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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.

THE USE OF MICROSCOPIC AND HARDNESS MEASUREMENT METHODS IN RECRYSTALLIZATION STUDIES

C.M. Kamma, Department of Mechanical Engineering, University of Lagos, Nigeria

and E. Hornbogen, Institut fur Werkstoffe, Ruhr-Universit~t, Bochum, W. Germany.

(Received May 15, 1983; Communicated by R. A. Huff.pins)

ABSTRACT The use of microscopic and hardness measurement methods as effective experimental techniques for recrystallization studies has been investigated. Consideration was given to the production of different starting-point microstructures of varying Fe~C-particle dispersion, known to be associated with the complications of recrystallization mechanism of second-phase materials. Each of the investigation method considered was used to study the various aspects of recrystallization phenomenon (recovery, in-situ-recrystallization, particle-controlled subgrain growth, partial recrystallization, grain growth) and the merits and limitations of application discussed.

INTRODUCTION The recrystallization mechanism in metallic materials is known

to be complicated (1). Of special note in this respect is the con- tributory influence of the nature of the starting-point microstructure. This is usually a homogeneous, super-saturated solid solution or a microstructure containing a second-phase in the form of particles. Precipitation in the super-saturated solution can take place before, during or after the recrystallization process (2,3). During the deformation stage, prior to the recrystallization annealing, the particles play the important role of influencing the distribution of dislocation-network in the deformed structure (4,5).

In consequence, the recrystallization behaviour of a normal metallic material is invariably associated with cases of combined reactions, whereby precipitation and growth of particles overlap the recrystallization processes. The complete course of recrystallization in a given material is accordingly seen to be made up of four stages: I) Recovery stage, involving very little annihilation of dislocations; II) Continuous combined reaction stage, in which particle growth and

particle inhibited subgrain growth occur; III) Primary recrystallization; IV) Further particle-and recrystallized grain-growth.

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1028 C.M. KAMMA, et al. Vol. 18, No. 8

There exists the common practice of interpreting recrystalliration phenomena with the help of results obtained from one particular investigation method. The information thus gained is likely to be unreliable, in view of the varying mechanisms associated with the different stages of the recrystallization process. In the present work, an investigation of the recrystailization mechanism of carbon steel has been undertaken with the view to assessing the suitability or otherwise of microscopic and hardness measurement methods in recrystallization studies. The experimental conditions (starting-point microstructures, %-deformation, annealing temperatures) have been so chosen to produce various types of possible complications in the course of the recrystallization process. The four stages of recrystallization have been systematically studied using light-, transmission electron- and x-ray microscopy as well as hardness measurement methods.

EXPERIMENTAL

Material and Spacemen Preparation

A carbon steel of the following composition (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.

TABLE I

Starting-point Microstructures (Samples)

Sample Thermal Pre-treatment Particle Particle dia (nm) shape

M 870°C,40 min/H20 -

M300 870°C,40 min/H20/300°C, 20 platelet lh/H20

M550 870°C,40min/H20/550°C, 200 sphere lh/H20

M650 870°C,40min/H20/650°C, 2.700 sphere 980h/H20

M signifies Martensite; M300, M550 and M650 are martensitic structures tempered for lh at 300°C, lh at 550°C and g80h at 650oc respectively. The specimens were cold-rolled; the amount of deformation ranged between 30% and 90% reduction in thickness. The mechanism of recrystallization was investigated for the three annealing temperatures 500oc, 550°C and 600°C.

Hardness Measurement

Vol. 18, No. 8 RECRYSTALLIZATION STUDIES 1029

The hardness of the specimens after the recrystallization heat- treatment operation was measured with a Leitz-micro hardness tester using a 5oop loading weight.

Microscopic Examination

The specimens for the light-microscopic examination were prepared by well-known standard metallographic techniques (6). Philips EM300 was used for the transmission electron microscopic investigation. The specimens were prepared by the thin-foil method (7). X-ray photographs were obtained with Philips PW1130 equipment using Fe-filter and Co-tube.

RESULTS AND DISCUSSION

Hardness vs Time Curve

During recrystal]ization annealing, variations in the ~e ~sity and Hi~tribution of dislocations take place, giving rise to changes in microstructure. The hardness of a material is an important structure-sensitive property, which can react very strongly to variations in the density and arrangement of crystal defects. Based on this fact, hardness measurement has been effectively utilized in recrystallization studies. Fig.1 shows the variation of hardness with the recrystallization treatment time for the three pre-treatments (starting-point microstructures) M(a.), M300 (b.) and M550(c.). The results shown refer to the given cold-work (30%,50%) and recrystallization treatment temperatures (500°C, 550°C, 600oc). For practical convenience, the time-axis has been represented in log-scale.

It can easily 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 recrystallization annealing, there is an interaction between the particles and the dislocations (segregation), resulting in an increase in hardness. At the same time, internal-stress relieving reactions take place. The net effect of this combined action is that in stage I, the hardness-time-curve can rise or fall (Fig.la), depending on which reaction outweighs the other. The shape of the curve in Stage I can therefore be used to assess the prominence to be given to segregation effects in a given recrystallization process. Stage II has a characteristic plateau-shape. It represents the combined, continuous reaction (subgrain- and particle-growth) normally associated with the recrystallization of second-phase materials. The duratio,, of this stage is an indication of the sluggishness of the recrystallization process arising from particle inhibited movement of subgrain-boundaries or recrystallization fronts. The mechanism involved here is associated with the so-called in-situ-recrystallization. The transition from stage II to III cqn be sharp(Fig~1b,550°C), showing the onset of primary recrystallization, or gradual (Fig.lb,500°C), indicative of a very sluggish onset of primary recrystallization in a predominantly in-situ-recrystallized matrix. The end of primary recrystallization is taken as the point in stage III from where the curve becomes parallel to the time-axis. Thereafter, the recrystallized grains and the particles continue to grow (stage IV).

1030 C . M . KAMMA, et al . Vol . 18, No. 8

Cold-work : 30% '] Recrysto, tlization : o-----o 5O0"C Treatment ~ 550°C

600 ~ 600 °C Pre-Treolment: M

50 0 ~ ~'°"'~ x : - : ~ - ' : ' ' , RB: Time to Begin of Recrystol l izotion

~c 200 RE RE ;'°°1 °o~'~ i io i io ,oo SOB ,ooo

S ra in h Log time la|

a o

Pre-treatment M

Cold - w o r k : 50 %

Recrystall izolion :0--.-o500 =C Treo.tment ~ 550

600 z~----~60O

Pre-Treatment : M 300

SO0

300 RB B

~o

='=1 > o ~ , li) i 1'0 ~ 1'0 tb0

mln h sl b) gOB 10'00 Log time

D o

Pre-treatment M300

?00

=lEE o LJ

200

Cold -- work ; 30 %

Recrystatl izotion : o ~ o 5OO'C Treot ment ~ 550

L>---,~600 Pre-Treotment : M 550

100. ~E

°o ~o i io i I"o 10o soo idoo $ min~ h Log time

(¢)

~o

Pre-treatment M550

FIG. I

Variation of hardness with recrystallization tretment time.


If the conditions are such that no primary recrystallization takes place at all owing to the influence of retarding forces, the h~rdness-time-curve shows only a continuous fall (Fig,lc,500oc). Tile hardness-measurement method is very effective in investigating the influence of cold-work, temperature, and starting-point microstructures on the kinetics of recrystallization. It can be used to

~) and end(t R ) of recrystalli- furnish the times to the beginning( R~nowledge of ~ PRB' zation in a given specimen. With th and by making use of the differential-hardness method (8), one can easily deter- mine the activation energy for recrystallization.

I

i~IG. 2

n • deformed condition

I : segregation

II : combined subgrain- and particle growth (in-situ-recrystallization)

III: discontinuous reaction (primary recrystallization)

IV :combined grain-and particle growth.

Log time

Hardness vs time curve showing the different stages of recrystallization.

Microscopic Investigation

Each of the microscopic methods was used in investigating the four stages of recrystallization. The difference in the results obtained (Fig.3) arose from the resolving power of each method.

Optical Microscopy

The results of the optical microscopic investigation are shown in Fig.3. From the three microscopic investigation methods, the optical microscope has the least resolving power. The maximum per- ceptible size of a recrystallized nucleus lies in the order of 2000 ~ (9). As can be seen from Fig.3a, this great handicap renders the optical microscope unsuitable for investigation in stages I and II involving segregation processes or formation and growth of subgrains. The method is therefore not capable of furnishing information about the actual mechanism responsible for the recrystallization phenomenon. On the other-hand, because of the large general view it provides, the optical microscope is quite effective in examining partially and fully recrystallized microstructures (Fig.3b,c,d). The ratio of

1032 C.M. KAMMA, et al. Vol. 18, No. @

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.

ao M650/90%/550°C,55 sec; X200

Stage I & II (segregation and subgrain/particle growth)

Do M650/50%/550°C,42 min; X64

Stage III (partial primary recrystallization)

el

M550/30%/550°C, 60 h; X80


d, M650/90%/550°C,I h; X320

Stage IV (fully recrystallized microstructure)

FIG. 3

Stages of recrystallization : Optical micrographs

Electron Microscopy The results of the investigation with the transmission electr-

on microscope are shown in fig 4. On account of its extremely high resolving power, 2 nm (I0i, the electron microscope is capable of furnishing unambiguous information about the real nature of recrystallization processes.

In stage I of the recrystallization process (Fig.4a), the special role of dislocations in the formation of diffuse cells, and the transformation of these into subgrains and subgrain-boundaries can easily be observed. Further development of the stage I structure into well-defined subgrains can be seen clearly in stage II (Fig.4b). The often observed lack of incidence of recrystallization or the extremely long recrystallization incubation time (Fig.lc) that could not be explained from the hardness measurement method, can be accounted for here. The efforts of the subgrain-boundaries to grow and assume the characteristics of large-angle boundaries are frustrated by the inhibitive action of the second-phase particles (Fig.4b). In this way, a mixture of continuous and discontinuous reaction, or at best, a particle-controlled primary recrystallization ensues. This situation is manifested in Fig.4c, showing an almost dislocation-free, recrystallized portion, separated from the substructure by a recrystallization-front. Thanks to its very high resolving power, the transmission electron microscope can be used to great advantage in the analysis of subgrain structures and second-phase particles, as well as establishing the orientation difference between subgrains and grain boundaries. Its disadvantages can be seen in the rather difficult specimen preparation and the small field of view, which is usually not representative of the whole sample.

X-Ray Microscopy

The application of x-ray microscopy in recrystallization studies is based on the differences in reflections produced by recrystallized and deformed microstructures. Undistorted, defect-free lattice of size 80 nm and more gives sharp, intensive reflections(11).

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a. M/90%/550°C, 46 sec. X22000

Stage I

b. M550/90%/550°C, 20 min. X22000 Stage II

c. MSSO/90%/SSO°C, 2h

Stage III+ IV

FIG. 4

Stages of recrystallization : Transmission electron micrographs

The effectiveness of x-ray reflection method in identifying microstructures can be deduced from the following equation (10) :

X B =

d.cos6

whereby, X wave-length of x-ray;~ average angle subtended by two crystallites; B diffraction line-spread; d diameter of the crystal z~ne capable of giving sharp reflections.

Vol . 18, No. 8 RECRYSTALLIZATION STUDIES 1035

M/90%/550oc, 20 sec. Stage I

i

b. M/50%/550°C, 10 min. Stage II

c. M300/50%/550°C, 2h d. M/50%/550°C, 30h Stage III Stage IV

FIG. 5

Stages of recrystallization : X-Ray back-reflection pictures

For deformed and subgrain structures, d= 20 nm, and 6 < I ° , thereby increasing the value of B. The x-ray interference lines are therefore strongly blurred and reflections from individual subgrain crystallites merge together and are incapable of resolu- tion. This situation is observed in the x-ray back-reflection pictures of the recrystallization stages I and II (Fig.5a,b). The interference line spread, B, in the equation above decreases in the order I, II, III, IV, with the deformed and segregation zone (stageI) having the most pronounced instance of broadening. With the formation of recrystallized grains (d >80 rim, 6 > 35o), sharp reflections start to appear on the background of a continuous ring in the x-ray photograph (stage III, Fig.5c). The special merit of the x-ray microscopy seems to lie in the de- termination of the end of primary recrystallization. This usually coincides with the disappearance of continuous Debye rings in the background of radiographs (stage IV, Fig. 5d).

Conclusion

It has been established that microscopic investigation and hardness-measurement methods are effective means of monitoring the progress of recrystallization and studying the mechanisms involved in the different stages. The scope of application of each method to the various aspects of recrystallization phenomenon has been found to be limited by, a) the inability to correlate hardness changes changes arising

1036 C.M. KAMMA, et al. Vol. 18, No. 8

from variations in density and arrangement of dislocations with the mechanisms responsible for the ensuing microstructures, b) the varying resolving powers arising from differences in the wave-lengths.

Generally, the transmission electron microscope is most suited to investigating recrystallization mechanisms involving in-situ recrystallization, subgrain- and particle-growth. The x-ray back reflection method is particularly good at establishing the onset and end of primary recrystallization. The best approach, however, is the complementary applifation of the various methods to a given situation, to ensure a balanced, fault- free interpretation of results.

References

I. E. Hornbogen, H. Kreye, Rekristallisation mehrphasiger Werkstoffe Institut fur Werkstoffe, Ruhr-Universit~t Bochum, 1975.

2. CoKamma, E. Hornbogen, J. Mat. Sci. 11 2340(1976).

3. U. K~ster, Met. Sci. 8 564(1974).

4. F.J. Humphreys, J.W. Martin, Phil. Mag. 17 (1968) 365

5. Van Drunen, G. Saimoto, Acta Met. 19 213(1971).

6. M. Beckert, H. Klemm, Handbuch der met. Atzverf., Leipzig(1966).

7. L. Reimer, Elektr. mikroskop. Untersuch-u. Pr~p. methoden, Berlin (1967).

8. S.S. Gorelik, Recrystallization in Metals and Alloys, MIR publishers, Moscow 454(1981).

9. E. Minuth, Bemerkungen zur metallographischen Untersuchung der Rekristallisation, Praktische Metallographie, ~ 197(1970).

10. Pinnow, Hornbogen, Die Untersuchung der Rekristallisation und Erholung am Beispiel der aush~rtbaren St~hle, Fortschr. Met.

11. B.D. Cullity, Elements of X-ray Diffractions, Addison-Wesley Publ. Co. 96(1967).

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