Titanium Metal Glossary
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Aging: A change in the properties of certain metals and alloys that occurs with time at ambient or moderately elevated temperatures after working or a heat treatment (natural or artificial aging) or after a cold working operation (strain aging). The change in properties is often, but not always, due to a phase change (precipitation), but it never involves a change in chemical composition of the metal or alloy.
Alloy: A substance having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
Alpha-beta structure: A microstructure containing alpha and beta as the principal phases at a specific temperature.
Alpha case: The oxygen-, nitrogen-, or carbon-enriched, alpha-stabilized surface resulting from elevated temperature exposure.
Annealing: A generic term denoting a treatment, consisting of heating to, and holding at, a suitable temperature followed by cooling at a suitable rate. It is used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to: improvement of machinability, facilitation of cold work; improvement of mechanical or electrical properties, and/or increase in stability of dimensions. When the term is used without qualification, full annealing is implied. When applied only for the relief of stress, the process is properly called stress relieving or stress-relief annealing. In nonferrous alloys, annealing cycles are designed to: (1) remove part or all of the effects of cold working (recrystallization may or may not be involved); (2) cause substantially complete coalescence of precipitates from solid solution in relatively coarse form; or (3) both, depending on composition and material condition. Specific process names in commercial use are final annealing, full annealing, intermediate annealing, partial annealing, recrystallization annealing, stress-relief annealing, anneal to temper.
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Beta transus: The minimum temperature above which equilibrium α does not exist. For β eutectoid additions, the β transus ordinarily is applied to hypoeutectoid compositions or those that lie to the left of the eutectoid composition.
Billet: (1) A solid, semi-finished round or square product that has been hot worked by forging, rolling or extrusion; usually smaller than a bloom. (2) A general term for wrought starting stock used in making forgings or extrusions.
Blank: A pressed, pre-sintered, or fully sintered compact, usually in the unfinished condition, to be machined or otherwise processed to final shape or condition.
Brinell hardness number (HB): A number related to the applied load and to the surface area of the permanent impression made by a ball indenter.
Brinell hardness test: A test for determining the hardness of a material by forcing a hard steel or carbide ball of specified diameter into it under a specific load. The result is expressed as the Brinell hardness number, which is the value obtained by dividing the applied load in kilograms by surface area of the resulting impression in square millimeters.
Cast or casting: To fabricate an item by pouring molten metal into a shaped cavity and permitting the metal to solidify. A cast can relate to the item or may be a synonym for heat, that is an identifiable chemistry lot.
Charpy test: An impact test in which a V-notched, keyhole-notched, or U-notched specimen, supported at both ends, is struck behind the notch by a striker mounted at the lower end of a bar that can swing as a pendulum. The energy that is absorbed in fracture is calculated from the height to which the striker would have risen had there been no specimen and the height to which it actually rises after fracture of the specimen.
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Coarse grains: Grains larger than normal for the particular wrought metal or alloy or of a size that produces a surface roughening known as orange peel or alligator skin in wrought alloys.
Cold pressing: The forming of a compact from powder at, or below, room temperature.
Cold working: Deforming metal plastically under conditions of temperature and strain rate that induce strain hardening. Usually, but not necessarily, conducted at room temperature.
Cold-worked structure: A microstructure resulting from plastic deformation of a metal or alloy below its recrystallization temperature.
Consumable electrode: A general term for any arc welding electrode made chiefly of filler metal.
Consumable electrode re-melting: A process for refining metals in which an electric current passes between an electrode made of the metal to be refined.
Corrosion: The deterioration of a metal by a chemical or electrochemical reaction with its environment.
Corrosion fatigue: Cracking produced by the combined action of repeated or fluctuating stress and a corrosive environment at lower stress levels or fewer cycles than would be required in the absence of corrosive environment.
Creep: Time-dependent strain occurring under stress. The creep strain occurring at a diminishing rate is called primary, or transient, creep; that occurring at a minimum and almost constant rate, secondary, or steady-rate creep; that occurring at an accelerating rate, tertiary creep. These rates are frequently represented graphically as 1, 2, 3, or as I, II, III.
Creep limit: (1) The maximum stress that will cause less than a specified quantity of creep in a given time. (2) The maximum nominal stress under which the creep strain rate decreases continuously with time under constant load and at constant temperature. Sometimes used synonymously with creep strength.
Creep rate: The slope of the creep-time curve at a given time determined from a Cartesian plot.
Creep recovery: Time-dependent strain after release of load in a creep test.
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Creep-rupture strength: The stress that will cause fracture in a creep test at a given time in a specified constant environment. Also known as stress-rupture strength.
Creep-rupture test: See stress-rupture test.
Creep strain: The time-dependent total strain (extension plus initial gage length) produced by applied stress during a creep test.
Creep strength: The stress that will cause a given creep strain in a creep test at a given time in a specified constant environment.
Creep stress: The constant load divided by the original cross-sectional area of the specimen.
Descaling: Removing the thick layer of oxides formed on some metals at elevated temperatures.
Double aging: Employment of two different aging treatments to control the type of precipitate formed from a supersaturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is usually carried out at higher temperature than the second.
Ductility: The ability of a material to deform plastically before fracturing. Measured by elongation or reduction of area in a tension test, by height of cupping in a cupping test or by the radius or angle of bend in a bend test.
Electrode: (1) The isolated sponge, master alloy and/or revert used in consumable vacuum arc melting. (2) The solidified ingot in cases when it is to be re-melted again in double and triple melting operations.
Electron beam cutting (EBC): A cutting process which uses the heat obtained from a concentrated beam composed primarily of high-velocity electrons which impinge upon the workpieces to be cut; it may or may not use an externally supplied gas.
ELI: Extra-low interstitial.
Elongated alpha: A fibrous type of structure brought about by unidirectional metal working. It may be enhanced by the prior presence of blocky and/or grain boundary alpha.
Elongated grain: A grain with one principal axis significantly longer than either of the other two.
Elongation: A term used in mechanical testing to describe the amount of extension of a test piece when stressed. In tensile testing, the increase in the gage length, measured after fracture of the specimen within the gage length, usually expressed as a percentage of the original gage length.
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Equiaxed structure: A polygonal or spheroidal microstructural feature having approximately equal dimensions in all directions. In alpha-beta titanium alloys, such a term commonly refers to a microstructure in which most of the minority phase appears spheroidal.
Fatigue strength: The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated.
Forgeability: Term used to describe the relative ability of material to flow under a compressive load without rupture.
Forging: (1) Plastically deforming metal, usually hot, into desired shaped with compressive force, with or without dies. (2) Reshaping a billet or ingot by hammering. (3) The process of placing a powder in a container, removing the air from the container, and sealing it. This is followed by conventional forging of the powder and container to the desired shape.
Fracture toughness: See stress-intensity factor.
Grinding: Removing material from a workpiece with a grinding wheel or abrasive belt. Also the machining process of turning larger diameter bars to smaller sizes.
Hardness: A measure of the resistance of a material to surface indentation or abrasion; may be thought of as a function of the stress required to produce some specified type of surface deformation. There is no absolute scale for hardness; therefore, to express hardness quantitatively, each type of test has its own scale of arbitrarily defined hardness. Indentation hardness may be measured by Brinell, Koop, Rockwell, Scleroscope, and Vickers hardness tests.
Heat treatment: Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition.
Hot working: Deforming metal plastically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening.
Ingot: A casting of simple shape, suitable for hot working or re-melting.
Machining: Removing material from a metal part, usually using a cutting tool, and usually using a power-driven machine.
Macrostructure: The structure of metals as revealed by macroscopic examination of a specimen. The examination may be carried out using an as-polished or a polished and etched specimen.
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Master alloy: An alloy, rich in one or more desired addition elements, that is added to a melt to raise the percentage of a desired constituent.
Mechanical properties: The properties of a material that reveal its elastic and inelastic (plastic) behavior when force is applied, thereby indicating its suitability for mechanical (load-bearing) applications. Examples are elongation, fatigue limit, hardness, modulus of elasticity, tensile strength, and yield strength.
Modulus of elasticity: A measure of rigidity or stiffness of a metal; the ratio of stress, below the proportional limit, to the corresponding strain. Specifically, the modulus obtained in tension or compression is Young's modulus (E), stretch modulus, or modulus of extensibility; the modulus obtained in torsion or shear is modulus of rigidity, shear modulus (G), or modulus of torsion; the modulus covering the ratio of the mean normal stress to the change in volume per unit volume is the bulk modulus.
Multiple annealing: The process of giving two or more heat treatments to a titanium alloy to enhance ductility and toughness at the expense of modest decreases in strength. STOA results from multiple annealing of an alloy. Historically, multiple annealing has been called duplex or triplex annealing. It occurs whenever a high-temperature solution heat treatment is followed by a second or third thermal treatment. An example might be a heat treatment of Ti-6A1-4V alloy: at 954 degrees C (1750 degrees F) for 2 hours and water quenched; plus 593 degrees C (1100 degrees F) for 2 hours and air cooled; plus 704 degrees C (1300 degrees F) for 2 hours and air cooled.
Pickling: Removal of the oxide film on a casting by a chemical process; pickling is sometimes used solely to show up defects.
Physical properties: Properties of a metal or alloy that are relatively insensitive to structure and can be measured without the application of force; for example, density, electrical conductivity, coefficient of thermal expansion, magnetic permeability, heat capacity, and lattice parameter. Does not include chemical reactivity.
Precipitation heat treatment: Artificial aging in which a constituent precipitates from a supersaturated solid solution.
Preheat: An early stage in the sintering procedure when, in a continuous furnace, lubricant or binder burnoff occurs without atmosphere protection prior to actual sintering in the protective atmosphere of the high heat chamber.
Premium grade: A term used to describe titanium alloys used for jet engines. Equivalent of aircraft quality, flight quality.
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Prior-beta grain size: Size of β grains established during the most recent β field excursion. Grains may be distorted by subsequent subtransus deformation. Beta grain boundaries may be obscured by a superimposed α – β microstructure and detectable only by special techniques.
Quench aging: Aging induced from rapid cooling after solution heat treatment.
Quenching: Rapid cooling. When applicable, the following more specific terms should be used: direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching and time quenching.
Recrystallization: (1) Formation of new, strain-free grain structure from the structure existing in cold-worked metal. (2) A change from one crystal structure to another, such as that occurring upon heating or cooling through critical temperature.
Reduction in area: (1) Commonly, the difference, expressed as a percentage of original area, between the original cross-sectional area of a tensile test specimen and the minimum cross-sectional area measured after complete separation. (2) The difference, expressed as a percentage of original area, between original cross-sectional area and that after straining the specimen.
Residual stress: Stress remaining in a structure or member as a result of thermal or mechanical treatment or both. Stress arises in fusion welding primarily because the weld metal contracts on cooling from the solidus to room temperature.
Rockwell hardness number (HR): A number derived from the net increase in the depth of impression as the load on an indenter is increased from a fixed minor load to a major load and then returned to the minor load. Rockwell hardness numbers are always quoted with a scale symbol representing the penetrator, load, and dial used.
Rotor grade: Titanium alloy material approved for the most stringent rotating-part applications in gas turbine engines.
Rupture stress: The stress at failure. Also known as breaking stress or fracture stress.
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Tensile strength: In tensile testing, the ratio of maximum load to original cross-sectional area. Also called ultimate strength.
Toll melting: Contracting with a company to melt a metal.
Transverse direction: Literally “across.” Usually signifying a direction or plane perpendicular to the direction of working. In rolled plate or sheet, the direction across the width is often called long transverse, and the direction through the thickness, short transverse.
Ultimate strength: The maximum stress (tensile, compressive or shear) a material can sustain without fracture, determined by dividing maximum load by the original cross-sectional area of the specimen. Also known as nominal strength or maximum strength.
Upset forging: A forging obtained by upset of a suitable length of bar, billet, or bloom.
Upsetting: Working metal so that the cross-sectional area of a portion or all of the stock is increased.
Vacuum arc re-melting (VAR): A consumable electrode re-melting process in which heat is generated by an electric arc between the electrode and the ingot. The process is performed inside a vacuum chamber. Exposure of the droplets of molten metal to the reduced pressure reduces the amount of dissolved gas in the metal.
Vacuum induction melting (VIM): A process for re-melting and refining metals in which the metal is melted inside a vacuum chamber by induction heating. The metal may be melted in a crucible, then poured into a mold. The process may also be operated in a configuration similar to that used in consumable electrode re-melting except that the heat is supplied by an induction heating coil rather than from the passage of electric current through the electrode.
Wire: A thin, flexible, continuous length of metal, usually of circular cross section, and usually produced by drawing through a die.
Yield: (1) As a noun: the weight of a finished casting divided by the total weight of metal needed to produce it, including running systems, etc. (2) As a verb: to deform plastically on the first instance in a tensile or compressive test.
Yield strength: The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An offset of 0.2% is used for many metals.
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