Hydrogen-permeable Alloy, And Hydrogen-permeable Film And Its Production Method

Abstract

A hydrogen-permeable Nb--Ti--Ni alloy having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, with an oxygen content of 1000 ppm or less in an as-cast state, which comprises (a) a hydrogen-permeable primary phase containing 70 atomic % or more of Nb and 10 atomic % or less of Ni, and (b) a eutectic phase having a particle phase comprising Nb and Ti as main components with a small Ni content and having an average particle size of about 5 .mu.m or less, which is dispersed in a matrix phase comprising 60 atomic % or more in total of Ni and Ti and having hydrogen embrittlement resistance, the alloy having a structure substantially free from an intermetallic compound phase.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a hydrogen-permeable alloy having high hydrogen permeability and hydrogen embrittlement resistance and good rollability, a hydrogen-permeable film formed by the hydrogen-permeable alloy, and its production method.

BACKGROUND OF THE INVENTION

[0002] Hydrogen used for fuel cells is at present produced by reforming methane, methanol, etc. However, this method generates impurity gases such as CO, CO.sub.2, H.sub.2O, etc. in addition to hydrogen. Among them, CO deactivates fuel cell electrodes. Therefore, impurity gases should be removed from hydrogen produced by reforming methods. Known as a method for easily purifying hydrogen is a separation method using a hydrogen-permeable metal film. Hydrogen-permeable films practically used at present are Pd--Ag alloy films. However, because the Pd--Ag alloy films disadvantageously contain expensive, rare Pd, it is expected that they cannot be supplied to meet all future demand of fuel cells. Accordingly, metal films substituting for the Pd--Ag alloy films are desired.

[0003] A hydrogen-permeable alloy having high hydrogen permeability and hydrogen embrittlement resistance was developed by combining V, Nb or Ta having high hydrogen permeability in a simple substance form, with other metals such as Ti, Zr, Hf, Ni, Co, etc. to a multi-phase alloy (Summary of the 2006, Autumn Meeting of The Japan Institute of Metals, page 171). As described in JP 2005-232491, JP 2006-274298, and "Materia Japan," Vol. 45, No. 3 (2006), pp. 186-191, Nb--Ti--Ni alloys have excellent hydrogen permeability and hydrogen embrittlement resistance. The Nb--Ti--Ni alloys suitable as hydrogen-permeable alloys are two-phase alloys having (a) a primary phase containing 70 atomic % or more of Nb [represented by (Nb,Ti)p because of small Ni content], and (b) a eutectic phase comprising a phase containing 60 atomic % or more in total of Ni and Ti (represented by NiTi because of small Nb content) and a phase containing a lot of Nb other than the primary phase [represented by (Nb,Ti)e because of small Ni content].

[0004] However, because the Nb--Ti--Ni alloy contains Nb and Ti extremely reactable with oxygen, brittle intermetallic compounds are likely formed by incorporating oxygen from an atmosphere in its mass-production method, in which it is cast in a large furnace, and heat-treated and rolled to a thin plate. With intermetallic compounds, the Nb--Ti--Ni alloy has low hydrogen permeability and hydrogen embrittlement resistance as well as low rollability. Also, the intermetallic compounds make cast alloy bodies brittle depending on their compositions.

[0005] To suppress the formation of intermetallic compounds in the Nb--Ti--Ni alloy, a high-vacuum melting atmosphere has conventionally been used. It has been found, however, that mere melting in vacuum fails to provide Nb--Ti--Ni alloys with such a low oxygen content as to be suitable for hydrogen-permeable alloys.

OBJECT OF THE INVENTION

[0006] Accordingly, an object of the present invention is to provide a hydrogen-permeable Nb--Ti--Ni alloy having a sufficiently low oxygen content to have excellent hydrogen permeability and hydrogen embrittlement resistance and improved rollability, a hydrogen-permeable apparatus formed by such Nb--Ti--Ni alloy, and a method for producing a hydrogen-permeable film.

DISCLOSURE OF THE INVENTION

[0007] As a result of intensive research in view of the above object, the inventors have found that to suppress the formation of brittle intermetallic compounds acting to lower rollability, it is necessary to highly reduce an oxygen content in the Nb--Ti--Ni alloy, and that to this end, it is necessary to reduce oxygen contents in alloy materials and an atmosphere as much as possible, and add a deoxidizer to an alloy material melt to remove oxygen therefrom. The present invention has been completed based on such finding.

[0008] Thus, the hydrogen-permeable Nb--Ti--Ni alloy of the present invention comprises a hydrogen-permeable phase and a hydrogen-embrittlement-resistant phase, and has an oxygen content (measured in an as-cast state) of 1000 ppm or less. It preferably has a primary phase having an oxygen content (measured by EPMA) of 2000 cps (counts per second) or less.

[0009] With the oxygen content of 1000 ppm or less, the cast alloy body has Vickers hardness of 270 HV or less and good rollability.

[0010] The hydrogen-permeable alloy preferably has a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %.

[0011] The hydrogen-permeable Nb--Ti--Ni alloy according to a preferred embodiment of the present invention has a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, with an oxygen content of 1000 ppm or less in an as-cast state, and comprises (a) a hydrogen-permeable primary phase containing 70 atomic % or more of Nb and 10% or less of Ni, and (b) a eutectic phase having a particle phase comprising

[0012] Nb and Ti as main components with a small Ni content and having an average particle size of about 5 .mu.m or less, which is dispersed in a matrix phase comprising 60 atomic % or more in total of Ni and Ti and having hydrogen embrittlement resistance, the alloy having a structure substantially free from an intermetallic compound phase.

[0013] The hydrogen-permeable film of the present invention can be obtained by heat-treating and rolling the above hydrogen-permeable alloy. The hydrogen-permeable film preferably has a thickness of 0.01-1 mm.

[0014] The method of the present invention for producing a hydrogen-permeable film comprises heat-treating and rolling a cast alloy body having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, and having an oxygen content of 1000 ppm or less.

[0015] While melting the cast alloy body of the present invention in vacuum or in a non-oxidizing atmosphere, oxygen is removed from an alloy material melt with a deoxidizer. The deoxidizer is preferably C, Al, Mg, Ca, etc. The amount of the deoxidizer introduced into the melt is preferably 30-1000 ppm based on Nb+Ti+Ni. To remove an oxygen gas from a melting atmosphere, a getter material is preferably used. The getter material is preferably metal V or Ti.

[0016] To reduce an oxygen content in the alloy, the heat treatment of the cast alloy body is conducted preferably in a hydrogen atmosphere.

[0017] The method for producing a hydrogen-permeable film having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, and a thickness of 0.01-1 mm according to a preferred embodiment of the present invention comprises the steps of (1) mixing alloy materials comprising Nb metal, Ti metal and Ni metal each having an oxygen content of 1000 ppm or less, with 30-1000 ppm of a deoxidizer based on the entire weight of the alloy materials, (2) melting the alloy materials in an inert gas atmosphere evacuated to 6.times.10.sup.-3 Pa or less to produce a cast alloy body having an oxygen content of 1000 ppm or less, and (3) repeatedly annealing and rolling the cast alloy body to a thickness of 0.01-1 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a graph showing the relation between an oxygen content and Vickers hardness in a cast Nb--Ti--Ni alloy.

[0019] FIG. 2 is a graph showing the relation between an oxygen content and elongation in a cast Nb--Ti--Ni alloy.

[0020] FIG. 3 is a graph showing the relation between an oxygen content and tensile strength in a cast Nb--Ti--Ni alloy.

[0021] FIG. 4 is a graph showing the X-ray diffraction patterns of the cast Nb--Ti--Ni alloys of Example 2 and Comparative Examples 2 and 4.

[0022] FIG. 5 is a graph showing the relation between an oxygen content and an NiTi.sub.2 phase content in a cast Nb--Ti--Ni alloy.

[0023] FIG. 6 is a graph showing the relation between an oxygen content and the relative intensity of a Nb.sub.40Ti.sub.15Ni.sub.45 phase peak in an X-ray diffraction pattern of a cast Nb--Ti--Ni alloy.

[0024] FIG. 7 is a SEM photograph showing the structure of the cast Nb--Ti--Ni alloy of Example 3.

[0025] FIG. 8 is a SEM photograph showing the structure of the cast Nb--Ti--Ni alloy of Comparative Example 4.

[0026] FIG. 9 is a graph showing the EPMA analysis results of the cast Nb--Ti--Ni alloy of Example 3.

[0027] FIG. 10 is a graph showing the EPMA analysis results of the cast Nb--Ti--Ni alloy of Comparative Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] Composition And Structure of Nb--Ti--Ni Alloy

[0028] The hydrogen-permeable Nb--Ti--Ni alloy of the present invention has a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, 10.ltoreq.y.ltoreq.50, by atomic %. When Ti is less than 10 atomic %, the cast alloy body is so brittle that it cannot easily be rolled. When Ti is more than 60 atomic %, the alloy has low hydrogen permeability. When Ni is less than 10 atomic %, the alloy has high hydrogen permeability, though it is easily embrittled with hydrogen. When Ni exceeds 50 atomic %, the cast alloy body becomes so mechanically brittle that its rolling is difficult. the Ti content x is preferably 20-40 atomic %, and the Ni content y is preferably 20-40 atomic %.

[0029] The hydrogen-permeable Nb--Ti--Ni alloy has a two-phase structure comprising (a) a primary phase [expressed by (Nb,Ti)p, wherein p means "primary phase"] containing 70 atomic % or more of Nb and 10% or less of Ni, and (b) a eutectic phase [expressed by NiTi+(Nb,Ti)e] comprising a phase (expressed by NiTi) containing 60 atomic % or more in total of Ni and Ti, and a phase [expressed by (Nb,Ti)e, wherein e means "eutectic"] containing Nb as a main component with a small Ni content.

[0030] The primary phase is a bcc crystal, in which hydrogen is dissolved and diffused, thereby exhibiting hydrogen permeability. The primary phase preferably has an average particle size of 7-20 .mu.m. The particle size of each primary phase is expressed by a diameter of a circle having the same area as that of the primary phase. The percentage of the primary phase in the alloy structure (corresponding to an area ratio determined on an electron photomicrograph) is preferably 30% or more. Though not restrictive, the upper limit of the percentage of the primary phase is practically 90%, particularly about 80%.

[0031] The NiTi phase constituting the eutectic phase matrix typically has a composition comprising 30-55 atomic % of Ni, 30-55 atomic % of Ti, and 5-15 atomic % of Nb, particularly 40-55 atomic % of Ni, 40-55 atomic % of Ti, and 5-15 atomic % of Nb, to exhibit hydrogen embrittlement resistance. The (Nb,Ti)e phase dispersed in the NiTi phase has a composition comprising Nb and Ti as main components, a Nb content being from 70 atomic % to about 40 atomic % like in the primary phase, with a small Ni content. In the cast alloy body, the eutectic phase has a laminar structure in which the NiTi phase and the (Nb,Ti)e phase are in laminar alignment, but after heat treatment, a (Nb,Ti)e particle phase having an average particle size of about 5 .mu.m or less, particularly 0.5-3 .mu.m, is dispersed in the NiTi phase as shown in FIG. 7.

[0032] 10 atomic % or less, preferably 5 atomic % or less, of Ni may be substituted by Ag, Cr, Cu, Ga, Zn or Fe. 10 atomic % or less of Ti may be substituted by other 4A-group elements. 10 atomic % or less of Nb may be substituted by other 5A-group elements.

[0033] When the oxygen content in the cast alloy body becomes higher than 1000 ppm, a phase (expressed by NiTi.sub.2) having a Ti/Ni atomic ratio about 2 times that in the NiTi phase, and a Nb.sub.40Ni.sub.15Ti.sub.45 phase appear in the eutectic phase. The NiTi.sub.2 phase generally has a composition comprising 20-40 atomic % of Ni, 40-60 atomic % of Ti, and 10-20 atomic % of Nb. These intermetallic compound phases harden the cast alloy body, resulting in extremely low elongation, so that the cast alloy body has extremely reduced rollability. Accordingly, substantially no intermetallic compound phases comprising the NiTi.sub.2 phase and the Nb.sub.40Ni.sub.15Ti.sub.45 phase preferably exist in the structure of the cast Nb--Ti--Ni alloy. The term "substantially no intermetallic compound phases" used herein means that the percentage of the intermetallic compound phases in the alloy structure is 5% or less by weight.

[0034] To meet the above structure conditions, the cast alloy body should have an oxygen content of 1000 ppm or less. The oxygen content of 1000 ppm or less suppresses the formation of the NiTi.sub.2 phase, resulting in high mechanical strength. The oxygen content of the cast alloy body is preferably 800 ppm or less, more preferably 500 ppm or less. Though the lower limit of the oxygen content is not particularly restricted, the oxygen content of less than 20 ppm is not practical because it requires increase in the number of steps, strict condition control, etc. in industrial mass production. When the hydrogen-permeable film is produced by heat treatment and rolling, increase in the oxygen content by heat treatment is 300 ppm or less.

[0035] The primary phase in the cast Nb--Ti--Ni alloy of the present invention should have an oxygen content (measured by EPMA) of 2000 cps (counts per second) or less. The measurement of the oxygen content is conducted as follows. Characteristic X-ray (K.alpha. line) generated by the measurement of a minor-polished alloy sample by an electron probe microanalyzer (EPMA-1610, available from Shimadzu Corporation) is diffracted by an analyzing crystal, and received by a proportional counter to count characteristic X rays inherent to oxygen, thereby measuring the oxygen content in the primary phase. The accelerating voltage is 20 kV, and the sample current is 50 nA. The analyzing crystal is pentaerythritol crystal for NbL.alpha. line, and LiF crystal for NiK.alpha. line and TiK.alpha. line, and artificial crystal LS7A (available from Shimadzu Corporation) for OK.alpha. line. The diameter of electron beams is 1 .mu.m. Measurement is conducted in a range of 50 .mu.m with measuring time of 1 second per one point and a step width of 0.2 .mu.m. Oxygen contents are measured in arbitrarily selected five primary phases each having a maximum diameter of 10 .mu.m or more, and averaged.

[2] Production Method of Nb--Ti--Ni Alloy

[0036] Alloy materials may be melted in vacuum or in an inert gas atmosphere by an arc-melting method, a high-frequency-induction-melting method, an electron-beam-melting method, a laser-melting method, a levitation-melting method, etc. Materials for a crucible used for melting the alloy materials are preferably ceramics such as zirconia, calcia and boron nitride, carbon, water-cooled copper, etc.

[0037] The alloy materials are preferably metals with as high purity as possible. The amount of oxygen in each of Nb, Ti and Ni metals is preferably 1000 ppm or less, more preferably 500 ppm or less. To reduce the oxygen content, each metal material may be heat-treated at 800.degree. C. to 1200.degree. C. for about 0.5-50 hours in hydrogen.

[0038] To reduce the oxygen content in the melting atmosphere, it is preferable to reduce the pressure of the atmosphere sufficiently, particularly to 6.times.10.sup.-3 Pa or less, before melting. More preferably, one or more steps of substituting the atmosphere with an inert gas such as Ar, and evacuating it are conducted after pressure reduction. The inert gas atmosphere, in which melting is conducted, may be atmospheric or reduced-pressure air, for instance, at about 40 kPa.

[0039] When the atmosphere has a large oxygen content, a lot of oxygen may be dissolved in the alloy melt. Accordingly, it is preferable to remove an oxygen gas from the atmosphere as much as possible. To this end, a getter material made of a metal easily absorbing oxygen (Ti, V, etc.) is melted in another vessel in a melting apparatus before melting, so that it can absorb an oxygen gas.

[0040] A deoxidizer such as C, Al, Mg, Ca, etc. is added to the melt to remove oxygen coming from the alloy materials. The deoxidizer dissolved in the melt reacts with oxygen to form slag, which floats on the surface. The amount of the deoxidizer added is preferably slightly smaller than a stoichiometric amount calculated from the oxygen content in the alloy material mixture (for instance, 90% or less), to prevent the deoxidizer from remaining in the resultant cast alloy body. Specifically, the amount of the deoxidizer added is preferably 30-1000 ppm, more preferably 50-300 ppm, based on the entire weight of the alloy materials. When the deoxidizer is less than 30 ppm, oxygen cannot sufficiently be removed from the melt. When it exceeds 1000 ppm, the deoxidizer remains in the resultant cast alloy body, deteriorating hydrogen permeability and rollability. The slag on the melt surface may be removed before solidification, or removed from the solidified cast alloy body surface by a grinder.

[0041] The cast Nb--Ti--Ni alloy thus obtained to have an oxygen content reduced to 1000 ppm or less is substantially free from intermetallic compounds deteriorating rollability, and has Vickers hardness of 270 HV or less, so that it can be easily rolled.

[3] Hydrogen-Permeable Film

[0042] To form a hydrogen-permeable film from the cast Nb--Ti--Ni alloy of the present invention, the cast alloy body is annealed and rolled. The cast alloy body may be hot-forged before rolling. The rolling may be a combination of hot rolling and cold rolling. In the case of cold rolling, a rolling ratio by one operation is preferably 30-70%. Because cold rolling causes hardening, annealing is conducted at a temperature of 900.degree. C. or higher, particularly 1000.degree. C. or higher, to provide the alloy with rollability by recrystallization. The annealing atmosphere is preferably hydrogen. A time period of one annealing operation may be about 0.1-10 hours. To conduct rolling and annealing alternately, the thickness of the cast alloy body can be reduced to 0.01-1 mm, suitable for a hydrogen-permeable film. The total rolling ratio [=(original thickness-final thickness)/original thickness] can be 70% or more, further 80% or more, particularly 90% or more.

[0043] The resultant hydrogen-permeable film is preferably heat-treated at 900-1100.degree. C. for 0.5-300 hours in vacuum or in a non-oxidizing atmosphere. This heat treatment provides the film with improved hydrogen permeability.

[0044] The present invention will be explained in further detail by Examples below without intention of restricting the present invention thereto.

EXAMPLE 1

[0045] As alloy materials, pure Nb metal (oxygen content: 10 ppm), pure Ti metal (oxygen content: 140 ppm) and pure Ni metal (oxygen content: 40 ppm) were mixed to a composition of Ni.sub.30Nb.sub.40Ti.sub.30 (atomic %), and metal Ca as a deoxidizer was added to the mixture in an amount of 200 ppm based on the alloy materials. The resultant mixture was charged into a first water-cooled copper crucible in a vacuum arc-melting apparatus. Metal Ti as a getter material for removing oxygen from an atmosphere was charged into a second water-cooled copper crucible in the vacuum arc-melting apparatus. The amount of the getter material was 70% by mass based on the alloy materials.

[0046] After reducing the pressure of the atmosphere in the vacuum arc-melting apparatus to 4.0.times.10.sup.-3 Pa, an Ar gas was introduced, and evacuation was conducted again to 4.0.times.10.sup.-3 Pa. Thereafter, an Ar gas (purity: 99.99%) at 40 kPa was introduced into the apparatus. The getter material was arc-melted to absorb an oxygen gas in the atmosphere. The alloy materials were then melted to a cast alloy body. To provide a uniform alloy composition, an operation of reversing, melting and solidifying the cast alloy body was repeated 5 times. The resultant cast alloy body was annealed at 1000.degree. C. in a hydrogen gas.

EXAMPLE 2

[0047] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 20 ppm), Ti metal (oxygen content: 250 ppm) and Ni metal (oxygen content: 40 ppm) as alloy materials, in the same manner as in Example 1 except for changing the amount of the getter material to 60% by mass, and the vacuum degree in the vacuum arc-melting apparatus to 5.0.times.10.sup.-3 Pa.

EXAMPLE 3

[0048] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 40 ppm), Ti metal (oxygen content: 250 ppm) and Ni metal (oxygen content: 60 ppm) as alloy materials, in the same manner as in Example 1 except for changing the amount of the getter material to 50% by mass, and the vacuum degree in the vacuum arc-melting apparatus to 5.0.times.10.sup.-3 Pa.

EXAMPLE 4

[0049] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 40 ppm), Ti metal (oxygen content: 250 ppm) and Ni metal (oxygen content: 60 ppm) as alloy materials, in the same manner as in Example 1 except for changing the vacuum degree in the vacuum arc-melting apparatus to 5.0.times.10.sup.-3 Pa without using a getter material.

COMPARATIVE EXAMPLE 1

[0050] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 1600 ppm), Ti metal (oxygen content: 1050 ppm) and Ni metal (oxygen content: 80 ppm) as alloy materials, in the same manner as in Example 1 except for changing the amount of the getter material to 50% by mass, and the vacuum degree of the atmosphere to 8.0.times.10.sup.-3 Pa, without using a deoxidizer.

COMPARATIVE EXAMPLE 2

[0051] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 2300 ppm), Ti metal (oxygen content: 500 ppm) and Ni metal (oxygen content: 80 ppm) as alloy materials, in the same manner as in Example 1 except for changing the amount of the getter material to 50% by mass, and the vacuum degree of the atmosphere to 6.7.times.10.sup.-3 Pa, without using a deoxidizer.

COMPARATIVE EXAMPLE 3

[0052] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 2300 ppm), Ti metal (oxygen content: 1050 ppm) and Ni metal (oxygen content: 80 ppm) as alloy materials, in the same manner as in Example 1 except for changing the amount of the getter material to 50% by mass, and the vacuum degree of the atmosphere to 9.3.times.10.sup.-3 Pa, without using a deoxidizer.

COMPARATIVE EXAMPLE 4

[0053] A cast Nb--Ti--Ni alloy body was produced from Nb metal (oxygen content: 2300 ppm), Ti metal (oxygen content: 1050 ppm) and Ni metal (oxygen content: 80 ppm) as alloy materials, in the same manner as in Example 1 except for changing the vacuum degree of the atmosphere to 6.7.times.10.sup.-2 Pa without using a deoxidizer and a getter material.

[0054] Each cast alloy body of Examples 1-4 and Comparative Examples 1-4 was melted in an inert gas, and its oxygen content was measured by an infrared absorption method. The X-ray diffraction pattern of each cast alloy body was measured. Assuming that each cast body of Examples 1-4 and Comparative Examples 1-2 had a structure having a (Nb,Ti)p phase, a NiTi phase, a (Nb,Ti)e phase, and a NiTi.sub.2 phase, the NiTi.sub.2 phase content was calculated from the X-ray diffraction pattern by Rietveld analysis. In Comparative Examples 3 and 4 in which a Nb.sub.40Ti.sub.15Ni.sub.45 phase appeared, a relative intensity of a Nb.sub.40Ti.sub.15Ni.sub.45 phase peak to a (Nb,Ti) peak was calculated. Further, each cast alloy body was measured with respect to Vickers hardness, and elongation and tensile strength by a tensile test at 900.degree. C. The measurement results are shown in Table 1 together with the production conditions of cast alloy bodies.

TABLE-US-00001 TABLE 1 Oxygen Content Getter Vacuum (ppm) in Material Degree of Alloy Materials (% by Atmosphere Deoxidizer No. Nb Ti Ni mass) (Pa) (ppm) Example 1 10 140 40 70 4.0 .times. 10.sup.-3 200 Example 2 20 250 40 60 5.0 .times. 10.sup.-3 200 Example 3 40 250 60 50 5.0 .times. 10.sup.-3 200 Example 4 40 250 60 None 5.0 .times. 10.sup.-3 200 Comparative 1600 1050 80 50 8.0 .times. 10.sup.-3 None Example 1 Comparative 2300 500 80 50 6.7 .times. 10.sup.-3 None Example 2 Comparative 2300 1050 80 50 9.3 .times. 10.sup.-3 None Example 3 Comparative 2300 1050 80 None 6.7 .times. 10.sup.-2 None Example 4 Oxygen Content (ppm) in Cast Vickers NiTi.sub.2 Content No. Alloy Body Hardness (HV) (% by weight) Example 1 110 206 1.37 Example 2 230 220 2.17 Example 3 400 235 3.91 Example 4 580 238 4.02 Comparative 1030 245 9.23 Example 1 Comparative 1920 253 16.69 Example 2 Comparative 2240 274 -- Example 3 Comparative 5000 449 -- Example 4 Relative Intensity of Tensile Test at 900.degree. C. Nb.sub.40Ti.sub.15Ni.sub.45 Peak Elongation Tensile Strength No. (arbitrary unit) (%) (MPa) Example 1 0 26 411 Example 2 0 23 402 Example 3 0 22 381 Example 4 0 22 370 Comparative 0 20 321 Example 1 Comparative 0 19 314 Example 2 Comparative 9.7 1.6 273 Example 3 Comparative 12.8 0.7 236 Example 4

[0055] FIG. 1 shows the relation between Vickers hardness and an oxygen content in the cast alloy bodies of Examples 1-4 and Comparative Examples 1-4. As the oxygen content increased, the Vickers hardness tended to increase. The cast alloy body of Comparative Example 3 had increased hardness because of the precipitation of a Nb.sub.40Ti.sub.15Ni.sub.45 phase even if the oxygen content was small. Increase in the Vickers hardness was presumably caused by the existence of brittle intermetallic compounds. FIGS. 2 and 3 respectively show the dependence of elongation and tensile strength on the oxygen content. As the oxygen content increased, the elongation and the tensile strength tended to decrease.

[0056] Each cast alloy body was hot-rolled to a 2-mm-thick test piece, which was cold-rolled at a rolling ratio of 50%, annealed at 1000.degree. C. for 1 hour, and cold-rolled again to a total rolling ratio of 80%. The resultant hydrogen-permeable film was as thick as 0.4 mm. The hydrogen-permeable films of Examples 1-4 were free from cracking, etc., though those of Comparative Examples 1-4 were cracked in their edges. Particularly the cast alloy bodies of Comparative Examples 3 and 4 could not be rolled at a rolling ratio of 70% or more.

[0057] FIG. 4 shows the X-ray diffraction patterns of the cast alloy bodies of Example 2 and Comparative Examples 2 and 4. EPMA measurement revealed that the NiTi phase had a composition of Ni.sub.48.7Ti.sub.42.5Nb.sub.8.8, and that the NiTi.sub.2 phase had a composition of Ni.sub.33.1Ti.sub.52.5Nb.sub.14.4. Although the cast alloy body of Example 2 contained substantially no NiTi.sub.2 phase, the cast alloy body of Comparative Example 2 contained a NiTi.sub.2 phase. The cast alloy body of Comparative Example 4 contained a Nb.sub.40Ti.sub.15Ni.sub.45 phase in addition to a NiTi.sub.2 phase.

[0058] FIG. 5 shows the relation between an oxygen content and the relative intensity of a NiTi.sub.2 phase in each cast alloy body. It is clear from FIG. 5 that decrease in the oxygen content leads to decrease in the NiTi.sub.2 phase content. FIG. 6 shows the relation between an oxygen content and the relative intensity of a Nb.sub.40Ti.sub.15Ni.sub.45 phase in each cast alloy body. Although the cast alloy bodies of Examples 1-4 and Comparative Examples 1 and 2 having an oxygen content of 1920 ppm or less did not have a Nb.sub.40Ti.sub.15Ni.sub.45 phase affecting rollability extremely adversely, those of Comparative Examples 3 and 4 having an oxygen content of 2240 ppm and 5000 ppm, respectively, had a Nb.sub.40Ti.sub.15Ni.sub.45 phase.

[0059] Although the cast alloy bodies of Comparative Examples 1 and 2 having an oxygen content of 1030 ppm and 1920 ppm, respectively, did not have a Nb.sub.40Ti.sub.15Ni.sub.45 phase, they contained a relatively large amount of a NiTi.sub.2 phase adversely affecting rollability. It is clear from above that to suppress the formation of the Nb.sub.40Ti.sub.15Ni.sub.45 phase and the NiTi.sub.2 phase, the oxygen content should be 1000 ppm or less.

[0060] FIGS. 7 and 8 are SEM photographs showing the cross section structures of the cast alloy bodies of Example 3 and Comparative Example 4. Although the cast alloy body of Example 3 having an oxygen content of 400 ppm had a primary phase (Nb,Ti)p and a eutectic phase [NiTi phase+(Nb,Ti)e phase], it had substantially no NiTi.sub.2 phase and Nb.sub.40Ti.sub.15Ni.sub.45 phase. On the other hand, the cast alloy body of Comparative Example 4 having an oxygen content of 5000 ppm had large amounts of NiTi.sub.2 and Nb.sub.40Ti.sub.15Ni.sub.45 phases, together with a primary phase (Nb,Ti)p and a eutectic phase [NiTi phase+(Nb,Ti)e phase].

[0061] The composition of the primary phase (Nb,Ti)p in the cast alloy bodies of Examples 1 and 3 and Comparative Example 2 was analyzed by SEM-EDX. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Ni Nb Ti Oxygen No. (atomic %) (atomic %) (atomic %) Content (ppm) Example 1 5.03 81.76 13.21 110 Example 3 4.48 84.50 11.02 400 Comparative 3.19 88.85 7.96 1920 Example 2

[0062] The comparison of the compositions of primary phases in Examples 1 and 3 and Comparative Example 2 revealed that as the oxygen content increased, the primary phase had a higher Nb concentration, resulting in drastic decrease in the Ti concentration. This appears to be due to the fact that as the oxygen content increased, Ti migrated from the primary phase to the eutectic phase. Presumably because Ti coming out of the primary phase reacted with a NiTi phase in the eutectic phase, the NiTi.sub.2 phase increased.

[0063] With respect to the cast alloy bodies of Example 3 and Comparative

[0064] Example 4, the EPMA analysis results are shown in FIGS. 9 and 10, in which the axis of abscissas represents a scanning distance, and the axis of ordinates represents the relative peak intensities of Nb, Ti, Ni and O. It should be noted that to include the data of all elements in one graph, the relative intensities do not have correlations with the concentrations of elements. The oxygen content (averaged cps determined from arbitrary 7 points) in the primary phase (Nb,Ti)p was 1849 cps in Example 3, and 2242 cps in Comparative Example 4.

[0065] In the cast alloy body of Example 3 having a small oxygen content, oxygen exists predominantly in the primary phase, while in the cast alloy body of Comparative Example 4 having a large oxygen content, the oxygen content in the primary phase increased only slightly. On the other hand, the oxygen content in a eutectic phase remarkably increased in Comparative Example 4. It is thus considered that a NiTi.sub.2 phase and a Nb.sub.40Ti.sub.15Ni.sub.45 phase capable of incorporating a large amount of oxygen were formed.

[0066] Among cast alloy bodies having a composition of Nb.sub.100-x-yTi.sub.xNi.sub.y (atomic %), an alloy in which x=20, and y=40, an alloy in which x=40, and y=20, an alloy in which x=20, and y=20, and an alloy in which x=40, and y=40 were evaluated with respect to rollability under the same conditions as in Example 1. As a result, it was found that alloys were rolled at high rolling ratios as long as they had an oxygen content of 1000 ppm or less.

EFFECT OF THE INVENTION

[0067] Because the hydrogen-permeable Nb--Ti--Ni alloy of the present invention has an oxygen content adjusted to 1000 ppm or less in an as-cast state, it has good rollability and excellent hydrogen permeability and hydrogen embrittlement resistance, with the formation of intermetallic compounds substantially suppressed. Accordingly, a thin, hydrogen-permeable film having excellent hydrogen permeability and hydrogen embrittlement resistance can be mass-produced by a low-cost method by which rolling is conducted after casting a Nb--Ti--Ni alloy. The Nb--Ti--Ni alloy with such a low oxygen content can be obtained by removing oxygen from its melt with a deoxidizer in an atmosphere containing a reduced amount of an oxygen gas.

Claims

1. A hydrogen-permeable Nb--Ti--Ni alloy comprising a hydrogen-permeable phase and a hydrogen-embrittlement-resistant phase, which has an oxygen content (measured in an as-cast state) of 1000 ppm or less.

2. The hydrogen-permeable alloy according to claim 1, which has a primary phase having an oxygen content (measured by EPMA) of 2000 cps or less.

3. The hydrogen-permeable alloy according to claim 1, which has Vickers hardness of 270 HV or less.

4. A hydrogen-permeable film produced by heat-treating and rolling the hydrogen-permeable alloy recited in claim 1.

5. A hydrogen-permeable Nb--Ti--Ni alloy having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, with an oxygen content of 1000 ppm or less in an as-cast state, which comprises (a) a hydrogen-permeable primary phase containing 70 atomic % or more of Nb and 10 atomic % or less of Ni, and (b) a eutectic phase having a particle phase comprising Nb and Ti as main components with a small Ni content and having an average particle size of about 5 .mu.m or less, which is dispersed in a matrix phase comprising 60 atomic % or more in total of Ni and Ti and having hydrogen embrittlement resistance, said alloy having a structure substantially free from an intermetallic compound phase.

6. A method for producing a hydrogen-permeable film having a thickness of 0.01-1 mm, comprising heat-treating and rolling a cast alloy body having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, and an oxygen content of 1000 ppm or less.

7. A method for producing a hydrogen-permeable film having a composition represented by Nb.sub.100-x-yTi.sub.xNi.sub.y, wherein 10.ltoreq.x.ltoreq.60, and 10.ltoreq.y.ltoreq.50 by atomic %, and a thickness of 0.01-1 mm, comprising the steps of (1) mixing alloy materials comprising Nb metal, Ti metal and Ni metal each having an oxygen content of 1000 ppm or less, with 30-1000 ppm of a deoxidizer based on the entire weight of said alloy materials, (2) melting said alloy materials in an atmosphere obtained by evacuating to 6.times.10.sup.-3 Pa or less and then introducing an inert gas, to produce a cast alloy body having an oxygen content of 1000 ppm or less, and (3) repeatedly annealing and rolling said cast alloy body to a thickness of 0.01-1 mm.