DTD 683
Just to set the record straight about the issues with the UK aluminium alloy DTD 683. This alloy was a refinement of an earlier alloy (DTD 363) and was a member of the 7xxx alloys (as designated today) and contained the major alloying elements zinc, copper and magnesium. The alloy was developed by High Duty Alloys, based in Slough where they had a large research and development centre. It was first shown to the public around 1937 and was given the trade name Hiduminium RR77. This alloy suffered from poor resistance to stress corrosion cracking (SCC) and was particularly prone to SCC as a result of quenched-in residual stresses. DTD 683 is strengthened by precipitation hardening - the formation of nano-meter sized intermetallic particles in the aluminium matrix by a combination of high temperature heat treatment, quenching and then ageing to form the precipitates. After quenching and ageing the residual stresses in the component were sufficient to drive SCC, hence the unused spars developing cracks during storage. It was this, rather than fatigue, that caused the failure of DTD 683 as used in the Valiant, although fatigue stresses in service obviously didn't help.
Note the USA developed a similar alloy, today known as AA7075, which contains a small chromium addition and Alcoa in America used this alloy successfully in the B29 Superfortress, amongst others. High Duty Alloys did not like the use of chromium as it tended to make the alloy less strong unless given a severe quench, which of course, increased the residual stresses. The UK alloy underwent various changes to composition and process, including chromium containing variants and boiling water quenching, but was ultimately dropped from use when over-aged tempers were developed which reduced the susceptibility of the American alloy to SCC, but did not work with the UK alloy due to the lower copper level used.
I understand the R&D effort of High Duty Alloys was led by one William Doyle who was apparently a very "colourful" gentleman in his time.