In view of the internal mechanism of coarsening of traditional aluminum alloy nano-precipitated phase at high temperature, the author’s team took microalloying as the basic regulation means, Based on the interaction between solute atoms and the artificial regulation of the thermodynamic kinetics of aging precipitation, a microstructure design strategy of coupling fast diffusion solute atoms (such as Cu, Zn, Mg, etc.) with slow diffusion solute atoms (such as transition group metals as Zr, Sc, Ti, etc.) is proposed to prepare high thermal stability and large volume fraction strengthened phases (FIG. 1).
A variety of new methods for thermal stabilization of aluminum alloys, such as precipitation phase interface atom polarization, heteroatom gap position order and multi-level heterophase interface co-lattice coupling, have been developed, and several new aluminum alloys with high temperature resistance and creep resistance have been developed.
Thus, the whole chain of organic combination between basic research and application research from mechanism research to organization design to process regulation, from performance breakthrough to amplification experiment to typical application is realized.
Fig.1 Temperature-dependent diffusivity of typical solutes (a) and 400 ℃ diffusivity vs excess solid solubility (Cmax-C400: the difference between the maximum solid solubility and the solid solubility at 400 ℃, atomic fraction, %) of typical solutes (b) in AI matrix
The coarsening of nano-precipitated particles in aluminum alloy at high temperature is a dynamic process in which small size particles dissolve, large size particles grow up, and the average particle size gradually increases, which is usually called Ostwald scalar, and its expression is:
r(t)n-r(to)n= k(t- to) (1)
Where r(t) and r(to) are the dimensions of the precipitated phase at time t and to respectively; n is the time index, which defines that the coarsening process is dominated by bulk diffusion (n= 3) or interface diffusion (n=2). k is the coaxing rate. For an ideal binary dilute solution, k can be abbreviated as:
k∝γD (2)
Where γ is the interfacial energy between the precipitate phase and the matrix. And D is the diffusion rate of solute atoms. It can be seen from the above equation that decreasing y river reduces the driving force of Ostwald ripening thermodynamically. The decrease of D can further inhibit the coarsening of precipitate particles dynamically.
The specific thermal stabilization measures are as follows: the fast diffusing solute atoms first form nano-precipitated phase particles with a large volume fraction, and then the slow diffusing solute atoms (microalloy elements) form partial polymerization at the interface between the precipitated phase particles and the matrix, reducing the interface energy, and preventing the fast diffusing solute atoms from crossing the interface, resulting in the dissolution or coaxing of the particles.
The thermal stabilization of nano-precipitated particles was realized from two aspects of thermodynamics and kinetics. The selection of solute atoms in CMO requires comprehensive consideration of the interaction between fast/slow diffusion solute atoms, atomic structure at the interface of precipitated phase particles, the association between solute atoms and vacancies and other factors, which can be screened and optimized by computational materials science.
Sc microalloyed Al-Cu alloy was used as a typical material. The fast diffusing solute atom Cu is used to form a large volume fraction (up to about 2.0%) of {100} oriented disk shaped ɵ’-Al2Cu nanoprecipitate phase. The slow diffusion atom Sc acts as an interface biased element.
The aging temperature of Al-Cu alloy is usually 170~190 C. In this temperature range, the diffusion capacity of Sc is insufficient, and it is difficult to form effective interface polarization. However, if the aging temperature is too high, it is easy to cause the rapid coarsening of ɵ’-Al2Cu nano-precipitated phase once it is precipitated, and the Sc atoms have no time to concentrate at the interface.
Therefore, three-stage heat treatment was used for Al-Cu-Sc alloy. While ensuring the fine dispersion distribution of ɵ’-Al2Cu nano-precipitated phase, the interface covers Sc atoms to play the role of “heat protection layer” (FIG.2). This “two-dimensional interface thermal stabilization” design significantly improves the high-temperature mechanical properties of Al-Cu-Sc alloy, and the steady-state creep rate at 300℃ is reduced by 2 to 3 orders of magnitude compared with reported aluminum alloys and even aluminum matrix composites (FIG.2b). The Al-Cu-Sc based alloy plate based on this material has been mass-produced, and applied cooperative research has been carried out in relevant aerospace units.
Fig.2 Representative high-angle annular dark field (HAADF) image and corresponding Cu and Sc mapping of a cross-sectioned ɵ’-Al2Cu nanoprecipitate in the Al-Cu-Sc alloy(a) and dependence of steady creep rate on creep strain at 300℃, in comparison with available data of other Al alloys and Al-based composites(b)
The heteroatoms on the interface of precipitated phase particles are still in the thermodynamic metastable state in essence, and thermal instability will occur at higher temperatures. Taking the Al-Cu-Sc alloy mentioned above as an example, although the ɵ’-Al2Cu precipitated phase with interfacial polarization of Sc atoms has good thermal stability at 300°C, when the temperature rises above 350 ℃, the originally uniformly distributed Sc atoms on the interface will locally form the second phase of Al3Sc through interfacial diffusion.
The region without Sc partial protection will dissolve and disconnect, resulting in the failure of “two-dimensional interface thermal stabilization”. If the solute atoms with slow diffusion and low solid solution can be introduced into the traditional precipitation enhanced phase particles and the periodic distribution can be realized, the dissolution barrier can be enhanced by enhancing the binding between atoms in the precipitation phase.
And the dissolution process can be slowed down by increasing the redissolution resistance of the slow diffusion solute atoms, the “3D structure thermal stabilization” design of the precipitation phase particles is expected to be realized.
The heat resistance characteristics of aluminum alloy can be roughly divided into three levels.
(1) After high temperature heat exposure, it still has room temperature strength equivalent to the original state, that is, the microstructure is very stable during high temperature heat exposure. Since thermal exposure involves only a single thermal field and no external load acts simultaneously, the material strength does not need to be too high. Therefore, Al3X type nanoparticles with high thermal stability formed by rare earth or transition group metals (such as Sc, Ti, Zr, etc., represented by symbol X) are the optimal choice of strengthening phase, although their volume fraction is too small and their strength at room temperature is too low.
(2) Excellent high temperature tensile strength. This is the ability of materials to resist dislocation slip under thermo-mechanical coupling external field and quasi-static loading. Therefore, nano-second phase particles with large volume fraction and high thermal stability are the preferred design principles for their microstructure, emphasizing the strong interaction between nano-second phase and dislocation in a short time.
(3) Excellent high temperature lasting strength. This is the ability of the material to resist creep deformation for a long time under the thermal coupling external field, and dislocation climbing is the main plastic deformation mechanism. At this time, the dispersed nano-second phase particles are no longer enough to constrain dislocation climbing, and the second phase connected by quasi-3D network is needed to form a space closed constraint.
In the easily generated quasi-3D network connected second phase, eutectic structure is a potential choice for high temperature creep resistant aluminum alloy: eutectic structure can not only effectively restrain dislocation climbing, but also be suitable for preparing heat resistant aluminum alloy castings due to good casting fluidity. In comparison, the two Al-Cu-based heat resistant alloys mentioned above have higher Cu content and poor casting fluidity, which are more suitable for the preparation of plates or forgings.
On the basis of the dislocation climbing constrained by the micron-scale eutectic structure, the coexistence of nano-scale precipitation phase can further constrain the dislocation slip, which is conducive to improving the high-temperature mechanical properties and realizing the coordinated regulation of room temperature strengthening and high-temperature strengthening. The disadvantage of quasi-connected eutectic structure is that it is difficult to plastic deformation and brittle fracture is easy to occur under external load, leading to early failure of the material. Improving the deformation coordination between the eutectic phase and the matrix through interface co-lattice is an effective way to improve the plastic deformation ability.
Fig.3 Representative SEM(a), TEM(b), and APT(c) images showing the multiscale microstructural features in the Al-Ce-Cu-Sc alloy, and dependence of steady creep rate on creep strain at 300℃, in comparison with its Sc-free Al alloys(d)
The future development of heat resistant aluminum alloy will still focus on the fine regulation of the second phase with large volume fraction and high thermal stability and multi-level composite. Combined with the new principle of material design and new preparation method, the temperature limit and service bottleneck will be further broken through. Among them, the following aspects deserve special attention:
(1) In terms of material design, new multi-element nano-precipitated phase particles should be developed, the second phase of high-entropy component nano and the second phase of super-large volume fraction co-lattice nano should be explored, and the structure of eutectic phase should be refined and toughened at the same time;
(2) In terms of material preparation, we focus on the fast cooling rate or sub-fast cooling rate preparation methods represented by additive manufacturing, and explore the strange solid state phase transformation behavior through the high solid solution of main alloying elements and transition microalloying elements and the substantial increase of vacancy and other crystal defect density;
(3) In terms of material service, the deformation and fracture behavior and its internal mechanism under high temperature are revealed, the coupling strengthening and toughening effect of multi-level characteristic microstructure is elucidated, and the dynamic evolution law of microstructure and its influence on service life under heat-force external field are clarified.