(An) interstitial aluminum phase developed by the decomposition of nanoscale ceramics

다국어 초록 (Multilingual Abstract)

Many countries are showing much interests to pursue and reach Carbon Neutrality no later than 2050, and they have come up with various proper countermeasures. According to the long-term strategy of the US, energy emission of CO2 coming from transportation sectors could be reduced by a large margin. Knowing that the lightweight frame of vehicles can significantly increase the fuel efficiency in transportation sector, demands of light-weight Al-based materials will eventually increase, especially in the transportation industries.

Light-weight aluminum alloys have been considered as excellent candidates in the transportation industry for various applications such as electric vehicles to reduce energy consumption and dissipate the accumulated heat in electronic devices. This adoption of conventional aluminum alloys is owing to their high specific strength and high thermal conductivity. However, they exhibit a moderate ductility, showing a low formability. This has frequently confined the industrial applicability of the alloys. Furthermore, from classical perspective of materials science and engineering, the strength-ductility trade-off is a long-standing issue in the aluminum alloy development. These issues originate from the inherent properties of aluminum with high stacking fault energy, leading to a limited dislocation cell size under the plastic flow.

Thus, this study introduces a new class of aluminum alloys containing the band in which interstitial oxygen atoms are dissolved in aluminum, termed as an interstitial aluminum alloys (I-Al). To develop the I-Al, we exploit a new processing method, non-metallic alloying method, which includes the decomposition of nano-sized ceramic particles containing non-metallic elements such as carbon, nitrogen, or oxygen atoms in the aluminum melt by the conventional casting technique, although generally non-metallic elements are hardly soluble in aluminum. I-Al exhibits both high-ductility that outperform inherent properties of conventional aluminum alloys, achieved by exclusive characteristics of the band. The band possesses unique microstructural features: i) coherent interface with the aluminum matrix, ii) pre-existing dislocations in the band even in an as-cast specimen, and iii) grain partitioning into numerous sub-grains after solidification. Furthermore, by exploiting I-Al as a starting material for alloying, I-Al-based alloys break the strength-ductility trade-off of conventional aluminum alloys. Therefore, this strategy opens a new pathway for achieving excellent performances and expanding the industrial applicability by using the non-metallic alloying method, which would be of significant importance to the relevant researchers.

Furthermore, this strategy of developing a new class of aluminum alloys apply to the development of aluminum-based composites, which is the first study of application of the concept of I-Al to composite materials. The band where interstitial oxygen are alloyed with aluminum shattered into many pieces during mechanical milling and the fragmented band structure are observed at the interface between the matrix and reinforcements along with lattice fringes. Interstitial oxygen in the fragmented band in the vicinity of interface induce additional chemical bonding via atomic bridging of non-metallic elements of ceramic reinforcements at the interface. This atomic bridging at the interface provide chemical bonding as well mechanical interlocking, enhancing mechanical properties of the I-Al-based composites.

국문 초록 (Abstract)

전 세계 많은 국가들은 최근 기후 위기 대응을 위해 2050 탄소 중립 합의에 발맞춰 다양한 정책 방안을 수립하고 이를 위한 기술 발전에 힘을 싣고 있다. 이에 대응하여 미국이 제시하는 장기적인 접근법들을 살펴보면, 이러한 정책방안을 실현하였을 때, 수송 분야에서 배출되는 이산화탄소량이 크게 감소할 것으로 예상하고 있어 수송 분야에서의 탄소 저감 전략이 크게 중요할 것으로 보인다. 특히, 차체의 경량화를 통해 연비를 높여 수송 분야에서의 탄소 저감이 가능할 것으로 보이며, 이에 전세계적으로 차체 적용 가능한 경량 소재인 알루미늄 합금에 대한 관심도가 높아지고 있다.

경량 합금으로 분류되는 알루미늄 합금은 전기차를 포함하여 차체 내 다양한 부품에 적용 가능한 우수한 물성을 가지는 재료로써, 재료의 경량화로 에너지 소비를 줄여 차체의 연비를 높일 수 있다. 열전도도 또한 우수한 소재로써, 최근 개발되는 첨단 수송기기 내 다양한 시스템 제어를 위해 장착되어 있는 전자기기 등에서 발생하는 열을 효율적으로 방출시킬 수 있어 차체의 내구성을 높일 수 있는 등 다양한 장점으로 인해 최근 주목받고 있다. 하지만, 이러한 장점에도 불구하고 알루미늄 합금의 경우 연신율이 철강 재료에 비해 높지 않고 낮은 성형성을 보여주는 등 현재 수송 기기 산업에의 적용은 쉽지 않다. 재료과학의 관점에서 바라볼 때, 알루미늄 합금을 개발하는 경우 강도를 향상시킬 수 있는 개발 재료의 경우 연신이 감소하는, 이른바 강도/연신 trade-off가 발생하는 현상을 쉽게 발견할 수 있다. 이는 적층 결함 에너지 등에 의해 알루미늄 고유의 물성에 의해 결정되며, 이 한계점은 알루미늄의 소성 변형 시 주로 관찰되는 전위 셀의 크기 등을 증거로 연관지을 수 있다.

이 연구에서는 알루미늄 내 침입형으로 산소를 고용시킨 밴드 구조를 형성시켜 개발된 알루미늄 합금을 새로이 제시하고, 이를 침입형 알루미늄 합금 (interstitial aluminum alloys), 즉 I-Al로 명명하였다. I-Al를 제조하기 위해서는 알루미늄 용탕 내 나노 크기의 세라믹 구성물을 분리시키는 비금속원소 합금법을 활용하였다. 이 합금법에 사용되는 나노 크기의 세라믹 구성물들은 탄소, 질소 또는 산소 등의 비금속원소를 포함하고 있어, 이를 용탕 내 분리시켰을 때, 비금속원소 원자가 분리되어 용탕 내 존재하게 된다. 일반적으로 비금속원소의 경우 알루미늄 내 평형 상태의 고용도가 현저히 낮지만, I-Al의 경우, 국부적으로 고용시킨 클러스터 형태의 밴드가 응고 이후 관찰됨을 알 수 있다. 산소가 상대적으로 다량 포함된 밴드 구조의 경우 알루미늄 기지와 정합 계면을 가지며, 주조 상태임에도 밴드 내 기 존재하는 전위들이 관찰되며, 또한 이러한 밴드들은 전체 그레인을 나누는 (partitioning) 역할을 하여 I-Al내 작은 크기의 아결정립이 많이 관찰된다. 이러한 밴드 구조는 소성 변형 시, 전위와의 상호작용을 통해 작은 크기의 전위셀을 형성시킬 수 있어 보다 균일한 소성 변형이 가능하다. 그 결과, I-Al는 재료의 연신 향상, 가공 경화능 지수 증가, 그리고 인장강도 향상을 가져와, I-Al는 일반적으로 관찰되는 강도/연신 trade-off를 돌파할 수 있는 합금으로 여겨진다. 또한, I-Al는 중력 주조로 제조가 가능하기 때문에 I-Al에 상용 합금 원소 등의 첨가를 통해, 밴드 구조를 포함하는 I-Al 시스템을 유지하면서도 다양한 기계적 물성을 나타내는 합금 설계가 가능하여 우수한 물성을 보임과 동시에 산업에의 적용 가능성을 보다 높일 수 있는 재료 설계라고 할 수 있다.

더불어, 본 연구에서는 I-Al 시스템을 복합 소재에 적용하기 위해 가스 아토마이징 공법을 활용하여 분말화 하였으며, 비금속원소가 포함된 알루미늄 분말의 특성을 연구하였다. 또한, 제조된 분말을 알루미늄 기지로 활용하고 SiC 분말을 강화재로 선정하여, 기계적 밀링법 및 핫프레싱 공법으로 복합 소재를 제조하였다. 비금속원소인 산소가 포함된 I-Al 합금을 기지로 활용한 I-Al/SiC 복합재의 경우 경도, 압축항복강도, 그리고 탄성 계수 등의 기계적 물성이 산소가 포함되지 않은 Al/SiC 복합재보다 우수한 것을 확인하였다. 기지와 SiC 강화재 사이 계면에서 I-Al 주조재에서 형성된 밴드 구조가 관찰되었으며, 이는 산소 농도가 높은 밴드 구조가 계면 결합을 향상시켜 강화 효과를 높임으로써 I-Al/SiC의 기계적 물성 향상을 가져왔다고 볼 수 있다. 이렇듯, I-Al는 종래에 크게 관심 받지 못한 비금속원소를 알루미늄에 합금 원소로써 활용하여 알루미늄 합금 개발의 범위를 확장했다고 여겨진다. 본 연구에서는 I-Al 합금에서 주요한 미세조직인 비금속원소가 고용된 밴드 구조를 나노 크기의 세라믹 구성물들을 활용해 알루미늄 용탕 내에서 분리 하여 형성시켰으며, 중성자 산란법 중 neutron total scattering 기법을 활용하여 I-Al내 산소의 고용을 실험적으로 증명하였기에 그 의의가 있다. 또한, 소성 변형 시 밴드 구조의 역할 뿐만 아니라 I-Al 시스템을 복합 소재로 확장하는 등 다양한 알루미늄 소재로의 적용 가능성을 보여 주어 추후 지속적으로 연구 가능한 알루미늄 합금 시스템으로써 I-Al를 소개하였다.

목차 (Table of Contents)

• List of Contents··············································································i
• List of Figures············································································· vii
• List of Tables···············································································xv
• List of Symbols ···········································································xvi
• List of Abbreviations ··································································xviii
• Abstract ····················································································xix
• Chapter 1. General introduction
• 1.1 Aluminum and aluminum-based materials··········································1
• 1.1.1 Alumium and aluminum alloys ·····················································1
• 1.1.2 Aluminum matrix composites (AMCs)············································6
• 1.1.3 Necessity of aluminum-based materials···········································9
• 1.2 Fabrication processes of aluminum-based materials······························13
• 1.2.1 Liquid state processes ·······························································13
• 1.2.1.1 Casting processes··································································13
• 1.2.1.2 Spray forming process ····························································17
• 1.2.2 Solid state processes·································································20
• 1.2.2.1 Powder metallurgy ································································20
• 1.2.2.2 Severe plastic deformation ·······················································23
• 1.3 Strengthening mechanisms in aluminum-based materials ·······················26
• 1.3.1 Strain hardening······································································26
• 1.3.2 Grain boundary strengthening ·····················································27
• 1.3.3 Solid solution strengthening ·······················································28
• 1.3.4 Precipitation hardening ·····························································28
• 1.4 Research Objective ····································································29
• References ··················································································32
• Chapter 2. Experimental procedures
• 2.1 Sample Preparation ····································································35
• 2.1.1 Materials ··············································································35
• 2.1.2 Fabrication processes ·······························································38
• 2.1.2.1 Gravity casting process ···························································38
• 2.1.2.2 Gas atomization process··························································38
• 2.1.2.3 Mechanical milling································································39
• 2.1.2.4 Consolidation processes ··························································39
• 2.1.2.5 Heat treatment ·····································································40
• 2.2 Structural characterization····························································40
• 2.2.1 X-ray diffraction ·····································································40
• 2.2.2 Neutron total scattering ·····························································41
• 2.2.3 X-ray photoelectron spectroscopy·················································44
• 2.3 Microstructure examination ··························································44
• 2.3.1 Optical microscopy··································································44
• 2.3.2 Scanning electron microscopy ·····················································45
• 2.3.3 Transmission electron microscopy················································45
• 2.3.4 Atom probe tomography····························································46
• 2.4 Mechanical tests········································································46
• 2.4.1 Hardness measurement ·····························································46
• 2.4.2 Elastic modulus measurement ·····················································47
• 2.4.3 Tension test ···········································································47
• 2.4.4 Compression test·····································································48
• 2.5 Thermal examination ··································································48
• 2.5.1 Difference scanning calorimetry ··················································48
• References ··················································································50
• Chapter 3. Structural analysis of the I-Al
• 3.1 Introduction ·············································································51
• 3.2 Sample preparation ····································································53
• 3.3 Results and discussion ································································57
• 3.3.1 Decomposition of ZnO nanoparticles in the aluminum ························57
• 3.3.2 Formation of the band structure in the I-Al······································68
• 3.3.3 In-situ stacking faults in the I-Al ··················································76
• 3.3.4 Neutron total scattering analysis of the I-Al ·····································82
• 3.4 Conclusion ··············································································89
• References ··················································································90
• Chapter 4. Mechanical behavior of the I-Al
• 4.1 Introduction ·············································································94
• 4.2 Sample preparation ····································································97
• 4.3 Results and discussion ································································99
• 4.3.1 Mechanical behavior of the WLAB in the I-Al ·································99
• 4.3.2 Mechanical behavior of SFs in the I-Al ········································109
• 4.4 Conclusion ············································································112
• References ················································································114
• Chapter 5. Characteristics of aluminum-intercalated oxygen in an
• interstitial aluminum alloy powder
• 5.1 Introduction ···········································································116
• 5.2 Sample preparation ··································································119
• 5.3 Results and discussion ······························································122
• 5.3.1 Structural analysis of gas-atomized I-Al powder ·····························122
• 5.3.2 Effect of mechanical milling on structure and mechanical properties of the IAl powder··················································································130
• 5.4 Conclusion ············································································137
• References ················································································138
• Chapter 6. Development of SiC particles reinforced aluminum matrix
• composites with intercalated oxygen
• 6.1 Introduction ···········································································140
• 6.2 Sample preparation ··································································145
• 6.3 Results and discussion ······························································147
• 6.3.1 Development of I-Al/SiC composites with microsized SiC particles ······147
• 6.3.2 Development of I-Al/SiC composites with nanosized SiC particles ·······158
• 6.4 Conclusion ············································································173
• References ················································································175
• Abstract in Korean ·····································································176

참고문헌 (Reference)

1 D. S. Huang, M. T. Lin, H. H. Cheng, "Microelectron", 52 905–911, 2012