HIGH TEMPERATURE TEXTURE GONIOMETER FOR IN SITU MEASUREMENTS OF TRANSFORMATION

A high temperature stage was designed for mounting onto a computer controlled four circle X-ray texture goniometer. This technique allowed to conduct in situ texture measurement, i.e. the determination ofthe texture evolution during rather than subsequent to annealing. The device was employed for temperatures up to 1000C. The furnace consisted of a resistance wire of Pt30Rh, which was isolated against the specimen with A1203 glue. The furnace with the specimen was covered by a hemispherical Kapton foil. Inside the hemisphere a reducing gas atmosphere was used to avoid oxidation of the sample surface.


INTRODUCTION
The a 7 phase transformation has been the subject of investigations for quite a long time, e.g.Bain published 'The Nature of Martensite' already in 1924  (Bain, 1924).The phase transformation of the most commercial steels occurs at high temperatures.Because of the experi- mental difficulties associated with the measurement of X-ray textures at elevated temperatures, there is virtually no information on austenite textures of commercial steel.For recording X-ray textures at high temperatures an existing four circle texture goniometer as described by Corresponding author G. BROCKNER et al.Hirsch et al. (1986) was modified (Reher et al., 1994).This allowed the determination of textures at different temperatures in one sample and therefore detailed investigations of e.g.transformation textures.

CONSTRUCTION
The furnace was heatod with a t0.5mmPt30% Rh resistance wire, which allowed a wire temperature of up to 1500C.The wire, bent to a spiral, was inserted in a ceramic body (Fig. I).The density of the loops decreased towards the centre of the furnace in order to obtain a homogeneous specimen temperature.The electrical contact of sample and resistance element was prevented by covering the wire with A1203 glue in the ceramic body.
To avoid damage to the Euler cradle due to thermal expansion or degradation of the lubricating grease, the temperature of the cradle should not exceed 180C.Therefore, the heat conduction to the Euler cradle was minimized by fastening the furnace onto low density ceramic pins, which were placed on aluminium cooling fins.This assembly kept the cradle temperature below 120C even at maximum furnace power.The specimen (35,24 mm2) was clamped down onto the furnace with metal claws (Fig. 1).The measuring surface of the sample was 14,24mm2.A Pt-Ptl0Rh thermocouple, which measured the tem- perature in the heating spiral was connected to a temperature con- troller.This controller itself was programmed and monitored by a computer, which set the entire thermal cycle for the experiment.This design allowed a quick and easy specimen change without changing the thermocouple.Further optional thermocouples attached to the speci- men were used to calibrate the furnace.Specimen temperatures in the centre and at the edge were measured and plotted as a function of the wire temperature in Fig. 2. The temperature gradients in the specimen were found to be less than 15C.The temperature in the wire and in the specimen differed by about 250C at maximum furnace power.
The furnace with the specimen was covered by a hemispherical Kapton TM foil with a thickness of 70pm.The hemisphere had been produced by deep drawing at 400C.The spherical shape ensured a constant absorption of the X-ray beam for all angular positions.The intensity loss due to the foil was measured for different temperatures.In Temperature gradients in the specimen did not exceed 15C.with CoK,-radiation.The X-ray intensity loss was found to be approximately 10%.The same result was obtained at 500C.
Within the hemisphere a reducing gas atmosphere with a flux of about 5-10 -3 m3/h had to be used to avoid oxidation of the sample surface.For the first experiments a N2 atmosphere was used.Because of the residual oxygen in the hemisphere the sample suffered strong oxidation, which could be measured in a 0-scan of the austenitic phase of a low carbon steel at 950C (Fig. 4(a)).An addition of 5% Hg_ reliably prevented an oxidation of the specimen (Fig. 4(b)).
In the Euler cradle the electrical wires and the gas supply of the hot stage were held in a U-shaped peripheral ring during the azimuthal rotation (Fig. 5).Every 360 the sense of rotation was reversed.For orthorhombic or higher sample symmetries the azimuthal rotation could be shortened to 90 , since only a quarter of the pole figure needed to be measured.This reduced the measurement time to 5 min for one pole figure and about 25 min for the determination of a full ODF (4 pole figures), respectively.This procedure could be useful for materials which undergo microstructural changes at high temperatures.
The reflected intensities could be recorded both by a scintillation detector and by a position sensitive detector.The latter is preferred when peak overlapping occur, e.g. in the ferrite / austenite region during the heating of carbon steels.(a) NE-gas atmosphere; (b) N2 q-5% H2-gas atmosphere.
FIGURE 5 Hot furnace in the Euler cradle: the peripheral ring leads the tube and the wires during azimuthal rotation, the sense of rotation changes every 360 (See Color Plate 1).

SPECIFICATIONS
specimen temperature temperature gradients in the specimen heating rate specimen size measuring surface gas atmosphere X-ray intensity loss due to Kapton

HIGH TEMPERATURE EXPERIMENTS
With the equipment described above pole figures of a microalloyed low carbon steel were measured at different temperatures using MoKradiation.At room temperature 3 incomplete BCC pole figures { 110}, {200} and { 112} were determined (Fig. 6).Subsequently the sample was automatically heated up to 950C in the high temperature X-ray goniometer.During heating the ferrit to austenit transformation took place.The { 111 }, {200} and {220} FCC pole figures were determined at 950C (Fig. 7).The ODF's for both phases were calculated to investigate the texture transformation.Results of the in situ measurements and modelling of transformation textures are published elsewhere (Briickner, 1996a,b).
FIGURESchematic arrangement of the resistance element.

Fig. 3 FIGURE 2
Fig. 3 the reflected intensities of the { 110} crystallographic planes were measured in a 0-scan with and without the foil at room temperature FIGURE 3 110} reflection peak of low carbon steel with and without Kapton TM foil at T-25C, CoKe-radiation.The intensity loss is about 10%.