JP4276853B2 - Niobium-based composite material - Google Patents

Niobium-based composite material Download PDF

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JP4276853B2
JP4276853B2 JP2003021265A JP2003021265A JP4276853B2 JP 4276853 B2 JP4276853 B2 JP 4276853B2 JP 2003021265 A JP2003021265 A JP 2003021265A JP 2003021265 A JP2003021265 A JP 2003021265A JP 4276853 B2 JP4276853 B2 JP 4276853B2
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temperature
niobium
strength
toughness
composite material
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JP2004232013A (en
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久男 田中
功 岩永
良平 田中
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Japan Ultra High Temperature Materials Research Institute JUTEM
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Japan Ultra High Temperature Materials Research Institute JUTEM
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Description

【0001】
【発明の属する技術分野】
本発明は、ガスタービン部材等の超高温用材料として使用される、高温クリープ強度と靭性に優れたニオブ基複合材料に関する。
【0002】
【従来の技術】
化石燃料の節減と地球環境保全の観点から、火力発電の熱効率の向上が求められ、ガスタービンの運転温度の一層の高温化が急務の課題となっている。現在、ガスタービン部材には主にNi基超合金が用いられているが、その耐熱温度の向上はほぼ限界に達している。そのため、より高温で使用しうる新たな耐熱材料が必要となっており、その一つとして高融点金属であるNbをベースにした材料が注目されている。
【0003】
ニオブシリサイド系の合金は高温強度が高くかつ密度も低いことから、既存のNi基超合金に替わりうる有望な高温材料の一つと考えられている。しかし、ニオブシリサイド自体は、他の金属間化合物と同様に室温での延性・靭性に乏しく、構造材料としての利用は大きく制限されている。このような欠点を解消する手段として、ニオブシリサイドを延性に富むニオブベースの母相中に分散させた複合材料を形成することが考えられ、本発明者らは、かかる複合材料について従来から種々の検討を行なってきた。
【0004】
本発明者らは、Nb−X−Si系材料(XはNbに固溶する元素)の高温強度について種々の検討を行い、先にNb−5〜30Mo−5〜15W−5〜20Si(数字はat%)なる組成を有するニオブ基複合材料を提案した(特開2001−226734号公報)。この材料は、Nb母相中にMo及びWを固溶させることによる固溶強化と、ニオブシリサイドを分散析出させることによる析出強化との複合強化により、高温強度と常温靭性を高めることを意図したものである。
【0005】
Nb−Si系の二元系合金は、その状態図から明らかなように、Nb:18.7at%付近に共晶点がある。一般に、Nbが18.7at%以下の亜共晶域では、マトリックス(連続相)が延性大なNb相であるのに対して、Nbが18.7at%以上の過共晶域では、延性の低いシリサイドがマトリックスになるため、硬くて脆い材料になり、靭性の確保が難しくなる。したがって、この材料はSi濃度をあるレベル以下に制限せざるを得ないという問題を有していた。
【0006】
本発明者ら上記の問題を解決する手段についても種々検討し、Nb−X−Si系の元素Xの種類や濃度を適切に選択することにより、過共晶域においても、Nb5Si3を主成分とするニオブシリサイド中に微細なニオブ固溶体が析出したミクロ組織にすることが可能なことを知見した。この知見に基づき、Mo:2〜10at%、Si:18.7〜26at%を含有するNb−Mo−Siの三元系合金を先に提案している(特願2002−116997)。この材料は、ニオブシリサイドの量を多くしているため高温強度が高く、かつ上記のようなミクロ組織であるため常温の靭性も低下しないという特徴を有している。
【0007】
また本発明者らは、Siを含有しないNb−Mo−W系固溶合金の機械的特性についても検討し、これにC又はCとHfを添加して、炭化物析出の効果により延性を改善した炭素添加ニオブ基複合材料を提案している(特願2002−116998)。
【0008】
【発明が解決しようとする課題】
上述のようなニオブシリサイド系の複合材料は、超高温域での引張強度や圧縮強度が高くかつ常温での靭性も兼ね備えており、高温ガスタービン等の構造材料として使用可能である。しかし、かかる目的に使用する材料は、例えば1400℃を超えるような温度域で、かつ強い応力が作用する条件下でも、十分な耐久性を有することが必要であり、上述のニオブシリサイド系複合材料も、とくにクリープ強度においてさらなる改善が望まれている。
【0009】
そこで本発明は、Nb−X−Si系材料において、高温強度や常温靭性に加えて、超高温下でのクリープ特性が改善されたニオブ基複合材料を提供することを目的とする。
【0010】
【課題を解決するための手段】
本発明者らは、ニオブシリサイド系の二相合金のマトリックス(連続相)や分散相内又はその粒界に微細な炭化物を析出させることにより、クリープ特性を改善させ得ることを知見した。また、析出炭化物を微細化するという観点から、炭素CとハフニウムHfを同時に添加するのが有効なことを知見した。
【0011】
この知見に基づく本発明のニオブ基複合材料の第一は、
Moを5〜30at%、Wを5〜30at%、Siを5〜18.7at%、Cを2〜20at%及びHfを2〜20at%含有し、残部がNbと不可避的不純物とからなるものである。
【0012】
また、本発明のニオブ基複合材料の第ニは、
Moを5〜30at%、Siを18.7〜26at%、Cを2〜20at%及びHfを2〜20at%含有し、残部がNbと不可避的不純物とからなるものである。
【0013】
なお、上記のいずれの複合材料においても、添加元素Mo,W,Si,C及びHfの合計量は、80at%以下であることが好ましく、より好ましくはこれを60at%以下とする。
【0014】
上記の複合材料の第一は亜共晶域の、第二は過共晶域のニオブシリサイド系二相合金であるが、後記実施例で示すように、いずれも微細な炭化物が析出したミクロ組織が形成され、これにより炭化物がない場合と比較して、クリープ強度を向上させることができる。
【0015】
【発明の実施の形態】
まず、Nb−X−Si系材料(XはMo又はMoとW)に、CとHfを添加した時のミクロ組織の変化について説明する。図1は、後記実施例に示す方法で調製した試験片の組織写真で、図1(a)はNb−5Mo−16Si(数字はat%、以下同じ)、図1(b)はNb−5Mo−15W−16Si、図1(c)はNb−5Mo−15W−16Si−5(C+Hf)の組織を示す。写真の倍率はいずれも500倍である。
【0016】
図1(a)及び(b)において、明るい部分がNb固溶相で、初晶のNb固溶相は大きく粒成長している。暗い部分はニオブシリサイドで、共晶のNb固溶相の中に細かく分散している。これに対して、CとHfを添加した図1(c)においては、初晶及び共晶のNb固溶相の中に、きわめて細かい炭化物(HfC)がほぼ一様に分散していることが認められる。また、この写真では十分明らかではないが、ニオブシリサイド相内やこれとNb固溶相との粒界にも極く微細な炭化物が析出している。
【0017】
CとHfを添加した場合の機械特性の変化については後記実施例で詳述するが、同一成分でCとHfを添加しないものと比較して、常温(圧縮)強度に若干の低下が見られるものの、常温靭性はほとんど変化せず、クリープ強度の改善が認められる。上述した微細な炭化物(大部分が1μm以下と思われる)の析出が、クリープ特性の改善に寄与しているものと考えられる。
【0018】
次に、本発明における成分範囲の設定根拠について説明する。本発明の複合材料の第一は、Nb−Si系で亜共晶域の材料で、Siの上限は共晶組成の18.7at%である。Siの下限を5at%とする理由は、これ未満ではニオブシリサイドの析出量が少な過ぎて、これによる高温強度の向上効果が十分でないためである。なお、より好ましいSiの下限値は10at%である。
【0019】
Nbに固溶する強化元素として、MoとWの双方を添加する理由は、いずれか一方のみの場合より高温強度と常温靭性のバランスのとれた材料を得易いためである。Mo添加量の下限を5at%と理由は、これ未満ではMoによるマトリックス(Nb固溶相)の固溶強化効果が不十分であり、上限を30at%とする理由は、これを超えると、マトリックス自体の靭性が著しく低下するためである。
【0020】
同様に、W添加量の下限を5at%と理由は、これ未満ではWによるマトリックスの固溶強化効果が不十分なためであり、上限を30at%と理由は、これを超えるとマトリックス自体の靭性が著しく低下するためである。なお、より好ましいWの上限は、15at%である。
【0021】
また、Cの下限を2at%と理由は、これ未満では微細な炭化物量が少なく、クリープ強度の改善効果が不十分なためであり、上限を20at%と理由は、これを超えると、内部欠陥が多くなり脆くなると同時に、クリープ強度の改善効果が頭打ちになるためである。また、Hfの濃度範囲を2〜20at%とする理由は、炭化物中のCに対応するHfの量を上記のCの範囲と同じにするためである。なお、より好ましいCとHfの範囲は、5〜10at%である。
【0022】
本発明の複合材料の第二は、Nb−Si系で過共晶域の材料で、Siの下限は共晶組成の18.7at%である。Siの上限を26at%とする理由は、これを超えるとシリサイドの体積分率が増え、延性維持に必要なNb固溶相が過少になって、靭性が急激に低下するためである。
【0023】
この第二発明の材料においては、Nbに固溶する強化元素としてMoのみを添加し、Wは添加しない。その理由は、Wを含む場合と含まない場合で、ミクロ組織に大きな差が生じるためである。すなわち、Moのみ添加した場合は、延性の大きいNb固溶相がマトリックス(連続相)となるため、材料の靭性が高い。これに対して、Wを添加すると延性の小さいニオブシリサイドが連続相になり易く、靭性が著しく低下するためである。
【0024】
Mo添加量の下限を5at%とする理由は、これ未満では、MoによるNb固溶相の強化効果が不十分になるためである。また、Mo添加量の上限を30at%とする理由は、これを超えるとマトリックスであるNb固溶相が脆くなり、材料の靭性が低下するためである。なお、より好ましいMoの上限は、20at%である。また、C及びHfの成分範囲の設定理由は、前述の第一発明材料の場合と同じである。なお、上記の第一及び第二発明の材料において、添加元素Mo,W,Si,C及びHfの残部は実質的にNbであればよく、不可避的不純物元素や少量のその他の添加元素を含んでいてもよい。
【0025】
次に、本発明のニオブ基複合材料の製造方法について述べる。
本発明のニオブ基複合材料の成形体は、溶解・凝固法によっても、粉末焼結法によっても製造することができる。溶解・凝固法としては、例えば高周波加熱スカル溶解法、プラズマアーク等によるハース溶解法や真空アーク、プラズマアーク、電子ビーム等を熱源として、原料を水冷鋳型内に連続的に供給して溶解しインゴットを形成する逐次溶解・凝固法等のいずれによってもよい。また、生成したインゴットを必要に応じて均質化熱処理をすればよい。
【0026】
粉末焼結法としては、配合原料をMA(メカニカルアロイイング)法等により所定の粒度まで粉砕・混合し、各種の粉末焼結法(例えば、ホットプレス、HIP処理、放電プラズマ焼結法など)により成形体を形成して、必要により熱処理を行えばよい。だだし、Nb,Mo,W,Si等はいずれも酸化され易く、酸素のピックアップは材料の強度特性等を著しく劣化させるから、粉末焼結法においては、粉砕−焼結−熱処理の各工程を不活性ガス雰囲気又は真空下で行うことが必要である。
【0027】
【実施例】
シリサイド分散型のNb−Mo−W−Si系複合材料において、CとHfの添加の有無による機械的特性の差を比較した。Siの添加量は、亜共晶域(16at%)と過共晶域(20at%と22at%)の両方で試験した。本発明材はCとHfの添加量をともに2.5at%と5at%の2水準とし、比較材はCとHf無添加で他の成分が略同一のものを用いた。亜共晶域でのMo及びWの添加量は、Moを5at%一定、Wを5,10,15at%の3水準とした。また、過共晶域では、Moのみ5,10at%の2水準で添加した。
【0028】
供試材は、所定の組成に配合された、いずれも純度99、9%以上の塊状Nb,Moと粒状W、純度99.999%以上の塊状Si、純度98%以上のスポンジ状Hf及び純度98%以上のNbCを原料として用い、これを溶解・急冷凝固させて成形体を製造した。この供試材を2073Kで48時間、Ar雰囲気下で加熱する均質化熱処理を行った。
【0029】
このようにして製造した、熱処理後の本発明材び比較材を所定の試験片形状に切り出し、高温の引張クリープ試験と常温の圧縮試験及び三点曲げ試験を行った。高温の引張クリープ試験の試験片の形状は、厚さ3mm、全長80mm、平行部寸法3mm厚×3mm幅×10mm長で、標点間距離は10mmである。引張クリープ試験は、当所が開発した超高温クリープ試験機HTT−3000を用い、1773K,Ar雰囲気で応力は20〜200MPaの範囲で行なった。
【0030】
常温の圧縮試験は、熱処理材から放電加工で切り出した3mm角、高さ6mmの試料を用いて、室温で歪速度3×10-4-1の条件で行い、得られた歪−応力曲線から0.2%耐力を判定した。また、常温の三点曲げ試験は、ASTM E−399に準ずる方法で、試験片は3mm幅×6mm高さ×30mm長さのものに、長手方向の中央に3mm深さのノッチの切り込みを入れた試験片について、支点間距離24mmで試験した。
【0031】
亜共晶域での高温クリープ試験の結果を図2に、過共晶域での結果を図3に示す。これらの図において、○,□,△の記号はCとHfを含有しない比較例であり、●,■の記号は本発明例である。この結果から、CとHfの添加により、Nb−X−Si系複合材料のクリープ特性が改善されることが明らかになった。
【0032】
一方、常温の強度(圧縮試験)と靭性(三点曲げ試験)の比較結果を図4に示す。この図は、横軸に常温強度(圧縮試験の0.2%耐力,MPa)、縦軸に三点曲げ試験の破壊靭性値(MPa・m1/2)をとって表示しており、○,△の記号はCとHfを含有しない比較例であり、●,▲の記号は本発明例である(亜共晶域と過共晶域のデータを同一図中に表示した)。この結果から、本発明例の常温強度はやや低下の傾向があるが、常温靭性は、本発明例と比較例でほとんど差がないことが知れる。なお、この材料の常温強度は十分であるから、この低下は実用上ほとんど問題とならない。
【0033】
【発明の効果】
本発明により、Nb−X−Si系材料の超高温下でのクリープ強度を改善することが可能になった。これにより、高温の引張、圧縮強度や常温靭性に加えて、高温のクリープ強度も大きいため、超高温域で用いる構造用材料として好適なニオブ基複合材料を提供することが可能になった。
【図面の簡単な説明】
【図1】本実施例の試験片のミクロ組織を示す写真である。
【図2】本実施例におけるCとHfの添加による高温クリープ強度の変化を示す図である。
【図3】本実施例におけるCとHfの添加による高温クリープ強度の変化を示す図である。
【図4】本実施例の材料のCとHfの添加の有無での常温の強度及び靭性の比較を示す図である。
[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a niobium-based composite material excellent in high-temperature creep strength and toughness that is used as a material for ultra-high temperatures such as gas turbine members.
[0002]
[Prior art]
From the viewpoint of fossil fuel saving and global environmental conservation, improvement in thermal efficiency of thermal power generation is required, and further increase in operating temperature of gas turbines is an urgent issue. At present, Ni-base superalloys are mainly used for gas turbine members, but the improvement of the heat resistant temperature has almost reached the limit. Therefore, a new heat-resistant material that can be used at a higher temperature is required, and as one of them, a material based on Nb, which is a refractory metal, has attracted attention.
[0003]
Niobium silicide-based alloys are considered to be one of the promising high-temperature materials that can replace existing Ni-base superalloys because of their high-temperature strength and low density. However, niobium silicide itself, like other intermetallic compounds, has poor ductility and toughness at room temperature, and its use as a structural material is greatly limited. As a means for eliminating such drawbacks, it is conceivable to form a composite material in which niobium silicide is dispersed in a ductile niobium-based matrix. I have studied.
[0004]
The present inventors have made various studies on the high-temperature strength of Nb—X—Si-based materials (X is an element that dissolves in Nb), and previously described Nb-5-30Mo-5-15W-5-20Si (numbers). Has proposed a niobium-based composite material having a composition of (at%) (Japanese Patent Laid-Open No. 2001-226734). This material was intended to increase the high-temperature strength and room temperature toughness by combined strengthening of solid solution strengthening by dissolving Mo and W in the Nb matrix and precipitation strengthening by dispersing and precipitating niobium silicide. Is.
[0005]
As is clear from the phase diagram, the Nb—Si binary alloy has a eutectic point near Nb: 18.7 at%. In general, in the hypoeutectic region where Nb is 18.7 at% or less, the matrix (continuous phase) is a ductile Nb phase, whereas in the hypereutectic region where Nb is 18.7 at% or more, the ductility is low. Since low silicide becomes a matrix, it becomes a hard and brittle material, and it becomes difficult to ensure toughness. Therefore, this material has a problem that the Si concentration must be limited to a certain level or less.
[0006]
The present inventors have also studied various means for solving the above-mentioned problems, and by appropriately selecting the type and concentration of the element X of the Nb—X—Si system, Nb 5 Si 3 can be reduced even in the hypereutectic region. It has been found that it is possible to obtain a microstructure in which fine niobium solid solution is precipitated in niobium silicide as a main component. Based on this knowledge, a ternary alloy of Nb—Mo—Si containing Mo: 2 to 10 at% and Si: 18.7 to 26 at% has been proposed previously (Japanese Patent Application No. 2002-116997). This material has the characteristics that the high-temperature strength is high because the amount of niobium silicide is increased, and the toughness at normal temperature does not decrease because of the microstructure described above.
[0007]
In addition, the present inventors also examined the mechanical properties of the Nb—Mo—W-based solid solution alloy containing no Si, and added C or C and Hf to improve the ductility by the effect of carbide precipitation. A carbon-added niobium-based composite material has been proposed (Japanese Patent Application No. 2002-116998).
[0008]
[Problems to be solved by the invention]
The niobium silicide-based composite material as described above has high tensile strength and compressive strength in an ultra-high temperature range and also has toughness at room temperature, and can be used as a structural material for high-temperature gas turbines and the like. However, the material used for this purpose needs to have sufficient durability in a temperature range exceeding, for example, 1400 ° C. and under a condition in which a strong stress acts, and the above-mentioned niobium silicide composite material However, further improvement in creep strength is desired.
[0009]
Accordingly, an object of the present invention is to provide a niobium-based composite material in which creep properties at an ultrahigh temperature are improved in addition to high-temperature strength and normal-temperature toughness in an Nb—X—Si-based material.
[0010]
[Means for Solving the Problems]
The present inventors have found that creep characteristics can be improved by precipitating fine carbides in the matrix (continuous phase) or dispersed phase of the niobium silicide-based two-phase alloy or in the grain boundaries thereof. Moreover, it discovered that it was effective to add carbon C and hafnium Hf simultaneously from the viewpoint of refining the precipitated carbide.
[0011]
The first of the niobium-based composite materials of the present invention based on this knowledge is
Containing 5 to 30 at% Mo, 5 to 30 at% W, 5 to 18.7 at% Si, 2 to 20 at% C, and 2 to 20 at% Hf, with the balance being Nb and inevitable impurities It is.
[0012]
In addition, the second of the niobium group composite material of the present invention,
It contains 5 to 30 at% Mo, 18.7 to 26 at% Si, 2 to 20 at% C, and 2 to 20 at% Hf, with the balance being Nb and inevitable impurities .
[0013]
In any of the above composite materials, the total amount of additive elements Mo, W, Si, C, and Hf is preferably 80 at% or less, and more preferably 60 at% or less.
[0014]
The first composite material is a hypoeutectic region and the second is a hypereutectic region niobium silicide two-phase alloy, but as shown in the examples below, the microstructure in which fine carbides are precipitated is used. As a result, the creep strength can be improved as compared with the case where no carbide is present.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
First, changes in the microstructure when C and Hf are added to an Nb—X—Si based material (X is Mo or Mo and W) will be described. FIG. 1 is a structural photograph of a test piece prepared by the method described in the examples below. FIG. 1 (a) is Nb-5Mo-16Si (numbers are at%, the same applies hereinafter), and FIG. 1 (b) is Nb-5Mo. FIG. 1C shows the structure of Nb-5Mo-15W-16Si-5 (C + Hf). The magnification of each photo is 500 times.
[0016]
In FIGS. 1A and 1B, the bright portion is the Nb solid solution phase, and the primary crystal Nb solid solution phase has a large grain growth. The dark part is niobium silicide and is finely dispersed in the eutectic Nb solid solution phase. On the other hand, in FIG. 1C in which C and Hf are added, extremely fine carbides (HfC) are dispersed almost uniformly in the primary and eutectic Nb solid solution phases. Is recognized. Further, although not sufficiently clear in this photograph, extremely fine carbides are precipitated in the niobium silicide phase and also at grain boundaries between this and the Nb solid solution phase.
[0017]
The change in mechanical properties when C and Hf are added will be described in detail in Examples below, but there is a slight decrease in normal temperature (compressed) strength as compared with the same component without addition of C and Hf. However, room temperature toughness is hardly changed, and an improvement in creep strength is observed. Precipitation of the fine carbides described above (mostly considered to be 1 μm or less) is considered to contribute to the improvement of creep characteristics.
[0018]
Next, the basis for setting the component range in the present invention will be described. The first of the composite materials of the present invention is an Nb—Si-based material in the hypoeutectic region, and the upper limit of Si is 18.7 at% of the eutectic composition. The reason why the lower limit of Si is 5 at% is that if it is less than this, the amount of niobium silicide deposited is too small, and the effect of improving the high-temperature strength is not sufficient. A more preferable lower limit of Si is 10 at%.
[0019]
The reason why both Mo and W are added as strengthening elements that dissolve in Nb is that it is easier to obtain a material having a balance between high-temperature strength and room-temperature toughness than either one of them. The reason why the lower limit of the amount of Mo added is 5 at% is that if it is less than this, the solid solution strengthening effect of the matrix (Nb solid solution phase) by Mo is insufficient. This is because its own toughness is significantly reduced.
[0020]
Similarly, the reason why the lower limit of the amount of W added is 5 at% is that if it is less than this, the effect of solid solution strengthening of the matrix by W is insufficient, and the reason that the upper limit is 30 at% exceeds the toughness of the matrix itself. This is because remarkably decreases. A more preferable upper limit of W is 15 at%.
[0021]
The reason why the lower limit of C is 2 at% is that if the amount is less than this, the amount of fine carbides is small and the effect of improving the creep strength is insufficient, and the upper limit is 20 at%. This is because the effect of improving the creep strength reaches its peak at the same time as the amount of slag increases. The reason why the Hf concentration range is set to 2 to 20 at% is to make the amount of Hf corresponding to C in the carbides the same as the above C range. A more preferable range of C and Hf is 5 to 10 at%.
[0022]
The second composite material of the present invention is an Nb—Si-based hypereutectic material, and the lower limit of Si is 18.7 at% of the eutectic composition. The reason why the upper limit of Si is 26 at% is that if it exceeds this, the volume fraction of silicide increases, the Nb solid solution phase necessary for maintaining ductility becomes too small, and the toughness rapidly decreases.
[0023]
In the material of the second invention, only Mo is added as a strengthening element dissolved in Nb, and W is not added. The reason is that a large difference occurs in the microstructure between the case where W is included and the case where W is not included. That is, when only Mo is added, the toughness of the material is high because the Nb solid solution phase having high ductility becomes a matrix (continuous phase). On the other hand, when W is added, niobium silicide having a small ductility easily becomes a continuous phase, and the toughness is remarkably lowered.
[0024]
The reason why the lower limit of the amount of addition of Mo is 5 at% is that if it is less than this, the strengthening effect of the Nb solid solution phase by Mo becomes insufficient. Moreover, the reason why the upper limit of the amount of Mo added is 30 at% is that if it exceeds this, the Nb solid solution phase as a matrix becomes brittle, and the toughness of the material decreases. A more preferable upper limit of Mo is 20 at%. The reason for setting the C and Hf component ranges is the same as in the case of the first invention material described above. In the materials of the first and second inventions described above, the remainder of the additive elements Mo, W, Si, C, and Hf may be substantially Nb, including inevitable impurity elements and a small amount of other additive elements. You may go out.
[0025]
Next, a method for producing the niobium-based composite material of the present invention will be described.
The molded body of the niobium-based composite material of the present invention can be produced by a melting / solidifying method or a powder sintering method. Examples of the melting / solidification method include a high-frequency heating skull melting method, a hearth melting method using a plasma arc, a vacuum arc, a plasma arc, an electron beam, etc. Any of the sequential dissolution and coagulation methods for forming the film may be used. Moreover, what is necessary is just to perform the homogenization heat processing for the produced | generated ingot as needed.
[0026]
As the powder sintering method, the blended raw materials are pulverized and mixed to a predetermined particle size by the MA (mechanical alloying) method or the like, and various powder sintering methods (for example, hot pressing, HIP treatment, discharge plasma sintering method, etc.) The formed body may be formed by the above, and heat treatment may be performed if necessary. However, Nb, Mo, W, Si, etc. are all easily oxidized, and oxygen pick-up significantly deteriorates the strength characteristics of the material. Therefore, in the powder sintering method, the steps of pulverization-sintering-heat treatment are performed. It is necessary to carry out under an inert gas atmosphere or under vacuum.
[0027]
【Example】
Differences in mechanical properties between the presence and absence of addition of C and Hf were compared in silicide-dispersed Nb—Mo—W—Si based composite materials. The amount of Si added was tested in both the hypoeutectic region (16 at%) and the hypereutectic region (20 at% and 22 at%). The material according to the present invention uses two levels of C and Hf, both 2.5 at% and 5 at%, and the comparative material is the same with no other components added with C and Hf. The addition amounts of Mo and W in the hypoeutectic region were set at three levels of Mo at a constant 5 at% and W at 5, 10, 15 at%. In the hypereutectic region, only Mo was added at two levels of 5, 10 at%.
[0028]
The test materials were blended in a predetermined composition, all of which were 99, 9% or more of massive Nb, Mo and granular W, 99,999% or more of massive Si, 98% or more of sponge-like Hf, and purity 98% or more of NbC was used as a raw material, and this was melted and rapidly solidified to produce a molded body. The sample was subjected to a homogenization heat treatment at 2073 K for 48 hours in an Ar atmosphere.
[0029]
The heat-treated present invention material and comparative material thus manufactured were cut into a predetermined test piece shape, and subjected to a high temperature tensile creep test, a normal temperature compression test, and a three-point bending test. The shape of the test piece for the high-temperature tensile creep test is 3 mm in thickness, 80 mm in total length, 3 mm in parallel dimension × 3 mm in width × 10 mm in length, and the distance between the gauge points is 10 mm. The tensile creep test was conducted using an ultra-high temperature creep tester HTT-3000 developed by the Institute, under a stress of 20 to 200 MPa in a 1773 K, Ar atmosphere.
[0030]
The compression test at room temperature was performed using a 3 mm square and 6 mm high sample cut out from the heat treated material by electric discharge machining at room temperature under a strain rate of 3 × 10 −4 s −1. 0.2% yield strength was determined. The normal temperature three-point bending test is a method according to ASTM E-399. The test piece is 3 mm wide x 6 mm high x 30 mm long, and a notch with a depth of 3 mm is inserted in the center in the longitudinal direction. The test pieces were tested at a fulcrum distance of 24 mm.
[0031]
FIG. 2 shows the results of the high temperature creep test in the hypoeutectic region, and FIG. 3 shows the results in the hypereutectic region. In these figures, the symbols ◯, □, and Δ are comparative examples that do not contain C and Hf, and the symbols ● and ■ are examples of the present invention. From this result, it has been clarified that the addition of C and Hf improves the creep characteristics of the Nb—X—Si based composite material.
[0032]
On the other hand, FIG. 4 shows a comparison result between normal temperature strength (compression test) and toughness (three-point bending test). In this figure, the horizontal axis represents normal temperature strength (0.2% proof stress of compression test, MPa) and the vertical axis represents fracture toughness value (MPa · m 1/2 ) of three-point bending test. Symbols △ and △ are comparative examples not containing C and Hf, and symbols ● and ▲ are examples of the present invention (data of hypoeutectic region and hypereutectic region are shown in the same figure). From this result, it is known that the normal temperature strength of the present invention example tends to be slightly lowered, but the normal temperature toughness is hardly different between the present invention example and the comparative example. In addition, since the normal temperature intensity | strength of this material is enough, this fall hardly causes a problem practically.
[0033]
【The invention's effect】
According to the present invention, it has become possible to improve the creep strength of an Nb—X—Si-based material at an ultrahigh temperature. As a result, in addition to high-temperature tensile and compressive strength and room-temperature toughness, high-temperature creep strength is also high, so that it has become possible to provide a niobium-based composite material suitable as a structural material used in an ultrahigh temperature range.
[Brief description of the drawings]
FIG. 1 is a photograph showing the microstructure of a test piece of this example.
FIG. 2 is a diagram showing a change in high-temperature creep strength due to the addition of C and Hf in this example.
FIG. 3 is a diagram showing a change in high-temperature creep strength due to the addition of C and Hf in this example.
FIG. 4 is a diagram showing a comparison of strength and toughness at room temperature with and without the addition of C and Hf in the material of this example.

Claims (2)

Moを5〜30at%、Wを5〜30at%、Siを5〜18.7at%、Cを2〜20at%及びHfを2〜20at%含有し、残部がNbと不可避的不純物とからなるニオブ基複合材料。Niobium containing 5 to 30 at% Mo, 5 to 30 at% W, 5 to 18.7 at% Si, 2 to 20 at% C and 2 to 20 at% Hf, with the balance being Nb and inevitable impurities Base composite material. Moを5〜30at%、Siを18.7〜26at%、Cを2〜20at%及びHfを2〜20at%含有し、残部がNbと不可避的不純物とからなるニオブ基複合材料。A niobium-based composite material containing 5 to 30 at% Mo, 18.7 to 26 at% Si, 2 to 20 at% C, and 2 to 20 at% Hf, with the balance being Nb and inevitable impurities .
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