Additive Manufacturing of L12 Precipitate Strengthened Nickel and Aluminum Alloys

Metal Additive Manufacturing (AM) technologies have enabled the manufacturing of parts with complex geometries that were previously not feasible with conventional manufacturing. Unfortunately, many commercial engineering alloys (with the exception of alloys for welding) were designed with conventional manufacturing in mind and can behave poorly in the rapid solidification and thermal conditions that go along with AM. Research is needed to optimize the higher performance engineering alloys, such as pre-cipitation hardened alloys, for AM. One important class of precipitates in both the nick-el-base and aluminum-base superalloys is the intermetallic L12 âstructured (Strukturbericht notation) intermetallics. Many scientific challenges must be overcome prior to widespread industrial usage of these alloys. AM of L12-strengthened Ni-base superalloys results in extensive micro-cracking and AM of L12-strengthened Al-base alloys has been successful but requires further research to understand the material be-havior. This thesis focuses on overcoming the scientific challenges for the AM of L12-strengthened alloys. A new Al-Mg-Zr alloy strengthened via L12-structured Al3Zr precipitates, AddalloyÂ®,was successfully fabricated for the first time with Laser-Powder Bed Fusion (L-PPF). The as-fabricated Al-Mg-Zr alloy displayed a unique bimodal grain structure: (i) the bottom of the melt pools consisted of fine equiaxed grains (~1 Âµm) and (ii) the top of the melt pool consisted of columnar grains (up to 40 Âµm long). The bimodal microstructure was at-tributed to the changing solidification conditions during the lifetime of the melt. A new microstructure modification strategy, laser rescanning, was shown to refine the micro-structure. The grain refinement was attributed to the reduced melt pool depths of the additional scans, thus breaking up the previously solidified columnar grains. Scanning transmission electron microscopy (STEM) investigations of peak-aged (400ËC, 8 h) samples reveal both continuous (~2 nm in diameter) and discontinuous (~5 nm wide and hundreds of nanometers long) coherent, secondary L12-Al3Zr precipitates. The Al-Mg-Zr in the peak aged (400ËC, 8 h) condition had a 385 MPa yield strength and a 19% elongation at fracture. The excellent combination of strength and ductility was attribut-ed to the bimodal microstructure. For short-term elevated temperature yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temper-atures above 150ËC (e.g., 87 vs 24 MPa at 260ËC). For longer term creep tests at 260 ËC, both as-fabricated and peak-aged samples displayed near-identical creep behavior. The reduced elevated temperature performance of the peak aged samples was attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to in-hibit grain-boundary sliding (GBS) of the fine equiaxed grains. The micro-cracking of CM24LC, a Ni-base superalloy, was attributed to both solidifica-tion and liquation cracking. Based on experimental evidence, reduction in the solidifica-tion interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. Sam-ples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with opti-mized processing conditions exhibit a reduction in crack density of 98%.

Development of High-Strength Aluminum Alloys for Additive Manufacturing

Marvin Lennard Schuster

Metal additive manufacturing (AM) offers the possibility to rapidly produce complex geometries that are not achievable with conventional manufacturing methods. The two most common technologies, Laser Powder Bed Fusion (LPBF) and Direct Metal Deposition (DMD), are characterized by high process temperatures, fast heating, and cooling rates, and an associated far-from-equilibrium solidification. This is problematic when processing materials that were developed and optimized many years ago for conventional manufacturing processes. Common difficulties are the occurrence of gas pores in the component, which are caused by the evaporation of volatile alloying elements during the process, the formation of hot cracks due to large solidification intervals, and the formation of metastable phases. For the processing of high-strength, precipitation-hardening Al-Cu aluminum alloys, this prevents the use of non-modified conventional alloys. However, the development of novel alloys tailored to these processes not only allows these difficulties to be overcome but also opens up the possibility of improving material properties through the targeted exploitation of process-inherent properties. This includes the processing of oxide particle-reinforced alloys, which can not be produced by conventional casting processes. The standard heat treatment usually performed for precipitation hardening, which has also been optimized for conventional materials, must be adapted accordingly to achieve optimum material properties. Within the scope of this work, the microstructure, precipitation, and defect formation after LPBF as well as the heat treatment behavior are thoroughly investigated based on an Al-Cu-Mg-Zr alloy for the first time. The obtained findings are applied to design a novel 2618 Al-Cu alloy for LPBF and DMD, with suitable heat treatment being developed for LPBF. The alloy is characterized with respect to microstructure, precipitation formation, and mechanical properties before, during, and after heat treatment. Furthermore, by modifying elemental Al-Zr powder blends with nm-sized Al2O3, an oxide dispersion strengthened alloy is produced by LPBF and investigated in detail.

EPFL2022

Modelling oriented investigations of primary radiation damage in ultra high purity Fe and FeCr alloys

Anna Prokhodtseva

In the quest of the structural materials for the future fusion reactor, it has been shown that ferritic/martensitic (F/M) steels are very promising candidates, with a good radiation resistance in terms of damage accumulation in the microstructure relative to for example austenitic steels. However, our ability to predict their response to irradiation in such harsh conditions is still not satisfactory. In this view, there is a critical need for information on the primary damage occuring in this materials class, as input to the multiscale modelling of the irradiation effects, including the impact of He and H. Despite numerous studies in the last 50 years, there is still lack of knowledge in many areas of the irradiation response of the microstructure of ferritic steels. This is due to the complexity of these materials, for they contain numerous alloying elements, grains boundaries and precipitates. The strategy nowadays to identify basic mechanisms of primary damage is to investigate so-called model alloys of those, which allows parametric studies in simplified structures. In this work ultra high purity Fe and Fe(Cr) model alloys were investigated in a transmission electron microscope (TEM) under ion irradiation in an attempt to better understand the fundamental mechanisms of radiation damage starting from the lowest doses, and the dependence on Cr content. Attention is also paid to the effects of He and H. Radiation induced dislocation loops and cavities were quantified. This study provides data for validation of the modelling efforts. Single, dual and triple beam ion irradiations were performed in JANNuS facility located on two sites, in Orsay and in Saclay in France. In Orsay, electron transparent thin foils of UHP Fe, Fe -5, -10 and -14Cr were irradiated in situ in TEM as a single beam experiment with 500 keV Fe+ ions and as a dual beam experiment with 500 keV Fe+ and 10 keV He+ ion beams, to a dose of 1 dpa with and without 1000 appm/dpa He at room and liquid nitrogen temperatures in order to observe the very first defects, desirably before their thermal evolution. Effects of dose rate on the produced damage were assessed in UHP Fe. The impact of He and Cr on defect accumulation was scrutinized in terms of the number density, size and Burgers vector of the visible defects. Special care was taken in the analysis of the TEM micrographs, for which new techniques were developed, with a particular one to determine the Burgers vector of a dense dispersion of fine nanometric defects. Emphasis was put on the identification of the loops Burgers vector, which is considered to be either a0〈100〉 or 1/2 a0〈111〉 in ferritic materials. 2 In order to study the effect of the free surfaces of the electron transparent thin specimens on the irradiation induced microstructure, the results obtained for TEM thin foils were compared to bulk samples that were irradiated ex situ in Saclay in single beam experiments with 24 MeV Fe8+ ions, and dual beam ones with 24 MeV Fe8+ to 1 dpa and 2 MeV He+ ions 1000 appm/dpa He at RT. To study synergistic effects of He and H a triple beam bulk irradiation with 24 MeV Fe8+, 2 MeV He+ and 0.6 MeV H+ ions, to 1 dpa, 1000 appm/dpa He and 4000 appm/dpa H at room temperature was preformed. From the bulk specimens thin lamellae were extracted by focused ion beam, which allowed the TEM observation of the damage through the whole implanted range of about 3.5 μm. In this way, the effect of irradiation with and without He and synergistic effects of simultaneous He and H implantation on the produced defects were assessed. In addition, radiation induced hardening was assessed by nano-indentation and related to the radiation induced microstructure. The analysis shows that in thin foils of UHP Fe the defect population is dominated by a0〈100〉 defects, while in presence of He it is dominated by 1/2 a0〈111〉 loops after irradiation to 1 dpa. It indicates that helium stabilizes mobile 1/2 a0〈111〉 loops, which in thin foils otherwise escape to 2 the free surfaces. Irradiation to the lowest dose of 0.05 dpa resulted in the production of either increased ratio of 1/2 a0〈111〉 loops, compared to the highest dose, or in the entire population of 2 1/2 a0〈111〉 loops in all materials in single and dual beam. In bulk UHP Fe and Fr(Cr) samples 2 after single and dual beam irradiation mainly 1/2 a0〈111〉 loops and few a0〈100〉 loops were 2 observed, but of smaller size than in thin foils, emphasizing the effects of free surfaces on the type of produced loops. It is thus inferred that 1/2 a0〈111〉 loops dominate the early loop 2 2 population and visible a0〈100〉 loops observed in UHP Fe and Fe(Cr) thin foils stem from addition and/or absorption reactions between 1/2 a0〈111〉 loops. It follows that these reactions 2 are reduced by the presence of He, which may eventually impede the formation of a0〈100〉 loops, leaving a loop population dominated by 1/2 a0〈111〉 loops. The same type of reactions 2 are valid for Fe(Cr) alloys, but the reduction in mobility of 1/2 a0〈111〉 loops by Cr in the single 2 beam case and the synergistic effects of He and Cr after dual beam irradiation result in a mixed population of 1/2 a0〈111〉 and a0〈100〉 type loops. This confirms the idea that the formation of 2 visible a0〈100〉 loops is promoted by the presence of free surfaces. Irradiation of UHP Fe thin foil at liquid nitrogen temperature confirmed the assumption of initial 1/2 a0〈111〉 loops. These then escape to the free surfaces after warming to RT, the more so 2 in the thinnest regions of the sample leaving a loop population dominated by a0〈100〉 loops. In single beam irradiated UHP Fe no dependence of the total density of defects or their Burgers vector on the rate of irradiation was observed. However, in the presence of He, only 1/2 a0〈111〉 2 loops formed under higher dose/implantation rates, while at the lowest dose rate results were similar to the single beam. This observation is attributed to the local increase of the effective He/defect ratio in the case of the high dose rate due to the increased temporal overlap of displacement cascades. Regarding mechanical properties, it appears that hardness of the as received materials increases with increasing Cr content. Following irradiation there is a significant hardening, which is not monotonous with Cr content, and which increases for all materials when irradiated together with He, and the more so when irradiated simultaneously with He and H. Hardening relates well to the observed microstructure and is the largest for Fe-5Cr.

EPFL2013

Development of High-Strength Aluminum Alloys for Additive Manufacturing

Marvin Lennard Schuster

EPFL2022

Modelling oriented investigations of primary radiation damage in ultra high purity Fe and FeCr alloys

Anna Prokhodtseva

EPFL2013