We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals.

https://science.sciencemag.org/content/sci/301/5638/1357.full.pdf

A Maximum in the Strength of Nanocrystalline Copper

The recent development in the field of superhard materials with Vickers hardness of ⩾40 GPa is reviewed. Two basic approaches are outlined including the intrinsic superhard materials, such as diamond, cubic boron nitride, C3N4, carbonitrides, etc. and extrinsic, nanostructured materials for which superhardness is achieved by an appropriate design of their microstructure. The theoretically predicted high hardness of C3N4 has not been experimentally documented so far. Ceramics made of cubic boron nitride prepared at high pressure and temperature find many applications whereas thin films prepared by activated deposition from the gas phase are still in the stage of fundamental development. The greatest progress has been achieved in the field of nanostructured materials including superlattices and nanocomposites where superhardness of ⩾50 GPa was reported for several systems. More recently, nc-TiN/SiNx nanocomposites with hardness of 105 GPa were prepared, reaching the hardness of diamond. The principles of de...

https://mediatum.ub.tum.de/doc/958917/file.pdf

The search for novel, superhard materials

Strain-heat methods of obtaining ultrafine-grained (UFG) metallic materials with grain sizes as small as 20 nm and peculiarities of their structure are considered. It is shown that intercrystalline boundaries are the main element of the structure of UFG materials and that they are typically in a non-equilibrium state. The formation of a special grain boundary phase, i.e. a thin near-boundary layer with high dynamic activity of atoms, has been found. This unusual structure leads to the manifestation of promising new elastic, strength, superplastic, damping and magnetic properties of UFG materials.

Structure and properties of ultrafine-grained materials produced by severe plastic deformation

An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

Intermetallics: Why is it so difficult to introduce them in gas turbine engines?

Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Influence of grain size on the mechanical behaviour of some high strength materials

A titanium-based intermetallic alloy having a high yield stress, a high creep resistance and sufficient ductility at ambient temperature has the following chemical composition as measured in atomic percentages: Al, from 16 to 26; Nb, from 18 to 28; Mo, from 0 to 2; Si, from 0 to 0.8; Ta, from 0 to 2; Zr, from 0 to 2; and Ti as the balance to 100; with the condition that Mo+Si+Zr+Ta>0.4%. Production, working and heat-treatment ranges adapted to the intended use of the material are also defined.

Titanium-based intermetallic alloys

A titanium-aluminum-niobium intermetallic phase alloy, which contains molybdenum, silicon, tantalum and/or zirconium and which has the composition given below, is new. A titanium based intermetallic phase alloy has the composition 16-26 at.% Al, 18-28 at.% Nb, 0-2 at.% Mo, 0-0.8 at.% Si, 0-2 at.% Ta, 0-2 at.% Zr and balance Ti, the sum of Mo + Si + Zr + Ta being greater than 0.4 at.%.

Alain Lasalmonie

Papers

Intermetallics: Why is it so difficult to introduce them in gas turbine engines?

Influence of grain size on the mechanical behaviour of some high strength materials

Titanium-based intermetallic alloys

Titanium-based intermetallic alloys of the Ti2AlNb type with high yield strength and good creep resistance

Intermetallic alloys based on titanium from Ti2AlNb type of high yield strength and good creep resistance