ULTRASONIC SHEARWAVE MEASUREMENTS OF PHASE TRANSITIONS IN

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1 ULTRASONIC SHEARWAVE MEASUREMENTS OF PHASE TRANSITIONS IN ZIRCALLOY AT ELEVATED TEMPERATURES D.R.Billsn and D.A.Hutchins Department f Engineering University f Warwick Cventry, CV4 7AL England INTRODUCTION CANDU reactrs each cntain several hundred zircnium-nibium pressure tubes. The zircnium-nibium ally is prne t hydrgen (and deuterium) absrptin, which can lead t the frmatin f brittle zircnium hydride platelets within the metal. These hydrides have caused a number f pressure tube leaks, and were the cause f a majr failure in ne early reactr. They have necessitated the retubing (r the scheduled retubing) f the earliest reactrs at a cst f several hundred millin dllars each. A nndestructive methd f measuring the hydride cncentratin in the metal wuld allw selective (rather than cmplete) retubing f the reactrs, thus saving large amunts f mney. N satisfactry nndestructive testing technique currently exists, s it is hped that the wrk presented here shws an effect n which a technique might be based. The maximum cncentratin f free hydrgen in the metallic lattice increases with temperature, and s as the ally is heated up, the hydride disslves. The temperature at which all the hydride has disslved depends n the initial cncentratin f the hydride in the ally, and thus if this temperature is knwn, the initial (rm temperature) hydride cncentratin can be calculated. It was thught that there might be a change in the material characteristics f the zircally at this transitin temperature, which shuld shw up as a change in ultrasnic Review f Prgress in Quantitative Nndestructive Evaluatin, Vl. 12 Edited by D.. Thmpsn and D.E. Chimenti, Plenum Press, New Yrk,

2 velcity. S a methd fr measuring the thrugh-transmissin time fr shear r lngitudinal waves thrugh the sample was devised. This wrks at temperatures up t 500 degrees Centigrade, using a pulsed laser as an ultrasund surce, and a specially devised EMAT as a receiver. DESIGN OF THE HIGH TEMPERATURE EMAT'S The EMATs were develped t fulfil tw criterin; firstly they had t perate clse (0.5mm r less) t the zircally samples (which were heated up t 400 degrees centigrade), and secndly they had t minimise thermal gradients within the samples. (We were lking fr a transitin at a specific temperature; having a variatin f temperatures thrughut the sample thickness wuld smth ut any results). brass -. steel ple pieces z cally sample laser ulse alcmaxiii temperature magnet s Fig.l Lngitudinal wave EMAT detectr. In earlier experiments, it was fund that water-cling the EMATs caused large thermal gradients which were unacceptable, and s EMATs were develped that culd run at the same temperature as the samples. The EMATs used alcmaxiii magnets which are rated fr use up t 550 degrees Centigrade, they give a much smaller magnetic field than the mre cmmnly used nedinium-irn-brn magnets, but are adequate fr this purpse. The EMAT and the sample were enclsed tgether within a brass husing t ensure that their temperatures were equal, thus reducing any thermal gradients within the sample. 1662

3 sample laser pulse --"'lcu_l... ake cil alcmaxiii high temperature magnet Fig.2 Shear wave EMAT detectr. The cils presented cnsiderable prblems as n sufficiently thin wire (O.lmm) was readily available that had insulatin capable f withstanding temperatures ver 150 degrees C. This was slved by cating the wires with a high temperature ceramic. The lngitudinal EMAT (fig.1) used a cil linearly wund rund a brass frmer. The Shear wave EMAT (fig.2) used a pancake cil which was sensitive t radial shear waves, enabling bth fast and slw shear waves t be detected simultaneusly. (the zircally was acustically birefringent). EXPERIMENTAL METHOD The experimental cnfiguratin was as in fig.3, the laser was a Lumnics 'mini-q' nedinium-yag, Q-switched laser, delivering 4nS pulses f 1060nm I.R. radiatin, at energies up t 120mJ. The laser beam was fcused with a 17cm fcal length lens thrugh an aperture in the brass husing, nt the sample. The sample face was 30cm frm the lens. The EMAT utput, after preamplificatin, was digitised n a Data Precisin scillscpe, and after averaging, the wavefrm was transferred nt an IBM cmputer and stred. The temperature f the sample was measured with a thermcuple attached directly t the sample face. The apparatus was set up, and the furnace was heated up t 400 degrees C, and left fr an hur t stabilise. The furnace was then switched ff and allwed t cl slwly. Whilst the furnace was cling, the measurements were taken. Cling was preferred fr the experiment as this gave a slw, even rate f temperature change, which culd nt be achieved n heating. The wavefrms were analyzed n a cmputer, and the psitins f the peaks were pltted with temperature. 1663

4 RESULTS A typical ultrasnic wavefrm is shwn in fig.4. The arrival times f the peaks were pltted against temperature, this is shwn in figure 5 (sample with 60ppm hydride) and in figure 6 (unhydrided sample). It is clear that there is a change in gradient n the hydrided sample which is nt present n the unhydrided sample. This was repeated fr a number f samples with different rm temperature hydride cncentratins, and a graph f transitin temperature against nminal hydride cncentratin is given in figure 7. p laser digital scillscpe IBM cmputer Fig.3 Experimental cnfiguratin. This effect has s far nly been bserved with the shear wave measurments. Further experiments are needed lking at lngitudinal waves, and are in prgress. The temperatures at which these gradient changes take place are cnsistent with results btained by ther wrkers in the field. It is t be nted that the gradient changes are due t velcity, rather than a thermal expansin changes. The thermal expansin f the zircally samples was measured with a dilatmeter, a large change in thermal expansin was nted at the transitin temperature fr the samples, but this was still tw rders f magnitude t lw t accunt fr the time f arrival changes. It is interesting t nte that the changes f velcity with temperature (gradients f the graphs) are cnsiderably different between the slw and the fast shear waves (the slw shear waves exhibit a much steeper gradient than d the fast nes). 1664

5 ,...,.0 $.., (!j Iil A :: E--<...:l P.. ::::s < r rrr"tt"rttttl-rrrr-r... rrrttl-rrrr-r"tt"'" TIME (MICROSECONDS) Fig.4 Typical ultrasnic shear wavefrm..... <0 I W '--' * <Il E += > 'C 2Q.40. ' ō.::.: a nn-rttt-.-r;n-rttt..,...,-,..,.,-rttttl...".,., Temperature (degrees C) Fig.5 variatin f shearwave arrival time with temperature - hydrided sample., til c I w it '--' III _I.OO C > c C III II.. 18JO.OO '!-nn-rttt"t"t""""""'-''''''''''''''''''''''''''''''''''''''''''' Temperature (degrees C) Fig.6 Variatin f shearwave arrival time with temperature - unhydrided sample. CONCLUSION A pulsed laser and specially designed EMATs have been used t investigate the change in ultrasnic velcity with temperature, f bth shear and lngitudinal waves, thrugh samples f zircally. This was dne between rm temperature and 400 degrees centigrade. The EMATs were develped t perate (uncled) at up t 500 degrees Centigrade. The shift in time f shear wave arrivals n cling f the sample was used t shw a phase transitin in hydrided zircally samples, the temperature at which this transitin ccurred is cnsistent with previus results, and may be used t estimate the rm temperature hydride cncentratin within the ally. 1665

6 ,.-, U s.. ;::l r 0.. S !=: < !1l r E-t r..-nr-tt..-nr-tt..-nrrrttl... rrl... TTTTrr-rrTrr-rrrrrrl Nminal hydr ide cncentrat in (ppm) Fig.7 variatin f transitin temperature with hydride cncentratin, REFERENCES 1. C.E.Cleman and J.F.R.Ambler, "Slubility limit f hydrgen in zircnium allys", Seminar n Fuel Channel Technlgy, Trnt, Ont., June D.A.Hutchins, "Ultrasnic generatin by pulsed lasers", in Physical Acustics Vl. XVIII (Academic Press, New Yrk, W.P.Masn and R.N.Thurstn, eds., 1979), pp V.Pervic, G.C.Weatherly and C.J.Simpsn, "The rle f elastic strains in the frmatin f stacks f hydride precipitates in zircnium allys", Scripta Metallurgica 16, pp (1982) 4. R.B.Thmpsn, "Physical principles f measurments with EMAT transducers", in Physical Acustics, Vl. XIX (Academic Press, New Yrk, W.P.Masn and R.N.Thurstn, eds., 1990) pp D.R.Billsn and D.A.Hutchins, "Laser-EMAT ultrasnic measurments f bnded metals", t be published in Nndestructive testing and evaluatin 1666