Grain boundary strengthening | EPFL Graph Search

In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmountable borders for dislocations and that the number of dislocations within a grain has an effect on how stress builds up in the adjacent grain, which will eventually activate dislocation sources and thus enabling deformation in the neighbouring grain as well. By changing grain size, one can influence the number of dislocations piled up at the grain boundary and yield strength. For example, heat treatment after plastic deformation and changing the rate of solidification are ways to alter grain size. Theory In grain-boundary strengthening, the grain boundaries act as pinning points impeding further dislocation propagation. Since the lattice structure of adjacent grains differs in orientation, it requires more energy for a dislocation to change

Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction

Stefan Brandstetter

Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unique physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, considerable effort has been put into uncovering the new deformation mechanisms that give rise to this outstanding response of nanocrystalline materials. Moreover, the production of nanocrystalline materials of reasonable sizes for structural applications remains a challenge. However, the size limitation is of no issue for their present application in the growing field of MEMS and NEMS devices. Ultimately, the reliability and lifetime prediction of these devices will depend on the accurate knowledge of their mechanical response. This dissertation addresses experimental and simulation procedures used to understand the fundamental deformation mechanisms operating in bulk nanocrystalline Nickel. Recent results from simulations suggested dislocations as a dominant carrier of plasticity in nanocrystalline materials. In contrast to coarse-grained materials, these dislocations are nucleated at grain boundaries and, after propagating through the nano grains, they are absorbed there as well. Deformation experiments during in-situ X-ray diffraction strengthened the predicted outcome from simulation but many open questions remained. Within this thesis a more extensive range of in-situ testing experiments are performed that aim to systematically investigate the nanocrystalline deformation mechanism in terms of both temperature and external loading conditions. The development of a low temperature tensile test set-up allowed to study temperature dependent behavior and revealed that dislocation activity in nanocrystalline Nickel is a strongly thermally activated process where propagation of dislocation seems to be as important as nucleation from dislocations at the grain boundary. This finding is further supported by strain-dip tests, which revealed that pinning points strongly influence dislocation propagation. Nanocrystalline Nickel exhibits, in its as prepared state, large internal stress variations. These stress variations and the small grain size are most likely responsible for the microplastic regime characterized by an extended macroscopic strain, making the usage of the classical definition of yield questionable. Furthermore it could be shown that upon annealing, which reduces the samples' internal stress, this extended microplastic regime was observed to be less pronounced. To study the structural stability of nanocrystalline Nickel at large strains, the material was investigated by compression experiments, revealing no changes in mean grain size. Furthermore, the three dimensional atomic probe technique was utilized for localizing impurity concentrations. The program was rounded by calculating diffraction peaks from simulated nanocrystalline structures with a single type of defect. This allowed investigating the characteristics of the diffraction pattern of nanocrystalline systems in a bottom up approach. Finally, the results of the thesis are discussed in terms of a thermally activated deformation mechanism that involves the nucleation, propagation and absorption of dislocation within the nanocrystalline environment.

EPFL2008

Helium effects on irradiation assisted stress corrosion cracking susceptibility of 316L austenitic stainless steel

Ignasi Villacampa Roses

The formation and growth of cracks by irradiation assisted stress corrosion cracking (IASCC) in light water reactor internals is a critical issue for a safe long-term operation of nuclear power plants. The IASCC susceptibility at relatively low dose is dominated by conventional mechanisms such as radiation-induced segregation and radiation hardening. However, the ageing of the nuclear fleet combined with the increase of their life-span reveals other mechanisms that could play an important role on IASCC susceptibility. Recent studies show that a huge amount of helium (He) can be accumulated in reactor internal components of pressurized water reactors (PWR) after long-term operation. This occurrence could significantly increase the IASCC susceptibility at high doses. The main objective was to investigate the He effects on IASCC susceptibility of the solution-annealed (SA) and 20% cold-worked (CW) austenitic steel 316L. SA and CW miniaturized tensile samples and a SA plate were homogeneously implanted up to 1000 appm He. He was implanted at ~45 MeV and 300°C at the cyclotron in CEMHTI/CNRS (France). Slow strain rate tests (SSRT) in air and in high-temperature water were then carried out on SA, CW, post-implantation annealed (PIA) and as-implanted samples; instrumented nanoindentation tests were performed at room temperature on non-implanted and implanted specimens; and scanning electron microscope and transmission electron microscope were employed for fractography and microstructural characterization. The results of SSRTs in high-temperature air and in hydrogenated high-temperature water showed that homogenized implanted He up to 1000 appm corresponding to a displacement damage of about 0.16 dpa (displacement per atom) does not produce intergranular cracking and IASCC. However, the deformation microstructure of non-implanted SA/CW is characterized by high dislocation density arranged in cell walls separated by relatively dislocation-free regions, while the as-implanted SA samples (> 0.05 dpa) exhibit a more planar deformation microstructure. Characterization of as-implanted CW and PIA samples did not show any significant implantation effect on the deformation microstructure. PIA of the He implanted plate was carried out from 650 to 1000°C for 1h to increase the helium grain boundary coverage, and to reproduce the observed microstructure of the replaced reactor internal components that showed IASCC and use the results for the PIA of tensile samples. The transmission electron microscope investigations showed that an increase of the annealing temperature causes an average bubble size increase and density decrease. The average He bubble size and distribution in the grain interior and on the grain boundary were similar in the range of temperatures studied. In both cases, bubbles grew by the Ostwald ripening mechanism. No preferential He build-up took place on the grain boundaries. The increase of yield stress produced by He bubbles was calculated with three distinct dislocation-localized obstacle and compared to the tensile test results. The bubble strength estimated for these models was used to assess their validity and to compare the results to published data. Finally, nano-indentation tests in SA, CW, as-implanted SA and PIA (650 to 850°C for 1h) samples did not show He effects on grain boundaries strength. However, the average hardness of the grain interior increased with the He implantation and decreased increasing the PIA temperature.

EPFL2018

Additive Manufacturing of L12 Precipitate Strengthened Nickel and Aluminum Alloys

Seth James Griffiths

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%.

EPFL2020