Heterogeneous deformation and the recrystallisation




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HETEROGENEOUS DEFORMATION AND THE RECRYSTALLISATION

OF IRON-BASE ODS PM2000 ALLOY


C. Capdevila1, Y. L. Chen2, N. C. K. Lassen3, A. R. Jones2 and H. K. D. H. Bhadeshia1


1 Department of Materials Science and Metallurgy

University of Cambridge, Pembroke Street, Cambridge CB2 3QZ

United Kingdom, www.msm.cam.ac.uk/phase-trans

2 Materials Science and Engineering

Department of Engineering, University of Liverpool, L69 3GH

United Kingdom, www.liv.ac.uk/mateng/home.html

3 Materials Research Department

Risø National Laboratory, DK-4000 Roskilde

Denmark, www.risoe.dk


Abstract


The recrystallisation behaviour of PM2000 oxide dispersion strengthened ferritic alloy has been investigated for samples which were cold deformed after extrusion. The evolution of the recrystallisation temperature TR, defined as the minimum temperature at which the sample begins to recrystallise, has been studied in detail as a function of the level of deformation. The microstructure was characterised using transmission electron and optical microscopy, together with microhardness measurements and local texture measurements using electron back-scattering technique. The results can be interpreted if it is assumed that anything which makes the microstructure heterogeneous, stimulates recrystallisation. In this sense, larger strain gradients lead to more refined and more isotropic grain structures. The way in which these results can be exploited for commercial applications is discussed.


Introduction


There is a commitment in Europe to renewable energy; biomass is likely to make a significant contribution to this type of power generation 1. This does not require radically new technologies when compared with alternative sources of renewable energy. However, the thermodynamic efficiency of the process is dependent on the maximum temperature which can be attained in the operating cycle. Technologies for developing biomass plant towards greater efficiencies are therefore vital. For example, it is planned to construct heat exchanger capable of gas operating temperatures and pressures of around 1100 C and 15-30 bar. This in turn requires metal tubing which can survive at temperatures up to 1150 C.


Mechanical alloying is a process in which powders of different compositions are deformed together to such as extent that a solid solution is formed.2 Stable oxides can also be incorporated into the alloy during this process.3, 4 Alloys produced in this way are available on a commercial scale and have many applications.5, 6, 7


The oxidation resistance and good creep performance of mechanically iron-base ODS alloys such as PM2000 and MA956, makes them prime candidates for the proposed heat exchangers.8, 9 As expected, the creep strength is influenced by grain size and shape.10 Only coarse recrystallised grains have adequate high temperature creep strength. The microstructure, however, following mechanical alloying and consolidation of the resulting powder (for example, by extrusion), consists of fine grains which have a width which is much less than a micrometer and which are cold-deformed during the consolidation process.11, 12, 13, 14 The material in this state is hard and contains an enormous amount of stored energy.15, 16 It has to be recrystallised into a coarse-grained microstructure before use. However, the recrystallisation behaviour of iron-base ODS alloys is peculiar. They recrystallise into a grain structure which resembles that obtained by directionally, with coarse, columnar grains which have their longest axes along the extrusion direction. Furthermore, recrystallisation usually does not occur until temperatures close to melting are reached.17, 18, 19


Although the coarse and directional grain structure is ideal for minimising creep along the axis of the columnar grains, the properties in the transverse direction are less than desired. For this reason, there is considerable interest in controlling the development of microstructure. Many of the methods which seek to alter the microstructure rely on some type of deformation of the consolidated metal before the recrystallisation heat treatment. Regle and Alamo20 studied the influence of cold deformation on the recrystallisation and obtained fascinating results in which the recrystallisation temperature and grain structure was radically altered by the degree of deformation. The purpose of the work presented in this paper was to study the role of a non-uniform deformation on the recrystallisation behaviour of PM2000 so as to propose a method to produce coarse grain structures was proposed. PM 2000 tubing is normally processed by unidirectional extrusion followed by heat treatment which produces the axially aligned microstructures. Naturally, these exhibit excellent axial creep properties. However, in pressurised tubes, the stress is larger along hoop direction where the creep strength is poorer. We wished to analyse the effect of deformation induced by an unusual torsional extrusion process known as flow forming, on the recrystallisation behaviour of tubes samples intended for the heat exchanger application.


Experimental procedure


The nominal composition of PM2000 used in this work is shown in Table 1. PM2000 is produced by mechanical alloying of the components in a high-energy mill to produce a solid solution which contains a uniform dispersion of yttria, in the form of coarse powder. The powder is then consolidated using hot isostatic pressing and these tube preforms are then extruded. The alloy was supplied by PLANSEE GmbH. The essential feature of PM2000 is that it contains 5.5 wt % of Al and 0.5 wt % of Y2O3. The aluminium enhances corrosion and oxidation resistance and it is claimed that PM2000 is better than other ODS materials in gaseous environments containing SO2 0.24%, CO2 15%, O2 4%, N2 to balance. 21, 22 The creep performance has been found to be optimum with a Y2O3 content of 0.5 wt %.


Table 1 Chemical composition of PM2000 (wt %).


Cr

Ti

Y2O3

Al

Fe

20

0.5

0.5

5.5

Bal.


In an attempt to produce microstructures which optimise the creep strength along the hoop direction, a novel processing route developed at MSR Metall-Spezialrohr GmbH was used to produce experimental tubes. The extruded tubes from Plansee are flow formed in a process of torsional extrusion at room temperature to achieve reductions in area of 72% (T1) and 90% (T2). The purpose of this torsional deformation is to cause a helical alignment of the oxide particles in the hope that the material then recrystallises into grains which twist along the extrusion axis and hence give better hoop strength. However, the twisted grain structure is not discussed in this paper, simply the effect of flow-forming on the development of microstructure.


Samples for metallographic examination were cut from the transverse direction of the tube. Optical microscopy was used to observe the microstructures both of as-flow formed and heat treated specimens. The etchant used was 2 g CuCl2, 40 ml HCl, and 40-80 ml ethanol. Transmission electron microscopy was carried out using a JEOL 2000FX microscope operated at 200 kV to evaluate the deformed microstructure in flow formed tubes. In order to minimise any defects introduced by sample preparation foils were extracted by spark erosion. Samples were ground and polished to ~ 30 - 50 m in thickness, then thinned to perforation by ion milling. Microhardness measurements were carried out in as flow formed samples by means of Mitutoyo Mvk-H2 hardness tester machine with a load of 200 g.


The crystallographic textures of T1 and T2 tubes were investigated using EBSP (electron back-scattering pattern) technique. This technique allows rapid, automated measurements of crystal orientations with high precision (< 1) and high spatial resolution (<100 nm) in the SEM. In this study, the technique was used for measuring local textures on the surface, in the middle, and on the inner surface of the tubes.


The sample preparation for local texture measurements using EBSP is describe as follow. A slice of approximately 1.5 cm was cut out form each of the two tubes investigated in this work (T1 and T2). Each ring was then cut up into 12 samples, each of size ~1.5 cm  0.5 cm (the 3rd dimension being the thickness of the tube) as it is shown in Fig. 1 and Fig. 2. The 12 samples were taken from positions well spread over the circumference of the ring as illustrated in Fig. 2. The samples of each tube were prepared for EBSP analysis using mechanical griding and polishing followed by electropolishing (A2 electropol.). For all the samples, the plane investigated by EBSP was the one containing the tube axis (TD) and the radial direction (RD), as illustrated in Fig. 1.






Figure 1. Illustrating the geometry of samples cut out of tubes for EBSP measurements. The plane investigated by EBSP contains TD and RD.


The samples were mounted on a special EBSP holder in pairs or triplets and inserted into a JEOL JMS-840 SEM equipped with a LaB6 filament. The accelerating voltage was either 15 kV or 20 kV and the beam current 1-2 nA. EBSPs were recorded on a high-quality detector from Nordif and analysed with a non-commercialised software developed at the Risø National Laboratory. Local textures were made by performing line-scans in the tube direction (TD) across the entire sample. The total distance covered by each line-scan was about 15 mm and the distance between sampled points chosen varied form 35 m to 150 m. Typically, 100-400 orientations where measured on each line scan. Line scans made near the surface and near the inside of the tubes where made at a distance of approximately 100 m from the edges.






Figure 2. Illustration of where the 12 samples for EBSP investigation were cut out from the ring.


The crystal orientation (texture) data were represented as (100)-pole figures which shows the stereographic projection of {100} planes in the crystals onto the TD-RD plane. The plots were made with specialised software developed at the Risø National Laboratory.


Throughout this work, there are two tubes studied, T1 and T2, with two different levels of reduction in area.


Recrystallisation Temperature (TR)


The temperature TR is defined as the minimum temperature at which recrystallisation takes place during one hour of heat treatment. Measured values of TR for the inner and outer surfaces of the tubes are listed in Table 2. Tube T2 shows a uniform behaviour whereas TR of the outer surface in T1 is much smaller than that of the inner surface. Furthermore, the recrystallisation behaviours of T1 and T2 are found to be quite different. In tube T1, recrystallisation begins close to the outer surface (Fig. 3a), whereas it initiates in the centre of the sample in the case of T2 (Fig. 3b). Moreover, the shapes of the recrystallised grains are very different in the two cases. More refined and equiaxed grains are obtained after recrystallisation in T1 when compared against T2, as is shown in Fig. 4.


Table 2 Measured values of TR temperatures.





Reduction in area / %

Outer surface

Inner surface

T1

72

835 C

1190 C

T2

90

1175 C

1200 C







Figure 3. Transverse sections of (a) T1 and (b) T2 recrystallised at 870 C for

30 min and 1180 C for 1h, respectively.






Figure 4. Transverse sections of (a) T1 and (b) T2, both recrystallised at 1380 C for 1h.


Surface per Unit Volume (SV)


Given the columnar grain structure, the surface per unit volume of recrystallised grain boundary was measured according to Bhadeshia et al. 23. Thus, the columnar grains was approximated as space-filling prisms of cross-sectional side length `a' and height `c', where c >> a. Since the recrystallised microstructure in PM2000 is anisotropic because the grain-growth velocity is much higher along the extrusion direction, the final recrystallised grains can be also approximated as space-filling prisms. The mean lineal intercept as measured on random sections is then given by:


. . . . . . . . . . (1)


for c >> a, this becomes


. . . . . . . . . . . (2)


The three different sections examined in the present work are illustrated in Fig. 5. Lineal intercept measurements were carried out from montages of micrographs taken at magnifications ranging from 50 to 100. The low magnification is necessary in order to ensure sufficient numbers of complete sections of recrystallised grains in the area examined. For directional microstructures, the lineal intercept is a function of scan orientation relative to the microstructure, whereas Eqns. (1) and (2) require that the test lines be randomly orientated with respect to the microstructure. Hence, each field was studied at 15 orientations of the scan direction.





Figure 5. Diagram illustrating the sections on which stereological measurements were carried out.


Mean lineal intercept measurements from the two longitudinal sections (LL1 and LL2) and the transverse section (LT) are presented in Table 3. The errors in the measurements arise due to the finite number (N) of tests and due to the variability in the size of features, as expressed by standard deviation . Both of these effects can be taken into account by calculating the standard error (SE) of the mean, given by 24


. . . . . . . . . . . (3)


Table 3 Mean lineal intercept measurements (in units of mm). SE values for are calculated from means of the SE values for the three different sections studied.





T1

T2



0.067

0.048



0.381

0.589



0.431

0.595



0.293

0.411



0.059

0.048

SE

0.024

0.019


Figure 6 shows the evolution of the surface per unit volume () for the recrystallised grains as a function of deformation. With increasing deformation, the grain shape tends to become more elongated and the grains also get longer, leading to the smaller intercepts for T2.






Figure 6. Evolution of recrystallised grain SV as a function of degree of deformation for PM2000


Grain Aspect Ratio (GAR)


The good high-temperature properties of ODS materials correlate directly with their coarse elongated grain structures. The grain aspect ratio (GAR) of a material can be as follows:


. . . . . . . . . . (4)


The grain structure developed in PM2000 can vary from micron sized grains to structures with high grain aspect ratios. The measured GAR values for recrystallised grains as a function of are listed in Table 4.


Table 4 Grain Aspect Ratio (GAR) values for recrystallised grains as a function of degree of deformation.





T1

T2

GAR

6.0

12.0



2.0

3.0


Through-Thickness Strain Distribution


Figure 7 shows a cross section of T1 and T2; it is evident that flow forming deformation does not fully penetrate the tube wall for T1 (Fig. 7a). T2 on the other hand shows a fairly homogeneous macrostructure, suggesting more homogeneous deformation (Fig. 7b).


In Fig. 8a, which is a transmission electron microscope (TEM) image, illustrates the inhomogeneous nature of the deformation in T1, with some regions where grains have a clearly elongated structure whereas other regions of the same sample show more equiaxed grain structures. By comparison, T2 and shows a uniform elongated grain structure (Fig. 8b).





Figure 7. Transverse section of (a) T1 and (b) T2 showing the pattern of the flow forming deformation





Figure 8. TEM micrographs of the transverse sections of (a) T1 and (b) T2 in the as-flow formed condition.


F
igure 9. Comparison of strain gradient across the wall-thickness

of the tube between T1 and T2.


The fact that the overall deformation is less homogeneous in T1 than in T2 is confirmed by the data presented in Figure 9, which shows the evolution of the hardness across the wall thickness of the two tubes. The hardness distribution into T2 is clearly more homogeneous than that in T1.


The crystallographic texture has also been measured as a function of depth in the deformed tube (Fig. 10). The outer surface of tube T1 has a fairly strong fibre texture with axis RD=[100] ('rotated cube' orientation). By contrast, the centre of the tube has a weak almost random texture, whereas the inner surface shows a strong texture dominated by a rotated cube component (TD=[110] RD=[001]). The misorientation distributions are illustrated in Fig. 11. The orientations have been measured on a rectangular grid with a sampling distance of 1um between grid points by means of EBSD. The y-axis shows the normalised counts of misorientation (a rotation angle between 0 and 62.8 degree for cubic crystals) between neighbour grains which falls within the selected intervals of 0.5 degrees. The solid line on the histograms shows the distribution for randomly oriented crystals (MacKenzie distribution). From this figure could be concluded that there is an increase in the fraction of low angle boundaries towards the inside of the tube. More deformation increases the fraction of high angle boundaries, as illustrated in Fig. 11.


The texture results of tube T2 are shown in Fig. 12. The three textures from the outside, centre and inside all show a maximum where RD=[100]. Three ideal orientations dominate the texture: Rotated cube (101)[110], and (1-11)[1-10] + (111)[1-10]. The textures become stronger when moving from outer surface towards inner surface. Near the outer surface the texture is fibre-like (fibre axis RD=[100]). The fibre-tendency decreases towards the inside.


There are similarities in the textures results of T1 and T2: rotated cube component, fibre-like on outside, strong texture on inside. There are also differences: the (1-11)[1-10] and (111)[1-10] components are not clearly observed in T1. Likewise, T2 shows a steady increase in texture strength from outside to inside, whereas T1 has a fairly weak texture in the centre. In general, it might be concluded that the texture is more homogeneous over the tube thickness for T2. This, of course, is expected given the more homogeneous distribution of strain in T2.


Histograms of the misorientation distribution across the wall-thickness of tube T2 (Fig. 13) show an increase in the number of low misorientation boundaries on moving from the outer towards the inner surface. The change from the outer surface to the centre is only minimal, but the change from centre to inner surface is quite significant. Comparing the data with that from tube T1 show several differences. For all layers, the shape of the histogram is quite different. In particular, the distribution of the low misorientation boundaries (smaller than 15 degree) is more concentrated at the really low misorientations (smaller than 3 degree) for T2 than that for T1 which has a more evenly spread distribution. For all layers, the fraction of boundaries with less than 3 degree misorientation is significantly larger on T2 than on T1. This concentration around really small-angle boundaries seems to be the most pronounced difference found between T1 and T2 from the misorientation distributions.


Recrystallisation "nucleates" by the bowing of grain boundaries. With the sub-micrometer grain size of mechanically alloyed metals, the grain junctions themselves act as severe pinning lines for grain boundary bowing. 25 The activation energy for the nucleation of recrystallisation is therefore very large, requiring exceptionally high recrystallisation temperatures. However, recrystallisation becomes much easier, and can occur at much lower temperatures, if a few grains happen to be slightly larger, i.e. if the grains are not uniform in size, or if there are local strain heterogeneities which assist nucleation.


Non-uniform strain must therefore enhance nucleation leading to a finer recrystallised grain size and a reduction in TR,, as is observed for tube T1. The same reasoning explains why TR is high for T2, and why its grain structure is coarse in spite of the higher degree of deformation during flow forming.


The crystallographic texture results are also consistent with this interpretation. Li reported that the diffusion of atoms between grains is more difficult when the grains are related by a low misorientation.26 A preponderance of low misorientation boundaries would therefore inhibit recrystallisation kinetics, including the motion of the grain boundary during the bowing stage of nucleation. The inner surface of the flow formed tube T1 has a high frequency of low misorientation boundaries; therefore, coarse grains are obtained on the inner tube-surface when compared with the outer surface. Similarly, there must be an inhibition of nucleation in T2 due to high fraction of low misorientation boundaries which explains the coarse grained structure.





Outer surface – Fibre texture (RD=[100])





Centre – Random





Inner Surface – Rotated Cube (TD=[110] RD=[001])

Figure 10. Texture at surface, centre, and inside of T1 prior to heat treatment




Outer surface



Centre




Inner surface


Figure 11. Misorientation distribution of T1 prior to heat treatment





Outer Surface – Weak fibre, peaking between

rotated cube (101)[110] and (1–11)[1–10]






Centre – peaks around

rotated cube (101)[110], (111)[1–10] and (1–11) [1–10]






Inner Surface – Strong peaks at rotated cube (101)[110] and (111)[1–10]


Figure 12. Texture at surface, centre, and inside of T2 prior to heat treatment






Outer Surface

C

entre


Inner Surface


Figure 13. Misorientation distribution of T2 prior to heat treatment


Conclusions


The influence of deformation on the recrystallisation of mechanically alloyed PM2000 has been studied. Tubes with two different levels of deformation due to flow-forming have been studied. As the reduction in area increases, a more homogeneous sub-micron microstructure and strain gradient across the wall thickness of the tube is observed. Likewise, the differences between the minimum temperature at which recrystallisation in the outer and inner surface of the tube takes place is reduced as deformation increase.


These results from hardness, microstructure and crystallographic texture are all consistent with the broad idea that anything which introduces heterogeneity into the microstructure, stimulates the nucleation of recrystallisation, giving a fine-grained microstructure.


Acknowledgements


C. Capdevila and H. K. D. H. Bhadeshia are grateful to Professor A. H. Windle for the provision of laboratory facilities at the University of Cambridge and the European Commission for funding the work via a BRITE-EURAM III project. It is a pleasure to acknowledge our project partners: Plansee GmbH, Metall-Spezialrohr GmbH (MSR), Sydkraft, and Mitsui Babcock Technology Centre.


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