金属晶体Metallic crystal

金属键没有方向性，金属原子倾向于高配位的密堆积：面心立方（Cu、Al、Au）、六方密堆积（Mg、Zn、Ti）或体心立方（Fe、Na、W）。自由电子在电场中定向移动产生导电性，并能高效传导热量；自由电子吸收和反射各种波长的可见光，使金属不透明而有光泽。受力时原子层可以相对滑移而金属键不被破坏，因此金属有延展性，面心立方金属滑移系最多、延展性最好。

Metallic bonding is non-directional, so metals favour high-coordination close packing: fcc (Cu, Al, Au), hcp (Mg, Zn, Ti) or bcc (Fe, Na, W). Free electrons drift in an electric field to carry current and transport heat efficiently; they absorb and re-emit visible light of all wavelengths, making metals opaque and lustrous. Under stress, layers of atoms slip past one another without breaking the metallic bond, so metals are ductile, and fcc metals, with the most slip systems, are the most ductile.

1900 年德鲁德提出自由电子气模型，成功解释了欧姆定律和维德曼-弗兰兹定律。1928 年索末菲用量子统计修正了这一模型。同年布洛赫证明周期势场中电子波函数呈布洛赫波，能带理论由此建立，解释了导体、半导体和绝缘体的区别。

In 1900 Drude proposed the free-electron gas model, explaining Ohm's law and the Wiedemann–Franz law. Sommerfeld corrected it with quantum statistics in 1928, and the same year Bloch showed that electrons in a periodic potential have Bloch-wave functions, founding band theory, which explains the difference between conductors, semiconductors and insulators.

实际金属的强度远低于理想晶体的理论值，原因是存在位错——1934 年泰勒、奥罗万和波拉尼分别提出位错理论。冷加工使位错增殖、相互缠结，金属变硬（加工硬化）；退火使其恢复。合金化、细化晶粒和析出强化都通过阻碍位错运动来提高强度。

Real metals are far weaker than the theoretical strength of a perfect crystal because they contain dislocations, a theory proposed independently by Taylor, Orowan and Polanyi in 1934. Cold working multiplies and tangles dislocations, hardening the metal (work hardening), and annealing reverses it. Alloying, grain refinement and precipitation hardening all strengthen metals by hindering dislocation motion.