contain aluminum or titanium as precipitation-hardening agents and are vacuum melted. Their resistance to oxidation and corrosion is excellent. These alloys have found wide application in rocket engine components such as: turbine housings, wheels, and blades; thrust chamber tubes and injectors; gas generators; high-temperature gas ducts, bolts, and fasteners.

Special Alloys

The ever-present extreme temperature condi- tions in liquid rockets calls for continued and intensive materials research, particularly with the advent of liquid hydrogen systems. In addition to the metals discussed in the preceding paragraphs, other metals and alloys are receiving increasing attention. Among these are:

Copper base alloys.-These metals exhibit excellent ductility and toughness at very low temperatures Typical representatives are Berylco-10. -25 alloys, and $\mathrm{Fe}-\mathrm{Si}$ bronze.

Cobalt base alloys.-The properties of these metals, such as Haynes-25, are similar to those of the nickel alloys.

Tantalum.-Tantalum, when pure, has good properties at both low and elevated temperatures.

Columbium.-This metal has been considered for cryogenic application, but is liable to become embrittled at very low temperatures.

Titanium-base alloys.-These alloys have attracted considerable attention because of their high strength-to-density ratios, particularly at very low temperatures.

Nonmetallic Materials

For gaskets, seals, lubricants, thread compounds, and the like, liquid rocket engines require compatible nonmetallic materials. A great variety of commercial products is available.

In advanced LOX-pump designs, as well as in liquid-hydrogen pumps. the pumped fluid is used as the lubricant.