合金元素对力学性能的影响.pdf

Influence of alloying elements on the mechanical propertiesof FeCo–V alloysR.S. Sundar, S.C. Deevi*Chrysalis Technologies Incorporated, Richmond, VA, USAAvailable online 9 April 2004Abstract‘More electric aircraft’ technology necessitated improving the high temperature mechanical properties of FeCo–V alloys whilemaintaining their excellent magnetic properties. FeCo–2V alloys offer the highest saturation magnetization but do not meet the yield strengthand creep resistance requirements. The aim of this paper is to review the influence of alloying elements, which enhance the mechanicalproperties without sacrificing the magnetic properties. Our alloy developmental efforts resulted in new FeCo–V alloys with uniquecombination of high tensile strength, good creep resistance and high electrical resistivity.q 2004 Elsevier Ltd. All rights reserved.Keywords: A. Magnetic intermetallics; B. Alloy design; B. Mechanical properties; B. Magnetic properties; B. Electrical properties1. IntroductionWorld wide efforts are underway to develop aircraftsubsystems based on ‘More Electric Aircraft (MEA)’technologies utilizing electric power as opposed to hydraulic,pneumatic, electric and mechanical subsystems [1,2]. Use ofelectric power is expected to result in significant weightsavings, increase reliability, maintainability and maneuver-ability, and reduce the need for ground equipment support.The development of more electric aircraft requires softmagnetic materials that can withstand the proposed designrequirements at high temperatures. For example, one of thecritical components of MEA is an internal starter/generatorunit operating in the range of 550–600 8C. A soft magneticmaterial with a yield strength of 500 MPa at 600 8C and aminimum creep rate of p 1028s21above 500 8C is needed.Soft magnetic materials based on FeCo alloys are best suitedfor this application since they exhibit the highest saturationmagnetization and high curie temperature (Table1) [3,4].High saturation magnetization of FeCo is advantageous asthe required magnetic force can be achieved with a lowerweight of component. Similarly, high Curie temperature ofFeCo helps to retain good magnetic properties at operatingtemperatures.Binary FeCo alloys are brittle at room temperature [5,6].Addition of 2 wt.% V improves the room temperatureductility as well as the cold workability of the alloy in thedisordered condition [7]. Currently, the cold rolled sheets ofFeCo–V alloys are used in various applications requiringhigh flux densities [8,9]. Mechanical and magnetic proper-ties of commercial FeCo–V alloys are sensitive to the finalheat treatment conditions [8,9]. For example, the cold rolledalloy is annealed at temperatures close to 700 8C to obtainhigh strength and good ductility. On the other hand, goodmagnetic softness (low coercivity and core loss) is achievedwhen the alloy is heat treated at high temperatures (.7208).Furthermore, commercial FeCo alloys lack the requiredstrength and creep resistance for use as magnetic com-ponents at elevated temperatures. Hence, there is a clearneed to develop new alloys based on FeCo to meet theemerging high temperature application requirements.2. Challenges in developing high strength FeCo alloysAttempts to improve the strength of FeCo alloys by grainrefinement, solid solution and precipitation hardening resultin the degradation of soft magnetic properties. Commercialgrade high-strength FeCo alloy relies on grain refinement toimprove its strength [8–10]. In the commercial alloys, finecarbide precipitates are formed due to the small addition ofcarbon and carbide forming elements like niobium andtantalum. Presence of these fine carbides restricts grain0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.intermet.2004.02.022Intermetallics 12 (2004) 921–927www.elsevier.com/locate/intermet*Corresponding author. Present Address: Research, Development andEngineering, Philip Morris USA, 4201 Commerce Road, Richmond, VA23234, USA. Tel.: þ1-804-274-1934; fax: þ1-804-274-2160.E-mail address: seetharama.c.deevi@pmusa.com (S.C. Deevi).growth during final annealing treatment and improves thestrength by retaining the fine grain size. However, highstrength achieved by either heat treating the alloy at a lowertemperature or restricting the grain size of the alloy by finecarbide precipitates results in an increase in the coercivity(Fig. 1) [12]. Similarly, fine-grained high strength alloy(50HS) has a higher creep rate and a lower rupture life overa magnetically soft alloy (50A) (Fig. 2) [10,11].Alloying elements added to improve strength andductility of FeCo invariably decreases saturation magneti-zation and increases coercivity of FeCo alloys [3,13–15].For example, addition of 2 wt.% vanadium to FeCo resultsin a 4% reduction in saturation magnetization Fig. 3 [14].Inaddition, vanadium addition decreases the permeability andincreases the coercivity of FeCo [3,16,17]. Table 2summarizes the effect of other alloying elements on thesaturation magnetization of FeCo [3,13,15]. The saturationmagnetization decreases from 1 to 5% for addition of eachalloying elements. Moreover, the solid solubility of most ofthe alloying elements is less than 1 at% in FeCo. Addition ofsuch alloying elements in excess of their solid solubilityresults in precipitation of paramagnetic phases, whichfurther increases the coercivity through pinning the domainwall movements [13,16].The mechanical and magnetic properties of FeCo alloysare highly susceptible to compositions, processing and heattreatments. Often, an improvement in the mechanicalproperties is achieved at the expense of magnetic properties.Hence, it is necessary to optimize the alloy composition andthermo-mechanical treatment to achieve the balancebetween the mechanical and magnetic properties.3. Strategies to improve mechanical properties throughalloying additionsIn this section, we summarize the effect of variousalloying elements which are beneficial in improving themechanical properties of FeCo alloys. The present discus-sion is limited to the improvement in strength and ductilityof FeCo alloys at ambient conditions. There is noinformation available in the literature regarding the effectof alloying addition on the high temperature strength and thecreep resistance of FeCo alloys.Ductility of cold worked FeCo–V alloys are verysensitive to the final annealing temperatures [18–21].Close control of the final annealing temperature is requiredFig. 1. Effect of grain size on the strength and coercivity of a FeCo–Valloy [12].Fig. 2. Creep resistance of commercial FeCo alloys at 600 8C (a) stress dependence of steady state creep rate and (b) rupture life in terms of Larson–Miller plot[10,11].Table 1Saturation induction of commercial soft magnetic alloysMaterial Saturation magnetiz. (T) Curie point (8C)Iron 2.15 770Low carbon steels 2.0–2.15 ,770Ferritic steels (Fe–Cr–Ni) 1.2–1.7 –Fe–(1–3%)Si alloys 1.9–2.0 730–750Ni–Fe alloys 0.8–1.6 250–500Fe3Al alloys 1.14 540Fe–(30–50)Co 2.3–2.45 920–985Amorphous Fe–Co–B 1.9 370–420R.S. Sundar, S.C. Deevi / Intermetallics 12 (2004) 921–927922to achieve an optimum combination of mechanical proper-ties [20,21]. However, it may be difficult to achieve suchclose control in industrial practice. Major et al. [22,23] haveshown that sensitivity of mechanical properties to heattreatment temperature can be minimized by the addition ofnickel. Fig. 4 summarizes the effect of final annealingtreatment on the room temperature strength and ductility ofFeCo–2V–4.5% Ni. For comparison, mechanical proper-ties of the standard Fe–49Co–2V alloy are also included inFig. 4. Unlike the Fe–49Co–2V alloy, the ductility of theNi-containing alloy remains high after heat-treating over awide temperature range. Yield strength of ternary alloy isgreater than that of nickel containing alloy after heattreating at temperatures below 680 8C, while the nickelcontaining alloy exhibits high strength at higher annealingtemperatures. Nickel addition to FeCo–V alloys results inprecipitation of g2phase (L12), rich in nickel and vanadium[24,25]. These precipitates are stable over the temperaturerange where cold rolled alloys are subjected to finalannealing treatment. The presence of stable g2phaseand refinement of grain or sub-grain size due to nickeladditions are responsible for the higher ductility in nickelcontaining alloys over a wide range of heat treatmentconditions [24,25].Vanadium addition is more effective in improving theductility of FeCo alloys in the disordered state than in theordered state. Introduction of order in the binary and inFeCo–V alloys results in ductility loss at room temperature[6,26]. Ordering results in planar slip and promotes cleavagefailure at low strain levels. Recently, George et al. [27]showed that ductility of FeCo–V alloy in the orderedcondition could be improved by adding boron and carbon(Fig. 5). Fine precipitates formed by these additions dispersethe slip in the ordered alloy and improves the ductility. Anoptimum amount of carbon or boron content leads to aductility of 15% in the ordered condition of FeCo–V alloy.In a series of studies, Kawahara et al. [15,26,28,29] haveidentified several alloying additions which increased thestrength as well as cold workability of FeCo alloys. Severalof the alloying additions are also shown to enhance theductility of the FeCo alloy in the ordered condition. Theycorrelated the effectiveness of a given alloying element (X)in improving the ductility to its ability to form orderedclusters whose composition is close to Co3X. Formation offine Co3X clusters results in the creation of localconcentrated disordered (LCD) zones in the matrix(Fig. 6). Furthermore, cold rolling of the alloy improvesthe ductility by uniformly distributing the ductile LCDFig. 3. Effect of vanadium addition on the saturation magnetization ofFeCo [14].Table 2Effect of alloying elements on the saturation magnetization of FeCo [3,15]Alloy (X) Saturation magnetizationof FeCo–2 X ðMsÞ; T% Loss in MsMo 2.34 2.5Nb 2.32 3.3W 2.37 1.2Ni 2.35 2.1Ti 2.33 2.9Mn 2.35 2.1C 2.35 2.1Cr 2.29 4.6Fig. 4. Effect of nickel addition and annealing temperature on the tensileductility (a) and yield stress (b) of FeCo–V alloy [22,23].R.S. Sundar, S.C. Deevi / Intermetallics 12 (2004) 921–927 923regions in the ordered matrix. The mechanical and magneticproperties of the promising compositions are presented inthe Table 3. Many of the alloying elements not onlyimproved the strength but also retained reasonable ductilityin the ordered condition. In many instances, the loss inmagnetic properties of FeCo due to these ternary additionsis less than that in conventional FeCo–V alloys. Forexample, the decrease in saturation magnetization andincrease in coercivity of FeCo due to vanadium and niobiumare shown in Fig. 7 [13,17]. As in FeCo–V alloys, saturationmagnetization decreases with increasing amounts of theparamagnetic second phase. However, the loss in saturationfor a given volume percent of second phase is less inniobium containing alloys. Similarly, for a given amount ofsecond phase, coercivity of niobium containing alloys is lessthan that of FeCo–V containing alloys. Thus, review of theliterature presented in this section suggests that mechanicalproperties of FeCo alloys could be improved withoutsacrificing much of the soft magnetic properties throughcareful alloying and thermo-mechanical treatment.Fig. 7. Effect of vanadium and niobium contents on the (a) saturationmagnetization and (b) coercivity of FeCo alloy [13,17].Table 3Mechanical and magnetic properties of FeCo–X alloys [15]Composition(at%)YS(MPa)% Elong. Saturationinduction (T)Coerciveforce (kA/m)FeCo–0.5C 458 15 2.36 1.0FeCo–2C 1050 9.0 2.37 2.5838 15.5 2.34 1.0FeCo–0.5Cr 1295 6.0 2.35 2.8FeCo–2Cr 1117 1.7 2.29 2.4FeCo–0.5Mo 1491 8.7 2.33 2.0FeCo–0.5W 1538 7.0 2.33 2.3FeCo–2W 1909 1.7 2.21 2.0FeCo–2W 2092 8.9 2.24 2.2FeCo–0.5Ta 1547 1.0 2.33 2.1FeCo–2Ta 1961 0.6 2.24 2.8FeCo–0.5Nb 1393 5.8 2.33 2.2FeCo–2Nb 1765 – 2.16 3.3FeCo–2Ni 1578 3.3 2.34 2.4Fig. 6. Schematic representation of (a) precipitate and surrounding localconcentration disordered region (LCD), (b) variation of cobalt content and(c) probability of order across a precipitate [26,28,29].Fig. 5. Room temperature ductility of FeCo–2V as a function of boron andcarbon additions [27].R.S. Sundar, S.C. Deevi / Intermetallics 12 (2004) 921–9279244. New generation of high strength and creepResistant FeCo AlloysThis section presents our alloy development efforts toimprove the mechanical properties of FeCo alloys to meetthe design requirements of MEA. We systematically variedCo/Fe ratio and alloying additions (Table 4) with theobjective of improving the mechanical properties as well asdecreasing the cost of FeCo alloy [30]. The alloys werevacuum-induction melted and processed into 30 mil sheetsby hot rolling [30]. Fig. 8 presents the yield strength of thealloys at room temperature as well as at 600 8C. Severalcompositions meet both the room temperature and 600 8Cyield strength requirement of MEA application. Moreover,the alloys have a ductility of at least 3% at roomtemperature. As mentioned earlier, the solid solubility ofmany of the alloying elements are less than 1 at% in FeCo.In majority of the alloys, the total solute content is morethan 5%. Hence, the alloys are amenable to age orprecipitation hardening treatment. The alloy exhibitsmaximum hardening when aged at 600 8C [31]. Isothermalage hardening response of selected alloys is shown inFig. 9a. The hardness of the alloy increases rapidly at thestart of aging treatment and the peak aging condition isreached with in an hour of aging. With further aging, thehardness of the alloy falls gradually during the over-agingcondition. Microstructure of one of the promising compo-sitions (SM9) after age hardening treatment is shown inFig. 9b. The alloy was initially solutionized at 1100 8C for10 min in the high temperature g (fcc) phase field. Duringquenching, the alloy undergoes martensitic transformationinto metastable bcc (a2) phase. When the alloy is aged at600 8C, the metastable a2phase decomposed into equili-brium ordered a0phase (B2) þ g2phase (fcc). In addition,the microstructure contains a few coarse (1–2 mm)precipitates which are rich in niobium. Age hardeningtreatment improved the hardness and enhanced the creepresistance of the alloy. Fig. 10 compares the creep resistanceof one of our promising alloys in the aged condition with thecommercial FeCo alloys at 600 8C [10,11]. The presentalloy exhibits longer rupture life and more than two ordersof magnitude lower creep rate than the commercial FeCo–Valloys. The alloys also exhibit high electrical resistivity andare expected to minimize the eddy current losses in the highfrequency soft magnetic applications [4]. Fig. 11 comparesthe electrical resistivity of our alloys with the commercialFeCo alloy. The commercial alloy has a resistivity of40 mV cm. The present alloys have 40–50% highe