Atomistic studies of dislocation-interface interactions in lamellar Ti-Al

高温合金的冶金在技术和基础层面上都提出了挑战。在高温下用于涡轮机的合金必须在长时间尺度内抵抗高应力以及有毒和腐蚀性气体的高流速。此外，它们经常出现在关键应用中，其中故障可能导致人类灾难。这是因为除了在发电方面的应用外，高温合金现在也被用于民用和军用喷气发动机的高温区。高温合金镍铝化物于1941年由蒙德镍公司发明并首次生产。它仍然是该行业的主力，尽管广泛寻找具有相同机械和化学性能的轻质合金。困难显然在于需要同时满足最严格的标准：高温抗氧化性和抗硫化性;高温强度和抗蠕变性;以及最后的低温延展性。后一项要求基本上是制造条件，而不是服务条件。除非金属具有至少百分之几的室温延展性，否则在制造和安装中很难使用。一个组件甚至可能无法生存下降到混凝土地板上！铝化钛最近已成为有前途的新的候选人，以取代优良的，但沉重的γ/γ '镍基高温合金。我们的工作是研究它们在室温下的塑性和延展性。虽然整体式TiAl金属间化合物非常脆，但通过某些热处理产生的显著合金显示出至少在百分之几水平的延展性。这种合金，称为（多合成孪晶）PST-TiAl，含有具有两相层（层状）微结构的晶粒（晶体），这些相之一（γ-TiAl）本身被分成具有不同晶体取向的层。这种三明治结构是稳定的，现在是世界范围内学术界和工业界的热门研究对象。我们在计算机中收集大量原子，通过某种原子间力模型（这可能是完全量子力学的，或者经典的球和弹簧，或者介于两者之间的任何东西）结合在一起。然后，我们观察它们的行为，因为它们在原子间力的影响下放松（分子静力学）或在一定温度下遵循牛顿运动方程（分子动力学）。在这里，我们做特别困难的模拟，即晶体位错。这些是二维晶体缺陷，金属中滑移（塑性变形）的载体。由于这些缺陷的复杂性，与纯金属相比，这些缺陷在合金中被大大放大;并且由于相关的长期应力的性质，因此这种模拟是苛刻的。成功模拟的回报是非常巨大的。我们可以在计算机中直接观察位错的运动。我们可以通过完美的晶体来跟踪它们的进展，当它们相互作用并可能穿透层状边界时。我们将了解滑移是如何在层间和层内传递的，并揭示PST钛铝化物具有韧性的秘密。我们建议使用原子模拟的结果来构建一个多堆模型来预测行为的顺序上的晶粒的大小在一个长度尺度。这被称为弥合长度尺度差距。有了这个模型，我们就可以提高延展性，我们就可以向实际的合金设计者提出这样的建议。进一步阅读：Alloys by Design，Tony Paxton，Physics World，vol 5，No 11，p35（1992年11月）合金设计中的电子理论，由D G Pettifor和A H Cottrell编辑，（金属研究所，1992）凝聚态物质中的原子间力，Mike Finnis，（OUP 2003）

The metallurgy of high temperature alloys presents challenges both at a technical and at a fundamental level. Alloys that are used in turbines at high temperature have to resist high stresses over prolonged timescales and high flow rates of noxious and corrosive gases. Moreover they are often found in critical applications where failure may lead to human distasters. This is because as well as finding applications in electricity generation, high temperature alloys are now found in the high temperature zones of jet engines, both civil and military.The superalloy nickel aluminide was invented and first produced by the Mond Nickel Company in 1941. It is still the workhorse of the industry despite extensive searches for lighter alloys having the same mechanical and chemical properties. The difficulty clearly resides in the need to meet the most stringent criteria simultaneously: high temperature oxidation and sulphidation resistance; high temperature strength and creep resistance; and finally low temperature ductility. The latter requirement is essentially a manufacturing, not a service condition. Unless a metal has at least a few percent room temperature ductility it's just too difficult to work with in manufacture and installation. A component may not even survive being dropped on a concrete floor!Titanium aluminides have recently emerged as promising new candidates to replace the excellent, but weighty gamma / gamma-prime nickel based superalloys. Our proposed work is concerned with their plasticity and ductility at room temperature. Although monolithic TiAl intermetallic is very brittle, a remarkable alloy produced by certain heat treatments shows ductility at least at the few percent level. This alloy, called (polysynthetically twinned) PST-TiAl contains grains (crystals) with a microstructure of layers ( lamellae ) of two phases, one of these phases (gamma-TiAl) being itself separated into layers having different crystal orientations. This sandwich structure is thermodynamically stable and is now the object of intense study in academia and industry worldwide.Our remit is atomistic simulation. We take large collections of atoms in a computer, bonded together by some model of interatomic forces (this may be fully quantum mechanical, or a classical ball and spring, or anything in between). Then we observe their behaviour as they relax under the influence of interatomic forces (molecular statics) or move at some temperature following Newton's equation of motion (molecular dynamics). Here we do particularly difficult simulations, namely of crystal dislocations. These are two dimensional crystal defects, the carriers of slip (plastic deformation) in metals. Such simulations are demanding because of the complexity of these defects, greatly amplified in alloys compared to pure metals; and because of the nature of the associated long ranged stresses. The rewards of a successful simulation are very great. We may view directly in the computer the motions of dislocations. We may follow their progress through perfect crystal and as they interact and possibly penetrate the lamellar boundaries. We will learn how slip is transmitted within and between lamellae and uncover the secret of why the PST titanium aluminide is ductile. We propose to use results of atomistic simulations to construct a multiple pile up model to predict behaviour at a length scale on the order of the size of a grain. This is called bridging the length scale gap. Armed with this model one should then be able to improve ductility and we shall be able to make such suggestions to the practical alloy designers.Further reading: Alloys by Design, Tony Paxton, Physics World, vol 5, No 11, p 35(Nov 1992) Electron Theory in Alloy Design, Edited by D G Pettifor andA H Cottrell, (Institute of Metals, 1992) Interatomic Forces in Condensed Matter, Mike Finnis, (OUP 2003)