Thermally Stable Nanocrystalline Nickel Alloys by Powder Metallurgy

Nanocrystalline (NC) materials have many enhanced room-temperature mechanical properties compared to their corresponding microcrystalline counterparts, including strength, hardness wear, and in some cases toughness. Despite these proven property benefits, the processing and usage of NC materials at elevated temperatures has been extremely limited so far. This is because with nanoscale grains there is a high density of rapid diffusion pathways in the material, which can lead to accelerated kinetic processes that tend to degrade the material, including grain growth or coarsening, and potentially creep as well. To process and use NC metals at high temperatures requires innovation in alloy design to thermodynamically stabilize the nanostructure, and shut down the kinetic evolution processes that cause coarsening and creep.

If stable NC alloys could be designed, it would dramatically open the prospect for processing bulk NC components; existing methods to make bulk materials have fallen short largely because the synthesis pathways for bulk NC materials are energy intensive and hard to scale up. Additionally, many current techniques achieve only “ultrafine” grain sizes, somewhat larger than are desirable, in very limited bulk forms that rarely surpass the cm scale, and with a very high processing cost.

A more cost-effective, scalable, and potentially broadly commercially applicable pathway to synthesize NC metals is through a powder processing route. In contrast to nanoparticle powders that have proven challenging to make and consolidate into polycrystals, NC powders (i.e., micro- scale particles with nanoscale internal grains) are relatively easy to produce even at full scale using high-energy powder milling techniques. Therefore, a main challenge of producing bulk NC parts by powder processing amounts to one of consolidation, which again relates to thermal stability; if NC powders can be sintered to full density while retaining their nanoscale grain size, it would enable scaled production not only for application, but for detailed study of NC materials in general. Retaining the nanoscale grain size in NC materials during their synthesis and processing at elevated temperatures is a fundamental hurdle from a thermodynamic standpoint, due to their inherent lack of thermal stability. However, a NC alloy designed for high thermal stability can exhibit excellent high-temperature processing as well as high-temperature mechanical properties, far exceeding those of its polycrystalline counterparts. What is more, NC alloys designed for thermal stability and rapid sintering, obtained by a powder metallurgy (PM) route, can then be integrated with 3D printing technologies, unlocking dramatic component shape advantages in combination with exceptional materials properties based on an NC structure.

Here we propose to partner with NASA's Marshall Space Flight Center to develop rapidly sintering and thermally stable NC Ni-based alloys, which would provide a lightweighting, performance and potentially a cost advantage compared with Inconel 625 and 718 in rocket engines and related components. This proposal specifically targets “Technologies Supporting Advanced Manufacturing, Structures and Materials” and will deliver an alloy powder and a preliminary laboratory scale processing route (densification and annealing schedules). As a collaborative effort between Marshall Space Flight Centers scientists/engineers and MIT, the alloy's microstructure stability, tensile strength and fracture toughness will be characterized at room temperature and elevated temperatures.

The PI's research group at MIT has more than a decade of experience in synthesizing and characterizing NC materials with special attention paid to stabilizing the nanostructure through alloying. Alloying is a key approach to stabilizing nanostructures, because it can contribute to stability in two independent but complementary ways:

• Kinetics: Alloying can dramatically slow the kinetics of nanostructure coarsening by causing various types of ‘drag’ forces. Second phase drag, solute drag and chemical ordering drag can retard or entirely inhibit grain boundary motion, and thus grain growth.
• Thermodynamics: Alloying elements can also directly reduce or eliminate the thermodynamic driving force for grain growth. This is possible by creating special configurations that stabilize grain boundaries and allow nanostructured grains to become the system's thermodynamically preferred state.

The PI is especially interested in the latter strategy, because it permits the design of new nanostructured alloys on the basis of thermodynamics, rather than merely relying on kinetic trapping of a nanostructured state. It also opens the door to much more stable nanostructures, which can be processed more easily at high temperatures without coarsening, and which may even permit high-temperature use of nanostructured metals with exceptional mechanical properties.

The premise of thermodynamic nanostructure stabilization was first realized through alloy modeling and simulations from the PI's group. Without going into too much detail, the group uses models that allow for the simulation of complex, non-regular alloy chemistries in which grain boundaries are permitted as part of the thermodynamic space of exploration.

An earlier example of the PI's modeling efforts is for the specific case of W-based alloys. A number of alloying elements, when added to W, favor nanocrystalline structures; these are shown in green. Also interesting are the alloys that lie in the blue region, which is a region where both a nanostructure and a reinforcing second phase are present. These alloys are very special; termed ‘nano-duplex’ alloys, they have both a stabilized nanostructure as well as a second phase that can be used to strengthen the material or facilitate its processing. Based on these results, the PI's group has studied as an example the W-Cr system: W-15 at. % Cr NC powder was produced by high-energy ball milling, which was subsequently annealed.

The reason that nano-duplex alloys (such as the example alloy of W-Cr above) are exciting for processing of bulk NC materials is that the presence of a second phase can fundamentally change the mechanisms and kinetics of sintering: a nano-duplex structure can densify very quickly thanks to the second phase, while the nano-duplex thermodynamics can retain a nanostructure within the sintering particles. The PI's group has recently explicitly proven this concept in the W-Cr system. This system reaches full density (>98%) when subjected to monotonic heating with zero applied pressure. Measurements of the relative density showed that the compact began to noticeably densify at 950 °C, far below the normal sintering temperature for W. A schematic and an actual TEM image of the sintered microstructure illustrates a Cr-rich phase precipitated from the supersaturated NC W, forming necks between the compact particles. These necks become rapid diffusional transport layers between the particles, leading to the accelerated consolidation.

An example of such a sintered material where the sample is bulk in every dimension (~cm scale) but the grain size is nanoscaled. This technology, for the specific case of W-Cr and a related Cr-based alloy, were successfully transitioned by the PI to Veloxint Corp., where the process is now run at larger, component scales. At those scales, it is possible to use 3D printing of the ‘green’ body, and then sinter net-shape components to high density.

It is the motivating hypothesis of this proposal that we should be able to design a similar process to produce a new family of rapidly-sintering NC Ni-based alloys. The interest in Ni as a base metal is motivated by the need for innovation in the metals used in engines, which are frequently based on nickel-based superalloys (such as Inconel 625 and 718) developed decades ago. An improved NC-Ni alloy with benefits in strength (room and cryogenic temperatures) and creep resistance could permit lightweighting through, e.g., topology optimization and 3D printing of leaner components. The ability of Ni to perform well in both the high temperature environments of engines as well as peripheral cryogenic environments presents an excellent starting point on which to build in the alloy design process. The approach of the PI is well-suited to explore Ni, which is in fact often produced through powder metallurgy and has extremely well-developed thermodynamic input data for many possible alloying additions. What will be new to the present work will be the design of NC-Ni alloys in which the alloying elements specifically (i) help support and stabilize a NC structure, and (ii) promote a nano-duplex structure that can accelerate sintering and open the door to 3D printed components produced through pressureless sintering.

As noted above, there are many thermodynamic data for Ni alloys that provide early guidance to the proposed project. If a nano-duplex alloy system can be designed, the second phase can have the effect of both accelerating the sintering and stabilizing a nanostructure to retain exceptional properties. Further, the roles of sintering accelerator and nanostructure stabilizer can be separated in practice, and addressed through the use of two different alloying elements in a ternary (or higher order) alloy. Such ternary strategies allow not only grain boundary energy reduction (thus reducing the equilibrium grain size), but can also be used to form thermally-stable nanoduplex structures. For example, it is well known through the PI's earlier work that W is a grain size stabilizer in NC- Ni4, although it is not a nano-duplex former that accelerates sintering. Thus a ternary combination of Ni-W-X, with X promoting a nano-duplex structure and rapid sintering, is one possible path for the research. It is a major deliverable of the proposed work to screen elements for such an alloy design process, and our preliminary assessment for Ni-based systems suggests a hypothesis—such nanostructured configurations and rapid sintering may be achievable with small additions of elements such as Al or Mg, which also contribute to lightweighting of the alloy.

We anticipate the benefits of such alloys to include the following:

• • Extraordinary strengths from cryogenic to elevated temperatures – Ni-based NC alloys are expected to have strengths of 2 GPa (~300 KSI) and higher, depending on the alloying elements and nanostructure. Thermal stability, low energy grain boundaries, and a homogenous distribution of nanoprecipitates all help to promote strength and creep resistance that are maintained at elevated operating temperatures. The primary aim for these alloys is to be competitive in strength and weight with Inconel 718 alloys for use in cryogenic environments, and at elevated temperatures pending follow-on work.
  • Increased strength and specific strength enables lightweighting and increased performance – Increased strength and specific strength are associated with the substantial reduction in grain size. However, the addition of alloying elements such as Al or Mg to Ni could unlock further improvements in both these properties. This is due to their lighter weight as well as the excellent mechanical properties of their associated intermetallic compounds at the nanoscale. As a result, weight reductions greater than 10-20% can be achieved at similar or greater strength. These alloying elements also contribute to cost reduction compared with conventional Ni-based alloys.
  • The extreme strength and creep resistance are obtained without sacrificing toughness, ductility or other material properties – In fact, with the PI's alloying approach, toughness may also be improved by fine-tuning the nanostructure (a combination of grain size and second phase distribution and size).
  • NC metals can be readily scaled via 3D printing and traditional PM techniques – These alloys can be rapidly prototyped by 3D printing of nanostructured components using a binder jet or bound-metal powder deposition (BMD) approach (i.e., non- laser based). The PI's earlier (non-Ni-based) nanostructured metal powders have already been printed by BMD, by compounding with a polymer to form a paste, and printing a green body through a nozzle system. Final consolidation is achieved by sintering, which is accelerated and improved in such NC alloys. Prior experience with Cr- and W-based NC alloys offers reason for optimism with a new family of NC-Ni alloys to be developed specifically in this project.

This is early/mid-stage (TRL 1) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

None of these are guaranteed paths for this specific project — TechPort itself doesn't have an "apply" button. Reaching out to the contact(s) above with a specific question is usually the fastest way to find out what's actually open.