Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

International audienceA coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intragrain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

A coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intra-grain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

Orientation gradients in rapidly solidified pure aluminum thin films: comparison of experiments and phase-field crystal simulations

Rapid solidification experiments on thin film aluminum samples reveal the presence of lattice orientation gradients within crystallizing grains. To study this phenomenon, a single-component phase-field crystal (PFC) model that captures the properties of solid, liquid, and vapor phases is proposed to model pure aluminium quantitatively. A coarse-grained amplitude representation of this model is used to simulate solidification in samples approaching micrometer scales. The simulations reproduce the experimentally observed orientation gradients within crystallizing grains when grown at experimentally relevant rapid quenches. We propose a causal connection between formation of defects and orientation gradients

Chromium-based bcc-superalloys strengthened by iron supplements

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials

Chromium-based bcc-superalloys strengthened by iron supplements

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials

Nopean jähmettymisen faasikenttämallinnusta ohutkalvoille ja materiaalia lisäävälle valmistukselle

The public defence will be organised via Zoom: https://aalto.zoom.us/s/64127861066 Zoom Quick Guide: https://www.aalto.fi/en/services/zoom-quick-guide The dissertation is publicly displayed as online display 10 days before the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/?lang=enSeveral advanced industrial manufacturing processes operate in rapid solidification conditions, including laser welding, thermal spray coating deposition, and additive manufacturing. These processes lead to materials with drastically altered properties, when compared to low solidification rate manufacturing methods such as casting. This is due to the unique microstructural features emerging in rapid solidification. Rapid solidification conditions alter growth dynamics, for example through kinetically selected metastable phases, notable interface attachment kinetics, and solute trapping. Therefore there is a strong motivation to adjust these manufacturing processes to target specific microstructures, in order to reach desirable material properties. These process-structure-property links can be established by computational modeling. In the past decades, the phase field method has become the state-of-the-art model to simulate solidification on microstructural scales. Its success in the materials science community can be attributed to its connection to statistical physics and thermodynamics, simplicity, and relative ease with which new physical phenomena can be implemented. In this thesis, a computationally efficient and quantitative phase field modeling framework is presented for the rapid solidification regime. The phase field model is made computationally efficient through adaptive mesh refinement and shared memory parallelization. A quantitative near-equilibrium alloy phase field model is extended to operate in the rapid solidification regime through matched interface asymptotics analysis, allowing for controllable solute trapping kinetics that follow the continuous growth model in the thin interface limit. The rapid solidification simulations are compared to thin film solidification experiments with time-resolved in-situ imaging. This phase field model is applied to the study of additive manufacturing, first for stainless steel to understand the process-microstructure relationships, and then as a method to investigate the effects of inoculation to alter the polycrystalline structures. The presented rapid solidification phase field modeling framework will assist systematic process-structure-properties based design of novel engineering materials.Useat teolliset metallien valmistusmenetelmät perustuvat nopeaan jähmettymiseen, kuten laserhitsaus, terminen ruiskutus ja materiaalia lisäävä valmistus eli 3D-tulostus. Näin syntyvien materiaalien ominaisuudet eroavat huomattavasti sellaisista materiaaleista, joita valmistetaan hitaan jähmettymisen menetelmillä kuten valamisella. Ero ominaisuuksissa johtuu nopean jähmettymisen aikaansaamista erityisistä mikrorakenteista, jotka riippuvat siitä miten nopea jähmettyminen muuttaa kasvudynamiikkaa. Esimerkkejä nopeasta jähmettymisestä ovat metastabiilien faasien valikoituminen, merkittävä rajapinnankiinnittymiskinetiikka (interface attachment kinetics) ja seosaineen loukkuuntuminen (solute trapping). Näiden ilmiöiden vaikutukset jähmettymiseen pitää ymmärtää perusteellisesti, jotta nopean jähmettymisen menetelmiä voidaan säätää hallitusti, luoda materiaaliin halutut mikrorakenteelliset piirteet, ja lopulta tuottaa toivottavat materiaaliominaisuudet. Näitä riippuvuuksia valmistusmenetelmän, mikrorakenteen ja ominaisuuksien välillä voidaan määrittää laskennallisella mallinnuksella. Viime vuosikymmeninä faasikenttämenetelmästä on tullut tärkein malli jähmettymisen simulointiin mikrorakenteen tasolla. Materiaalitiedeyhteisössä faasikenttämenetelmän suosio perustuu sen yhteydellä statistiseen fysiikkaan ja termodynamiikkaan sekä sillä, että uusia fysikaalisia ilmiöitä voidaan ottaa mukaan näihin malleihin suhteellisen helposti. Tässä väitöskirjassa esitellään laskennallisesti tehokas ja tarkka faasikenttämallinnusmenetelmä joka soveltuu nopeaan jähmettymiseen. Faasikenttämallista tehdään laskennallisesti tehokas mukautuvalla verkon tihennyksellä ja jaettuun muistiin perustuvalla rinnakkaistuksella. Kvantitatiivinen tasapainoa lähellä toimiva faasikenttämalli laajennetaan nopean jähmettymisen alueelle käyttäen menetelmää nimeltä Matched Interface Asymptotics Analysis. Näin seoksen ansautumisen kinetiikkaa voidaan säätää hallitusti vastaamaan niin sanottua Continuous Growth -mallia. Tässä väitöskirjassa kehitettyä nopean jähmettymisen faasikenttämallia sovelletaan materiaalia lisäävään valmistukseen. Ensin menetelmää sovelletaan ruostumattomaan teräkseen valmistusmentelmä-mikrorakenne -riippuvuuksien ymmärtämiseen. Lopuksi tutkitaan inokuloinnin vaikutusta monikiderakenteisiin. Tässä väitöskirjassa kehitetyn nopean jähmettymisen faasikenttämalli edistää uusien teknillisten materiaalien systemaattista suunnittelua