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

高温合金的冶金在技术和基础上都面临着挑战。用于高温涡轮机的合金必须在长时间内承受高应力和高流量的有毒和腐蚀性气体。此外，它们经常出现在故障可能导致人类灾难的关键应用程序中。这是因为除了在发电方面的应用之外，高温合金现在也被发现用于民用和军用喷气发动机的高温区域。高温合金镍铝化物是由蒙德镍公司于1941年发明并首次生产的。尽管人们广泛寻找具有相同机械和化学性能的更轻的合金，但它仍然是该行业的主力。显然，困难在于需要同时满足最严格的标准：耐高温氧化和抗硫化；高温强度和抗蠕变；最后是低温延展性。后一种要求本质上是制造条件，而不是使用条件。除非金属具有至少几个百分点的室温延展性，否则在制造和安装中很难使用。一个组件甚至可能被扔到混凝土地板上而无法存活！近年来，钛铝化物作为有前途的新候选材料取代了性能优异但重量较大的γ / γ素基镍基高温合金。我们提出的工作是关注它们在室温下的塑性和延性。虽然整体TiAl金属间化合物非常脆，但通过某些热处理产生的一种显著的合金至少在几个百分点的水平上显示出延展性。这种合金被称为（多合成孪晶）PST-TiAl，它包含的晶粒（晶体）具有两相的微观结构，其中一相（γ - tial）本身被分成具有不同晶体取向的层。这种夹层结构具有热稳定性，是目前国内外学术界和工业界研究的热点。我们的研究范围是原子模拟。我们在计算机中收集大量的原子，通过某种原子间相互作用的模型连接在一起（这可能是完全的量子力学，或者一个经典的球和弹簧，或者介于两者之间的任何东西）。然后我们观察它们在原子间作用力（分子静力学）的影响下放松或在牛顿运动方程（分子动力学）下在一定温度下运动时的行为。这里我们做了特别困难的模拟，即晶体位错。这些是二维晶体缺陷，是金属滑移（塑性变形）的载体。由于这些缺陷的复杂性，这种模拟要求很高，与纯金属相比，合金中的缺陷大大放大；而且由于相关的长期压力的性质。成功模拟的回报是非常大的。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跟踪它们的进展，当它们相互作用并可能穿透片层边界时。我们将学习滑动是如何在片层内部和片层之间传递的，并揭开PST铝化钛具有延展性的秘密。我们建议使用原子模拟的结果来构建一个多堆模型，以预测颗粒大小的长度尺度上的行为。这就是所谓的弥合长度差距。有了这个模型，我们应该能够提高延展性，我们将能够向实际的合金设计师提出这样的建议。进一步阅读：合金设计，Tony Paxton，物理世界，第5卷，第11期，第35页（1992年11月）。合金设计中的电子理论，由D G Pettifor和H Cottrell编辑，（金属研究所，1992年）。

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)