Nano twins make super strong copper and stainless steel
A Chinese research group has succeeded in making copper more than 10 times as strong as it is normally, without adversely affecting its electrical conductivity and a popular grade of stainless steel up to five times stronger, by commercially viable manufacturing techniques.
The trick is to produce 'twins' which are areas of crystalline structure within metal grains that are oriented slightly differently to each other. Dislocations – imperfections within the crystal structure - have difficulty traversing the boundaries of the twins, making the metal stronger, but not so much difficulty that the material becomes brittle.
The breakthrough achieved by Professor K Lu and his colleagues at the Shenyang National Laboratory for Materials Science is in discovering viable methods of producing such effects on a 15nm or smaller scale in usable pieces of metal.
The team started by using pulsed electrodeposition to produce foils up to 30µm thick. However, in the course of giving the Kelly lecture to the Department of Materials Science at Metallurgy at Cambridge on June 15th 2010, he revealed that the team had moved on to producing the effect by dropping a hammer onto copper samples in liquid nitrogen from a height of 7m. The top strength performance achieved so far is a yield stress of more than 600MPa, as compared with 70MPa for conventional annealed conductivity copper, while retaining 97% of the electrical conductivity.
Prof Lu said that Chinese electronic companies were eagerly interested, presumably in order to make connector pins that are difficult to become bent. He then went on to say that the team had applied the same type of forming technique to AISI 316L stainless steel. The low temperature forming process in this case produces nano sized grains as well as nano twins, which make the material brittle, but this problem can be overcome by annealing the metal in such a way that the grains recrystallise to larger size, without making the twins thicken significantly. If this is done, ultimate tensile strength can be raised to 900 to 1400 MPa instead of 260 MPa. Prof Lu's latest work is to apply the same methodology to iron 25% manganese steel.
The Kelly lecture is named after Professor Tony Kelly, widely considered to be the father of all modern fibre reinforced composites.
