Why pure metal is not enough
Pure metals are generally soft, low in strength and weak against wear. The yield strength of pure iron is around 100 MPa; add carbon and alloying elements to that same iron and the figure can rise above a thousand MPa.
Alloying is not done for strength alone. Corrosion resistance, high-temperature performance, machinability, weldability, magnetic properties and the melting range are all tuned with alloying elements.
The internal structure of alloys
- Solid solution
- Atoms of the alloying element take up places in the crystal lattice of the base metal. If the atomic radii are close a substitutional solid solution forms; if the atom is very small, an interstitial one. In steel, chromium and nickel dissolve substitutionally, carbon and nitrogen interstitially.
- Intermetallic compound
- The elements combine in fixed proportions to form a compound with a crystal structure of its own. The γ′ (Ni₃Al) phase in nickel-based superalloys is the example, and it is the main source of their high-temperature strength.
- Multi-phase structure
- Elements that do not fully dissolve in one another exist as separate phases. The mixture of ferrite and austenite in duplex stainless steels belongs to this group.
What alloying elements do
| Element | Principal effect |
|---|---|
| Chromium (Cr) | Forms a passive oxide film, giving corrosion and oxidation resistance. Stainless behaviour requires at least 10.5% chromium by weight. |
| Nickel (Ni) | Stabilises the austenite phase, increasing toughness and low-temperature strength. |
| Molybdenum (Mo) | Provides strength and creep resistance at high temperature; effective against pitting corrosion. |
| Vanadium (V) | Forms carbides, refining the grain and giving precipitation hardening. |
| Niobium (Nb) | Pins grain boundaries and prevents sensitisation after welding. |
| Titanium (Ti) | Ties up carbon and so prevents chromium carbide formation; contributes to the γ′ phase in superalloys. |
| Silicon (Si) | Acts as a deoxidiser and lowers magnetic losses in electrical steel. |
| Manganese (Mn) | Ties up sulphur, prevents hot shortness and increases hardenability. |
| Boron (B) | In very small amounts, markedly increases grain boundary strength and hardenability. |
The effect of an alloying element is not linear. Its useful range can be narrow; excess addition brings brittleness, unwanted phase formation or loss of machinability.
How alloys are produced
- Melting. The base metal is melted in an electric arc furnace, an induction furnace or a vacuum induction furnace (VIM). For critical alloys, vacuum prevents oxygen and nitrogen pick-up from the atmosphere.
- Alloy addition. The elements are added as ferroalloys or master alloys. The order and temperature of addition follow each element's tendency to oxidise.
- Secondary metallurgy. The composition is brought to its final state in a ladle furnace, with deoxidation, desulphurisation and inclusion modification.
- Remelting. In critical applications such as aerospace, cleanliness and homogeneity are improved by VAR (vacuum arc remelting) or ESR (electroslag remelting).
Why master alloys are used
Adding an element to a melt in its pure form is often inefficient. Molybdenum melts at 2620 °C; dropping pure molybdenum into a steel bath means a long dissolution time and uneven distribution. Instead the element is added pre-alloyed with iron or nickel.
- The melting point drops and dissolution is faster.
- Burn-off loss falls, so the target composition is reached at higher yield.
- The element distributes homogeneously through the bath, reducing the risk of segregation.
- Volatile elements such as magnesium can be added in a controlled way.
Iron-based ones are called ferroalloys; nickel- or aluminium-based ones are called master alloys. Nickel-boron, nickel-magnesium and nickel-hafnium are common examples of the latter group.
Buying alloys
- A standard number alone is not enough; write the lower and upper limits of the critical elements.
- State the impurity limits (S, P, O, N) separately; the standard may leave them wide.
- Ask for the form and particle size that suit your addition method.
- Require a certificate of analysis for every lot and, where needed, an EN 10204 3.1 inspection certificate.
Frequently asked questions
What is an alloy?
An alloy is a metallic material formed by two or more elements combining, at least one of which is a metal. Almost every metal used in industry is an alloy rather than a pure metal; alloying is done to tune strength, corrosion resistance, high-temperature performance and machinability.
What is the difference between an alloy and a mixture?
In an alloy the elements combine at atomic level: a solid solution, an intermetallic compound or a multi-phase structure forms, and the result is a single material with properties of its own. In a mixture the components merely sit side by side and keep their own properties.
What does master alloy mean?
It is an alloying element supplied pre-alloyed with iron, nickel or aluminium. Using a master alloy instead of adding the element to the melt in pure form lowers the melting point, speeds up dissolution, reduces burn-off loss and gives homogeneous distribution through the bath.
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