| In this paper, the transfortions of the microstructures and mechanical properties of the Cu-rich in Super304H austenitic heat resistant stainless steel and the Ni2(Cr,Mo) in newly-developed C-22HS alloy at a nano-scale are investigated by means of advanced electronic microscopy. The early precipition behavior of Cu-rich phase and Ni2(Cr,Mo) have been analyzed detailedly.The special character of Super304H heat resistant steel is the addition of3wt%Cu. Super304H with3wt%Cu has excellent corrosion resistance duo to the exiting of the Cr, Ni element. Because of the precipitation of fine and dispersive Cu-rich phase, Super304H also has high creep strength after long aging process (10000h). Thus, it is crucial to investigate the precipitation behavior, growth mechanism and strengthening mechanism. The results show that it was found that Cu atoms accumulated to sphere particles, while the main matrix elements of Fe, Cr and Ni atoms were excluded outside of the Cu-rich phase during aging. There are two factors that dominate the growth kinematics and thus the strengthening effect of precipitates:(1) interfacial energy between the precipitates and the matrix;(2) diffusion coefficient. The coherent interfacial energy between Cu-rich precipitate and austenitic matrix was determined to be0.017-0.086J/m2based on two approximations, resulting in a slow growth of the Cu precipitates in austenite matrix. The excellent strengthening effect of Cu-rich precipitates resulted from the coherent interfacial structure and highly stable microstructure.C-22HS alloy is a new type Ni-Cr-Mo superalloy, which has excellent both high temperature strength and high corrosion resistance due to the precipitation of Ni2(Cr,Mo) superlattice under the two-step heat treatment. Compare to others Ni-base alloys, the preicipitation mechanism of Ni2(Cr,Mo) in C-22HS is not from short-range ordered to long-range orderd. The results show that a typical orientation relationship between the precipitates and the Ni-Cr-Mo matrix is determined as (200)matrix//(103)Ni2(Cr,Mo),[001]matrix//[010]Ni2(Cr,Mo). The Ni2(Cr,Mo) precipitates are enriched in Mo a little, compared with Ni-based matrix. The weak streak fringes along (311)*directions were observed in the EDP along the [112] zone axis, with the maximum intensity at the1/2(311)*due to slight enrichment of Mo atoms. Thermodynamically, the behavior of molybdenum atoms’ nano-aggregation at the early stage of aging and the subsequent formation of LRO Ni2(Cr,Mo) superlattice could contribute to the decrease of the entropy and the Gibbs free energy of the system, further promoting the phase transition to strengthe the alloy. Thus, it is determined that nano-aggregation of Mo atoms along1/2{311}Ni-base would occur first every two{311}Ni-base planes at a high temperature (705℃) which might increase the transition temperature of LRO Ni2Cr phase from590℃to higher temperature (i.e.605℃or even higher). As the second step of heat treatment proceeds at605℃, Ni2Cr phase may form and then Ni2(Cr,Mo) particles precipitate correspondingly by the substitution of Mo atoms for Cr atoms every three{311}Ni-base planes in Ni2Cr phase with the continuous long-range diffusion of Mo atoms.The phase has an enhanced stability at the composition of Ni2Cro.67Moo.33due to the appearance of unstable Ni2Cr0.5Mo0.5, which is at a local energy maximum in the energy profile. The independent elastic constants of Ni2Cr1-xMox superlattices are calculated to study the elastic properties. The results show that all the Ni2Cr1-xMox phases are mechanically stable. However, both the strength and directionality of the chemical bonds in Ni2Cr0.5Mo0.5are reduced abnormally, which agrees with experimental observations pretty well. |
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