chromium-free nickel alloys for hot sulfuric and sulfur environments

7
Chromium-free nickel alloys for hot sulfuric and sulfur environments Joseph W. Newkirk a, *, JenHsien Hsu a , Richard K. Brow a , Thomas Lillo b a Materials Science & Engineering, Missouri University of Science and Technology, 223 McNutt Hall, Rolla, MO 65409-0340, United States b Material Sciences, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 3415-2218, United States article info Article history: Received 21 December 2009 Received in revised form 19 May 2010 Accepted 4 June 2010 Available online 23 February 2011 Keywords: NieSieNb G-phase (Ni 16 Si 7 Nb 6 ) Corrosion Cold rolling abstract There are few adequate materials available for severe corrosion conditions, like those of the SeI thermochemical cycle. High Si, Ni-alloys have excellent corrosion resistance, especially in mineral acids, but have typically been limited by poor mechanical properties or difficult fabrication issues. The ductility of nickel silicide, Ni 3 Si, can be improved through a combination of micro- and macro-alloying. Nb and other minor alloying elements yield a cast alloy with excellent corrosion resistance to sulfuric acid and good mechanical properties. In this paper, efforts to optimize the alloys performance are pre- sented along with progress toward the development of a wrought version of the material. It was found that an appropriate heat treatment provides the largest improvement in the cast NieSi alloy microstructure. Trials have resulted in more than a 50% reduction by the cold rolling process. This process not only increases homogenization but also results in a more uniform distribution of G-phase particles, which is beneficial for the improvements in ductility and corrosion resistance. These alloys have great potential for use in future hydrogen production as well as fossil energy combustion. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction The SulfureIodine thermochemical cycle has been proposed as a method for producing large quantities of hydrogen gas from water using the waste heat of a nuclear reactor. Sulfuric acid is decomposed at temperatures up to 850 C to form SO 2 , which is combined with iodine and water to form HI mole- cules and reform sulfuric acid. The HI molecules decompose into iodine and hydrogen. The iodine and the sulfuric acid are re-used in this closed loop cycle. This cycle has a number of advantages, including a high thermodynamic efficiency and the ability to produce hydrogen directly from water. One potential drawback is the extreme corrosion conditions which limit the possible materials for construction. The sulfuric acid decomposition loop, which includes a re- concentration of the sulfuric acid after the reaction to form HI, is a particularly difficult area to find suitable construction materials. The high temperatures combined with the highly corrosive nature of sulfuric acid at these concentrations and pressures leave almost no materials available for use. Fig. 1 and Table 1 show corrosion rates of several commercial corrosion resistance alloys in different concen- trations of boiling sulfuric acid at atmospheric and high pressure conditions. Duriron (Fe-14.5 wt% Si) has the satis- factory corrosion resistance over the entire concentration range indicated. However, the high Si content and the D0 3 structure make it extremely brittle. The precious metals, like Pt, have great corrosion resistance, but the price is too high to * Corresponding author. Tel.: þ1 573 341 4725; fax: þ1 574 341 6934. E-mail address: [email protected] (J.W. Newkirk). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 36 (2011) 4588 e4594 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.007

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    Article history:

    Received 21 December 2009

    Received in revised form

    19 May 2010

    acid is decomposed at temperatures up to 850 C to form SO2,

    advantages, including a high thermodynamic efficiency and

    the ability to produce hydrogen directly from water. One

    potential drawback is the extreme corrosion conditions which

    limit the possible materials for construction.

    pressures leave almost no materials available for use.

    factory corrosion resistance over the entire concentration

    range indicated. However, the high Si content and the D03structure make it extremely brittle. The precious metals, like

    Pt, have great corrosion resistance, but the price is too high to

    * Corresponding author. Tel.: 1 573 341 4725; fax: 1 574 341 6934.

    Avai lab le at www.sc iencedi rect .com

    w.

    i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4E-mail address: [email protected] (J.W. Newkirk).which is combined with iodine and water to form HI mole-

    cules and reform sulfuric acid. The HI molecules decompose

    into iodine and hydrogen. The iodine and the sulfuric acid are

    re-used in this closed loop cycle. This cycle has a number of

    Fig. 1 and Table 1 show corrosion rates of several

    commercial corrosion resistance alloys in different concen-

    trations of boiling sulfuric acid at atmospheric and high

    pressure conditions. Duriron (Fe-14.5 wt% Si) has the satis-1. Introduction

    The SulfureIodine thermochemical cycle has been proposed

    as a method for producing large quantities of hydrogen gas

    from water using the waste heat of a nuclear reactor. Sulfuric

    The sulfuric acid decomposition loop, which includes a re-

    concentration of the sulfuric acid after the reaction to formHI,

    is a particularly difficult area to find suitable construction

    materials. The high temperatures combined with the highly

    corrosive nature of sulfuric acid at these concentrations andAccepted 4 June 2010

    Available online 23 February 2011

    Keywords:

    NieSieNb

    G-phase (Ni16Si7Nb6)

    Corrosion

    Cold rolling0360-3199/$ e see front matter Copyright doi:10.1016/j.ijhydene.2010.06.007There are few adequate materials available for severe corrosion conditions, like those of

    the SeI thermochemical cycle. High Si, Ni-alloys have excellent corrosion resistance,

    especially in mineral acids, but have typically been limited by poor mechanical properties

    or difficult fabrication issues. The ductility of nickel silicide, Ni3Si, can be improved

    through a combination of micro- and macro-alloying. Nb and other minor alloying

    elements yield a cast alloy with excellent corrosion resistance to sulfuric acid and good

    mechanical properties. In this paper, efforts to optimize the alloys performance are pre-

    sented along with progress toward the development of a wrought version of the material. It

    was found that an appropriate heat treatment provides the largest improvement in the cast

    NieSi alloy microstructure. Trials have resulted in more than a 50% reduction by the cold

    rolling process. This process not only increases homogenization but also results in a more

    uniform distribution of G-phase particles, which is beneficial for the improvements in

    ductility and corrosion resistance. These alloys have great potential for use in future

    hydrogen production as well as fossil energy combustion.

    Copyright 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.a r t i c l e i n f o a b s t r a c tChromium-free nickel alloys fenvironments

    Joseph W. Newkirk a,*, JenHsien Hsu a, RichaaMaterials Science & Engineering, Missouri University of Science anbMaterial Sciences, Idaho National Laboratory, P.O. Box 1625, Idah

    journa l homepage : ww2010, Hydrogen Energy Phot sulfuric and sulfur

    K. Brow a, Thomas Lillo b

    echnology, 223 McNutt Hall, Rolla, MO 65409-0340, United States

    lls, ID 3415-2218, United States

    e lsev ie r . com/ loca te /heublications, LLC. Published by Elsevier Ltd. All rights reserved.

  • would be a single-phase material consisting of b-phase exclu-

    sively. In order to produce an optimized single b-phase micro-

    structure, three major directions were studied in the present

    work: 1. A homogenization treatment could result in the elim-

    ination of the high temperature phases. 2. Control the Nb

    content to maximize its effect on ductility while reducing the

    likelihoodof formingG-phase. 3. Coldworking thealloyprior to

    homogenization should increase the rate and also break up the

    G-phase particles, resulting in a finer, more uniform distribu-1

    1.5

    2

    2.5ro

    sion

    rate

    (mm/

    yr.)

    NiMoFeZr

    i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4589be an engineering material. The Ni(Si,Nb) alloy has excellent

    corrosion resistance in different concentrations of sulfuric

    acid. It easily meets the target of 2.54 e

    NiSi20Nb3B0.5, Ni Base 0.07 0.10

    a Tested on 37.4 atm.The compression test was done with a Gleeble system.

    Rockwell C hardness testingwas donewith an InstronWilson/

    pressure boiling sulfuric acid [1,2].

    80 wt% 96 wt%

    1 atm 1 atm 13.8 atm

    e 2.03 8.00

    >2.54 0.76 3.00

    w0 e 0.10ation. The effects of these three approaches on the microstruc-

    ture and properties of several NieSieNb alloys are described.

    2. Experimental procedures

    The alloys were melted in an induction furnace under an

    Argon atmosphere. Themelt was initially heated to 1500 C. Itwas held for about 15 min to homogenize the melt. The

    temperature was then lowered to 1350 C and the melt washeld for about 5 min at this temperature before pouring. The

    melt was poured into one of two graphite molds. One mold

    was a rectangular mold which held 15 lbs of metal. The other

    was a cylindrical bottlemoldwhich held 20 lbs ofmetal. The Si

    plus Nb contents were kept close to a combined 23 atomic

    percent to increase the chances of a stable b-phase. Table 2

    shows the composition, hardness and ultimate strength of

    alloys prepared for this study.

    An Electro-Discharge Machine (EDM) was used to cut

    15 mm long cylindrical compression specimens with a 10 mm

    diameter and rolling specimens with dimensions of

    30 10 3mm from the homogenized (950 C 4 days in argon)ingots. The compression specimen surfaces were abraded by

    SiC paper before the test. The tests were conducted in an

    argon atmosphere in the temperature range between room

    temperature to 950 C. The strain rate used was 3.3 104 s1.For the cold work process, the samples were rolled either at

    room temperature or at 300 C with intermediate anneal of950 C for 5 h. The rolling reduction per pass was set to 10%.The hardness and reduced dimensions were recorded at each

    step and the microstructures were evaluated after various

    steps.

    For corrosion tests, rectangular samples were cut to

    10 7 3mmand ground by #1200 SiC paper. The corrosiontests were done by immersing the samples in 70 wt% H2SO4 at

    the boiling temperature (165 C). After each immersion,

  • Rockwell hardness tester. Microstructure was observed by

    optical microscopy and Hitachi S570 SEM. Differential

    Thermal Analyses (DTA) was done using a PerkineElmer

    DTA7 under argon at a heating rate of 10 C/min.

    3. Results and discussion

    22 and 23 at%, so the stable structure of those alloys should be

    single b-phase. So, a homogenization heat treatment could be

    used.

    Fig. 4 shows the DTA results indicating the phase trans-

    formation temperatures in each alloy. After comparing these

    results to the NieSi phase diagram, 950 C was chosen as anappropriate homogenization temperature for these alloys.

    a b eutectic, see Fig. 6. The changes in microstructure arerelated to the improvement in corrosion resistance, see Table

    Table 2 e Mechanical properties of alloys [3].

    Alloy Hardness (RC) UTS (Mpa) Elogation (%)

    As cast 900 C 1 Day 950 C 4 Day

    NiSi18Nb5B0.5 e 45.0 1.4 39.7 2.4 e 0.7bNiSi20Nb2B0.5 46.4 2.7 43.3 1.5 42.5 1.5 e eNiSi20Nb3B0.5 e 44.4 1.9 41.2 1.2 876 17a 3.6 1.6aNiSi21Nb1.6B0.5 e e 39.6 1.3 810 33a 1.9 0.6a

    a Heat treatment: 950 C 4 Day.b Heat treatment: 900 C 1 Day.

    i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44590The goal of this work is to determine howmicrostructure and

    alloy composition affect the ductility and corrosion properties

    of NieSi alloys that could be used for components of the SeI

    thermochemical cycle for hydrogen production. The effects of

    heat treatments, alloy modifications and cold working in

    these properties are discussed.

    3.1. Heat treatment

    For the NieSieNb alloys studied in this paper, the as-cast

    microstructure usually includes a b eutectic, b, G and g-phase, see Fig. 2. G and g-phase are brittle, and the a beutectic corrodes rapidly in strong acids [3]. Further, the

    galvanic cell created between phases accelerates corrosion.

    Fig. 3 shows the corrosion attack seems to have been focused

    on the interdendritic regions. So, a single b-phase micro-

    structure should be ideal. There is no complete NieSieNb

    phase diagram. An assumption that small additions of Nb and

    B do not significantly change the NieSi diagram was made.

    The SiNb content of those alloys listed on Table 2 is betweenFig. 2 e Microstructure of as-cast NiSi20Nb3B0.5. It contains

    needle-like g-phase, aDb eutectic (black), b-matrix and

    G-phase (bright) [3].3, and reduction in hardness as shown in Table 2.

    3.2. Decreasing Nb content

    After the homogenization heat treatments, the G-phase was

    still prevalent in the microstructure. G-phase is apparently

    a stable phase for these compositions. As G-phase is created

    when the Nb content exceeds the solubility of Nb in b-phase,

    decreasing the Nb content could eliminate or reduce G-phase

    in the microstructure. However, Nb was added to improveThe peritectoid transformation, a b2/ b1, may happen forsome alloys at temperatures over about 1000 C. This trans-formation should be avoided because it separates b-phase into

    two phases. Fig. 5 shows themicrostructure for 1 day at 950 Cand it is evident that the alloy was not completely homoge-

    nized; a high Si area produced from the dissolved g-phase is

    still obvious. Homogenization for four days at 950 C wasfound to be necessary to dissolve the g-phase and most ofFig. 3 e Optical Micrograph of NiSi20Nb3B0.5 heat treated at

    900C for 1 day, after 352 hrs of exposure in boiling 96%sulfuric acid at 37.4 atm. The attack seems to have been

    focused on the interdendritic regions.

  • 850 900 950 1000 1050 1100 1150Temperature (C)

    En

    do

    th

    erm

    ic

    E

    xth

    erm

    ic

    1012C

    1092C 1111C

    1097C1015C

    1092C1112C

    1022C 1101C1137C

    NiSi20Nb3B0.5

    NiSi18Nb5B0.5

    NiSi20Nb2B0.5

    NiSi21Nb1.6B0.5

    Fig. 4 e Phase transformation temperatures of Ni-Si alloys

    from DTA [3]. All transformation temperatures in these

    Ni-Si alloys are greater than 1000 C, so heat treatment at950C was chosen for homogenization.

    i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4591ductility by increasing Ni3Si(b) matrix grain boundary

    strength. In order to produce a single-phase alloy, the total

    SiNb appears to be limited to the range 22e24.5 at%. In orderto maximize ductility, adding enough Nb to just meet the

    solubility limit seems prudent. Additionally, keeping Si levels

    high should maintain the stability of the silica film.

    Fig. 7 shows the area fraction of G-phase of Ni3(Si,Nb) alloys

    with different Nb contents after homogenization at 950 C forfour days, including alloys not reported on here. There is an

    apparent linear relationship between Nb content and G-phase

    area fraction. Extrapolating from this data, the Nb solubility in

    the Ni3Si(b) matrix might bew1.2 at%.

    3.3. Rolling process

    The heat treatment alone was not able to remove the G-phase

    from alloys which exceeded the Nb solubility limit. Even theFig. 5 e Backscattered micrograph of NiSi20Nb2B0.5 heat

    treated at 950C for 1 day. The alloy didnt completelyhomogenize; the high Si area where the needle-like

    g-phase dissolved is still obvious.alloy whose Nb content was close to the extrapolated solu-

    bility of the matrix still contained G-phase. Local variations in

    the chemistry due to segregation could lead to the presence of

    G-phase, and b-phase with less than themaximum amount of

    Nb. Cold working should produce a more complete homoge-

    nization process, more uniform microstructure, and finer b-

    grain size. The rolling annealing process should break upthe G-phase particles and distribute the resulting smaller

    particles more uniformly. Mechanical properties are expected

    to improve due to the shorter crack paths associated with the

    small brittle particles and smaller grain size. Additionally, the

    fabricability of the material can be evaluated.

    3.3.1. Rolling reductionsNiSi18Nb5B0.5, NiSi20Nb2B0.5 and NiSi21Nb1.6B0.5 alloys were

    deformed more than 50% without cracks, using multiple

    Fig. 6 e Microstructure of NiSi21Nb1.6B0.5 heat treated at

    950C for 4 days. g-phase and most aDb eutectic havedissolved, but G-phase remains.passes. The reduction per roll pass is about 8% for these three

    alloys when rolled at 300 C, using a 10% reduction setting.The compression test results, Fig. 8, show behavior consistent

    with L12 crystal structure intermetallic compounds. The yield

    stress increases with temperature from RT up to a peak

    strength, before dropping off at higher temperatures. The

    alloy still retains significant strength (>200 MPa) up to 900 C.Based on these characteristics, rolling effectiveness may be

    increased by lowering the temperature to less than 300 C,which many studies recommended [6e9]. However, using the

    same (10%) reduction setting, the reduction achieved per roll

    pass is smaller when samples were rolled at room tempera-

    ture, shown on Fig. 9. On the other hand, the alloys appear to

    be able to endure higher pressure without cracking at room

    temperature. In the case of NiSi21Nb1.6B0.5, when reduction

    was set at 12%, samples rolled at 300 C cracked but onesrolled at room temperature survived. The reason might be

    Ni3(Si,Nb) alloys have higher ductility at room temperature.

    3.3.2. Area fraction and distribution of G-phaseFor the highest Nb alloy, NiSi18Nb5B0.5, comparing Fig. 10 and

    Fig. 11, it is obvious that the microstructure of the alloy was

    extended along the rolling direction. After 10 roll anneal

  • passes, the average size of the G-phase particles decreased

    and the number increased, see Table 3, indicating that the

    G-phase was being fractured by the process. However, the

    relative area of the phase, a b eutectic, did not changesignificantly. For the lowest Nb alloy, NiSi21Nb1.6B0.5, seen in

    Figs. 6 and 12, after 10 roll anneal passes, the a b eutectic

    recovered. However, hardness did not decrease much after

    a more severe corrosion environment than a high concen-

    tration of sulfuric acid [1].

    Fig. 14 shows a comparison of the normalized weight loss

    with sulfuric acid exposure for Ni3(Si,Nb) alloys in various

    conditions. After a period of time, the weight loss rate

    decreased significantly for all samples under different condi-

    tions. Two important data, average corrosion rate and

    passivation time, were obtained from the figure. In this paper,

    Table 3 e Corrosion rate and microstructure analysis of alloys.

    NiSi18Nb5B0.5 NiSi20Nb2B0.5 NiSi21Nb1.6B0.5

    HT Rolled HT Rolled HT Rolled

    Corrosion rate (mm/yr.) 1.14 0.64 0.05 0.05 0.18 0.08

    Passivation time (min) 370 700 300 300 450 90

    G phase Size (um) 8.3 4.4 5.3 3.7 4.3 3.6 4.2 3.4 5.5 3.8 2.3 2Counts/mm2 2500 6100 4700 5200 1330 4700

    Area fraction 16 1.5 14 2 5 0.5 4 1 1.4 0.5 2 0.5Eutectic area fraction 27 3 25 2 1 w0 1 w0

    0

    1000

    1200

    -300 0 300 600 900 1200Temperature (C)

    NiSi20.4Nb3B0.5

    NiSi18Nb5B0.5

    NiSi22

    Fig. 8 e Compressive yield stress of the Ni3(Si,Nb) alloys as

    a function of test temperature. Tested in Argon

    atmosphere at strain rate of 3310-4 s-1 [5].

    0.12

    0.15RT roll; set 10%300C roll; set 10%

    i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44592several rolling and annealing passes. The reason might be G-

    phase did not dissolve in matrix after several rolling pass,

    although G-phase distribution became more uniform, see

    Table 3.

    4. Corrosion test

    After the cold rolling process, the distribution of G-phase was

    more uniform for all alloys, see Table 3. Both the heat treated

    and rolled alloys were tested in 70% boiling sulfuric acid to

    investigate the effect of rolling on the corrosion behavior. For

    Ni3(Si,Nb) alloys, medium concentration sulfuric acid is

    12

    16

    20

    ac

    tio

    n(%

    )dissolved into the matrix and the distribution of G-phase was

    more uniform. The quantitative microstructure analysis of all

    alloys is given in Table 3.

    3.3.3. HardnessFig. 13 is an example of how the alloys hardness changes

    during the rolling annealing process. Annealing successfullyremoved strain hardening and the hardness of alloys was0

    4

    8

    0 1 2 3 4 5 6Nb (at%)

    G p

    ha

    se

    a

    re

    a fr

    Fig. 7 e Linear relationship between Nb addition and area

    fraction of G-phase in heat treated (950C 4 d) alloys,including alloys not reported on here. Extrapolating from

    this data, the Nb solubility in the Ni3Si(b) matrix is about

    1.2 at%.200

    400

    600

    800

    Yield

    stress (M

    Pa)0

    0.03

    0.06

    0.09

    0 3 6 9 12

    Rolling pass

    Red

    uctio

    n

    Fig. 9 e The reduction achieved on NiSi21Nb1.6B0.5 versus

    number of rolling passes is higher at 300C than at roomtemperature.

  • Fig. 10 e G-phase, aDb eutectic and b-phase in

    i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4593the passivation was defined as the alloys corrosion rate

    decreased significantly. When the normalized weight loss

    with exposure time was demonstrated in the natural loga-

    rithmic plot, the change of corrosion rate, which relates to the

    slope, can be clearly recognized. And the passivation timewas

    defined as the exposure time that the alloys begin to passivate.

    The corrosion rate, was calculated on the basis of weight

    losses (Dm, g), testing time (t, year), density (r, g/cm3), and the

    original surface area (S, cm2), after alloys passivated, using

    following equation:

    Corrosion rate mm=year; 39:37 mpy Dm=10 S r t

    microstructure of NiSi18Nb5B0.5 heat treated at 950C4 days.Table 3 compares the corrosion rate and passivation timewith

    microstructure analysis of alloys. Several important observa-

    tions can be made: (a) after rolling, the corrosion rate

    improved and all alloys meet the corrosion resistance target

    Fig. 11 e The microstructure of NiSi18Nb5B0.5 sectioned

    parallel to rolling direction, after 10 roll(300C)Dannealpasses, total reduction was 52%.(

  • tion and G-phase uniformity which are positive to corro-

    Acknowledgements

    This work was supported by NERI-DOE project, DE-FC07-

    06ID14753. The authors would like to thank Dr. Lillo, Idaho

    National Lab, for the compression tests and high pressure

    corrosion tests.

    r e f e r e n c e s

    [1] Davies Michael. Materials selection for sulfuric acid. 2nd ed.Materials Technology Institute; 2005.

    [2] ThomasLillo M, KarenDelezene-Briggs M. Commercial alloysfor sulfuric acid vaporization in thermochemical hydrogencycles. AIChE Annu Meet; 2005.

    [3] SanHong Zhang. The development of nickel silicide based

    0

    0.005

    0.01

    0.015

    0.02

    0.025

    0.03

    0 2000 4000 6000 8000 10000

    Time (min)

    Wei

    ght l

    oss

    / Are

    a(g/cm

    2 )

    HT44% reduction61% reduction

    Fig. 14 e The normalized weight loss with boiling 70%

    sulfuric acid exposure for NiSi21Nb1.6B0.5 for homogenized

    and rolled conditions.

    i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44594sion resistance.

    5. The eutectic constituent is detrimental to corrosion

    resistance.

    6. The solubility limit of Nb in Ni3Si at 950 C is approximatelypassivation time and the microstructure analyses of alloys is

    not clear just from the limited data. It needs more study to

    understand the relationship.

    5. Conclusions

    1. After appropriate heat treatment, 950 C for 4 days, mostunstable and detrimental phases were eliminated in the

    microstructure of the Ni3(Si,Nb) cast alloys.

    2. The homogenized alloys have lower hardness and can be

    deformed more than 50% by multiple cold rolling passes.

    3. All homogenized and rolled Ni3(Si,Nb) alloys in this paper,

    except the low Si alloy, meet the corrosion resistance target

    (