The use of microscopic and hardness measurement methods in recrystallization studies

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<ul><li><p>Mat. Res. Bull., Vol. 18, pp. 1027-1036, 1983. Printed in the USA. 0025-5408/83 $3.00 + .00 Copyriff.ht (c) 1983 Pergamon Press Ltd. </p><p>THE USE OF MICROSCOPIC AND HARDNESS MEASUREMENT METHODS IN RECRYSTALLIZATION STUDIES </p><p>C.M. Kamma, Department of Mechanical Engineering, University of Lagos, Nigeria </p><p>and E. Hornbogen, Institut fur Werkstoffe, Ruhr-Univers i t~t, Bochum, W. Germany. </p><p>(Received May 15, 1983; Communicated by R. A. Huff.pins) </p><p>ABSTRACT The use of microscopic and hardness measurement methods as effect ive exper imental techniques for recrysta l l i zat ion studies has been invest igated. Considerat ion was given to the product ion of dif ferent start ing-point microstructures of varying Fe~C-part ic le dispersion, known to be associated with the compl icat ions of recrysta l l i zat ion mechanism of second-phase materials. Each of the invest igat ion method considered was used to study the various aspects of recrysta l l i zat ion phenomenon (recovery, in-s i tu-recry- stal l izat ion, part ic le-contro l led subgrain growth, partial recrystal l izat ion, grain growth) and the merits and l imitat ions of appl icat ion discussed. </p><p>INTRODUCTION The recrysta l l i zat ion mechanism in metal l ic mater ia ls is known </p><p>to be compl icated (1). Of special note in this respect is the con- tr ibutory inf luence of the nature of the start ing-point microstructure. This is usual ly a homogeneous, super-saturated solid solut ion or a microstructure containing a second-phase in the form of part icles. Prec ip i tat ion in the super-saturated solut ion can take place before, during or after the recrysta l l i zat ion process (2,3). During the de- formation stage, prior to the recrysta l l i zat ion annealing, the part ic les play the important role of inf luencing the distr ibut ion of d is locat ion-network in the deformed structure (4,5). </p><p>In consequence, the recrysta l l i zat ion behaviour of a normal metal l ic material is invariably associated with cases of combined reactions, whereby prec ip i tat ion and growth of part ic les overlap the recrysta l l i zat ion processes. The complete course of recrysta l l i zat ion in a given material is accordingly seen to be made up of four stages: I) Recovery stage, involving very l itt le annihi lat ion of dis locat ions; II) Cont inuous combined react ion stage, in which particle growth and </p><p>part ic le inhibited subgrain growth occur; III) Pr imary recrysta l l izat ion; IV) Further part ic le-and recrysta l l i zed grain-growth. </p><p>1027 </p></li><li><p>1028 C.M. KAMMA, et al. Vol. 18, No. 8 </p><p>There exists the common practice of interpret ing recrystal l i rat ion phenomena with the help of results obtained from one part icular invest igat ion method. The information thus gained is l ikely to be unreliable, in view of the varying mechanisms associated with the different stages of the recrystal l izat ion process. In the present work, an invest igat ion of the recrysta i l izat ion mechanism of carbon steel has been undertaken with the view to assessing the suitabi l i ty or otherwise of microscopic and hard- ness measurement methods in recrystal l izat ion studies. The ex- perimental condit ions (start ing-point microstructures, %-defor- mation, anneal ing temperatures) have been so chosen to produce various types of possible compl icat ions in the course of the recrystal l izat ion process. The four stages of recrystal l izat ion have been systematical ly studied using light-, transmission electron- and x-ray micros- copy as well as hardness measurement methods. </p><p>EXPERIMENTAL </p><p>Material and Spacemen Preparation </p><p>A carbon steel of the fol lowing composit ion (in wt.%) was used for the investigation: 0.23 C; 0.37 Si; 0.53 Mn. In a systematic thermal pre-treatment operation, four different types of starting point microstructures were prepared. The details of the specimens are given in Table 1. </p><p>TABLE I </p><p>Start ing-point Microstructures (Samples) </p><p>Sample Thermal Pre-treatment Particle Part icle dia (nm) shape </p><p>M 870C,40 min/H20 - </p><p>M300 870C,40 min/H20/300C, 20 platelet lh/H20 </p><p>M550 870C,40min/H20/550C, 200 sphere lh/H20 </p><p>M650 870C,40min/H20/650C, 2.700 sphere 980h/H20 </p><p>M signif ies Martensite; M300, M550 and M650 are martensit ic structures tempered for lh at 300C, lh at 550C and g80h at 650oc respectively. The specimens were cold-rol led; the amount of deformation ranged between 30% and 90% reduction in thickness. The mechanism of recrystal l izat ion was invest igated for the three anneal ing temperatures 500oc, 550C and 600C. </p><p>Hardness Measurement </p></li><li><p>Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1029 </p><p>The hardness of the specimens after the recrysta l l i zat ion heat- treatment operat ion was measured with a Lei tz -micro hardness tester using a 5oop loading weight. </p><p>Microscopic Examinat ion </p><p>The specimens for the l ight -microscopic examinat ion were pre- pared by wel l -known standard meta l lographic techniques (6). Phi l ips EM300 was used for the t ransmiss ion electron microscopic invest igat ion. The specimens were prepared by the thin-foi l method (7). X-ray photographs were obtained with Phi l ips PW1130 equipment using Fe-f i l ter and Co-tube. </p><p>RESULTS AND DISCUSSION </p><p>Hardness vs Time Curve </p><p>During recrysta l ] i zat ion annealing, var iat ions in the ~e ~sity and Hi~tr ibut ion of d is locat ions take place, giving rise to changes in microstructure. The hardness of a material is an important s t ructure-sens i t ive property, which can react very strongly to var iat ions in the density and arrangement of crystal defects. Based on this fact, hardness measurement has been ef fect ively ut i l ized in recrysta l l i zat ion studies. Fig.1 shows the variat ion of hardness with the recrysta l l i zat ion treatment time for the three pre- t reatments (start ing-point micro- structures) M(a.), M300 (b.) and M550(c.). The results shown refer to the given cold-work (30%,50%) and recrysta l l i zat ion treatment temperatures (500C, 550C, 600oc). For pract ical convenience, the t ime-axis has been represented in log-scale. </p><p>It can easi ly be seen that the general shape of all the curves is of the form shown in Fig.2. The deformed stage corresponds with the point O. Usually, in the deformed stage, and shortly after the start of the recrysta l l i zat ion annealing, there is an interact ion between the part ic les and the d is locat ions (segregat ion), result ing in an increase in hardness. At the same time, internal -s t ress rel ieving react ions take place. The net effect of this combined action is that in stage I, the hardness- t ime-curve can rise or fall (Fig.la), depending on which react ion outweighs the other. The shape of the curve in Stage I can therefore be used to assess the prominence to be given to segregat ion effects in a given recrysta l l i zat ion process. Stage II has a character is t ic p lateau-shape. It represents the combined, cont inuous reaction (subgrain- and part ic le-growth) normal ly associated with the recrysta l l i zat ion of second-phase materials. The duratio,, of this stage is an indicat ion of the s luggishness of the recrysta l l i zat ion process aris ing from part ic le inhibi ted movement of subgra in-boundar ies or recrysta l l i zat ion fronts. The mechanism involved here is associated with the so-cal led in -s i tu - recrysta l l i zat ion . The transi t ion from stage II to III cqn be sharp(F ig~1b,550C) , showing the onset of primary recrysta l l izat ion, or gradual (F ig. lb,500C), indicat ive of a very sluggish onset of pr imary recrysta l l i zat ion in a predominant ly in -s i tu - recrysta l l i zed matrix. The end of pr imary recrysta l l i zat ion is taken as the point in stage III from where the curve becomes paral lel to the t ime-axis. Thereafter , the recrysta l l i zed grains and the part ic les continue to grow (stage IV). </p></li><li><p>1030 C .M. KAMMA, et al. Vol. 18, No. 8 </p><p>Cold-work : 30% '] Recrysto, tlization : o-----o 5O0"C Treatment ~ 550C 600 ~ 600 C </p><p>Pre-Treolment: M </p><p>50 0~ ~'"'~ x : - :~- ' : ' ' , RB: Time to Begin of Recrystollizotion </p><p>~c 200 RE RE ;'1 o~'~ i io i io ,oo SOB ,ooo </p><p>S rain h Log time la| </p><p>ao </p><p>Pre - t reatment M </p><p>Cold -work : 50 % </p><p>Recrystallizolion :0--.-o500 =C Treo.tment ~ 550 </p><p>600 z~----~60O Pre-Treatment : M 300 </p><p>SO0 </p><p>300 RB B </p><p>~o </p><p>='=1 &gt; o~, li) i 1'0 ~ 1'0 tb0 </p><p>mln h sl b) gOB 10'00 Log time </p><p>Do </p><p>Pre - t reatment M300 </p><p>?00 </p><p>=lEE o LJ </p><p>200 </p><p>Cold -- work ; 30 % </p><p>Recrystatlizotion :o~o 5OO'C Treot ment ~ 550 </p><p>L&gt;---,~600 Pre-Treotment : M 550 </p><p>100. ~E </p><p>o ~o i io i I"o 10o soo idoo $ min~ h Log time </p><p>() </p><p>~o </p><p>Pre - t reatment M550 </p><p>FIG. I </p><p>Var ia t ion of hardness wi th recrys ta l l i za t ion t re tment t ime. </p></li><li><p>Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1031 </p><p>If the condit ions are such that no primary recrysta l l i zat ion takes place at all owing to the inf luence of retarding forces, the h~rdness- t ime-curve shows only a cont inuous fall (F ig, lc,500oc). Tile hardness-measurement method is very effect ive in invest igat ing the inf luence of cold-work, temperature, and start ing-point micro- structures on the kinet ics of recrystal l izat ion. It can be used to </p><p>~) and end(t R ) of recrystal l i - furnish the times to the beginning( R~nowledge of ~ PRB' zation in a given specimen. With th and by making use of the d i f ferent ia l -hardness method (8), one can easily deter- mine the act ivat ion energy for recrystal l izat ion. </p><p>I </p><p>i~IG. 2 </p><p>n deformed condit ion </p><p>I : segregat ion </p><p>II : combined subgrain- and part ic le growth ( in-s i tu-recry- stal l izat ion) </p><p>III: d iscont inuous re- action (primary recrysta l l izat ion) </p><p>IV :combined grain-and part ic le growth. </p><p>Log time </p><p>Hardness vs time curve showing the dif ferent stages of recrystal l izat ion. </p><p>Microscopic Invest igat ion </p><p>Each of the microscopic methods was used in invest igat ing the four stages of recrystal l izat ion. The dif ference in the results obtained (Fig.3) arose from the resolving power of each method. </p><p>Optical Microscopy </p><p>The results of the optical microscopic invest igat ion are shown in Fig.3. From the three microscopic invest igat ion methods, the optical microscope has the least resolving power. The maximum per- ceptible size of a recrysta l l i zed nucleus lies in the order of 2000 ~ (9). As can be seen from Fig.3a, this great handicap renders the optical microscope unsuitable for invest igat ion in stages I and II involving segregat ion processes or formation and growth of sub- grains. The method is therefore not capable of furnishing infor- mation about the actual mechanism responsible for the recrystal l i - zation phenomenon. On the other-hand, because of the large general view it provides, the optical microscope is quite effect ive in examining part ia l ly and fully recrysta l l i zed microstructures (Fig.3b,c,d). The ratio of </p></li><li><p>1032 C .M. KAMMA, et al. Vol. 18, No. @ </p><p>r~-crystallized to non-recrystallized portions can conveniently be found quantitatively; the two portions are easily differetiated frqm each other on account of the relatively, easily etchable nature of the non-recrystallized microstructure. </p><p>ao M650/90%/550C,55 sec; X200 </p><p>Stage I &amp; II (segregation and subgrain/particle growth) </p><p>Do M650/50%/550C,42 min; X64 </p><p>Stage III (partial pri- mary recrystallization) </p><p>el M550/30%/550C, 60 h; X80 </p></li><li><p>Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1033 </p><p>d, M650/90%/550C, I h; X320 </p><p>Stage IV (fully recry- stal l ized microstructure) </p><p>FIG. 3 </p><p>Stages of recrysta l l i zat ion : Optical micrographs </p><p>Electron Microscopy The results of the invest igat ion with the transmiss ion electr- </p><p>on microscope are shown in fig 4. On account of its extremely high resolving power, 2 nm (I0i, the electron microscope is cap- able of furnishing unambiguous information about the real nature of recrysta l l i zat ion processes. </p><p>In stage I of the recrysta l l i zat ion process (Fig.4a), the spe- cial role of d is locat ions in the formation of diffuse cells, and the transformat ion of these into subgrains and subgra in-boundar ies can easily be observed. Further development of the stage I stru- cture into wel l -def ined subgrains can be seen clearly in stage II (Fig.4b). The often observed lack of incidence of recrysta l l i zat ion or the extremely long recrysta l l i zat ion incubation time (Fig.lc) that could not be explained from the hardness measurement method, can be accounted for here. The efforts of the subgra in-boundar ies to grow and assume the character is t ics of large-angle boundaries are frustrated by the inhibit ive action of the second-phase parti- cles (Fig.4b). In this way, a mixture of cont inuous and disconti- nuous reaction, or at best, a part ic le -contro l led primary recry- sta l l izat ion ensues. This s i tuat ion is manifested in Fig.4c, show- ing an almost d is locat ion-free, recrysta l l i zed portion, separated from the substructure by a recrysta l l i zat ion- f ront . Thanks to its very high resolving power, the transmission electron microscope can be used to great advantage in the analysis of sub- grain structures and second-phase particles, as well as establ i - shing the or ientat ion di f ference between subgrains and grain boun- daries. Its d isadvantages can be seen in the rather dif f icult speci- men preparat ion and the small f ield of view, which is usual ly not representat ive of the whole sample. </p><p>X-Ray Microscopy </p><p>The appl icat ion of x-ray microscopy in recrysta l l i zat ion studies is based on the di f ferences in ref lect ions produced by recrysta- l l ized and deformed microstructures. Undistorted, defect- free latt ice of size 80 nm and more gives sharp, intensive ref lect ions(11). </p></li><li><p>1034 C .M. KAMMA, et al. Vol. 18, No. 8 </p><p>a. M/90%/550C, 46 sec. X22000 </p><p>Stage I </p><p>b. M550/90%/550C, 20 min. X22000 Stage II </p><p>c. MSSO/90%/SSOC, 2h </p><p>Stage I I I+ IV </p><p>FIG. 4 </p><p>Stages of recrysta l l i zat ion : Transmiss ion electron micrographs </p><p>The ef fect iveness of x-ray ref lect ion method in ident i fy ing micro- structures can be deduced from the fol lowing equation (10) : </p><p>X B = </p><p>d.cos6 </p><p>whereby, X wave- length of x-ray;~ average angle subtended by two crystal l i tes; B di f f ract ion l ine-spread; d diameter of the crystal z~ne capable of giving sharp ref lect ions. </p></li><li><p>Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1035 </p><p>M/90%/550oc, 20 sec. Stage I </p><p>i </p><p>b. M/50%/550C, 10 min. Stage II </p><p>c. M300/50%/550C, 2h d. M/50%/550C, 30h Stage III Stage IV </p><p>FIG. 5 </p><p>Stages of recrysta l l i zat ion : X-Ray back-ref lect ion pictures </p><p>For deformed and subgrain structures, d= 20 nm, and 6 &lt; I , thereby increasing the value of B. The x-ray interference lines are therefore strongly blurred and ref lect ions from individual subgrain crystal l i tes merge together and are incapable of resolu- tion. This s i tuat ion is observed in the x-ray back-ref lect ion pictures of the recrysta l l i zat ion stages I and II (Fig.5a,b). The inter...</p></li></ul>

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