CHARACTERIZATION OF MICROSTRUCTURE AND TEXTURE OF A TITANIUM ALLOY CORONA 5

Both parameters, microstructure and texture, are studied on several samples for different processes. 
The same route, thermomechanical treatment (forging or rolling) followed by solutioning and ageing 
is carried out with various temperature and deformation amount on an identical titanium alloy CORONA 5.


INTRODUCTION
The CORONA 5 is an (ot/fl) titanium alloy interesting for aerospace and naval applications because of a good combination of a corrosion resistance in chlorinated environment, a long fatigue life and a high fracture toughness.
It is well known that mechanical properties of titanium alloys are very sensitive to variations in microstructure and texture which depend on thermomechanical and heat treatments (Wanhill 1978, Bowen 1988).
The present paper deals with the characterization of the evolution of both microstructure and texture according to different routes.

MATERIAL AND EXPERIMENTAL PROCEDURES
The CORONA 5 alloy (Ti-4.5A1-5Mo-I.5Crweight %) was transformed by CEZUS.From an identical initial microstructure eight routes consisting of a forging or rolling followed by a solutioning treatment, all the structures have undergone the same ageing treatment, Table (Benhaddad 1992).For metallographic studies the samples were cut perpendicularly to the bar axis D and electropolished using an electrolyte of 30 cm perchloric acid + 175 cm n butanol + 300 cm methanol at -40C and 24V.Then samples were etched in a solution of 2 cm hydrofluoric acid, 3 cm nitric acid + 95 cm water.
The pole figures 0002 }, 1010 and 1012 were measured by X-ray diffraction in reflection, in the case of the sample F having coarse grains, the pole figures were measured by neutron diffraction in transmission at laboratoire L. Brillouin in Saclay.The orientation distribution function (O.D.F) was calculated using Roe notation (Roe 1965).This function describes quantitatively the texture: dV /Vo K.F(g).dg (1) Vo: sample volume (orientation volume) K: normalization constant and F(g): orientation distribution function (O.D.F).The function F(g) is calculated from four poles figures.The pole density q(t, r/) at a given point of a {hkil} pole figure is related to O.D.F by: q(t, r/)= l-_F(g)d), (2) and r/: spherical coordinates of the normal to diffracting plane in the specimen frame.
d?': rotation around the normal to the diffracting plane.
The O.D.F is calculated by inversing this integral, the principle of the calculation consists in expanding each member of this equation (2) on spherical harmonics basis and finding relations between the coefficients of both expansions (Bunge 1969).
The F(g) maximums represent the orientation density of the different components present in the studied polycrystal.Therefore, the texture can be represented by the preferential orientations with their respectives F(g).

Microstructure
The microstructure is governed by: the forging temperature the deformation amount the solutioning temperature *According to the forging temperature, three kinds of ct phase morphology are observed (Figure 1): the equiaxed or globular morphology is obtained by forging in the (ct + fl) field, route A.
the acicular microstructure is obtained after forging in the fl field, the ct phase precipitates at prior fl grain boundaries and in the fl matrix, routes D, E and F. The acicular structure is also observed after rolling in the fl field, routes R and CR.
the bimodal microstructure is obtained after forging in the (ct + fl) field near the fl transus.
*The deformation amount acts upon the structure of the D and the F process.A high degree of deformation, D, leads to: a decrease of prior fl grain size.a broken morphology of prior fl grain.
an increase of the number of needles *The solutioning treatment in high a/fl field (B) leads to a small amount of the fl stable phase, contrary to a treatment at lower temperature (C) after cooling.The fl metastable phase is transformed in ct/fl acicular.

Texture
The texture is represented by the principal orientations {hkil} <uvtw>: {hkil} is the crystallographic plane parallel to the reference plane (T, N) and <uvtw> the crystal- lographic direction parallel to the transverse direction T, N is the perpendicular axis to the bar axis D (Figure 2).The {0002} pole figures are shown (Figure 3).
All the {0002} pole figures exhibit similar location of the maximum in the form of a cross in the principal planes (T, N) and (D, N) with sometimes a reinforcement at the center of the pole figure.The texture presents a mixed character with two components CON and CDr The component CDN corresponds to basal planes with their axes <0001> in the (D, N) plane.The second component CDr correponds to basal planes with their axis <0001> in the (T, N) plane (Figure 4).
The F(g)max of these two components varies with the temperature, the forging amount, and the temperature of solutioning treatment.
All CDN and CDr components, their intensity F(g)max and their corresponding angle ), (angle between axes <0001> of each component and the D axis) are given in the Table 2  Oprocess CR Niveau 1) 2) 33 6") Figure 3 The recalculated pole figures.

Effect of deformation mode
For a same temperature different types of texture are obtained after each deformation mode.A cross rolling (CR) in the fl phase field leads to a basal texture.On the contrary, the forging (F) or the unidirectional rolling (R) leads to two components near to the basal/transverse texture type (Peters et al., 1980).
Figure 4 The texture components.

Effect of solutioning temperature
For a same temperature and amount of, (process 1 or 2) the solutioning treatment at low temperature (process C or E) does not change the texture.However the solutioning treatment at high temperature in the (o + fl) phase field (process B or D) transforms the texture.The evolution of texture after a high solutioning treatment is: the axes c of the component CDN are near to the deformation axis D. the axes c of the component CDT are near to the transverse direction T.

Effect of forging temperature
The Figures 5a and b exhibit evolution of Cos and CDT intensity as a function of forging temperature.The results are separated in "high treated" and "low treated".The same evolution is observed for the two classes: at high temperature, the intensity of CDN is equal or lightly greater than the CDT intensity.
at low forging temperature (process A), the CoN component is preponderant.The low intensity of texture of the sample F is essentially due to a decrease of the deformation amount of the F process comparatively to the D process.
The evolution of angle between the axes <0001> of each component and the deformation axis D, with the forging temperature is plotted Figures 6a and b.
It can be noted that: the angle ?' of the CDN component increases with forging temperature.the angle ), of the Cox component decreases when the forging temperature is increased.
The tendency is the formation of transverse texture 1010} < 1210> at high temperature with eventually a component of basal texture {0001} <uvtw>.
Comparative Evolution of Microstructure and Texture The evolution of both microstructure and texture with different processes shows that it is possible to obtain several combinations of these two parameters: 30S.BENHADDAD, C. QUESNEAND R. PENELLE

Figure 1
Figure 1 Microstructure of the different samples.

Figure 2
Figure2The forging bar reference.D: forging bar axis N: normal axis T: transverse axis

Figure 6
Figure 6 Evolution of angle between the axes <0001> and the deformation axis D, with the forging temperature.

Table 1
Roues of treatments.

Table 2
Texture results.Evolution of O.D.F as function of forging temperature.