Ductility

A type of plasticity, which refers to the ease with which a material can be stretched. The most common way to show this is to pull a bar or plate specimen of a fixed cross-sectional shape in the axial direction at a constant speed, and show the amount of elongation after fracture. Pure metals are generally highly ductile. This is because metal crystals (the order of atomic arrangement) have a close-packed structure, and there are many atomic planes that are easy to slide on. Even metals with high ductility have an elongation rate of about 40% or less, but some iron and titanium alloys can show elongation rates of over 100%, which are called superductile or superplastic. This is because these alloys allotropically transform into a different harder crystal above a certain stress, making it difficult for local contraction to occur before fracture. Some ceramics that undergo allotropic transformation also show superplasticity at high temperatures. In materials that undergo such processing-induced transformation, if the stress dependence of the transformation is nearly reversible, they show superelasticity, returning to their original length after unloading, and if it is irreversible, they show shape memory, returning to their original shape when reheated. There are also amorphous polymeric materials that exhibit similar properties such as localized superductility, shape memory, and superelasticity (non-rebound), but these abnormal properties are due to the arrangement of the molecular chains.

[Hajime Sudo]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

塑性の一種で、材料の延ばしやすさをいう。これを示すもっとも一般的な方法は、断面形状一定の丸棒や板の試験片を軸方向に一定速度で引っ張り、破壊後の延び量で示すことである。純金属は一般に延性が大きい。これは金属の結晶（原子配列の規則性）が密充填(じゅうてん)構造で、すべりやすい原子面が多いことによる。延性が高い金属でもその伸び率はおよそ40％以下であるが、鉄やチタンの合金には100％以上の延び率を示すものもあり、これを超延性もしくは超塑性という。これらの合金では、ある応力以上でより硬い別の結晶に同素変態して、破断の前におこる局部収縮がおこりがたくなるためである。同素変態するセラミックスにも高温で超塑性を示すものがある。このような加工誘起変態をおこす物質で、その変態の応力依存性が可逆的に近い場合は除荷後に元の長さに戻る超弾性を示し、不可逆的なものは再加熱によって元の形状に戻る形状記憶性を示す。非結晶質の高分子材料でも局部超延性、形状記憶、超弾性（無反発）等の類似の性質を示す材料があるが、これらにおける異常性質は分子鎖の配列状態に起因する。

［須藤　一］

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