METAL ALLOYING ELEMENTS

What are alloying elements in metals?

Alloying elements are chemical elements intentionally added to a base metal (such as iron, aluminum, copper, or titanium) during the production of metal alloys to modify and enhance specific properties. These additions alter the alloy's microstructure, phase stability, and behavior under various conditions, improving characteristics like strength, hardness, ductility, corrosion resistance, electrical conductivity, or high-temperature performance.

Alloying elements can be major (high percentages, e.g., 10-50%) or minor/micro-alloying (low percentages, e.g., 0.01-1%), and their effects often depend on concentration, interactions with other elements, and processing methods like heat treatment.

Common alloying elements are primarily used in ferrous (iron-based) and non-ferrous alloys. Below is a list summarizing some of the most widely used alloying elements and their primary effects.

Aluminum (Al)

• Aluminum effects are highly concentration-dependent (typically 0.005-12% in alloys) and often synergize with elements like silicon or zinc for optimized results in transportation, construction, and electronics industries.

• Deoxidation and Inclusion Control: In steel production, aluminum acts as a powerful deoxidizer by forming stable alumina (Al₂O₃) inclusions, which minimize oxygen-related defects, improve cleanliness, and enhance ductility and fatigue resistance. It also controls sulfur inclusions for better machinability.

• Corrosion and Oxidation Resistance: Aluminum forms a protective oxide layer that boosts resistance to atmospheric oxidation and scaling at high temperatures (up to 600-800°C) in stainless and heat-resistant steels. In magnesium alloys, it improves seawater corrosion resistance, while in aluminum bronzes, it enhances overall durability.

• Strength and Grain Refinement: As a micro-alloying element (0.01-0.05% in steels), it refines grain size during heat treatment, increasing yield strength, toughness, and low-temperature impact resistance without significantly affecting weldability. However, higher levels (>1%) can reduce hardenability by promoting ferrite formation.

Carbon (C)

• Carbons effects are highly sensitive to concentration, cooling rates, and interactions with elements like iron or silicon.

• Carbon (C) is the primary alloying element in iron-based metals, defining steels (typically 0.02-2% C) and cast irons (>2% C), where it fundamentally governs phase diagrams, heat treatment responses, and mechanical behavior.

• Principal hardening agent in steel

• Hardness and Strength: Carbon dramatically increases hardness, tensile strength, and wear resistance by forming hard iron carbides (e.g., cementite, Fe₃C) during solidification or heat treatment. In medium- to high-carbon steels (0.3-1.0% C), it enables quenching to produce martensite for superior load-bearing capacity in tools and springs.

• Hardenability and Heat Treatment: It enhances the depth and uniformity of hardening by shifting phase transformation temperatures (e.g., raising the eutectoid point), allowing complex shapes to achieve full hardness. However, very high levels (>1.5% C) promote graphitization in cast irons for improved machinability but reduced strength.

• Ductility, Toughness, and Formability: Low carbon content (<0.25% C) preserves high ductility, elongation, and impact toughness, ideal for welding and cold-forming in structural steels. Higher concentrations reduce these properties, increasing brittleness and susceptibility to cracking, which requires balancing with elements like manganese.

• Wear and Corrosion Resistance: Boosts abrasion and erosion resistance in high-carbon alloys (e.g., for blades or rails), but can lower corrosion resistance in steels by promoting carbide networks that act as initiation

• In most alloy steels, carbon has about the same effect upon hardness as it does in the plain carbon steels

Chromium (Cr)

• Increases hardenability and strength in low ranges

• Improves corrosion and oxidation resistance in higher ranges around 12-30%

• Forms a passive oxide on surface that protects against rust, pitting, and high temperature oxidation

• Enhances wear resistance to abrasion and erosion

• Effects vary by alloy type (other elements) and chromium concentration

Cobalt (Co)

• Primarily used in super alloys

• High Temperature Strength and Hardness (up to 1000 C / 1800 F or higher)

• Wear Resistance, Corrosion Resistance

• Improves creep resistance

• Imparts characteristic or the ability to remain hard when red hot “red hardness”

Copper (Cu)

• As an alloy only beneficial in very low levels; 0.20% or less. Detrimental to welding see below

• Improves atmospheric corrosion resistance

• Improvement in electrical or thermal conductivity

• Improvement in ductility, formability, or machinability

• In Welding copper has a tendency to cause

  • Embrittlement or cracking (hot-shortness, embrittlement in HAZ)

  • Reduced toughness, decreases Charpy toughness

Manganese (Mn)

• In low levels up to 0.80% manganese is generally intended to combine with sulfur to offset embrittlement

• In levels between 1-12% manganese increases the toughness and hardenability without decreasing ductility

• Wear and Toughness Resistance: In high-manganese steels (e.g., 12-14% Mn in austenitic Hadfield steel), it dramatically improves work-hardening ability, abrasion resistance, and impact toughness under dynamic loads, making it ideal for wear parts like crusher jaws.

• Austenite Stabilization and Corrosion Resistance: Manganese stabilizes the austenitic phase in stainless steels, reducing the need for nickel and improving resistance to deformation at high temperatures. It also enhances pitting and atmospheric corrosion resistance when combined with other elements.

Molybdenum (Mo)

• Molybdenum effects are concentration-dependent (usually 0.1-8% in alloys) and often synergize with chromium or nickel for optimal results in for metals used in the energy and refining industries.

• Boosts hardenability by slowing the transformation from austenite to ferrite/pearlite, allowing deeper and more uniform hardening through sections of steel (deep hardening)

• Increases tensile/yield strength and toughness through solid solution strengthening and carbide formation, reducing brittleness during tempering

• Molybdenum raises creep and rupture strength at elevated temperatures (up to 600-1000 C / 1100-1800 F) by stabilizing carbides and inhibiting grain growth in superalloys and heat-resistant steels.

• Corrosion Resistance: In stainless steels, molybdenum (typically 2-6%) enhances resistance to pitting, crevice, and stress corrosion cracking in chloride environments, making alloys like 316L more robust for marine or chemical use. It also improves atmospheric and sulfuric acid resistance.

• It promotes fine carbide dispersion for better abrasion and erosion resistance in tool steels, while improving fatigue life under cyclic loading by refining microstructure.

• Prevents temper embrittlement of low chrome alloys

Nickel (Ni)

• Nickels effects are concentration-dependent (typically 1-70% in alloys) and often synergize with chromium or molybdenum for enhanced performance in marine, energy, and petrochemical industries.

• Enhances corrosion resistance, toughness, and high-temperature stability

• Toughness and Ductility: It enhances low-temperature toughness and impact resistance by lowering the ductile-to-brittle transition temperature.

• Steels with more than 24% nickel are non-magnetic

• Strength and Hardenability: As a solid solution strengthener, nickel boosts tensile strength, yield strength, and fatigue resistance.

• In alloy’s such as Inconel or Hastelloy with 40-70% nickel, nickel provides excellent creep resistance at high temperatures.

• Nickel significantly improves corrosion resistance to acidic, alkaline, and chloride environments by forming a passive oxide layer and stabilizing the austenitic structure in stainless steels (e.g., 8-12% Ni in 304/316 grades), reducing pitting, crevice corrosion, and stress corrosion cracking.

• At 10-22% nickel strongly stabilizes the face-centered cubic austenite phase of stainless steels with reduced chromium needs and better high-temperature oxidation resistance (800-1000 C / 1472-1800 F).

Niobium (Nb)

• Improves strength properties at higher temperatures without significantly reducing ductility

• Grain refinement; leads to smaller grain size which improves tensile strength, toughness, and fatigue resistance

• Carbide stabilizer in austenitic grades of stainless steel

• Tempering and Hardenability Resistance: In martensitic and tool steels, it increases resistance to softening during tempering, maintaining hardness at elevated temperatures, while in some cases slightly lowering overall hardness to balance toughness.

• Corrosion and Wear Resistance: Niobium promotes dispersion hardening through carbide precipitates in stainless steels, enhancing resistance to pitting, intergranular corrosion, and abrasion in wear-resistant alloys. It also improves weldability by stabilizing the microstructure in the heat-affected zone.

• High-Temperature Performance: In nickel- and iron-nickel-base superalloys, additions stabilize phases and improve creep resistance and oxidation behavior at temperatures exceeding 800 C / 1472 F.

Nitrogen (N)

• These effects are highly concentration-dependent (typically 0.001-0.6% in alloys) and often controlled via stabilizers like aluminum or titanium to avoid brittleness.

• Corrosion Resistance: Nitrogen stabilizes the austenitic phase (reducing nickel needs) and improves resistance to pitting, crevice corrosion, and chloride stress cracking in duplex and super-austenitic stainless steels, making alloys more robust in aggressive environments like seawater or acids.

• Surface Hardening: During nitriding processes, nitrogen diffuses into the surface to form hard nitride layers, dramatically increasing case hardness (up to 1000 HV) and wear/erosion resistance while maintaining a ductile core, ideal for gears and shafts.

Oxygen (O)

• Oxygen (O) is generally considered an impurity rather than an intentional alloying element in metals, as it reacts readily with most metals to form oxides during melting, casting, or processing and these oxides can degrade mechanical properties.

• In steelmaking oxygen is used to burn out excess carbon and shorten melting time.

Phosphorus (P)

• Phosphorus (P) is typically treated as an impurity rather than an intentional alloying element in most metals, particularly steels, where it is strictly controlled (often <0.04%) due to its detrimental effects on ductility and toughness.

• Corrosion and Segregation Issues: Phosphorus can promote grain boundary corrosion in acidic environments or exacerbate hydrogen-induced cracking. It also increases hot shortness when combined with sulfur, complicating hot rolling.

• Ductility and Toughness Reduction: Phosphorus causes cold shortness by segregating to grain boundaries, forming brittle phosphide phases that embrittle the metal at low temperatures, reducing impact toughness and elongation. This increases susceptibility to cracking during cold working or in service, especially in weld heat-affected zones.

• Minor Strength Enhancement: It provides solid solution strengthening, slightly increasing tensile and yield strength in low-carbon steels without significantly affecting hardness. However, this gain is often outweighed by the loss in toughness, limiting its use in high-strength applications.

• Castability and Machinability: In cast irons (e.g., 0.5-3% P in white or gray irons), phosphorus improves molten fluidity and feeding during casting, reducing defects. It also enhances machinability by forming soft phosphide networks that act as chip breakers, though at the cost of reduced strength.

Sulfur (S)

• Sulfur (S) is generally regarded as an impurity in most metals, particularly steels, where it is tightly controlled (typically <0.03-0.05%) due to its detrimental effects on hot workability and ductility.

• Intentional sulfur additions (0.08-0.35%) are used in free-machining steels and some cast irons to improve machinability. Its impacts are most significant in iron- and steel-based alloys for applications in automotive, toolmaking

• Ductility and Toughness Impairment: Even in small amounts, sulfur inclusions act as stress raisers, reducing elongation, impact toughness, and fatigue resistance, especially in cold conditions. It promotes anisotropic properties in rolled products, increasing transverse brittleness.

Silicon (Si)

• Silicon (Si) is a common alloying element and deoxidizer primarily added to steels, cast irons, and non-ferrous alloys like aluminum and copper to refine microstructure, enhance mechanical properties, and improve resistance to environmental degradation.

• The effects of silicon are highly concentration-dependent (typically 0.1-4% in alloys) and often synergize with elements like manganese or aluminum for balanced performance.

• Strength and Hardness: As a solid solution strengthener, silicon increases tensile strength, yield strength, and elastic modulus without greatly reducing ductility at low levels (0.2-0.8% in structural steels).

• ER70S-6 is a carbon steel welding electrode classified under AWS A5.18, with a specified silicon content of 0.80-1.15% (maximum). Typical values in commercial products are around 0.90-0.95%. This higher silicon level acts as a deoxidizer to reduce dissolved oxygen, minimize porosity, and improve weldability on mildly contaminated or rusty base metals.

Titanium (Ti)

• Titanium (Ti) is a reactive alloying element primarily used as a micro-alloy (0.005-0.1%) in steels, stainless steels, and aluminum alloys to refine microstructure, enhance strength, and improve resistance to corrosion and fatigue.

• Titanium’s effects are highly concentration-dependent and often synergize with elements like nitrogen, vanadium, or aluminum for optimized performance.

• Grain Refinement and Microstructure Control: Titanium forms stable carbides (TiC) and nitrides (TiN) that act as nucleation sites during solidification and heat treatment, inhibiting grain growth and resulting in finer, more uniform microstructures. This boosts toughness, yield strength, and low-temperature impact resistance in HSLA steels while improving weldability by reducing heat-affected zone coarsening.

• Corrosion Resistance: In stainless and duplex steels, titanium stabilizes carbides and prevents chromium depletion, reducing intergranular corrosion and sensitization during welding. It also improves pitting resistance in aggressive environments like chlorides.

• High-Temperature Stability: Titanium enhances creep resistance and oxidation behavior at elevated temperatures (up to 800°C) by dispersing fine precipitates in superalloys and heat-resistant steels, though excessive additions can lead to brittleness.

Tungsten (W)

• Tungsten (W) is a refractory alloying element primarily added to high-speed steels, tool steels, and superalloys to enhance high-temperature performance, hardness, and wear resistance.

• The effects of tungsten are highly concentration-dependent (typically 1-20% in alloys) and often synergize with elements like cobalt or vanadium for optimized performance.

• High-Temperature Strength and Stability: Tungsten significantly improves creep resistance, red hardness (retaining hardness at elevated temperatures up to 600-800 C / 1100-1472 F), and thermal stability by solid solution strengthening and forming stable carbides, making it essential for high-speed steels (e.g., 4-18% W in T1 or M2 grades) and nickel-based superalloys.

• Hardness and Wear Resistance: It boosts overall hardness, tensile strength, and abrasion/erosion resistance through the formation of hard tungsten carbides (WC), which provide excellent cutting and drilling performance in tools, though it can reduce toughness if overused.

Vanadium (V)

• Vanadium (V) is a micro-alloying element primarily added to high-strength low-alloy (HSLA) steels, tool steels, stainless steels, and superalloys to refine microstructure, enhance mechanical strength, and improve resistance to wear and fatigue. It is particularly effective in small amounts (0.01-0.5%).

• The effects of vanadium are highly concentration-dependent and often synergize with elements like nitrogen, titanium, or niobium for optimized results.

• Grain Refinement and Strengthening: Vanadium forms fine, stable carbides (VC) and nitrides (VN) that act as grain boundary pinners during heat treatment, resulting in finer austenite grains and improved yield strength, tensile strength, and toughness. This solid solution strengthening also enhances fatigue resistance in HSLA steels, allowing higher strength at lower carbon levels.

• Hardenability and Temper Resistance: It boosts hardenability for deeper and more uniform hardening during quenching, while increasing resistance to softening during tempering, making tool steels retain hardness at elevated temperatures (up to 500-600 C / 932-1100 F).

• Wear and Corrosion Resistance: Vanadium promotes dispersion of hard precipitates that improve abrasion and erosion resistance in wear-resistant alloys. In stainless steels, it enhances pitting and crevice corrosion resistance, especially in chloride environments, by stabilizing the passive layer.

• High-Temperature Performance: In superalloys and heat-resistant steels, vanadium improves creep strength and oxidation resistance at temperatures exceeding 700 C / 1300 F by refining phases and inhibiting coarsening.

Definitions:

Creep: in the context of metals, refers to the slow and progressive permanent deformation (or plastic flow) of a material under constant mechanical stress, typically at elevated temperatures and stresses below the material's yield strength. This phenomenon can lead to failure over time if not accounted for in design, particularly in high-temperature applications like turbine blades or boilers.