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第 5 章 結言
電子ビーム溶融法（EBM）により作製したIN 718について，積層造形材特有の組織を観 察することで，組織形成メカニズムを調査し，また，熱処理が機械的特性に与える影響を調 査したことで以下の知見を得た．
1. EBMにより造形されたIN 718 as-built材の織観察の結果，表面近傍のコントア領域では 溶融池中心でエピタキシャル成長による積層方向に[001]を向いた柱状粒が形成し，溶融 池端部で粗大な柱状粒と微細な等軸粒の混粒組織が形成することを明らかにした．また，
コントア領域では溶融池端部の微細な等軸粒により，中心部のハッチ領域に比べ，クリ ープ特性が劣る．
2. EBMにより造形されたIN 718 as-built材の組織観察の結果，中心部のハッチ領域では積 層方向に[001]を向いた柱状粒が支配的な組織が形成し，ところどころに粉末の不完全溶 融によると軸粒が密集した領域と，過冷却による単独で存在する等軸粒が形成する．
3. 本研究のEBM材は，高温下で造形するため，複雑な熱履歴により，as-built材で強化相
であるγ″相が析出する．STA材では，溶体化処理によりγ″相が微細化し，DA材では時
効により粗大化した．熱処理材で得られる組織は，造形時間や冷却速度に影響を受け，
EBM材の熱処理を考える際は，造形条件も考慮に入れる必要性を明らかにした．
4. EBM as-built材では，粒界と粒内に微小δ相が観察され，982 ˚Cの溶体化処理によって
粒界δ相は粗大化し，1000 ˚C以上の溶体化処理によって固溶した．1095 ˚Cで溶体化処 理を施した後，955 ˚Cで3.5時間時効処理を施すことで，粒界に粗大なδ相が析出し，
溶体化温度とその後の時効処理によって，粒界δ相の大きさを制御することができるこ とを明らかにした．
5. EBM as-built材では，熱処理により，粒界δ相を適当な大きさで析出させることで，粒
界すべりが抑制されるため，粒界が強化され，90˚ 材のクリープ特性が向上することを 明らかにした．
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86
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