Fred Starr recollects… supersonic progress

Materials World magazine
,
1 Oct 2017

Mouldering away in the attic is my final year thesis. Something, as the UK police might say, proves I have ‘form’ with the Concord project. As my recent letters to Materials World have shown, I confess to having spent 50 years refining my arguments, and am happy to meet anyone, anytime, to debate whether or not Concorde was an engineering failure. 

Roots of my opposition were actually metallurgical, the title of my thesis being Titanium for the Supersonic Transport. Unlike Concorde’s aluminium alloys, which were on the borderline for creep, I took it as read that titanium would withstand the temperature regimes of supersonic flight. Indeed, the existence of the all-titanium Lockheed SR71 Blackbird, the world’s fastest plane, had been announced early in 1966, just before I embarked on the thesis. 

The subject was my personal choice, in contrast to previous years, where students had been told to review a specific topic. But I can only remember two discussions with my supervisor. When I first went to see him, he told me, ‘Pity you are not doing something on high strength steels’. On handing in the completed effort, he then refined his opinion to ‘I really wished you had done something on high-strength steels’. 

That was the last I heard from someone who spent much of his lecture time on the Massive Phase - an aspect of the physical metallurgy of steel even Professor Google has forgotten about. 

Google wasn’t around fifty years ago, and when I plunged into the subject, I found it pretty confusing. The only clear thing was that the workhorse of the titanium alloys, the alpha-beta, Ti-6Al-4V, formulation didn’t have the strength for flight at Mach 3. 

American researchers were pushing the beta phase alloys, and the alpha alloys seemed to be becoming the also-rans. I struggled with the vast range of heat treatments, some designed to modify the grain structure, others to induce age hardening. There were even titanium martensites to worry about.

What was being published was brand new, and probably not properly understood by the authors. Certainly not for publication were the hiccups in manufacture and application of these new materials. 

We now know that the SR71 Blackbird was made from the complex beta alloy, B-120 VRC (Ti-13V-11Cr-3Al). It had never been produced in sheet form. The designer of the Blackbird has since admitted that first samples were so brittle, they broke when they fell off his desk. It was a matter of faith, and more money, to go on, he said.

It was becoming the same with me. I did the best I could with what I read, and asked a good friend, Jack Roberts, if he would look through the thesis before I handed it in. Jack wasn’t exactly overwhelmed, but said the interesting bit was right at the end. 

There, I suggested, the big issue for titanium was cost, which could wreck the economics of a supersonic transport. Titanium was being extracted in the same crude way as aluminium, using sodium to displace the metal from the appropriate salt. It led to aluminium once being priced above gold. This now vital metal only came into common use with the invention of the Hall-Heroult electrolytic process. What titanium needed, I wrote, was a Hall-Heroult analogue. I kept this thought to myself ever afterwards.  

Amazingly, a few months back, I met Professors Derek Fray and George Chan, two of the inventors of the FFC Cambridge process, which does the business electrolytically, using molten CaCl2, but in a far more sophisticated way than I could ever have imagined (read more about this process in MW Nov 2009). I met them at the Molten Salts Discussion Group, where I had been inveigled into giving the after dinner speech. The technology depends on some peculiar characteristics of the titanium oxides, discovered by Fray in the course of his blue skies research. But the FFC process is no laboratory-based effort, having passed beyond the ill-fated commercial venture stage, with a British company, Metalysis (read more on their work on alloys in MW Sep 2017), halving the cost of titanium. Also promised is the production of titanium alloys, directly, electrolytically. Are these not supersonic developments of a different kind? 

Watch a video interview with Derek Fray at bit.ly/2wU1IM3