Periodic Reporting for period 2 - TheSBIE (Thermodynamic Stabilization by Interface Engineering)
Okres sprawozdawczy: 2019-03-01 do 2020-02-29
This GF project concerns nano-crystalline (NC) metallic alloys, unique materials having extremely small crystals (grains)
which exhibit significantly improved mechanical properties over their conventional coarse-grained counterparts. Yet their
inherently-large fraction of internal interfaces (grain boundaries, GBs), associated with excess energy, leads to coarsening of
their structure at elevated temperatures during either fabrication, processing or service life. This results in a rapid
deterioration of their properties, rendering them unsuitable for many applications. Compared with conventional, kinetic
stabilization of NC alloys, which is limited and temporary in nature, the approach proposed here is of ‘Thermodynamic
Stabilization by Interface Engineering’ employing solute segregation: alloying with elements which preferentially migrate to
GBs to substantially reduce their excess energy, leading to a stable, tunable nano-scale grain size even at high
temperatures. Employing a thermodynamic approach for engineering the structure and chemistry of interfaces in these
materials stands a good chance of overcoming their fundamental stability hurdle with nature’s blessing. The main materials
to be studied are iron-based alloys. In particular, NC iron-magnesium alloys have the potential for exceptional absolute and
specific strength, exceeding that of the hardest steels. Experiments will be combined with mesoscale and atomistic
simulations of thermodynamic, kinetic and mechanical properties. This international interdisciplinary research involves MIT
(USA), Technion (Israel) and WWU (Germany), bridges physical metallurgy, nanotechnology and interface science. It will
result in a deeper fundamental understanding of energetics and kinetics in NC alloys; tools for designing stable NC alloys
with tailored mechanical properties; and commercialization of successful alloys. It shall thus strengthen the EU "metallurgical
infrastructure" according to the EC’s Metallurgy Road Map.
The conclusions of this action are that thermodynamic stabilization of nanocrystalline alloys is indeed possible and scientifically-sound. However, in many cases, several stabilization mechanisms are at play and it may be difficult to distinguish between their individual contributions. We were able to prove both points by employing two unique alloy systems - Fe-Mg and Fe-Au.
In addition, not only are the alloys we developed thermally stable against grain growth (retaining a grain growth of about 100nm or less at elevated temperatures), but they can also be sintered from powders to dense material which can then be used to make parts in industry.
which exhibit significantly improved mechanical properties over their conventional coarse-grained counterparts. Yet their
inherently-large fraction of internal interfaces (grain boundaries, GBs), associated with excess energy, leads to coarsening of
their structure at elevated temperatures during either fabrication, processing or service life. This results in a rapid
deterioration of their properties, rendering them unsuitable for many applications. Compared with conventional, kinetic
stabilization of NC alloys, which is limited and temporary in nature, the approach proposed here is of ‘Thermodynamic
Stabilization by Interface Engineering’ employing solute segregation: alloying with elements which preferentially migrate to
GBs to substantially reduce their excess energy, leading to a stable, tunable nano-scale grain size even at high
temperatures. Employing a thermodynamic approach for engineering the structure and chemistry of interfaces in these
materials stands a good chance of overcoming their fundamental stability hurdle with nature’s blessing. The main materials
to be studied are iron-based alloys. In particular, NC iron-magnesium alloys have the potential for exceptional absolute and
specific strength, exceeding that of the hardest steels. Experiments will be combined with mesoscale and atomistic
simulations of thermodynamic, kinetic and mechanical properties. This international interdisciplinary research involves MIT
(USA), Technion (Israel) and WWU (Germany), bridges physical metallurgy, nanotechnology and interface science. It will
result in a deeper fundamental understanding of energetics and kinetics in NC alloys; tools for designing stable NC alloys
with tailored mechanical properties; and commercialization of successful alloys. It shall thus strengthen the EU "metallurgical
infrastructure" according to the EC’s Metallurgy Road Map.
The conclusions of this action are that thermodynamic stabilization of nanocrystalline alloys is indeed possible and scientifically-sound. However, in many cases, several stabilization mechanisms are at play and it may be difficult to distinguish between their individual contributions. We were able to prove both points by employing two unique alloy systems - Fe-Mg and Fe-Au.
In addition, not only are the alloys we developed thermally stable against grain growth (retaining a grain growth of about 100nm or less at elevated temperatures), but they can also be sintered from powders to dense material which can then be used to make parts in industry.
The work performed during the project thus far, includes the design, preparation, processing and investigation of Iron-based nanocrystalline alloys in powder form. In particular, we have found several alloying candidates for Iron that should lead to a stable nanostructure, and have then narrowed-down this list based on practical considerations, such as safety of use, cost, etc. Ultimately we have studied two primary alloys - Iron-Magnesium (Fe-Mg) and Iron-Gold (Fe-Au). We studied the alloying process, performed by ball milling, as well as the consequent thermal stability using various microscopy methods, to observe the nanostructure of the material, as it evolves with time and temperature. We identified the stabilizing mechanism(s) in each case, and have optimized the alloy composition. We then moved on to study the sintering (agglomeration) of the powders into a dense, nanocrystalline metal part. For Fe-Mg, we have added Chromium, creating a ternary Fe-Mg-Cr alloy with excellent thermal stability, and good sintering characteristics. In the case of Fe-Au, we found that, as expected, this alloy is highly sinterable without any further additions. A central results of our work is that we were able, for the first time, to design and prepare a nanocrystalline alloy which decreases its grain size to a lower (but stable) value, with an increase in temperature. This results side-steps a basic paradigm of materials science: "grains always grow with increased thermal exposure". This unique result has been published in the prestigious journal, Physical Review Letters (PRL). One final results we obtained was that, again, contrary to common thinking, mechanical alloying during ball milling does not always result in a homogenous alloy - it is quite possible to attain a nanocrystalline alloy with grain boundary segregation already in the as-milled state. This is a unique finding which we explained by studying the competition between ballistic and thermal diffusion, and published in the journal Scripta Materialia.
* We have studied in detail and identified, for the first time, the interplay between the two commonly accepted stabilization mechanisms in nanocrystalline alloys: thermodynamic and kinetic stabilization.
* We have designed and prepared a nanocrystalline alloy which refines its grain size with increased temperature by harnessing the benefits a structural phase transformation in Iron. This is a significant advancement in the field, much beyond the state of the art.
* We expect to achieve full- or nearly-full density nanocrystalline alloys at the end of the project.
* The wider implications of these results are that these alloys could replace current metal parts while being both lighter and stronger. This, in turn, has a substantial beneficial effect on the ecological footprint of heavy industries (e.g. auto, aero-space, electronics). These new alloys, some of which have been previously developed at MIT, are already in use in commercial applications.
* We have designed and prepared a nanocrystalline alloy which refines its grain size with increased temperature by harnessing the benefits a structural phase transformation in Iron. This is a significant advancement in the field, much beyond the state of the art.
* We expect to achieve full- or nearly-full density nanocrystalline alloys at the end of the project.
* The wider implications of these results are that these alloys could replace current metal parts while being both lighter and stronger. This, in turn, has a substantial beneficial effect on the ecological footprint of heavy industries (e.g. auto, aero-space, electronics). These new alloys, some of which have been previously developed at MIT, are already in use in commercial applications.