자료유형  

 학위논문

Control Number  

0017161548

International Standard Book Number  

9798382761756

Dewey Decimal Classification Number  

620

Main Entry-Personal Name  

Ekaputra, Clement Nevin.

Publication, Distribution, etc. (Imprint  

[S.l.] : Northwestern University., 2024

Publication, Distribution, etc. (Imprint  

Ann Arbor : ProQuest Dissertations & Theses, 2024

Physical Description  

361 p.

General Note  

Source: Dissertations Abstracts International, Volume: 85-11, Section: B.

General Note  

Advisor: Dunand, David C.

Dissertation Note  

Thesis (Ph.D.)--Northwestern University, 2024.

Summary, Etc.  

요약This thesis aims to develop new, cast and additively-manufactured, aluminum alloys for lightweight, high-temperature structural applications, based on combinations of coarsening-resistant strengthening phases for high thermal stability and creep resistance. Alloy compositions were designed, then fabricated by casting and laser powder-bed fusion (L-PBF) additive manufacturing. Then, the evolution of their microstructure and mechanical properties during thermal exposure at 300-400°C was experimentally characterized as a function of both composition and processing.The first half of this work details the development and characterization of a new, cast Al-Ce-Ni-Mn-Sc-Zr alloy system. Combinations of the various elements are studied systematically, to understand the effects of each alloying element on the resulting microstructure and mechanical properties. Compositions are designed to maximize the effects of the strengthening phases in the alloy while avoiding unwanted, harmful phases resulting from interactions between the various alloying elements.Firstly, in cast Al-Ce-Sc-Zr-(Er) alloys, it is found that such alloys consist of micron-scale, Al11Ce3 platelets formed during eutectic solidification in the Al-Ce system, and nano-scale, L12-Al3(Sc,Zr,Er) precipitates formed during secondary aging from Sc, Zr, and Er addition. The Al11Ce3 and L12 phases form mostly independently of each other, except in a Zr-rich and Er-containing alloy where scavenging of Er and Si by Al11Ce3 on solidification reduces the kinetics of precipitation and number density of the L12 precipitates. In an Sc-rich alloy, however, the creep strain rate at 300°C is reduced by as much as five orders of magnitude in the Al11Ce3- and L12-strengthened alloy compared to alloys containing only L12 precipitates. This improvement is due to the combination of load transfer and precipitation-strengthening from the Al11Ce3 micro-platelets, and precipitation strengthening from the L12-Al3(Sc,Zr) nanoprecipitates.Next, the effects of Mn on the Al-Ce system are studied. Small Mn additions (~0.4 at.%) remain in solid solution in the α-Al matrix, and excess Mn results in coarse, primary Al20CeMn2 precipitates. These Mn solutes provide solid-solution strengthening both at room temperature and during creep. Furthermore, an Al-Ce-Mn-Sc-Zr alloy combines all three of the aforementioned strengthening mechanisms for enhanced creep resistance.In the final part of the cast alloy work, the effects of Ni additions on the microstructure and mechanical properties of the previously-designed Al-Ce-Mn-Sc-Zr alloy are studied. The primary and eutectic phase formation are found to vary with both Ni content and cooling rate. For higher Ni content and slower solidification rates, Ni forms an Al27Ce3Ni6 phase (both primary and eutectic), while for lower Ni content and faster solidification rates, Ni forms an Al9(Ni,Mn,Fe)2 phase. An alloy containing 3.2 wt.% Ni is selected for further study, and the resulting alloy contains four strengthening constituents: micron-scale (i) Al11Ce3 and (ii) Al27Ce3Ni6 or Al9(Ni,Mn,Fe)2 platelets, all formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed during secondary aging, and (iv) Mn solute atoms in the α-Al matrix. This alloy shows higher hardness during aging and creep resistance than alloys containing subsets of the four strengthening constituents, indicating that cumulative strengthening can be achieved by combination of these strengthening mechanisms. This final alloy also shows a high dislocation creep threshold stress of 62 MPa at 300°C.The second half of this work focuses on the design of aluminum alloys for L-PBF additive manufacturing. The effect of rapid solidification, inherent to the L-PBF process, on the microstructure and mechanical properties is emphasized, with comparison to the respective cast alloys. Furthermore, attention is given to alloying elements which are typically not useful in cast alloys, but may provide beneficial properties under the extreme L-PBF processing conditions.A similar Al-Ce-Ni-Mn-Sc-Zr alloy to the previously designed cast alloy is processed by L-PBF, and contains the same four strengthening constituents as in the cast alloy. However, the eutectic Al11Ce3 and Al27Ce3Ni6 phases are greatly refined by the rapid solidification process, resulting in greatly enhanced strength and creep resistance. Non-stoichiometric phase compositions are also found in the peak-aged state, which shift to equilibrium compositions during over-aging. During early stages of aging at 300-400°C, the strength of the alloy rapidly decreases due to fragmentation of the continuous eutectic network structure, then gradually declines due to the subsequent particle coarsening. The importance of the various strengthening mechanisms in the alloy are also explored via analytical and numerical modelling, and it is found that direct precipitation strengthening from the eutectic precipitates is the dominant creep strengthening mechanism in the alloy.To study the possibility of enhanced solid-solution strengthening in L-PBF processed alloys, simpler, ternary Al-Zr-X alloys are studied, where X = Mn, Cr, V, Mo, and W are slow-diffusing transition metal elements. These alloys contain both L12-Al3Zr nanoprecipitates formed during aging at 400°C, and solid solutions of the ternary alloying element in amounts well beyond their equilibrium solubility limits. These extended solid solutions markedly enhance strength and creep resistance at 400°C compared to the binary Al-Zr alloy. While the rate of decomposition of these solid solutions during aging varies based on their relative diffusivities, their contribution to the room-temperature strength and high-temperature creep resistance is independent of the solute diffusivity.Lastly, these slow-diffusing solute elements (Mn, Cr, V, Mo, and W) are studied in L-PBF processed ternary Al-Ce-X alloys. These ternary Al-Ce-X alloys consist of a hypereutectic microstructure, containing a high volume fraction of interconnected, eutectic Al11Ce3 precipitates and submicron, equiaxed Al20CeX2 precipitates. These Al20CeX2 precipitates are isomorphous among the five ternary alloys. The Al20CeX2 precipitates are also highly coarsening-resistant due to the extremely slow diffusivity of the ternary element, resulting in greater retention of strength during thermal exposure at 400°C. Furthermore, these coarsening-resistant Al20CeX2 precipitates also substantially improve alloy creep resistance, increasing the threshold stress for dislocation creep at 300°C from ~32 MPa for the binary Al-Ce alloy to ~77-100 MPa for the ternary Al-Ce-X alloys, and at 400°C from 40 MPa for the ternary Al-Ce-V alloy.The direct scientific outcome of this work is to further understand a relatively new alloy system (Al-Ce), and the impact of alloying additions (Sc, Zr, Er, Mn, Ni, Cr, V, Mo, W) and processing technique (casting vs. L-PBF) on the resulting evolution at high temperatures of microstructure and mechanical properties. This work also results in new, cast and additively-manufactured alloys with extreme high-temperature mechanical properties, for improved performance of lightweight, elevated-temperature applications. Future directions are proposed to (i) further improve understanding of composition and processing on the properties of the Al-Ce-Ni-Mn-Sc-Zr alloy system, (ii) design new, L-PBF specific alloys with improved high-temperature mechanical properties based on the strengthening mechanisms explored here, and (iii) improve fundamental understanding of structure-property relationships in complex eutectic-structured and hierarchically-strengthened alloys.

Subject Added Entry-Topical Term  

Engineering.

Subject Added Entry-Topical Term  

Materials science.

Subject Added Entry-Topical Term  

High temperature physics.

Index Term-Uncontrolled  

Alloy design

Index Term-Uncontrolled  

Sluminum alloys

Index Term-Uncontrolled  

Creep

Index Term-Uncontrolled  

High-temperature applications

Index Term-Uncontrolled  

Mechanical properties

Index Term-Uncontrolled  

Microstructures

Added Entry-Corporate Name  

Northwestern University Materials Science and Engineering

Host Item Entry  

Dissertations Abstracts International. 85-11B.

Electronic Location and Access  

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Control Number  

joongbu:658200