Engineering Alloys: Challenges for the Future a Lecture by Prof. Barbara Shollock
Engineering Alloys: Challenges for the Future, 3rd November, 2015
Oh the joy! For once the audience at a lecture of the local affiliated branch of the UK professional society for materials (that's the Institute of Materials, Minerals and Mining to give its full title) listened to a lecturer who uttered the words "yield stress", "creep strength" and "fatigue response" within minutes of starting her address. This was the second lecture of the 2015-16 session and it was delivered by Professor Barbara Shollock who holds the Tata Steel Research Chair in Advanced Characterisation and Coatings at the University of Warwick. First, she outlined the three stages of her talk, namely 1 what do we expect of engineering alloys, 2 current challenges and 3 future opportunities. That's when she introduced the metallurgical terms mentioned above as primary factors in defining what we expect of engineering alloys. She then went on to review the secondary factors of corrosion and oxidation resistance which so often limit performance in service. Using turbine discs in aircraft engines as her example she argued that high temperature oxidation not only degrades the product, but the stresses induced by oxidation affect the overall process. Carefully avoiding any mention of alloy composition beyond acknowledging its nickel base, Professor Shollock outlined the importance of grain size in determining material properties and having provided a general introduction to her subject then went on to give an insight into her own research. A slide of a Focused Ion Beam (FIB) microscope showed it to be superficially similar to a conventional electron microscope, but with a stream of ions in place of electrons. Professor Shollock's unit is linked to a mass spectrometer which provides compositional data and comparing FIB and SEM images of the same areas of oxidised metal provides a wealth of information. Static oxidation, oxidation under bending loads and oxidation during bend testing were all investigated with some samples initially subjected to shot peening whilst the surfaces of others were unadulterated.
Now your reporter is not going to pretend that he followed the detail of Professor Shollock's subsequent revelations, for much of what she had to say was illustrated by pairs of FIB and SEM photographs that were of necessity shown briefly, but it was apparent that a number of pre-existing ideas are challenged. For example, using an isotopic tracer it is possible to distinguish between "old" and "new" oxide, the latter following on from rupture of the primary oxide layer. Also, inwards diffusion of oxygen explains the greater damage caused by surface shot peening whilst oxide inclusions are surprisingly sometimes seen ahead of the tip of a fatigue crack. Finally, very small amounts of lanthanum (~30ppm ) can inhibit oxidation and thereby influence the materials' mechanical properties.
What are the implications for this novel approach to high temperature oxidation? Obviously FIB is a valuable new tool for research in this area. More speculatively, it might be possible to exploit surface changes, both compositional and mechanical, to influence alloy microstructure and improve turbine blade performance in service.
One thing is certain. For the old-fashioned metallurgists in the audience this was a thoroughly enjoyable evening in the familiar territory of physical metallurgy even if the technology utilised was outwith their immediate experience.