【MSE A 2016】Effect of aging on microstructures and properties of Mo-alloyed Fe-36Ni invar alloy

Effect of aging on microstructures and properties of Mo-alloyedFe–36Ni invar alloyHongwang Liu a,Zhonghua Sun b,n,Gongkai Wang a,Xiaoran Sun b,Jianxin Li b,Feng Xue b,Huifen Peng a,n,Yunfei Zhang ba School of Materials Science and Engineering,Hebei University of Technology,Tianjin300130,Chinab Hebei Iron and Steel Technology Research Institute,Hebei Iron and Steel Group,Shijiazhuang052165,Chinaa r t i c l e i n f oArticle history:Received23July2015Received in revised form5December2015Accepted11December2015Available online12December2015Keywords:Invar alloyMo alloyingStrengtheningThermal expansionAginga b s t r a c tThe Mo-alloyed invar alloy was prepared and its microstructures,mechanical properties,and thermalexpansion behavior were investigated.It was found that precipitation of the Mo2C secondary phase couldbe tailored by adjusting the aging processes,accordingly,the highest strength of820MPa and goodelongation of35%,together with a low coefficient of thermal expansion(CTE)value of3.37Â10À6/°Cwere obtained upon solution-treated at1050°C for1h and then aged at525°C for3h.The aggregationand growth of the Mo2C secondary phase at the grain or sub-grain boundaries of the austenite could beobserved with the increase of the aging time,resulting in deterioration in strength and thermal ex-pansion of the materials.&2015Elsevier B.V.All rights reserved.1.IntroductionFe–Ni binary alloy with the nickel content around36wt%istermed as Invar alloy,which is well known as its extremely lowcoefficient of thermal expansion(CTE,α25$100°C r1.5Â10À6/°C)below Curie temperature(T c$230°C).The invar alloy plays an in-dispensable role in a wide range offields such as precision instru-ments,TV kinescope,large electronic telescope base positioningdevices,core wires of long-distance power cables,etc.[1–4].In therecent years,it is necessary to develop high mechanical strengthinvar alloys together with a low CTE value to meet the rapidgrowing demand for applications of high-capacity power trans-mission wires,containers of liquid natural gas(LNG)and aeronauticcomposite-material moulds.As strategically rising materials,thestudy of the invar alloys has attracted striking attention[5–9].Conventional strengthening methods,such as solid-solutionstrengthening[10],grain refinement[5],work hardening[8,11]and precipitation hardening[12–16],have been developed in invaralloys,e.g.Fe–36Ni[5,11],Fe–Ni–Co[12,17,18],Fe–Ni–Ti[19]andFe–Ni–Mo[10,16],etc.Vinogradov et al.[5]reported that thetensile strength of the base Fe–36Ni alloy can be increased up to912MPa when its mean grain size was refined to0.18μm by theequal-channel angular pressing(ECAP).Additionally,Morikawaet al.[11]found that the base Fe–36Ni invar alloy cold-rolled atliquid nitrogen temperature can be greatly strengthened,the cor-responding tensile strength was about1200MPa owing to therefinement of austenite grains caused by formation of the de-formation twins.However,it is really hard to realize by this kind ofgrain refinement technique in the current industrial enterprises.Therefore,the precipitation strengthening integrated with thework hardening is suggested in order to obtain high-strength invaralloy.The Fe–36Ni–6.5Co–2.5Ti–1.0Al alloy exhibited the highesttensile strength of1500MPa by means of precipitation of inter-metallics such asγ′-Ni3(Al,Ti),coupled with the subsequent colddeformation[19].It is notable that the precipitation of inter-metallics could result in an undesirable increase in its CTE value[12].This drawback can be offset by introduction of more Co al-loying elements,which apparently will increase the productioncost of the materials.On the other hand,the Co-free invar alloy has recently beenpaid much attention by means of carbide precipitates,e.g.VC,NbCand Mo2C[13,14,20].Nakama et al.[13]reported that the invaralloy through precipitation of VC and TiC particles in the austenitematrix,together with subsequent50%cold deformation presentedthe highest tensile strength of1100MPa,which is two times largerthan that of the base Fe–36Ni alloy.Meanwhile,this alloy shows alow CTE value of3Â10À6/°C in the temperature range of30–150°C.Zhang et al.[10]and Lu et al.[16]studied the effect ofsolid-solution treatment and carbide precipitates on mechanicalContents lists available at ScienceDirectjournal homepage:/locate/mseaMaterials Science&Engineering A/10.1016/j.msea.2015.12.0180921-5093/&2015Elsevier B.V.All rightsreserved.n Corresponding authors.E-mail addresses:sunzhonghua@(Z.Sun),peng@(H.Peng).Materials Science&Engineering A654(2016)107–112properties of the Fe–Ni–Mo–C and Fe–Ni–Mo–V–C invar alloys, respectively.However,little information on size,morphology and distribution of the precipitates during aging was reported.In this paper,effect of aging on microstructures and properties of the Mo-alloyed invar alloy was studied.Specially,variation in size,mor-phology and distribution of the Mo2C precipitates during aging and the correlation with the mechanical properties and the ther-mal expansion ones were also investigated.2.Experimental procedureA40kg ingot of Fe–36.7wt%Ni–4.41wt%Mo–0.26wt%C invar alloy was prepared by vacuum induction melting furnace(DDVIF-50-100-2.5)under the protection of argon.After homogenizing at 1250°C for1h,the ingot was hot-rolled to plate of6mm in thickness,then cooled to room temperature.The hot-rolled plate was solution treated at1050°C for1h to dissolve the primary carbides into austenite matrix,then aged at the temperature of 475–575°C for1–9h for the purpose of precipitation strengthening.X-ray diffraction was conducted at room temperature with a Rigaku Dmax2500PC diffractometer operated at40kV and150mA using a Cu-Kαradiation.The XRD patterns were recorded at a scanning rate of1°minÀ1in the2θrange of30–95°.Samples wereetched in the FeCl3supersaturated solution.Their microstructures were observed under OLYMPUS-BH metalloscope,and grain size of the austenite was measured by Nano Measurer1.2software.Fur-ther microstructure observation was conducted in ZEISS ULTRA-55 FE-SEM at30KV and in PHILIPS Tecnai G2F20HRTEM at200KV. Chemical compositions of the phases were measured by the at-tached EDX apparatus.Hardness of the samples was measured on HXD-1000Vickers tester at a load of100g.Measurements on mechanical properties were conducted on SHT-5305material testing machine at a load-ing rate of2mm/min.Thermal expansion of the alloys was mea-sured in the temperature range ofÀ196°C to300°C by a modified Leica J11dilatometer,and the linear CTE was calculated by the Eq.(1).α=Δ⋅Δ() LL T11 0whereαis the mean CTE,ΔL/L0is the relative change in length of the samples,L0is the original length of the samples,ΔT is the temperature variation.3.Results3.1.Mechanical behaviors of invar alloy under different agingconditionsFig.1a shows the variation of hardness for the present samplesunder different processes.Thefitted curves show the same char-acteristics that the hardness climbs gradually and reaches a peakvalue then declines with an increase in either aging temperatureor aging time.The highest hardness of280HV could be obtainedfor the sample treated at525°C for5h,which is60.9%larger thanthat of the solution treated one.Stress–strain curves in Fig.1bindicate that the hot-rolled sample presents the tensile strength of690MPa,which is almost200MPa higher than that of the baseinvar alloy without any Mo addition[5,6].This result suggests thatthe Mo alloying plays an important role in enhancing strength ofthe invar alloy.Solution treatment at1050°C results in a decreasein strength,this phenomenon should be related to the dissolutionof the primary carbides into the austenite matrix during heating[10].Aging at525°C produces an apparent increase in strength,and the sample aged for3h exhibits the highest strength of827MPa.This value is about39%higher than that of the solutiontreated one,which is similar to what is observed in Fig.1a.Dif-ferent from the variation in hardness,further prolongation inaging time leads to a decrease in strength.3.2.Microstructural analysis of Invar alloy under different agingconditionsIn order to check the variation in the abovementioned me-chanical properties,the microstructures of the samples in Fig.1were observed by metallographic microscopefirstly.The hot-rol-led sample consists of thefine austenite with grain size in a rangeof10–40μm and a few number of primary secondary phases uniformly distributed in the austenite matrix(Fig.2a).Solid-so-lution treatment at1050°C causes most of the secondary phasesdissolving into the austenite(Fig.2b).Therefore,an abrupt growthin the austenite grain size after the solid-solution treatment isobserved due to the weakness of the pinning effect by the pre-cipitates[21],concomitant with an apparent decrease in hardness.Aging at525°C leads to re-precipitation of the secondary phases.However,only a littlefine precipitates are observed in some aus-tenite grains of the3h-aged sample(Fig.2c).It means that thesecondary phases withfine particle size precipitated during ashort aging time are difficult to be noticed just through theoptical Fig.1.Variation in mechanical behaviors of the samples treated at different processes,(a)hardness,(b)stress–strain curves.H.Liu et al./Materials Science&Engineering A654(2016)107–112108microscope.Further prolongation in aging time results in an ap-parent increase in the amount and size of the precipitates,which are easy to be observed and almost uniformly disperse in the matrix,indicated by circles in Fig.2d,for the 5h-aged sample.Those results suggest that both nucleation sites and size of the precipitates increase with an increase in the aging time.It is no-teworthy that much bigger precipitates are observed in grain boundaries or sub-grain boundaries,as indicated by arrows in Fig.2e,concomitant with the disappearance of some precipitates in grains.It reveals that aggregation of the precipitates occurs during long aging time.3.3.Thermal expansion behaviors of invar alloy under different aging conditionsThermal expansion curves of the samples under different processes are shown in Fig.3.The hot-rolled sample shows the lowest CTE value of α¼1.54Â10À6/°C,almost the same as that of the base Fe –36Ni alloy [2],in the temperature range of À100to 100°C.However,the CTEs for the solid-solution treated and aged samples are approximately two times larger than that of the hot-rolled sample.The CTE value is about α¼4.01Â10À6/°C for the solid-solution treated sample.Aging at 525°C results in a slight decrease in the CTEs,which are about 3.37Â10À6/°C and 3.48Â10À6/°C,respectively,for 3h and 5h-aged samples.The variation in CTEs is closely related to the changes in micro-structures observed in Fig.2.Maximum CTE should be derived from the dissolution of the secondary phases during solid-solution treatment,whereas a concomitant decrease in the CTE is observed in the aged sample due to re-precipitation of the secondaryphases.Fig.2.Microstructures of the samples under (a)hot-rolling,(b)solid-solution treatment at 1050°C and aging at 525°C for (c)3h,(d)5h (circles)and (e)7h (arrows).H.Liu et al./Materials Science &Engineering A 654(2016)107–1121094.Discussion4.1.X-ray diffraction and TEM analysis of invar alloy under different aging conditionsIn order to further investigate the abovementioned variation in microstructures and properties of the samples under different processes,the XRD patterns were measured accordingly,which are shown in Fig.4.The samples without aging are primarily com-posed of austenite-single phase.And secondary phase like Mo 2C,in additional to the austenite,is observed in samples with the aging time longer than 3h.Enhancement in intensity of the Mo 2C peaks indicates the increase in the amount of Mo 2C precipitates with the increase in the aging time.TEM results shown in Fig.5further demonstrate that the particles located at intersections of the grain boundaries are the Mo 2C phase that presenting the h.c.p.structure with the lattice constants of a ¼0.3012nm and c ¼0.4735nm.4.2.SEM analysis of invar alloy under different aging conditions Additionally,the characteristic of precipitates does not change with the change of aging,the variation in properties of the ma-terials shown in Figs.1and 3should be attributed to the dis-tribution and size of the secondary phases precipitated during aging.Due to the limit information of the variation in precipitate obtained from the metallographic microstructure shown in Fig.2,we double checked the microstructures of the samples through SEM accordingly.Some residual precipitates,indicated by arrows in Fig.6a,are observed in the matrix,probably due to the in-complete dissolution of the Mo 2C particles during heating [10].However,three kinds of the Mo 2C particles with the size of 50–170nm,150–390nm and 350–500nm,respectively,are observed in austenite interior as shown in Fig.6b.The very finer Mo 2C particles should be attributable to those precipitated in austenite interior,the intermediate ones mainly exist along sub-grain boundaries of the austenite,and the largest ones are along the austenite grain boundaries.According to distribution of the Mo 2C particles,it is deduced that those particles are inclined to migrate from sub-grain boundaries to grain boundaries of the austenite,where the Mo 2C particles aggregate and grow.As a result of par-ticle growth,several large-sized Mo 2C particles ($0.9μm)are found at austenite grain boundaries,indicated by circles in Fig.6b,simultaneously,some Mo 2C particles in a size of 160–290nm are observed to precipitate along the austenite grain boundaries,in-dicated by arrows also in this figure.Longer aging time of 5h re-sults in a remarkable increase in the amount of the Mo 2C particles aggregating at grain boundaries (Fig.6c),consequently,the Mo 2C particles exhibit an average grain size larger than 0.8μm.It is the aggregation of the Mo 2C particles along the austenite grain-boundaries that causes the weakening of the pinning effect on the sub-grain boundaries by the secondary phases,concomitant with a sharp decrease in the number of sub-grain boundaries.Accord-ingly,the Mo 2C particles that have not enough sites to precipitate along the sub-grain boundaries begin to aggregate in the austenite interior and prefer to re-unite in the form of “chains ”,indicated by arrows in Fig.6c,larger than 4.5μm in length.The EDS result in Fig.4d indicates that the precipitate consists of only Mo and C elements.Based on the SEM results,we carefully analyze the character-istic of the Mo 2C particles for the samples aging at different stages.Various parameters of those particles,together with hardness,tensile strength and CTE value for the corresponding samples are listed in Table 1.The results clearly indicate that the 3h-aged sample with more fine Mo 2C particles distributing in grain interior and sub-boundaries of the austenite presents the maximum ten-sile strength of 827MPa.Shifting of the Mo 2C particles from the grain interior or sub-boundaries to grain boundary,concomitant with a increase in the sizes,results in an unexpected decrease in tensile strength,even though the maximum hardness for the 5h-aged sample was obtained.Generally,hardness linearly correlates with tensile strength of the metal materials,the former measures the resistance of a sample to material deformation at a constant compression and the latter is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking.Accordingly,the hardness peak is observed in the longer aged sample with the bigger Mo 2C particles presenting high deformation resistances,while the tensile strength peak is observed in the shorter aged sample with the smaller ones exhibiting high adhesive strength between the Mo 2C and the austenite matrix.The CTE value of Mo 2C is about 7.8Â10À6/°C reported in the literature,which is much higher than that of the austenite.Ac-cordingly,more large Mo 2C particles dispersing along grain boundaries of the austenite should be responsible for theincreaseFig.3.Thermal expansion curves of the samples under differentprocesses.Fig.4.XRD patterns of the samples treated under different processes,(a)hot-rolling,(b)solid-solution treatment at 1050°C,and then aging at 525°C for (c)3h,(d)5h and (e)7h,respectively.H.Liu et al./Materials Science &Engineering A 654(2016)107–112110in the CTE value when aging time from 3h to 5h,as indicated in Table 1.Additionally,the solid-solution treated sample with the largest CTE (α¼4.01Â10À6/°C)mainly results from the Mo and C atoms occupying at the positions of lattices and vacancies,re-spectively,concomitant with the dissolution of the Mo 2C particles.The magneto-lattice effect from the magneticmomentFig.5.TEM morphology of the Mo 2C particle after aging at 525°C for 3h (a),SAD pattern and the indexing (b)and EDS result(c).Fig.6.SEM morphologies of the samples under solution treatment at 1050°C for 1h (a)and then aging at 525°C for 3h (b),5h (c).EDS result of the precipitate (d).H.Liu et al./Materials Science &Engineering A 654(2016)107–112111interactions between Fe and Ni atoms in the lattices is partially destroyed by the introduction of Mo and C atoms [4].Accordingly,the temperature dependence of thermal expansion or contraction can be hardly compensated by the corresponding contraction or expansion originated from the magneto-lattice effect [22,23].5.ConclusionsEffect of aging on microstructures and properties of the Mo-alloyed invar alloys was investigated.It was found that the amount,size,morphology and distribution of the Mo 2C secondary phase could be tailored by adjusting aging processes.The highest strength of 820MPa and good elongation of 35%,together with low CTE value of 3.37Â10À6/°C,was obtained after solution treatment at 1050°C for 1h and then aging at 525°C for 3h.This process produced a uniformly distributed fine Mo 2C secondary phase in the austenite matrix.The aggregation and growth of the Mo 2C secondary phase at the grain or sub-grain boundaries of the austenite could be observed with the increase of the aging time,resulting in deterioration in strength and thermal expansion of the materials.AcknowledgmentsThis work was financially supported by Hebei Natural Science Foundation –Iron and Steel Joint Funds (No.E2014318003)and the Open Project of Key Laboratory of Cryogenics,Technical In-stitute of Physics and Chemistry (TIPC),Chinese Academy of Sci-ences (CAS)(No.CRYO 201503).References[1]M.Inaba,K.Teshima,E.Higashinakagawa,Y.Ohtake,IEEE T.Electron Dev.35(1988)1721–1729.[2]M.Shiga,Curr.Opin.Solid State Mater.Sci.1(1996)340–348.[3]M.V.Schilfgaarde,I.A.Abrikosov,B.Johansson,Nature 400(1999)46–48.[4]M.Mstsushita,S.Endo,K.Miura,J.Magn.Magn.Mater.265(2003)352–356.[5]A.Vinogradov,S.Hashimoto,V.I.Kopylov,Mater.Sci.Eng.A 355(2003)277–285.[6]X.Wang,J.F.Zhang,Y.H.Zhang,L.S.Fu,J.L.Zhang,X.L.Wang,Chin.J.Rare Met.33(2009)670–674.[7]K.Hamada,Y.Yamaoka,H.Tsabono,Y.Kawabata,Wire 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Res.2(2006)31–34(inChinese).[20]K.Nakama,Sanyo Tech.Rep.21(2014)38–46.[21]D.Y.Cai,M.Yao,P.L.Nie,W.C.Liu,Mater.Lett.57(2003)3805–3809.[22]S.Khmelevskyi,P.Mohn,Phys.Rev.B 69(2004)140404(R).[23]A.V.Ruban,M.I.Katsnelson,W.Olovsson,S.I.Simak,I.A.Abrikosov,Phys.Rev.B 71(2005)054402.Table 1Parameters of the Mo 2C particles in the invar alloys under different processes,and their corresponding properties.Mo 2C particles HVTensile strength /MPaCTE/10À6K À1Average size /nmSitesNumber Hot-rolled ///189.8690 1.54Solid-solution ///170.6595 4.01Aging for 3h50–170Grain interiorMore 254.68273.37150–390Sub-grain boundaries More 350–700Grain boundaries A little $900Grain boundaries Less Aging for 5h 4800Grain boundaries More 280.8789 3.4844500ChainsA littleAging for 7h/Grain boundariesMuch more277.4//H.Liu et al./Materials Science &Engineering A 654(2016)107–112112。