An Experimental Study On Elevated Aging Of GH625 Alloy

To strengthe n the advanced applicat ion of the domestic superalloy in the civil field,and according to the practice of the power system need to vigorously deve loping the ultra-supercrit ical coal-fired unit（A-USC）to meet the development of economy demand for less pollut ing energy in the current stage of C hina.Therefore,in this thesis the superalloy GH625 is studied as one of the candidate materials which has been used in high temperature components of A-USC at 700℃.By means of scanning electron microscopy,energy-dispersive spectrometry,X-ray diffract ion,transmiss ion electron microscopy,room-temperature tens ile and hardness tests,the evolut ions of microstructure and mechanical properties occurring upon therma l aging of GH625 was studied at 720℃,it aims to provide reference and basis for the practical applicat ion of domestic GH625.Through aging test,the result shows as follows:（1）The evolut ion o f age-precipitated phases and microstructure in GH625 alloy is investigated by therma l aging test 1 × 103,3 × 103 and 5 × 103 hours at 720℃.The experimenta l results shown that the changes of grain boundary configurat ion are affected by decompositio n and precip itatio n of carbides,the connect ions of γ" phase and the precip itat ion and growth of δ phase,and the configurat ion of grain boundary showed the trend of coarsening-thinning-coarsening w ith the prolonging of aging time.The ma in intragranular precip itated phases were precipitated in the order γ’ → γ" → δ phase,and it was observed that the metastable γ" phases progressive ly transform into the stable δ phases wit h the prolonging of aging time,the coarsening of γ" phases is the ma in influence on this phase transit io n.The γ’ and γ" phase are uniformly distributed in the matr ix for thermal agings to 1 × 103 hours,the three kinds of precipitat ions which is the γ’,γ",δ phase are precip itated uneve nly for aged 3 × 103 hours and the precip itation after aging for 5 × 103 hours is mainly composed of δ phase.（2）The phase structrues of oxidation layer at var ious aging stages are analyzed wit h X-ray diffraction.The results ind icated that the sur face oxidation layers are composed of NiO,Cr₂O₃ and NiCr₂O₄.（3）According to O stward ripening t heory,the therma l stabilit y and coarsening process of γ" phases were analyzed.The results ind icated that the coarsening process of γ" phase accord with t he description of O stwald theory.At the direction of faster coarsening rate,the γ" phases size tended to be consistent and the maximum average size of γ" phases is 0.24253μ m after aging for 4.541 × 103 hours.The morphology size distribut ion of precipitated γ" particle coinc ide with t he testing quasi-stationary distribut ion cur ves of LSW theory.In addit ion,the phase-transit ion process of γ" phase agree wit h the descript ion,whic h is kinet ics curve of Avrami equation and the Avrami index n ≈ 1 is consistent with experimental observation.（4）The relat ion between metallographic structure and tensile properties of alloy were analyzed through the mechanica l properties testing and observation of fracture morphology and based on modification Hollomon model to obtain the curves of work hardening rate-strain and hardening exponent.The results ind icated that the mechanical properties of alloy are influenced by the content of main strengthe ning phase;the dis location dens ity was calc ulated in beginning of the hardworking,which is controlled by cross-slip and the va lue of the dis location dens ity is 2.55858 × 1011cm-2.It is also found that the hardening exponent is associated with strain in this model.（5）Under the same product ion techno logy and heat treatment condit ion,compare the mechanical properties of t he domestic GH625 and the Inconel 625 which is produced by Internat ional N ickel C ompany.The results indicated that there is no existed dist inct differences between GH625 and Incone l 625 on mechanica l properties indexes wit hout aging,but when GH625 aging for lo ng t ime under similar condit ions,the tens ile strengt h and plastic ity are not as good as the Inconel 625.From the precipitat ion process of strengt hening phase,the ma in factor for the gap is that the γ" phases in GH625 has faster coarsening rate than Incone l 625,so the therma l stabilit y of GH625 is not as good as the Inconel 625,this caused a large amount of δ phase precipitated during earlier period of aging,the content of solid solut ion ele ments in matrix continuous to decrease and the solution strengthening effect was weaken.（6）Compare wit h the European standard require ments of high-temperature metallic materia ls in 700 ℃ A-USC.The results indicated that a rough calculat ion of unit oxidation weight loss after 105 hours aging at 720℃ is 4.45 × 10-3 g / mm2,which meet applicabilit y requirements of A-USC at 700℃,but the stabilit y of microstructure of GH625 still need to be enhanced when GH625 is used as structural components of A-USC for long time under the high temperature.