Interaction solute atom dislocation during steady-state creep in ferritic 2.25Cr1Mo steel

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1 Intraction solut atom dislocation during stady-stat crp in frritic 2.25Cr1Mo stl Albrto C. Picasso (1,2) 1 Laboratorio d Mtalurgia y Tcnología Mcánica, Dpartamnto d Ingniría, Univrsidad Nacional dl Sur, Av. L. N. Alm 1253, (8000) Bahía Blanca, Argntina. 2 Comisión d Invstigacions Cintíficas d la Provincia d Bunos Airs, Argntina. albrto.picasso@uns.du.ar Abstract Unusual bhaviour was rcordd at stady stat crp curv for th frritic stl 2.25Cr1Mo tstd at T = 873K and undr an applid strss of 103 MPa. Th mchanism of dformation is intrprtd from th suprposition of two mchanisms: a) intraction of mobil dislocations with solut atoms, and b) viscous crp. Th activation nrgy associatd was 246 kj/mol, which is consistnt with th activation nrgy by diffusion of Cr atoms in th frrit phas. Kywords: crp, viscous glid, srratd flow, intraction solut atom- dislocations 1. INTRODUCTION: At homologous tmpraturs highr than about 0.4, th solut atoms do not act as point obstacls for dislocation glid. Nvrthlss, thy can mak th dislocation glid vlocity lowr, sinc thy form atmosphrs round thm which must b draggd by th dislocations or, sinc thy form a short rang ordr, which must b dstroyd by moving dislocations [1]. Th drag of an atmosphr formd round a dislocation du to th lastic intraction btwn th dislocation and solut atoms was originally analyzd by Cottrll and Jaswon [2]. Ths authors proposd that th dragging procss is th sgrgation of solut atmosphrs to moving dislocations. Thus, th dislocation spd is limitd by th rat of migration of th solut atoms. It has bn suggstd that during th so-calld Thr-Powr-Law rgim, th dformation mchanism that controls th strain rat by crp during th stady-stat is th viscous glid of dislocations [3]... ss 0.35 æ s G ç A è G ø 3 whr ss is th minimum crp-rat, G is th shar modulus, A is an intraction paramtr which charactrizs th particular viscous drag procss controlling dislocation glid and s th applid strss. Mohamd and Langdon [4] considrd th dragging mchanism proposd by Cottrll-Jaswon and drivd th following rlationship, (1) 1

2 . ss p (1 -n ) kt Cb G æ s ö ç è G ø 3 (2) whr is th solut-solvnt siz diffrnc ( = (r-r o )/r, with r, th solvnt radius, r o, th solut radius), C is th concntration of solut atoms (C = C o xp (W/kT), whr C o is th avrag concntration and W th binding nrgy), n is th Poisson s cofficint, b _ th Burgr s vctor, T th absolut tmpratur and D is th diffusion cofficint for th solut atoms. Th aim of this papr is to show xprimntal vidnc of th xistnc of a solut-dislocation intraction mchanism in th stl 2.25Cr 1Mo, during th stady-stat on th curv of crp for a tmpratur and strss applid. Th rsults found hr ar consistnt with a crp strain mchanism controlld by th moving of dislocations intracting with atmosphr of solut. 2. EXPERIMENTAL: Th matrial usd for th crp tst was 2.25Cr1Mo stl. This is a crp-rsistant stl, which is oftn usd as r-hating tubs in th boilrs in powr plants. Th chmical composition of th matrial is shown in Tabl 1. TABLE-1: Chmical composition of th stl usd in this study (wt. %) C Mn Si S P Cr Mo F Bal. Crp sampls 25 mm in gaug lngth and 5 mm in gaug diamtr wr machind dirctly from th tub. Th crp tst was prformd in air with a loading bam capabl of maintaining a constant strss, within 1% of th initial strss, up to strains of th ordr of 10% [5]. Th xprimnt was prformd at 873K undr an applid strss of s = 103 MPa. Th tst tmpratur was monitord with two thrmocoupls touching th spcimn gaug sction and hld constant within ± 1.5 K. Th longation of th spcimn was masurd by attaching fusd silica rods, which xtndd out of th furnac, to th uppr and lowr grips. Sinc th lowr grip was fixd, a masurmnt of th rlativ displacmnt of th fusd silica rods, using an LVDT, givs th tru longation of th spcimns, liminating any influnc of th loading train. Th signals of th thrmocoupls and th LVDTs fd an A/D card connctd to a slot of a PC. Data ar rcordd using a customizd acquisition program. 3. RESULTS AND DISCUSSION: An anomalous bhavior during th crp tst was rcordd, through a diagram tru strain by crp vrsus tim at a tmpratur T = 873 K and applid strss, s = 103 MPa. Fig. 1 shows th whol crp curv obtaind during th tst crp; whil Fig. 2 illustrats th xtndd rgion of th stady stag, whr variation of dformation ovr tim during this stag is shown. 2

3 FIGURE:1 0,06 0,05 Crp by tnsil : 2.25Cr1Mo stl s = 103 MPa T = 873 K 0,04 0,03 Stady-stat with srrations 0,02 0,01 0, t (h) Fig-1:Diagram tru strain () vs. tim (t) for s = 103 MPa y T = 873K FIGURE:2 0,0125 s = 103 MPa 0,0100 0,0075 0,0050 0, t (h) Fig-2:Stady stat nlargd rgion showing th srrations on th xprimntal curv (s Fig. 1). 3

4 As can b sn, th stady stag shows a variation of th dformation ovr tim in th form of an irrgular wav altrnating, ovrlappd on an avrag slop, which rprsnts th dformation spd. Aftr confirming and assuring that th bhavior is charactristic of th matrial and not of som instrumnt or th nvironmnt, vidnc shows that this is a dformation mchanism, which prsnts hardning by dformation and rcupration, similar to thos obsrvd during th tnsion tsts, probably du to th ffct of th intraction btwn mobil dislocations and solut atmosphrs. This kind of bhavior is unusual in crp diagrams. From th prvious diagram a minimum spd of dformation or stady stag o = 1.5x10-8 s -1 was obtaind. Maruyama t al. [6], xamind th maps of dformation mchanism by crp in th stl 2.25Cr-1Mo for dformation spds btwn and 10-6 s -1 and a tmpratur ranging from 723 to 923 K and found that th minimum dformation spd by crp is rlatd to th strss applid by a powr-law with 3 < n < 5. Fig. 3 shows th diagrams obtaind by Maruyama t al. and th rsult of th prsnt work. FIGURE:3 Fig-3: Minimum crp rats d/dt as a function of crp strss s normalizd by Young s modulus E for two hat tratmnts obtaind by Maruyama t al. In addition, brokn lin indicats th xprimntal point corrsponding to this study. 4

5 From such rsults, w can infr that undr our tst conditions, th rlationship btwn th tnsion applid and th minimum strain rat by crp is associatd with a dislocation glid mchanism assistd by diffusion. Howvr, th activation nrgis by crp obtaind by Maruyama t al. rangd from 370 to 420 kj/mol K. Chalmrs [7] obsrvd a typ of slow crp ( micro-crp ) on tin crystals which occurrd at tnsil strain rats blow 10-8 s -1 and at tnsil strsss blow 1.5 MPa. This author concludd that critical crp rat rquirs that D 10 s -8-1 (3) Intraction of solut atoms round moving dislocations: Undr an xtrnal forc, th spd of a dislocation surroundd by a solut atmosphr is rstrictd to th spd of migration of solut atoms in atmosphr. Sinc diffusion spds ar slow compard with normal dislocation spds, this procss is likly to b important only in slow crp xprimnts. Cottrl has stimatd that th critical vlocity rquird to lav bhind th solut atmosphr can b dscribd [8] by v c = 4D / l (4) whr th charactristic lngth l (=A/kT) may b rgardd as th radius of th atmosphr and D th diffusion cofficint. If th dnsity of ths dislocations is r th corrsponding critical crp rat is dg / dt = vrb = 4Drb / l (5) In our cas, w might stimat th dnsity of dislocations using th valu that w hav dtrmind o xprimntally for th minimum spd of dformation by crp, = 1.5x10-8 s -1 and th diffusion cofficint D. Thus, to calculat th diffusion cofficint w us th D o = 8.52x10-4 m 2 s -1 [9] and a valu for th activation nrgy Q= 246 ± 25 kj/mol K [10]. Thus, r» 1x10 10 m -2 ; which drivs from a rasonabl valu for dformation by crp in th stady stag. It is intrsting to highlight that th valu of th activation nrgy by crp that w hav usd is vry clos to that obtaind by Bown t al. for th atom diffusion of Cr in alpha iron [9], which was Q D = 250 kj/mol o K. Png t al. [11] studid th ffct of dynamic strain aging (DSA) on th proprtis at high tmpraturs in th austnitic stainlss stl 18-8 and attributd such bhaviour to th intractions of atmosphrs (C,Ni) and (C,Cr) with dislocations, at low and high tmpraturs, rspctivly. Taking into account th activation nrgy by crp obtaind in this matrial, it sms rasonabl to infr that th dformation mchanism by crp in th stady stag, in our cas, would b composd of a viscous dformation mchanism, on which bhaviour of th DSA typ is ovrlappd and whos atmosphr is probably composd of Cr atoms. 4. CONCLUSIONS: Th anomalous bhavior obsrvd in th stady stag rgion of th crp curv obtaind at T = 873K and σ = 103 MPa in th stl 2.25Cr1Mo, may b intrprtd from th following obsrvations: 1.- Th stady stag rgion of th crp curv by tnsil, obtaind at T = 873 K and s = 103 MPa, showd an anomalous bhavior of th DSA typ 2.- Th anomalous bhavior would b associatd with an anchorag mchanism and rlas of mobil dislocations controlld by an atmosphr of Cr atoms. 5

5. ACKNOWLEDGEMENTS This work was partially supportd by SCyT-Univrsidad Nacional dl Sur and th Comisión d Invstigacions Cintíficas d la Provincia d Bunos Airs (CICPBA). 6. REFERENCES [1] Čadk J.

6 5. ACKNOWLEDGEMENTS This work was partially supportd by SCyT-Univrsidad Nacional dl Sur and th Comisión d Invstigacions Cintíficas d la Provincia d Bunos Airs (CICPBA). 6. REFERENCES [1] Čadk J. (1988) Crp in Mtallic Matrials, Elsvir, Amstrdam, p [2] Cottrll, A.H. and Jaswon, M.A., Proc R. Soc. A, 1949, 199, 104. [3] M.E. Kassnr and M.T. Pérz-Prado, Fundamntals of Crp in Mtals and Alloys, Elsvir (2004). [4] Mohamd, F.A. and Langdon, T.G. Acta Mtall. 22, 779(1974). [5] A.C. Picasso, PhD Thsis, Univrsity of th Cntr of Bunos Airs Stat (1996). [6] K. Maruyama, K. Sawada, J. Koik, H. Sato, K. Yagi, Matr. Sci. Eng. A (2009) [7] Chalmrs B., Proc. Roy. Soc. A, 1936, 156, 427. [8] Cottrll, A.H. in Dislocations and Plastic Flow in Crystals, Ed. by Oxford Univrsity prss, Amn Hous, London E.C.4, 1953, [9] A.W. Bown, G.M. Lak Mt. Trans. A, 1, 1695 (1970). [10] G. Gonzálz, A. Juan, A. Garófoli, L. Moro, A. Picasso, Primr Congrso Argntino d Ingniría Mcánica (I CAIM 2008), Octubr 2008, Bahía Blanca, Argntina. [11] Kaiping Png, Kuangwu Qian, Wnzh Chn, Mat. Sc. and Eng. A379 (2004) AUTHOR S BRIEF BIOGRAPHY: Dr.Albrto C.Picasso:H holds a PhD in physics, Indpndnt Rsarchr CICPBA, currntly hads th rsarch group "kintic volution and mchanical proprtis rsistant alloys at high tmpraturs" at th Laboratory of Mtallurgy and Mchanical Tchnology, Dpartmnt of Enginring at th Univrsidad Nacional dl Sur (UNS) in th city of Bahia Blanca, Provinc of Bunos Airs, Argntina; H has publishd svral articls in intrnational journals of spcialization and has dirctd svral studnts in its undrgraduat and graduat dgr in Matrials Scinc. In addition, h is Associat Profssor in th Dpartmnt of Enginring at th UNS, whr h tachs Prof. th chairs Dr. Albrto of mtalforming C. Picasso and Mtallic Matrials Profssor. 6

Mechanical Properties

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