The microstructure and mechanical behavior of modern high

The Microstructure And Mechanical Behavior Of Modern High-Free PDF

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ln 1 ln 1 1 ln 1 1 ln 1 ln 1 ln 1, where and are Boltzmann s constant the number of ways of mixing and the gas constant 8 314. mole respectively According to Eq 1 reaches its maximum when the alloy has an equi. atomic ratio This explains why equal atomic fraction is preferred in the alloy design for HEAs In. general Eq 1 may hold for alloy systems above their melting point If one further assumes that the. same amount of configurational entropy of mixing still remains after solidification HEAs the alloy. system with equal atomic fractions thus arise which are excepted to possess an intrinsic structural. disordeness or entropy much greater than conventional alloys with one or two base elements. Indeed Cantor et al 2004 might be the first one discovering the effect of high mixing entropy. Contrary to the common notion that intermetallics would be formed by mixing together different. metallic elements with negative enthalpy of mixing Cantor et al 2004 found a single phase solute. solution structure in the FeCrMnCoNi alloy system without any obvious intermetallics phase Later on. this behavior of forming solid solution rather than intermetallics in a multi component alloy was. attributed to the high mixing entropy effect by Yeh et al 2004a According to the literature Yeh et. al 2004b Yeh 2013 HEAs would possess the following distinctive structural thermodynamic. attributes as compared to conventional alloys high mixing entropy sluggish diffusion lattice. distortion and cocktail effects out of which the high mixing entropy is expected to play the essential. role in the alloy design and mechanical properties of HEAs. Despite the fact that the alloy discovered by Cantor et al 2004 is single phased most of the HEAs. currently studied are of a multi phased solid solution structure Crystallographically the typical atomic. structure in HEAs includeface centered cubic FCC and or body centered cubic BCC crystal. structure s with or without nano precipitates Yeh et al 2004a Tong et al 2005 The complexity. in the atomic structure of HEAs as accompanied by heavy solid solution strengthening binding. enhancement and fine grain strengthening can lead to superior mechanical properties at high. temperatures such as relatively high hardness Huang et al 2007 Lin et al 2011 and mechanical. strengths Zhou et al 2007a Yang et al 2012 good thermal stability Sriharitha et al 2014 and. work hardenability Varalakshmi et al 2008 excellent anticorrosive properties Chou et al 2010a. b and wear resistance Wang et al 2013 2011 and unique magnetic properties Tariq et al 2013. Liu et al 2012 Such a combination of impressive mechanical properties renders HEAs one of the. most promising candidate structural materials for future engineering and industrial applications such. as sport goods nuclear technology aerospace engineering superconductorand hydrogen storage 2014. Otto et al 2013a Zhang et al 2013 Tsai et al 2013a Sheng et al 2013 Tsai et al 2013b Ng et. al 2012 Zhuang et al 2012 Shun et al 2012b Hemphill et al 2012 Zhang et al 2012a Senkov. Woodward 2011a Hsu et al 2011, In practice HEAs can be synthesized by various experimental methods including mechanical. alloying vacuum arc melting sputtering and splat quenching Sriharitha et al 2014 Sheng et al 2013. Qiu Liu 2013 Zhang et al 2012b Casting route have been widely used for HEAs preparation. among the aforementioned processes and it usually produced typically dendrite structure with some. segregation in dendrite and interdendrite regions Chen et al 2006 Yeh et al 2004a Tong et al 2005. On the other hand it is reported that there will be formed a meta stable phases during sputtering or. splat quenching however it may become stable after annealing at high temperature Yeh et al. 2004a Some studies have shown that mechanical alloying lead to a HE alloy with more homogenous. and stable nano crystalline microstructures Chen et al 2009 a b In spite of the intense research. endeavors however a few fundamental issues still remain unresolved such as phase stability atomic. scale deformation mechanisms and the structure property relation At the present these constitute the. central theme of research in the area of HEAs In this article we would like to summarize the recent. research efforts dedicated to resolve these outstanding issues. A T Samaei et al Engineering Solid Mechanics 3 2015 3. 2 Microstructure of High Entropy Alloys,2 1 Thermodynamic and Phase Stability. In the case of HEAs there has been presented a very few theories based on the thermodynamic. properties for predicting the microstructure formation and phases Zhang et al 2008 Hsieh et al 2009. Wang et al 2009b Lin Tsai 2010a The morphological stability and the nature and dynamic. behavior of the phases in the formation of HEAs are the essential challenges to understand their. microstructure and predict their physical and mechanical properties for designing an optimized alloy. system for special application The maximum number of equilibrium phases at constant pressure can. be predicted based on the Gibbs phase rule by using the relation. where and are degree of freedom number of components and number of phase respectively. However the number of phases appeared in multi element HEA systems is not in accordance with the. aforementioned rule and it usually is far less than the maximum prediction due to the high mixing. entropy effect which caused to restrain the formation of intermetallic compounds Yeh et al 2004b. For example the number of components was six in the AlCoCrCuFeNi HEA system but the actual. number phase reported by Tung et al 2007 was only three included FCC BCC HCP and ordered. BCC which is far less than the highest value of phases 7 A few studies have thermodynamically. investigated the phase formation rule for illustration the decreasing of Gibbs energy. where and are the Gibbs energy the enthalpy of mixing the entropy. of mixing and the absolute temperature respectively of the solid solution at elevated temperatures led. to the formation of simple solid solutions against intermetallic compounds due to forming random solid. solution easily and more stable than the other phases in HEA systems Based on the completely random. mixing the entropy of mixing of multi component alloy systems with element is. where and are the gas constant and the mole percentage of the th component and also. 1 Due to this mixing entropy which becomes a maximum for equi atomic alloys like. AlCoCrFeNiTi HEA systems have much higher mixing entropies than the conventional. multicomponent alloys Zhou et al 2007b In addition the slow diffusion kinetics accompanies the. formation of simple solid solutions because of the difficult cooperation among the migrants of various. elements and the activation energies related to the number of composing elements in the matrix of. HEAs correspondingly are higher than the conventional multicomponent alloys Tsai et al 2013b. Recently some investigations have conducted to determine the rules governing the phase stability in. high entropy alloys regarding relevancy to the formation of solid solutions intermetallic and. amorphous phases based on a statistical analysis of the constituent elements in a large database of these. alloys Guo et al 2013 have highlighted two types of factors controlling the aforementioned phase. selection 1 topological mainly the atomic size for example the atomic size difference for solid. solution phases is small against amorphous phases 2 chemical the electronegativity electron. concentration or the mixing enthalpy as instance the mixing entropy and mixing enthalpy of. amorphous phases are more negative and smaller than those of the solid solution phases For the. multicomponent alloys Zhang et al 2012c firstly defined three parameters which are the atomic size. difference which plays an essential role in the glass transition of hard sphere colloidal systems. Guo et al 2013 the mixing enthalpy and the mixing entropy to characterize the. collective behavior of constituent elements as, where is atomic radius of the th element The mixing enthalpy is given as. where 4 is the mixing enthalpy of binary liquid AB alloys Takeuchi. Inoue 2000 assuming a random solid solution of atom A in B is obtained by using the. Miedema s model Egmai Waseda 1984 considered only the chemical contribution and it implies. that the electronegativity effect is included in this model. For a series of equiatomic or nearly equiatomic alloys the mixing entropy the mixing. enthalpy and atomic radius difference were calculated based on above equations and. listed in Table 1 These alloys were prepared by the arc melting injection melting copper mold casting. and melt spinning and the phases mentioned in Table 1 are mostly referring to the as cast state which. most of the solid solution phases in HEAs are quite stable near to the equilibrium state Wen et al. 2009 Lin et al 2010 b c However the cooling rates the kinetic nature of transition and other. thermodynamic parameters are determinant for the formation amorphous phases It proposed that the. fundamental thermodynamic parameters and properties of the elements in preparation of alloys do. make a difference on forming the phases such as solid solution amorphous and intermetallic phases. for example any alloy can form amorphous phases from liquid state by using a sufficient cooling rate. Cohen Turnbull 1961 On the other hand there are some investigations shown some equiatomic or. near equiatomic alloys cannot form the amorphous phases even by aforementioned conditions Cantor. et al 1976 The atomic size difference of SrCaYbMgZn AM ErTbDyNiAl AM ZrHfTiCuNi. AM ZrHfTiCuFe AM WNbMoTa SS WNbMoTaV SS FeCoNiCrCu SS FeCoNiCrCuAl0 3. SS AlCoCrCuFe0 5Ni SS CoCrFeNiTi IM NbCrFeMnCoNi IM Cantor et al 2004. andTi2CrCuFeCoNi IM was obtained to be 15 25 13 74 10 32 10 42 2 31 3 15 1 03 3 42 5 4 6 68. 5 49 and 7 24 respectively which AM SS and IM denote to amorphous solid solution and. intermetallic phases respectively Gao et al 2011 Ma et al 2002 Senkov et al 2010 Tang et al. 2007 It implies that severe lattice distortion may lead to amorphous tendency due to large atomic size. difference Tsai et al 2013a It is found that the phases tends to form in the domain defined as. follows 1 solid solution SS phases 0 8 5 22 7 kJ mol and 11. 19 5 kJ mol 2 the amorphous AM phases 4 5 18 5 50 8 kJ mol and 7. 17 5 kJ mol and 3 intermetallic IM phases 4 11 5 35 4 kJ mol. and 13 17 5 kJ mol, Fig 1 shows the presented data in Table1 to illustrate the three main parameters governing the.
phase stability such as mixing entropies mixing enthalpy andthe atomic radius. difference indicating the collective behavior of the constituent elements for all three mentioned. phases and also presenting the phase formation regions as defined above From Fig 1 it is found that. the above phases form in approximately separate regions from each other 1 the solid solution SS. phases high mixing entropy mostly positive mixing enthalpy and small atomic size difference 2 the. intermetallic IM phases moderately high mixing entropy negative mixing enthalpy and moderately. high atomic size difference 3 the amorphous AM phases smaller mixing entropy more negative. mixing enthalpy and high atomic size difference Therefore the formation of these three phases can. be determined by noting to the mixing entropies and mixing enthalpy in terms of the atomic size. difference of alloys For example when the mixing entropy is high and the mixing enthalpy is. correspondingly very negative the phase is suggested amorphous one based on the statistical analyses. indicated in the Table 1 and Fig 1 For the phase selection or prediction in alloys especially high entropy. alloys it has been shown that the two alloy composition physical parameters atomic size difference. and mixing enthalpy are necessary but not sufficient conditions by preparation some new. alloys using the melt spinning method Guo et al 2013. A T Samaei et al Engineering Solid Mechanics 3 2015 5. Table 1 Calculated parameters atomic size difference mixing enthalpy mixing entropies. and phases AM Amorphous SS Solid Solution and IM Intermetallic used in Fig 2 a and b. Material 100 H S Phase Reference,ErTbDyNiAl 13 74 37 6 13 38 AM Gao et al 2011. ZrHfTiCuNi 10 32 27 36 13 38 AM Ma et al 2002,ZrHfTiCuCo 10 23 23 52 13 38 AM Ma et al 2002. ZrTiNiCuBe 12 51 30 24 13 38 AM Takeuchi and Inoue 2001. Cu46 Zr42 Al 7Y5 11 84 24 88 8 79 AM Li et al 2007. Ca65 Mg15 Zn20 13 47 14 26 7 37 AM Li et al 2007,FeCoNiCrCu 1 03 3 2 13 38 SS Tong et al 2005. FeCoNiCrCuAl0 3 3 42 0 16 14 43 SS Tong et al 2005. FeCoNiCrCuAl0 5 4 17 1 52 14 7 SS Tong et al 2005,FeCoNiCrCuAl1 0 5 28 4 78 14 9 SS Tong et al 2005. FeCoNiCrCuAl1 5 5 89 7 05 14 78 SS Tong et al 2005. FeCoNiCrCuAl2 0 6 26 8 65 14 53 SS Tong et al 2005. FeCoNiCrCuAl2 3 6 4 9 38 14 35 SS Tong et al 2005,AlCo0 5 CrCuFeNi 5 45 4 5 14 7 SS Tung et al 2007.
AlCoCr0 5 CuFeNi 5 22 5 02 14 7 SS Tung et al 2007. AlCoCrCu0 5FeNi 5 51 7 93 14 7 SS Tung et al 2007,AlCoCrCuFe0 5Ni 5 4 5 55 14 7 SS Tung et al 2007. AlCoCrCuFeNi0 5 5 43 3 9 14 7 SS Tung et al 2007,CoCrCu 0 5FeNi 0 84 0 49 13 15 SS Tung et al 2007. AlCoCrCu0 5FeNi 5 51 7 93 14 7 SS Ke et al 2006,AlCrCu0 5FeNi 5 92 7 7 13 15 SS Ke et al 2006. AlCoCrCu0 5FeNi 5 51 7 93 14 7 SS Ke et al 2006, AlCo2CrCu0 5 FeNi 5 17 7 67 14 23 SS Ke et al 2006. AlCo3CrCu0 5 FeNi 4 88 7 67 14 23 SS Ke et al 2006. The microstructure and mechanical behavior of modern high temperature alloys properties for predicting the microstructure formation and phases

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