Structural refinement of Cr-75Nb alloy subjected to air blast shot peening

Transcription

1 2nd Interntionl Conference on Mchinery, Mterils Engineering, Chemicl Engineering nd Biotechnology (MMECEB 2015) Structurl refinement of Cr-75Nb lloy subjected to ir blst shot peening b Hizhong Zheng, Qinho Luo, Xioyong Shu, Guif Li, Lingling Xiong School of Mteril Science nd Engineering, Nnchng Hngkong University, Jingxi , Chin. zhznchu@126.com, bluoqinho163@163.com Keywords: Cr-75Nb lloy, shot peening tretment, deformed lyer, microstructure, structurl refinement Abstrct. In this pper, the ir blst shot peening on Cr-75Nb lloy ws crried out t the pressure of 0.6 MP nd processing durtion of 13 min. The result showed tht severe plstic deformtion lyer ws gined on the surfce of Cr-75Nb lloy. The thickness of deformed lyer ws bout 50 µm, nd the structurl scles of Nb-bsed solid solutions (Nbss) of top surfce lyer is refined to bout 22 μm. The hrdness of the surfce lyer is enhnced significntly in nrrow depth, which might be ttributed to the structurl refinement nd/or work-hrdening. 1. Introduction In recent yers, numerous investigtions hve been focused on the Lves phse NbCr2 bsed intermetllic compounds due to their excellent mechnicl performnce such s high melting temperture (1730 ), reltively low density (7.7 g/cm3) nd pprecible creep resistnce [1-3]. Therefore, these excellent properties project it s promising cndidte mteril for high-temperture structurl pplictions. However, the oxidtion resistnce is in need of improvement for further ppliction in the eroengine hot end components in the erospce industry. The oxidtion pest phenomenon hs been observed in niobium lloys t 600, resulting in bd oxidtion resistnce of Cr-75Nb lloy in the temperture rnge of [4]. Over the pst decdes, mny reserches [5,6] hve been performed to develop protective coting on NbCr2 bsed lloys in order to improve the oxidtion resistnce bove However, the pck cementtion processes to prepre the coting re performed t high tempertures for long durtion, which my induce serious deteriortion of the substrte. The problems my be solved by the occurrence of surfce nnocrystlliztion (SNC) [7,8]. Nnocrystlline mterils possess ultrfine grins with lrge number of grin boundries which my ct s fst tomic diffusion chnnels [9,10]. Numerous investigtions demonstrte tht surfce structurl refinement cn be chieved by severe strining of the mteril [11,12]. However, little hs been investigted in the open litertures on the surfce of Cr-75Nb. In this work, Cr-75Nb lloy ws treted by ir blst shot peening tretment (ABSP). The microstructurl evolution nd the hrdness chnge were chrcterized by mens of different nlyticl techniques. The microstructure of Cr-75Nb lloy ws nlyzed. 2. Experimentl The s-cst Cr-75Nb (t. pct) lloy plte used in this work ws prepred by rc melting in the WK-Ⅱvcuum furnce. The rc melted ingot ws remelted five times nd nneled in vcuum t 1573 K for 10 hours nd cooling to mbient temperture in the furnce for obtining homogeneous structure, the structurl scle of Nbss of Cr-75Nb lloy ws bout 90 µm, s shown in Fig.1. Smple of mm3 ws prepred for the ABSP tretment. Before the ABSP tretment, the Cr-75Nb lloy smple surfce ws polished with #1200 silicon crbide ppers. Then, the surfce of the smple ws peened by the flying stinless shots (dimeter of 1mm) with high energy. The distnce between the treted smple nd the shot jet ws 30 mm. The processing durtion ws The uthors - Published by Atlntis Press 699

2 min. The cross section of Cr-75Nb lloy smple ws prepred for metllogrphic exmintion. The microstructure of Cr-75Nb lloy ws chrcterized by scnning electron microscope (SEM). The phse constitutions in the treted surfce lyer were identified by using X-ry diffrction (XRD) instrument with Cu-Kα rdition. And hrdness mesurements were performed by using Vickers indenter with 200 gf lod nd interior up to 500µm with 50 µm intervls in order to investigte the hrdness vritions long the depth. 200 μm Fig.1 The opticl microgrph of Cr-75Nb before ABSP 3. Results nd discussion 3.1 Microstructure The cross-sectionl opticl microgrph of the Cr-75Nb lloy treted for 13 min is shown in Fig.2. As in Fig.2, severe plstic deformtion occurred in the surfce lyer of Cr-75Nb lloy. In the deformed lyer, the ner-equixed Nbss become stremlined. It is worth noting tht the region A (shown in Fig.2) is obviously impcted by the blls t high speed nd smll prt of the mteril begn to come off. The reson of the mss loss is tht Lves phse NbCr 2 is n intermetllic compound with topologiclly close pcked (TCP) crystl structure. Due to its high brittleness ttributed to the difficulty in moving disloctions by process clled synchrosher [13], repetitive nd multidirectionl loding on the Cr-75Nb lloy leds to the mteril loss. A deformtion lyer 200 μm Fig.2 Opticl observtion on the deformtion lyer of the smple treted for 13min Figure 3 shows tht cross-sectionl SEM observtion of Cr-75Nb shot-peened with 13 min. As seen in Fig.3 (low mgnifiction), the Cr-75Nb lloy fter the ABSP tretment shows primry Nb-rich solid solution dendrites surrounded by more or less continuous thin lyer of Nb/NbCr 2 eutectic structure nd the verge structurl scle of Nbss is bout 90 μm. In the Fig.3 b (high mgnifiction), it occurs sever plstic deformtion on the top surfce of Cr-75Nb treted by ABSP. The thickness of the deformtion lyer is bout 50 μm. With tretment durtion incresing to 13 min, the structurl scle is refined to bout 22 μm. 700

3 b Nbss eutectic eutectic Nbss Fig.3 Cross-sectionl SEM imge of Cr-75Nb shot-peened with 13 min() low mgnifiction; (b) high mgnifiction The X-ry diffrction of the top surfce lyer of untreted nd treted smple re shown in Fig.4. It is noticed tht the untreted nd the ABSP treted Cr-75Nb lloy smples re consisted of Nb nd NbCr 2 phse, without ny evidence of new phse. It differs from the Ni 3 Al intermetllic of which the crystl structure chnged during the SMAT [14]. It my be implied tht the strins in the treted smple during ABSP re not lrge enough to induce new phse trnsformtion. The diffrction peks of Nb nd NbCr 2 shift to the high diffrction ngle due to the solid solution of Cr. And there is evident brodening of the Brgg reflection profiles on the top surfce lyer of the shot-peened smples, which cn be ttributed to structurl refinement nd microstrin [15]. NbCr 2 Nb intensity,.u 13 min untreted θ, deg Fig. 4 XRD pttern of the untreted nd shot peened Cr-75Nb lloy. 3.2 Hrdness Figure 5 shows the hrdness vrition long the depth of the smple treted for 13 min. It cn be seen tht the hrdness of s-treted Cr-75Nb lloy increses significntly in the top surfce lyer. In the region of bout 50 µm deep from the top surfce lyer, the hrdness shows shrp drop nd then decreses till to stble level with further increse of the depth which mens pproching the hrdness of the mtrix. For the smple treted for 13 min, the mximum hrdness of the deformed lyer is incresed to 4.3 GP, while the hrdness of the mtrix is 3.2 GP. The present results show tht the hrdness of Cr-75Nb lloy cn be significntly improved by ABSP, which might be owing to the structurl refinement of Nbss nd/or work-hrdening. 701

4 4.5 Hrdness / Gp Depth / m Fig. 5 Hrdness vrition long the depth of the smple treted for 13 min. 4. Conclusions The ABSP tretment of Cr-75Nb lloy hs been conducted t the ir pressure of 0.6 MP nd the tretment durtion of 13 min. A severe plstic deformtion lyer structure ws fbricted on the surfce lyer of Cr-75Nb lloy subjected to the ABSP tretment. The thickness of the deformed lyer of Cr-75Nb is bout 50 µm. For the smple shot-peened for 13 min, the mximum hrdness of the top surfce lyer is ttched to 4.3 GP, which is enhnced significntly compred with tht of mtrix. The structurl scle of Nbss in the top surfce lyer of Cr-75Nb treted for 13 min could be refined to bout 22 µm. Acknowledgments: This work ws supported by the Aeronuticl Science Foundtion of Chin (2013ZE56014). References [1] S.Q. Lu, H.Z. Zheng, M.W. Fu. The frcture toughness of hot-pressed NbCr 2 lloys doped by rre erth yttrium. Script Mter. Vol. 61 (2009) No. 2, p [2] H.Z. Zheng, S.Q Lu, J.Y. Zhu, G.M. Liu. Effect of Al dditions on the oxidtion behvior of Lves phse NbCr 2 lloys t 1373 K nd 1473 K. Int. J. Refrct. Met. Hrd Mter. Vol. 27 (2009) p [3] A.V. Kzntzis, M. Aindow, I.P. Jones, et l. The mechnicl properties nd the deformtion microstructures of the C15 Lves phse Cr 2 Nb t high tempertures. Act Mter. Vol. 55 (2007) No. 3, p [4] H.Z. Zheng, S.Q. Lu, Y. Hung. Influence of grin size on the oxidtion behvior of NbCr 2 lloys t Corros. Sci. Vol. 51 (2009) p [5] Y.Q. Qio, M.Y. Li, X.P. Guo. Development of silicide cotings over Nb-NbCr 2 lloy nd their oxidtion behvior t Surf. Cot. Technol. Vol. 258 (2014) No. 15, p [6] P. Zhng, X.P. Guo. A comprtive study of two kinds of Y nd Al modified silicide cotings on n Nb-Ti-Si bsed lloy prepred by pck cementtion technique. Corros. Sci. Vol. 53 (2011) p [7] K. Lu, J. Lu. Surfce nnorystlliztion of metllic mterils - presenttion of the concept behind new pproch. J. Mter. Sci. Technol. Vol. 15 (1999) No. 3, p [8] G. Liu, J. Lu, K. Lu. Surfce nnocrystlliztion of 316L stinless steel induced by ultrsonic shot peening. Mter. Sci. Eng. A. Vol. 286 (2000) p

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