DUCTILITY

A ductile material is one that is considered capable of being easily DRAWN OUT, for example as COLD DRAWN WIRE. Metals and alloys that lack ductility tend to WORK HARDEN very quickly and therefore crack or fracture easily when drawn through DIES. Exceptionally ductile metals (GOLD and SILVER for example), can undergo severe drawing before WORK HARDENING (to the point where ANNEALING is necessary). Other metals and alloys with good ductile properties include WROUGHT IRON, COPPER, ALUMINIUM (higher purity), and some BRASSES. Whilst LEAD would suggest ductile properties, structural weakness makes it difficult to draw without breaking and it is not considered an especially ductile metal.

TOUGHNESS

Also known as IMPACT RESISTANCE, toughness is perhaps best described as a combination of STRENGTH and DUCTILITY. Good strength and ductility help a metal to withstand the shock of a heavy impact. Unsurprisingly this quality is tested and measured by an instrument that strikes a metal test sample with known force (IZOD and CHAPRY IMPACT TESTS for example), the result of these tests is expressed in JOULES. CAST IRON is typically a very strong material, however manner in which CARBON (in the form of GRAPHITE) is distributed through it’s metallurgical structure as FLAKES (especially in GREY CAST IRON); means that many cast irons have a poor ability to resist shock impact, easily shattering under force. Unlike most traditional cast iron materials, DUCTILE CAST IRON is formulated to significantly improve DUCTILITY (by encouraging the formation of graphite NODES instead of flakes). This makes ductile cast iron a significantly tougher material than most traditional cast iron products. By these measures, most MILD (LCS) STEELS can be described as tough, though this quality can vary as the CARBON content in the alloy increases (as in LOW ALLOY STEELS), and ductility decreases.

MALLEABILITY

Together with ductile qualities, malleability tends to be a characteristic found in high purity metals with GOLD and SILVER being the most malleable. Instead of DRAWING, the ability of a material to be HAMMERED determines it’s malleability. Most of us are familiar with the ability of fine gold and silver to be worked into a LEAF sheet as thin as 0.0001mm (see also GILDING), this degree of processing is made possible by the exceptional malleability of these metals. WORK HARDENING can occur when malleable metals are severely COLD WORKED, therefore ANNEALING or NORMALISATION of a design may need to be carried out to prevent cracking. REPOUSSÉ working fine gold, silver and copper is a technique that relies heavily on the ability of these metals to be hammered out with relative ease, other metals considered to have excellent malleability include ALUMINIUM (high purity), TIN and LEAD. Unlike cast iron, WROUGHT IRON is considered a very malleable material.

CREEP

Creep is the tendency for a metal to distort over a period of time. This is can be demonstrated by suspending a LEAD pipe between two points some distance apart. Over time it likely that the pipe would start to sag in the middle. A metal like LEAD is especially prone to this because it’s RE-CRYSTALLISATION temperature is lower than ambient room temperatures, this means that the metal is more or less in a permanently soft or ANNEALED condition. Creep is less evident in metals with high TENSILE STRENGTHS, though even these can be subject to creep if inadequately supported. In extreme cases creep can lead to PLASTIC DEFORMATION in the affected design which could eventually cause it to structurally fail.

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