JP2004099941A

JP2004099941A - Magnesium-base alloy and production method

Info

Publication number: JP2004099941A
Application number: JP2002260465A
Authority: JP
Inventors: Akihisa Inoue; 井上　明久; Kenji Amitani; 網谷　健児
Original assignee: Japan Science and Technology Corp
Current assignee: Japan Science and Technology Agency
Priority date: 2002-09-05
Filing date: 2002-09-05
Publication date: 2004-04-02

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a magnesium-base alloy having excellent strength and elongation and usable in various industrial fields. <P>SOLUTION: The average composition of the magnesium-base alloy is represented in atomic percentage by the composition formula: Mg<SB>100-a-b-c</SB>Ln<SB>a</SB>Zn<SB>b</SB>(wherein, Ln stands for one or more rare earth elements selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and misch metal; and 0.5≤a≤5, 0.2≤b≤4 and 1.5≤a+b≤7 are satisfied). The crystals of the matrix are composed of Mg having a hexagonal structure and ≤5 μm average grain size, and concentration modulation partially exists in the crystals. As to the concentration modulation, the sum of Ln metals is increased by ≥1 to ≤6 atomic% and/or Zn is increased by ≥1 to ≤6 atomic% compared to the average composition of the whole alloy. This magnesium-base alloy is produced by the rapid solidification process. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、強度及び伸びに優れ、産業上の種々の分野に利用可能なマグネシウム基合金の製造方法に関する。
【０００２】
【従来の技術】
溶融状態の合金を急冷することにより種々の組成において、非晶質合金又は非常に微細な結晶を有する合金が得られることが知られている。これらの合金は急速凝固合金と呼ばれ、特にナノメートルサイズの微細な結晶からなる合金は、高い冷却速度が容易に実現できる単ロール法によって製造される場合が多く、Ｆｅ系、Ａｌ系、又はＭｇ系合金について数多くの急速凝固合金材料が得られている。なかでも、Ｍｇ系急速凝固合金は他の急速凝固合金に比べて低比重で軽量であり、種々の分野への応用が期待されている。このようなＭｇ系急速凝固合金としてＭｇ−Ａｌ−Ｍ（ＭはＧａ、Ｓｒ、Ｂａから選ばれる少なくとも１種以上）系急速凝固合金がある（特許文献１）。
【０００３】
しかし、単ロール法によって作製できるＭｇ系急速凝固合金の形状は薄帯に限られており、薄帯形状のままでは応用範囲が限定されるため、棒状などの種々の形状が可能である急速凝固合金材料を開発することが求められている。そのため、アトマイズ法を用いて、粉末形状の急速凝固合金を作製し、ホットプレスや押出し成型等により目的形状に固化成型が容易な合金が開発されている（特許文献２、３）。
本発明者らは、Ｍｇ基合金の組成と、その結晶構造を限定し、長周期六方構造を出現させることにより高強度と高延性を兼ね備えたＭｇ基合金が得られることを見出し、特願２００１−６０９７８号として出願するとともに、論文として報告した（非特許文献１）。
【０００４】
【特許文献１】
特開平５−１７１３３１号公報
【特許文献２】
特開平７−３３７５号公報
【特許文献３】
特開平７−９０４６２号公報
【０００５】
【非特許文献１】
Ａｋｉｈｉｓａ　Ｉｎｏｕｅ　ｅｔ　ａｌ．，「Ｎｏｖｅｌ　ｈｅｘａｇｏｎａｌ　ｓｔｒｕｃｔｕｒｅ　ａｎｄ　ｕｌｔｒａｈｉｇｈ　ｓｔｒｅｎｇｔｈ　ｏｆ　ｍａｇｎｅｓｉｕｍ　ｓｏｌｉｄ　ｓｏｌｕｔｉｏｎ　ｉｎ　ｔｈｅ　Ｍｇ−Ｚｎ−Ｙ　ｓｙｓｔｅｍ」，．Ｍａｔｅｒ．Ｒｅｓ．，Ｍａｔｅｒｉａｌｓ　Ｒｅｓｅａｒｃｈ　Ｓｏｃｉｅｔｙ，Ｊｕｌｙ　２００１，Ｖｏｌ．１６，Ｎｏ．７，ｐ．１８９４−１９００
【０００６】
【発明が解決しようとする課題】
特開平７−３３７５号公報や特開平７−９０４６２号公報に開示されているＭｇ系急速凝固合金は５５０ＭＰａ前後の高い引張強度を有しているが、室温に保持した状態で脆化する現象が見られ、また、伸びも２．５％程度であり、従来のマグネシウム合金に比較して延性が良好であるとは言えなかった。そのため、構造材料としての応用範囲が狭く、実用化の観点から、粉末冶金の手法を用いた場合で５００ＭＰａ以上程度の引張強度を有し、延性が良好なＭｇ基合金が強く求められていた。
【０００７】
さらに、特開平７−９０４６２号公報に開示されているようなＭｇ基急速凝固合金は、通常、Ｍｇが９５原子％未満であるために比重が高く、Ｍｇの軽量という特性を阻害しており、Ｍｇを９３原子％以上、より好ましくは９６原子％以上含有し、軽量で、高強度と高延性を兼ね備えたＭｇ基合金が求められている。
【０００８】
【課題を解決するための手段】
本発明者らは、これらの課題に鑑みて、Ｍｇ含有量が９３原子％以上であり、高強度、高延性のＭｇ基急速凝固合金材料を提供することを目的として、先の発明（特願２００１−６０９７８号）の合金について更に鋭意検討を行なったところ、長周期六方構造を生じなくとも、前記のＭｇ基合金と同等以上の強度を有する場合があることを見出し、その高強度が得られる条件についてさらに詳細に検討を行った。
【０００９】
その結果、マグネシウム基合金において、マグネシウムに希土類元素及びＺｎを加えて、その組成を特定し、さらに、母相の結晶である六方晶構造のＭｇ中に希土類元素及びＺｎの濃度変調部分を生じさせることにより高強度と高延性を兼ね備えたマグネシウム基合金が得られることを見出し、本発明を完成するに至った。
【００１０】
すなわち、本発明は、合金全体の平均組成が原子％による組成式Ｍｇ_{１００−ａ−ｂ}Ｌｎ_ａＺｎ_ｂ（式中、Ｌｎは、Ｙ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｐｍ、Ｓｍ、Ｅｕ、Ｇｄ、Ｔｂ、Ｄｙ、Ｈｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、又はミッシュメタルから選ばれる１種以上の希土類元素、０．５≦ａ≦５、０．２≦ｂ≦４、及び１．５≦ａ＋ｂ≦７である）であり、母相の結晶が平均粒径５μｍ以下の六方晶構造を有するＭｇから形成され、該結晶中の一部に濃度変調が存在し、その濃度変調が合金全体の平均組成と比べて、Ｌｎの合計が１原子％以上６原子％以下及び／又はＺｎが１原子％以上６原子％以下増加していることを特徴とするマグネシウム基合金である。
【００１１】
本発明のマグネシウム基合金において、その合金全体の平均の組成において、Ｙ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｐｍ、Ｓｍ、Ｅｕ、Ｇｄ、Ｔｂ、Ｄｙ、Ｈｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、又はミッシュメタルから選ばれる１種以上の希土類元素の含有量は０．５原子％以上５原子％以下、好ましくは１．０原子％以上３原子％以下である。
【００１２】
本発明において、ミッシュメタル（Ｍｍ）とはＣｅを主成分とする希土類金属の混合体を意味し、安価に希土類金属を用いることができる。Ｙ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｐｍ、Ｓｍ、Ｅｕ、Ｇｄ、Ｔｂ、Ｄｙ、Ｈｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、又はミッシュメタルから選ばれる１種以上の元素が０．５原子％未満であると所望の濃度変調を得ることができず強度が低下し実用に供せない。また、５原子％を超えると、材料の脆化がみられるとともに比強度が低下してしまう。
【００１３】
Ｚｎは０．２原子％以上４原子％以下であり、好ましくは０．５原子％以上２原子％以下である。０．２原子％未満であるとＺｎの添加効果が見られず強度が低い。４原子％を超えると伸びが低下する。
【００１４】
ＬｎとＺｎの総和は１．５原子％以上７原子％以下であり、好ましくは２原子％以上４．５原子％以下、さらに好ましくは２．５原子％以上４原子％以下である。１．５原子％未満であると、濃度変調を得ることができずに強度が低下してしまい、７原子％を超えると脆化が認められるために実用に供せない。
【００１５】
本発明において、形成されるマグネシウム基合金の母相の結晶粒径は５μｍ以下の六方晶構造を有するＭｇから形成されている必要がある。母相の結晶粒径が５μｍを超えて粗大化していると強度の低下が顕著であり従来のマグネシウム合金に比べて顕著な強度増加がなくなる。なお、結晶粒径はどのような測定法を用いても構わないが、１μｍ以上であれば偏光光学顕微鏡により観察を行い、また、１μｍ未満であれば透過型電子顕微鏡により結晶粒径を観察することにより求めることが可能である。平均の結晶粒径は、各々の結晶が球であると仮定した場合の平均値である。本件明細書において、結晶粒径は、かかる測定法によるものである。
【００１６】
さらに、本発明においては、母相の結晶中の一部に濃度変調が存在し、その濃度変調が合金全体の平均組成と比べて、Ｌｎの合計が１原子％以上６原子％以下及び／又はＺｎが１原子％以上６原子％以下増加している必要がある。好ましくは、合金全体の平均の組成に比べてＬｎの合計が１原子％以上６原子％以下及び／又はＺｎが１原子％以上６原子％以下の増加であり、濃度変調している部位の溶質組成がＬｎの合計で２原子％以上８原子％以下かつＺｎが１原子％以上８原子％以下、残部Ｍｇである。
【００１７】
本発明で、濃度変調とは、新たな化合物を析出することなしに結晶粒内で濃度変化することを意味する。この濃度変調の領域は母相の結晶中の一部に存在する必要があるが、本発明において領域の大きさは、通常、体積割合で母相結晶中の１０％から５０％の領域である。この濃度変調の部分において、Ｌｎの濃度が平均の組成と比べて２原子％未満の増加及びＺｎの濃度が平均の組成と比べて２原子％未満の増加である場合、この濃度変調による強度増加が顕著でなく、従来のマグネシウム合金に比べて大きな強度増加が得られない。また、Ｌｎの合計が６原子％を超えた増加及び／又はＺｎが６原子％を超えて増加した部位が存在する場合は、伸びが急激に低下するため実用に供せない。
【００１８】
本発明の上記の濃度変調の部分において好ましい結晶構造は、濃度変調を生じている部分の一部が長周期六方構造を有していることである。長周期六方構造を有している領域は、体積割合で濃度変調の部分の５０％〜９０％の範囲で存在することが好ましい。濃度変調の部分において長周期六方構造を有することにより、濃度変調の領域が安定して存在し、高強度と高延性を兼ね備えたマグネシウム合金となる。この長周期六方構造とは、マグネシウム単位胞ｃ軸長さ（０．５２ｎｍ）の整数倍を１周期として構造をなすものをいう。通常、その周期は３から７の範囲である。
【００１９】
さらに好ましい結晶構造は、上記長周期六方構造において、その長周期の中に原子層レベルで濃度変調が存在することである。例えば、長周期が７周期である場合には、Ｚｎ及び／又はＬｎが多い周期が２周期存在し、その他の５周期はＺｎ及び／又はＬｎが多くない周期となっている。合金の結晶中の濃度変調は、一般に、走査型電子顕微鏡（ＳＥＭ）又は透過型電子顕微鏡（ＴＥＭ）により試料を観察し、顕微鏡付属のＥＤＳなどの分析装置によって測定を行ない、観察することが可能である。
【００２０】
例えば、透過型電子顕微鏡で試料を観察する場合は、合金より１ｍｍ程度の破片をサンプリングした後、ＴＥＭに供する観察試料を作製し、明視野像により観察を行ない、その視野の中にある結晶粒から任意に１つを選択し、さらに分析モードで結晶粒全体の成分をＥＤＳにより測定し、さらに結晶粒内の任意の点についても３０ｎｍφ程度の測定スポットにより測定を行なう。結晶粒内の任意の点については、濃度変調がある部位には積層欠陥が生じ易いという特徴を利用し、明視野で観察される積層欠陥の付近を分析することにより、濃度変調がある部位について測定が行ない易くなる。
【００２１】
好ましい結晶形態としての長周期六方構造を観察する場合は、上記明視野観察した際に電子回折図形を得た後に、結晶格子のｃ軸が晶帯軸と直交する、つまり、電子線入射軸方向と直交する方位となるよう結晶粒を試料傾斜装置を用いて傾斜させる。さらに、得られた回折図形から結晶格子の（０００１）面に対応する回折斑点を見出し、回折図形中心と（０００１）回折斑点との間において、これを何等分かに内分する位置に回折斑点があれば、ｃ軸方向に長周期構造を持っていると判断できる。また、その周期の長さは、マグネシウム単位胞のｃ軸長さと内分の積である。
【００２２】
本発明のマグネシウム基合金は、急速凝固又は成形や加工などにより目的形状にした後に、結晶粒径及び濃度変調が上述の要件を満たしていれば、延性の改善や強度の上昇のために、熱処理や鍛造などの追加の加工を行なうことが可能であり、従来のＭｇ合金と同等に成型及び加工ができることから、本発明のマグネシウム基合金は工業的に有益である。
【００２３】
また、本発明のマグネシウム基合金は、急速凝固時に化合物を生じない程度にＮｉ，Ｃｏ，Ｇａ，Ｃｕ，又はＡｇの元素を１原子％以下の範囲で添加し、強度や延性をさらに向上させた材料を提供することができる。
【００２４】
本発明のマグネシウム基合金を作製する際の急速凝固の方法は、１０^２Ｋ／ｓｅｃ以上の冷却速度で凝固さえ行えれば特に限定されず、例えば、単ロール法によりリボン状の急速凝固合金を作製し、粉砕機により粉末形状の合金にした後、該合金を加工しながら成型することにより作製することが可能であり、また、金型鋳造を用いて溶融状態から急速に凝固させてバルク形状の急速凝固合金を作製し、該合金を目的形状に加工することにより作製することも可能である。また、ガスアトマイズ法を用いて、粉末形状の急速凝固合金を作製しても構わない。
【００２５】
例えば、代表的な単ロール法においては、孔径０．３〜２ｍｍの黒鉛製ノズルを用い、合金をノズル中で、アルゴン雰囲気下で溶融した後、アルゴン雰囲気中で、２００ｒｐｍから２０００ｒｐｍで回転している直径２０ｃｍ程度の銅ロールの回転面上に噴出圧０．５〜２．０ｋｇ／ｃｍ^２で噴出し、急速凝固させることによりリボン状の合金を得ることができる。さらに、リボン状の合金はローターミルなどの粉砕機により粉体状の合金にする。粉砕中は、粉砕による発熱を防ぐために液体窒素などにより冷却を行ないながら粉砕することが望ましい。粉末状の合金は、最終製品にするための固化成型を容易に行なうために、平均粉末粒径を３０μｍ程度にすることが望ましい。
【００２６】
さらに、粉末形状の合金を押出し容器に充填した後、加熱を行ないながら押出し比３〜２０の押出し成型を行なうことにより容易に本発明のマグネシウム基合金からなる成型材を作製することができる。押出し比が３未満であると熱間で成型しても粉末が固化されず、押出し比が２０を超えると押出し容器の破壊などにより成型が困難になるので好ましくない。
【００２７】
さらに、本発明のマグネシウム基合金は製造方法に特に限定されず、前記以外の液体急冷法である双ロール法、溶融抽出法などを用いて、薄帯状やフィラメント状などの目的形状に近い製造方法を選択し、さらに、加工及び熱処理を施すことにより、種々の形状を有する本発明のマグネシウム基合金が容易に得られる。
【００２８】
さらに、本発明においては熱処理によっても母相の結晶中の一部に濃度変調を生じさせ、本発明のマグネシウム基合金を作製することができる。すなわち、合金全体の平均組成が原子％による組成式Ｍｇ_{１００−ａ−ｂ}Ｌｎ_ａＺｎ_ｂ（式中、Ｌｎは、Ｙ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｐｍ、Ｓｍ、Ｅｕ、Ｇｄ、Ｔｂ、Ｄｙ、Ｈｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、又はミッシュメタルから選ばれる１種以上の希土類元素、０．５≦ａ≦５、０．２≦ｂ≦４、及び１．５≦ａ＋ｂ≦７である）のマグネシウム合金を溶融状態から１０^４Ｋ／ｓｅｃ以上の冷却速度で急速凝固を行ない、母相の結晶が平均粒径５μｍ下の六方晶構造を有するＭｇから形成される合金を作製した後に、１５０〜４００℃の温度において熱処理を行ない、結晶中の一部に濃度変調を生じさせるとともに、その濃度変調が合金全体の平均組成と比べて、Ｌｎの合計が１原子％以上６原子％以下又はＺｎが１原子％以上６原子％以下増加にすることを特徴とするマグネシウム基合金の製造方法である。
【００２９】
本発明の組成を有する合金を急速凝固を行なう際に冷却速度を上げると濃度変調をきたした部位が減少する傾向にあり、上記の温度範囲で熱処理を行なうことにより、濃度変調の部位を増加させることが可能である。熱処理温度が１５０℃未満であると熱処理の効果が見られず、また４００℃以上であると急激に軟化するため高強度のマグネシウム基合金を得ることができない。熱処理の温度の好ましい範囲は、２００℃〜３５０℃である。また、熱処理時間は５分から２４時間が好ましい。熱処理時間が５分未満であると合金の内部と外部の熱処理の効果が異なり中心部分まで熱処理を施すことができなくなる。また、２４時間を超えると化合物が析出し脆化する傾向にある。
【００３０】
【実施例】
次に、実施例及び比較例により本発明をさらに具体的に説明する。
実施例１
Ｍｇ_９７Ｙ_２Ｚｎ_１（原子％）からなる合金を、アルゴン雰囲気中、先端に孔径１ｍｍのオリフィスを有した黒鉛製ノズル中で１０ｇ溶融した後、２０００ｒｐｍのロール回転数で回転している直径２０ｃｍの銅ロールの回転面上に、６５０℃でアルゴン雰囲気下、噴出圧０．５ｋｇ／ｃｍ^２でノズルから溶湯を噴出させ、急速凝固させて、幅３ｍｍ厚さ５０μｍの連続した薄帯形状の急速凝固合金を作製した。次に、薄帯形状の急速凝固合金をローターミルにより粉末粒径が平均で５０μｍの粉末状にして押出容器に装入し、押出し成型を行なった。押出しの温度は５７３Ｋ、押出し比１０で成型を行なった。成型後の試料は機械加工により容器部分を削除し成型材とした後に、各種試験及び観察に供した。
【００３１】
引張り試験にはインストロン引張試験機を用い、ひずみ速度５×１０^−４ｓ^−１で試験し、強度及び伸びを測定した。結晶粒径及び長周期六方構造の測定は、試料中から５ヵ所サンプルを行い観察試料とし、透過型電子顕微鏡（日本電子製ＪＥＭ−３０００Ｆ）を用いて観察を行った。結晶粒径については各々の観察試料から２００００倍の観察において５視野の観察を行い結晶粒径とした。
【００３２】
また、濃度変調においては、合金全体の平均組成を求めるために２万倍の視野でＥＤＸによる組成分析を行った後の観察試料から結晶を任意に選択し、結晶中の積層欠陥が観察される部位の周辺を５点抽出し、ＥＤＸによる組成分析を行なった。測定の結果、平均の結晶粒径は１２０ｎｍであり、組成はＭｇ_９７Ｚｎ_１Ｙ_２（原子％）であった。電子線を２０ｎｍにしてさらにＥＤＸの分析を行なったところ、結晶粒内の積層欠陥が観察される部位でＹ及びＺｎの濃度が一番高い部分はＭｇ_８６Ｙ_７Ｚｎ_７（原子％）であった。その他、Ｍｇ_９０Ｙ_５Ｚｎ_５やＭｇ_９３Ｙ_３Ｚｎ_４の組成を有する部分も観察された。
【００３３】
さらに、ＹとＺｎの濃度が高かった部位について制限視野電子回折図形を得た。この回折図形を見ながら、Ｍｇのｃ軸が電子線入射方向と直交するよう結晶粒を透過電顕の試料傾斜装置を用いて傾斜させた。［０１０］入射となるよう傾斜させ、カメラ定数を１５０ｃｍとして回折図形を撮影した。濃度がＭｇ_８６Ｙ_７Ｚｎ_７及びＭｇ_９０Ｙ_５Ｚｎ_５の組成である部分から得られた回折図形からは、回折図形中心と（００１）Ｍｇ回折斑点との間において、これを３等分に内分する位置に回折斑点が見出された。
【００３４】
これよりＭｇのｃ軸方向に長周期六方構造を持っており、その周期の長さはＭｇ単位胞ｃ軸長さの３倍（１．５６ｎｍ）であることが判った。それ以外の部位については、長周期構造が観察されなかった。以上の試験及び観察より、濃度変調が存在する部位はＹが１〜５ａｔ％、Ｚｎが３〜６ａｔ％の濃度増加を示していた。また、最大の濃度変調があった部位には３周期の長周期六方構造を有しており本発明のマグネシウム基合金であった。引張り試験の結果、実施例１で作製した材料の降伏強度は５９０ＭＰａであり延びは４％であった。
【００３５】
実施例２〜４及び比較例１〜４
実施例１と同様に表１に示す組成について、単ロール法で薄帯を作製し粉砕して粉末状の急速凝固合金を作製した後に、押出し温度５７３Ｋ、押し出し比１０の条件にて押出し加工を行ない成型材を作製し、各種試験及び観察に供した。
【００３６】
【表１】

【００３７】
比較例５
実施例１と同様にＭｇ_９７Ｙ_２Ｚｎ_１の組成を有する合金を単ロール法で薄帯を作製し粉砕して粉末状の急速凝固合金を作製した後に、押出し温度４２３Ｋ、押出し比５の条件で押出し加工を行ない成型材を作製し、各種観察及び試験を行なった。比較例５の場合、透過型電子顕微鏡により微小分析を行なったが、顕著な濃度変調が見られなかった。また、降伏強度は３００ＭＰａであり、実施例１と比べて降伏強度が高いとは言えない。
【００３８】
比較例６
市販のマグネシウム合金（ＡＺ９１）を実施例１と同じ工程で成型材を作製し、引張り試験を行なった。降伏強度は２４０ＭＰａ、伸びは３．８％であった。
【００３９】
実施例１〜４及び比較例１〜６から明らかなように、実施例１〜４の成型材は、本発明のマグネシウム基合金であるので、５００ＭＰａ以上の降伏強度を有し、かつ３％以上の伸びを有しており、比較例６に示す従来のマグネシウム合金に比べて高強度を有しており、延性も保持している。比較例１〜４は本発明のマグネシウム基合金の組成範囲から逸脱しているため、強度が低く、また、比較例５は、濃度変調が見られず、本発明のマグネシウム基合金ではないため３５０ＭＰａ程度の強度しか得られなかった。
【００４０】
実施例５
Ｍｇ_９７Ｚｎ_１Ｙ_２の組成からなる合金を溶融し、単ロール法により幅１ｍｍ厚さ２５μｍである急冷凝固リボン材を作製した。リボン材を作製する条件は、ロール：Ｃｕ製２００ｍｍφ、ロール回転速度：６０００ｒｐｍ、ノズル孔径：０．５ｍｍφである。
【００４１】
その後、急速凝固リボン材を２００℃、３００℃、４００℃の各温度で２０分の熱処理を行なった後、強度の指標としてビッカース硬度計により硬度（Ｈｖ）を測定した。また、密着曲げ試験により延性の評価を行なった。密着曲げ試験はリボン材をマイクロメータにＵの字状にはさみこみ、破断するまで間隔を狭めることにより試験を行い、最後まで破断せずに１８０度密着曲げできたものを密着曲げ可と判断した。また、実施例１と同様に濃度変調が最大の部位についてＥＤＸによる分析も行なった。その結果を表２に示す。
【００４２】
【表２】

【００４３】
実施例５〜７の合金は硬度（Ｈｖ）１４０以上を有し、高強度とともに延性を兼ね備えていることが分かる。比較例８の合金は、熱処理温度が本発明の製造方法より低いために、急冷材に比べて濃度変調が少なく硬度も低い、また、比較例９の合金は温度が高いために軟化を起こし硬度が低くなる。
【００４４】
【発明の効果】
以上説明した通り、本発明のマグネシウム合金は高強度と高延性を兼ね備えているため、従来のマグネシウム合金では使用が不可能であった部位などにおいても本発明の合金が使用できるとともに、従来、マグネシウム合金を使用していた部位においても小型化が可能になる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a magnesium-based alloy which has excellent strength and elongation and can be used in various industrial fields.
[0002]
[Prior art]
It is known that an amorphous alloy or an alloy having very fine crystals can be obtained in various compositions by rapidly cooling a molten alloy. These alloys are called rapidly solidified alloys. In particular, alloys composed of fine crystals of nanometer size are often produced by a single roll method that can easily realize a high cooling rate, and are Fe-based, Al-based, or Many rapidly solidifying alloy materials have been obtained for Mg-based alloys. Above all, the Mg-based rapidly solidified alloy has a lower specific gravity and lighter weight than other rapidly solidified alloys, and is expected to be applied to various fields. As such a Mg-based rapidly solidified alloy, there is a Mg-Al-M (M is at least one selected from Ga, Sr, and Ba) -based rapidly solidified alloy (Patent Document 1).
[0003]
However, the shape of Mg-based rapid solidification alloys that can be produced by the single roll method is limited to ribbons, and the application range is limited if the ribbon shape is used, so that various shapes such as rods are possible. There is a need to develop alloy materials. For this reason, a powdery rapidly solidified alloy has been produced using an atomizing method, and an alloy which can be easily solidified and formed into a target shape by hot pressing or extrusion molding has been developed (Patent Documents 2 and 3).
The present inventors have found that a Mg-based alloy having both high strength and high ductility can be obtained by limiting the composition of the Mg-based alloy and the crystal structure thereof and exhibiting a long-period hexagonal structure. 60978 and filed a paper (Non-Patent Document 1).
[0004]
[Patent Document 1]
JP-A-5-171331 [Patent Document 2]
JP-A-7-3375 [Patent Document 3]
JP-A-7-90462
[Non-patent document 1]
Akihisa Inoue et al. , "Novel hexagonal structure and ultrahigh strength of magnesium solid solution in the Mg-Zn-Y system",. Mater. Res. , Materials Research Society, July 2001, Vol. 16, No. 7, p. 1894-1900
[0006]
[Problems to be solved by the invention]
The Mg-based rapid solidification alloys disclosed in JP-A-7-3375 and JP-A-7-90462 have a high tensile strength of about 550 MPa, but the phenomenon of embrittlement while being maintained at room temperature is not observed. In addition, elongation was about 2.5%, and it could not be said that ductility was better than that of a conventional magnesium alloy. Therefore, the range of application as a structural material is narrow, and from the viewpoint of practical use, there has been a strong demand for a Mg-based alloy having a tensile strength of about 500 MPa or more and a good ductility when powder metallurgy is used.
[0007]
Further, Mg-based rapid solidification alloys as disclosed in Japanese Patent Application Laid-Open No. 7-90462 usually have a high specific gravity because Mg is less than 95 atomic%, which hinders the lightweight properties of Mg. There is a demand for a Mg-based alloy containing 93 at% or more, more preferably 96 at% or more of Mg, which is lightweight and has both high strength and high ductility.
[0008]
[Means for Solving the Problems]
In view of these problems, the inventors of the present invention aimed at providing a high-strength, high-ductility Mg-based rapidly solidified alloy material having a Mg content of 93 atomic% or more and a prior invention (Japanese Patent Application Further investigations on the alloy of No. 2001-60978) have revealed that even if a long-period hexagonal structure is not generated, the alloy may have a strength equal to or higher than that of the above-mentioned Mg-based alloy, and the high strength can be obtained. The conditions were examined in more detail.
[0009]
As a result, in the magnesium-based alloy, the rare-earth element and Zn are added to magnesium to specify the composition, and further, the concentration-modulated portion of the rare-earth element and Zn is generated in the hexagonal structure Mg that is the crystal of the mother phase. As a result, they have found that a magnesium-based alloy having both high strength and high ductility can be obtained, and have completed the present invention.
[0010]
That is, the present invention has an average composition in the composition formula _{_{Mg 100-a-b Ln a}} Zn b ( wherein by atomic% of the total alloy, Ln is, Y, La, Ce, Pr , Nd, Pm, Sm, Eu, At least one rare earth element selected from Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or misch metal, 0.5 ≦ a ≦ 5, 0.2 ≦ b ≦ 4, and 1.5 ≦ a + b ≦ 7), and the crystal of the parent phase is formed of Mg having a hexagonal structure with an average particle size of 5 μm or less, and concentration modulation is present in a part of the crystal, and the concentration modulation is A magnesium-based alloy characterized in that the total of Ln is 1 atomic% or more and 6 atomic% or less and / or Zn is 1 atomic% or more and 6 atomic% or less as compared with the average composition.
[0011]
In the magnesium-based alloy of the present invention, in the average composition of the entire alloy, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or The content of one or more rare earth elements selected from misch metal is 0.5 to 5 atomic%, preferably 1.0 to 3 atomic%.
[0012]
In the present invention, misch metal (Mm) means a mixture of a rare earth metal whose main component is Ce, and a rare earth metal can be used at low cost. One or more elements selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or misch metal are less than 0.5 atomic%. If it is, the desired density modulation cannot be obtained, and the strength is reduced, so that it cannot be put to practical use. On the other hand, if it exceeds 5 atomic%, the material becomes brittle and the specific strength is lowered.
[0013]
Zn is at least 0.2 at% and at most 4 at%, preferably at least 0.5 at% and at most 2 at%. When the content is less than 0.2 atomic%, the effect of adding Zn is not seen and the strength is low. If it exceeds 4 atomic%, the elongation decreases.
[0014]
The total of Ln and Zn is 1.5 atomic% or more and 7 atomic% or less, preferably 2 atomic% or more and 4.5 atomic% or less, and more preferably 2.5 atomic% or more and 4 atomic% or less. If the content is less than 1.5 atomic%, the concentration cannot be modulated, and the strength is reduced. If the content is more than 7 atomic%, embrittlement is observed, so that it cannot be put to practical use.
[0015]
In the present invention, the matrix phase of the magnesium-based alloy to be formed must be made of Mg having a hexagonal crystal structure of 5 μm or less. If the crystal grain size of the parent phase exceeds 5 μm, the strength is remarkably reduced and the strength does not increase remarkably as compared with the conventional magnesium alloy. The crystal grain size may be measured by any method, but if it is 1 μm or more, observation is performed with a polarizing optical microscope, and if it is less than 1 μm, the crystal grain size is observed with a transmission electron microscope. Can be obtained by The average crystal grain size is an average value assuming that each crystal is a sphere. In the present specification, the crystal grain size is based on such a measuring method.
[0016]
Further, in the present invention, concentration modulation exists in a part of the crystal of the matrix, and the concentration modulation is such that the total of Ln is 1 atomic% or more and 6 atomic% or less and / or compared to the average composition of the entire alloy. Zn must be increased by 1 atomic% or more and 6 atomic% or less. Preferably, the sum of Ln is 1 atomic% or more and 6 atomic% or less and / or Zn is 1 atomic% or more and 6 atomic% or more as compared with the average composition of the entire alloy, and The composition is 2 atomic% or more and 8 atomic% or less in total of Ln, Zn is 1 atomic% or more and 8 atomic% or less, and the balance is Mg.
[0017]
In the present invention, the term “concentration modulation” means that the concentration changes within a crystal grain without depositing a new compound. The region of the concentration modulation needs to be present in a part of the matrix crystal, but in the present invention, the size of the region is usually 10% to 50% by volume in the matrix crystal. . In the part of the concentration modulation, when the concentration of Ln is less than 2 atomic% as compared with the average composition and the concentration of Zn is less than 2 atomic% as compared with the average composition, the intensity increase due to the concentration modulation Is not remarkable, and a large increase in strength cannot be obtained as compared with a conventional magnesium alloy. Further, when there is a site where the total of Ln exceeds 6 atomic% and / or there is a site where Zn increases beyond 6 atomic%, the elongation is sharply reduced and thus cannot be put to practical use.
[0018]
A preferred crystal structure in the above-mentioned concentration modulation portion of the present invention is that a portion of the concentration modulation portion has a long-period hexagonal structure. It is preferable that the region having the long-period hexagonal structure exists in a range of 50% to 90% of the density-modulated portion by volume ratio. By having a long-period hexagonal structure in the portion of concentration modulation, a region of concentration modulation is stably present, and a magnesium alloy having both high strength and high ductility is obtained. The long-period hexagonal structure refers to a structure in which one cycle is an integral multiple of the length of the magnesium unit cell c-axis (0.52 nm). Usually, the period is in the range of 3 to 7.
[0019]
A more preferable crystal structure is that in the long-period hexagonal structure, concentration modulation exists at the atomic layer level in the long period. For example, when the long period is seven, there are two periods with a large amount of Zn and / or Ln, and the other five periods are periods with a small amount of Zn and / or Ln. In general, the concentration modulation in the crystal of the alloy can be observed by observing the sample with a scanning electron microscope (SEM) or a transmission electron microscope (TEM) and measuring it with an analyzer such as an EDS attached to the microscope. It is.
[0020]
For example, when observing a sample with a transmission electron microscope, after sampling a fragment of about 1 mm from the alloy, an observation sample to be subjected to a TEM is prepared, observed using a bright-field image, and crystal grains in the visual field are observed. Arbitrarily, a component in the entire crystal grain is measured by the EDS in the analysis mode, and an arbitrary point in the crystal grain is also measured using a measurement spot of about 30 nmφ. At any point in the crystal grain, by utilizing the characteristic that stacking faults are likely to occur at sites where there is concentration modulation, by analyzing the vicinity of stacking faults observed in bright field, it is possible to analyze the region where there is density modulation. Measurement becomes easier.
[0021]
When observing a long-period hexagonal structure as a preferred crystal form, after obtaining an electron diffraction pattern during the above-described bright-field observation, the c-axis of the crystal lattice is orthogonal to the crystal zone axis, that is, the direction of the electron beam incident axis. The crystal grains are tilted by using a sample tilting device so as to have an orientation orthogonal to the above. Further, a diffraction spot corresponding to the (0001) plane of the crystal lattice is found from the obtained diffraction pattern, and the diffraction spot is located between the center of the diffraction pattern and the (0001) diffraction spot at a position where the diffraction spot is equally divided. If there is, it can be determined that it has a long period structure in the c-axis direction. The length of the cycle is the product of the c-axis length of the magnesium unit cell and the internal division.
[0022]
The magnesium-based alloy of the present invention is heat-treated to improve ductility and increase strength if the crystal grain size and concentration modulation satisfy the above-mentioned requirements after rapid solidification or forming into a target shape by molding or processing. The magnesium-based alloy of the present invention is industrially useful because it can perform additional processing such as forging and forging, and can be formed and processed in the same manner as a conventional Mg alloy.
[0023]
Further, in the magnesium-based alloy of the present invention, the element of Ni, Co, Ga, Cu, or Ag is added in an amount of 1 atomic% or less to the extent that no compound is generated during rapid solidification, and the strength and ductility are further improved. Material can be provided.
[0024]
The method of rapid solidification when producing the magnesium-based alloy of the present invention is not particularly limited as long as it can be solidified at a cooling rate of 10 ² K / sec or more. It is possible to produce the alloy in powder form with a pulverizer, and then mold the alloy while processing it.Also, it can be rapidly solidified from the molten state using mold casting to form a bulk It is also possible to produce a rapidly solidified alloy of No. 1 by processing the alloy into a target shape. Also, a powdery rapidly solidified alloy may be produced by using a gas atomizing method.
[0025]
For example, in a typical single roll method, a graphite nozzle having a pore diameter of 0.3 to 2 mm is used, and the alloy is melted in an argon atmosphere in the nozzle, and then rotated at 200 rpm to 2000 rpm in an argon atmosphere. A ribbon-shaped alloy can be obtained by ejecting the solution onto a rotating surface of a copper roll having a diameter of about 20 cm at an ejection pressure of 0.5 to 2.0 kg / cm ² and rapidly solidifying it. Further, the alloy in the form of a ribbon is turned into a powdery alloy by a pulverizer such as a rotor mill. During the pulverization, it is desirable to perform the pulverization while cooling with liquid nitrogen or the like in order to prevent heat generation due to the pulverization. The powdered alloy desirably has an average powder particle size of about 30 μm in order to easily perform solidification molding to obtain a final product.
[0026]
Further, after the powdery alloy is filled in the extrusion container, the molding material made of the magnesium-based alloy of the present invention can be easily produced by performing extrusion molding at an extrusion ratio of 3 to 20 while heating. If the extrusion ratio is less than 3, the powder will not be solidified even when hot molded, and if the extrusion ratio exceeds 20, molding becomes difficult due to breakage of the extrusion container, which is not preferable.
[0027]
Further, the magnesium-based alloy of the present invention is not particularly limited to a production method, and a production method close to a target shape such as a ribbon shape or a filament shape using a twin roll method other than the above-mentioned liquid quenching method, a melt extraction method, or the like. Is selected and further subjected to working and heat treatment, whereby the magnesium-based alloy of the present invention having various shapes can be easily obtained.
[0028]
Further, in the present invention, even in the heat treatment, concentration modulation is caused in a part of the crystal of the parent phase, whereby the magnesium-based alloy of the present invention can be produced. That is, the average composition in the composition formula _{_{Mg 100-a-b Ln a}} Zn b ( wherein by atomic% of the total alloy, Ln is, Y, La, Ce, Pr , Nd, Pm, Sm, Eu, Gd, Tb, At least one rare earth element selected from Dy, Ho, Er, Tm, Yb, Lu, or misch metal, 0.5 ≦ a ≦ 5, 0.2 ≦ b ≦ 4, and 1.5 ≦ a + b ≦ 7 Is rapidly solidified from the molten state at a cooling rate of 10 ⁴ K / sec or more to produce an alloy in which a matrix crystal is formed of Mg having a hexagonal structure with an average grain size of 5 μm or less. , A heat treatment is performed at a temperature of 150 to 400 ° C. to cause concentration modulation in a part of the crystal, and the concentration modulation causes the total of Ln to be 1 atomic% or more and 6 atomic% or less as compared with the average composition of the entire alloy. Or, Zn is at least 1 atomic% and 6 A method for producing a magnesium-based alloy, characterized in that the content is increased to not more than atomic%.
[0029]
Increasing the cooling rate during rapid solidification of the alloy having the composition of the present invention tends to decrease the concentration-modulated sites, and performing heat treatment in the above temperature range increases the concentration-modulated sites. It is possible. If the heat treatment temperature is lower than 150 ° C., the effect of the heat treatment is not seen. If the heat treatment temperature is higher than 400 ° C., the alloy is rapidly softened, so that a high strength magnesium-based alloy cannot be obtained. A preferable range of the temperature of the heat treatment is 200 ° C to 350 ° C. The heat treatment time is preferably from 5 minutes to 24 hours. If the heat treatment time is less than 5 minutes, the effects of the heat treatment inside and outside the alloy are different, so that the heat treatment cannot be performed up to the central portion. If the time exceeds 24 hours, the compound tends to precipitate and become brittle.
[0030]
【Example】
Next, the present invention will be described more specifically with reference to Examples and Comparative Examples.
Example 1
10 g of an alloy made of Mg ₉₇ Y ₂ Zn ₁ (atomic%) was melted in a graphite nozzle having an orifice with a hole diameter of 1 mm at the tip in an argon atmosphere, and then a diameter of 20 cm rotating at a roll rotation speed of 2000 rpm. The molten metal is ejected from a nozzle at an ejection pressure of 0.5 kg / cm ² at 650 ° C. in an argon atmosphere on a rotating surface of a copper roll, and is rapidly solidified to form a continuous thin strip having a width of 3 mm and a thickness of 50 μm. A solidified alloy was produced. Next, the ribbon-shaped rapidly solidified alloy was made into a powder having an average particle diameter of 50 μm by a rotor mill, charged into an extrusion container, and extruded. Molding was performed at an extrusion temperature of 573K and an extrusion ratio of 10. The molded sample was subjected to various tests and observations after removing the container portion by machining to obtain a molded material.
[0031]
The tensile test was performed using an Instron tensile tester at a strain rate of 5 × 10 ⁻⁴ s ⁻¹ , and the strength and elongation were measured. For the measurement of the crystal grain size and the long-period hexagonal structure, five samples were taken from the samples to obtain observation samples, which were observed using a transmission electron microscope (JEM-3000F manufactured by JEOL Ltd.). The crystal grain size was determined by observing five visual fields at a magnification of 20,000 from each observation sample.
[0032]
In the concentration modulation, a crystal is arbitrarily selected from an observation sample after performing composition analysis by EDX in a view of 20,000 times in order to obtain an average composition of the entire alloy, and stacking faults in the crystal are observed. Five points around the site were extracted and subjected to composition analysis by EDX. As a result of the measurement, the average crystal grain size was 120 nm, and the composition was Mg ₉₇ Zn ₁ Y ₂ (atomic%). EDX analysis was further performed with an electron beam of 20 nm. As a result, Mg ₈₆ Y ₇ Zn ₇ (atomic%) was found in the portion where the stacking faults in the crystal grains were observed, where the concentration of Y and Zn was the highest. Was. In addition, a portion having a composition of Mg ₉₀ Y ₅ Zn ₅ or Mg ₉₃ Y ₃ Zn ₄ was also observed.
[0033]
Further, a selected area electron diffraction pattern was obtained for a portion where the concentrations of Y and Zn were high. While observing the diffraction pattern, the crystal grains were tilted using a sample tilting device of a transmission electron microscope so that the c-axis of Mg was orthogonal to the electron beam incident direction. [010] The diffraction pattern was photographed with the camera inclined at an incidence and a camera constant of 150 cm. From the diffraction pattern obtained from the portion where the concentration is the composition of Mg ₈₆ Y ₇ Zn ₇ and Mg ₉₀ Y ₅ Zn ₅ , the distance between the diffraction pattern center and the (001) Mg diffraction spot is divided into three equal parts. Diffraction spots were found at the position of internal division.
[0034]
From this, it was found that Mg has a long-period hexagonal structure in the c-axis direction, and the period length is three times (1.56 nm) the length of the Mg unit cell c-axis. No long-period structure was observed in other sites. From the above tests and observations, the site where the concentration modulation exists showed an increase in the concentration of Y of 1 to 5 at% and Zn of 3 to 6 at%. Further, the portion where the maximum concentration modulation occurred had a long-period hexagonal structure with three periods and was the magnesium-based alloy of the present invention. As a result of the tensile test, the yield strength of the material produced in Example 1 was 590 MPa, and the elongation was 4%.
[0035]
Examples 2 to 4 and Comparative Examples 1 to 4
In the same manner as in Example 1, for the composition shown in Table 1, a ribbon was produced by a single roll method and pulverized to produce a powdery rapidly solidified alloy, and then extruded at an extrusion temperature of 573 K and an extrusion ratio of 10. Performed molding materials were prepared and subjected to various tests and observations.
[0036]
[Table 1]

[0037]
Comparative Example 5
After producing the the same manner as in Example 1 Mg ₉₇ Y ₂ alloy having a composition of Zn ₁ to prepare a thin ribbon by a single roll method milled to powder form rapidly solidified alloy, the extrusion temperature 423 K, conditions of extrusion ratio 5 Extrusion was performed to produce a molded material, and various observations and tests were performed. In the case of Comparative Example 5, microanalysis was performed using a transmission electron microscope, but no significant concentration modulation was observed. Further, the yield strength was 300 MPa, and it cannot be said that the yield strength was higher than that of Example 1.
[0038]
Comparative Example 6
A molded material was produced from a commercially available magnesium alloy (AZ91) in the same process as in Example 1, and a tensile test was performed. The yield strength was 240 MPa and the elongation was 3.8%.
[0039]
As is clear from Examples 1 to 4 and Comparative Examples 1 to 6, since the moldings of Examples 1 to 4 are magnesium-based alloys of the present invention, they have a yield strength of 500 MPa or more and 3% or more. , And has higher strength than the conventional magnesium alloy shown in Comparative Example 6, and also maintains ductility. Comparative Examples 1 to 4 deviated from the composition range of the magnesium-based alloy of the present invention, so that the strength was low. Comparative Example 5 did not show concentration modulation and was not a magnesium-based alloy of the present invention. Only a degree of strength was obtained.
[0040]
Example 5
An alloy having a composition of Mg ₉₇ Zn ₁ Y ₂ was melted to produce a rapidly solidified ribbon material having a width of 1 mm and a thickness of 25 μm by a single roll method. The conditions for producing the ribbon material are as follows: roll: 200 mmφ made of Cu, roll rotation speed: 6000 rpm, nozzle hole diameter: 0.5 mmφ.
[0041]
Thereafter, the rapidly solidified ribbon material was subjected to a heat treatment at 200 ° C., 300 ° C., and 400 ° C. for 20 minutes, and the hardness (Hv) was measured by a Vickers hardness meter as an index of strength. The ductility was evaluated by a contact bending test. In the close contact bending test, a test was performed by inserting the ribbon material into a U-shape in a micrometer and narrowing the interval until breakage occurred, and it was judged that a material that could be tightly bent 180 ° without breaking to the end was considered to be capable of close contact bending. Further, similarly to Example 1, the analysis by EDX was also performed on the portion where the concentration modulation was maximum. Table 2 shows the results.
[0042]
[Table 2]

[0043]
It can be seen that the alloys of Examples 5 to 7 have a hardness (Hv) of 140 or more and have both high strength and ductility. Since the heat treatment temperature of the alloy of Comparative Example 8 was lower than that of the manufacturing method of the present invention, the concentration modulation was lower and the hardness was lower than that of the quenched material. Becomes lower.
[0044]
【The invention's effect】
As described above, the magnesium alloy of the present invention has both high strength and high ductility, so that the alloy of the present invention can be used even in a region where the conventional magnesium alloy cannot be used. It is possible to reduce the size of the part where the alloy was used.

Claims

Average composition in the composition formula _{_{Mg 100-a-b Ln a}} Zn b ( wherein by atomic% of the total alloy, Ln is, Y, La, Ce, Pr , Nd, Pm, Sm, Eu, Gd, Tb, Dy, One or more rare earth elements selected from Ho, Er, Tm, Yb, Lu, or misch metal; 0.5 ≦ a ≦ 5, 0.2 ≦ b ≦ 4, and 1.5 ≦ a + b ≦ 7) The crystal of the parent phase is formed of Mg having a hexagonal structure with an average grain size of 5 μm or less, and concentration modulation exists in a part of the crystal, and the concentration modulation is smaller than the average composition of the entire alloy. A magnesium-based alloy, wherein the total of Ln is 1 atomic% or more and 6 atomic% or less and / or Zn is increased 1 atomic% or more and 6 atomic% or less.

The magnesium-based alloy according to claim 1, wherein a part of the portion where the concentration modulation occurs has a long-period hexagonal structure.

Average composition in the composition formula _{_{Mg 100-a-b Ln a}} Zn b ( wherein by atomic% of the total alloy, Ln is, Y, La, Ce, Pr , Nd, Pm, Sm, Eu, Gd, Tb, Dy, One or more rare earth elements selected from Ho, Er, Tm, Yb, Lu, or misch metal; 0.5 ≦ a ≦ 5, 0.2 ≦ b ≦ 4, and 1.5 ≦ a + b ≦ 7) Is rapidly solidified from the molten state at a cooling rate of 10 ⁴ K / sec or more to prepare an alloy in which the crystals of the parent phase are formed of Mg having a hexagonal structure with an average grain size of 5 μm or less. A heat treatment is performed at a temperature of about 400 ° C. to cause concentration modulation in a part of the crystal, and the concentration modulation causes the total of Ln to be 1 atomic% or more and 6 atomic% or less and / or compared to the average composition of the entire alloy. Or, Zn is 1 atom% or more and 6 atoms % Of magnesium alloys.