Evaluating parent grain reconstruction in Titanium using high temperature in-situ EBSD

Introduction

The microstructure and physical properties of many high-performance materials are strongly dependent on their high temperature microstructures; this is particularly true for materials that undergo displacive phase transformations during the cooling stage of their thermomechanical processing. Therefore, a prerequisite to engineer such materials is to understand the microstructure of the high temperature parent phases. The direct measurement of the parent microstructure, for example by electron backscatter diffraction (EBSD), requires in-situ heating experiments which may be challenging and are often limited by experimental restrictions. The Oxford Instruments AZtecCrystal EBSD data processing software has a parent grain reconstruction tool that uses orientation data measured on the low temperature ("child") phase to reconstruct the high temperature ("parent") microstructure, based on the orientation relationship between the two phases. The reconstruction algorithm is based on work published by Huang et al. (2020) and can be applied to any system that has undergone a displacive phase transformation (e.g. austenite to martensite in steels, β to α in Ti-, Zr- or Nb- alloys). To test the accuracy of this approach, we performed in-situ EBSD heating experiments in the electron microscope on pure titanium, measuring both the true high temperature microstructure of the parent β-Ti phase and the final child α-Ti microstructure at room temperature. This permitted a direct comparison between the as-measured and reconstructed β-Ti microstructure and therefore an evaluation of the efficacy of the parent grain reconstruction process.

Experimental Details

The experiments presented were all conducted on a 7 x 10 x 1 mm commercially pure Ti sample. The sample was polished using diamond paste down to 0.5 µm and then with an additional step using colloidal silica (OPS). Finally, the sample surface was etched using a mixture of glycerine and aqua regia (HNO₃+3 HCl) for about 10 seconds. Electron backscatter diffraction patterns (EBSPs) were collected using the Symmetry S2 fibre-optic-based CMOS EBSD detector. The front of the detector was equipped with a new design of high T phosphor screen, with an in-built interference filter for effectively filtering out the incident infra-red radiation resulting from the heating of the stage and the sample. In contrast to conventional high T phosphor screens that use a thick metal coating on the surface of the phosphor screen, the use of an interference filter maintains the excellent sensitivity of the Symmetry S2 detector, enabling high speed analyses using low electron doses even at high sample temperatures.

The experiment was carried out using a Carl Zeiss Gemini SEM 450 field emission scanning electron microscope. During all experiments the accelerating voltage was set to 20 kV with a beam current of about 25 nA, operating at a working distance of 16.7 mm. The microscope was equipped with a heating stage (Heating Module 1050°C, Kammrath & Weiss, Schwerte, Germany) capable of generating temperatures up to 1050 °C. The measurement setup is shown in figure 1 at both ambient conditions and at a sample temperature of ~850 °C.

Fig 1. Measurement setup in the scanning electron microscope, viewed using the SEM chamberscope. The heating stage is tilted 70° with respect to the electron beam towards the Symmetry S2 EBSD camera. Left: Setup at ambient conditions. Right: Setup at ~850 °C, showing the significant infra-red radiation.

Parent Grain Analysis

The temperature of the Ti sample was increased until almost all of the α-Ti in the area of interest had been transformed into β-Ti, achieved at a sample temperature of ~920 °C. Whilst maintaining this sample temperature, a low magnification EBSD map was collected from an area of 2.87 x 2.15 mm using a step size of 5 µm (~240,000 pixels at an indexing rate of ~95 %). The analyses were collected at ~1000 patterns per second (pps), with a total collection time of about 4 minutes. The resulting data are presented in figure 2.

Subsequently, the sample was cooled down to ambient conditions over a period of 12 hours and measured again. The EBSD analysis covered the same area but with a smaller step size of 2.5 µm, at an analysis rate of 2,300 pps (7 minutes total collection time) and with an indexing rate of >96%. The results are shown in figure 3. Only a small amount (~2 %) of the high temperature β-Ti phase is retained and the phase map is dominated by the low temperature α-Ti phase, exhibiting a typical lath structure.

The orientation information of the α-Ti was then used to reconstruct the parent β-Ti microstructure, using the parent grain reconstruction tool within AZtecCrystal. This was based on the typical Burgers orientation relationship between α- and β-Ti:

{110}_β || {0001}_α and <-11-1>_β || <2-1-10>_α

The results are shown in figure 4. The direct comparison of the reconstructed IPF maps and pole figures with those measured directly at high temperature (figure 2) reveals an excellent agreement between the 2 parent microstructures. This includes the overall grain structure, the measured orientations and even the positions of twin boundaries and therefore confirms the veracity of the reconstruction capability of AZtecCrystal.

Fig 2. Measurement results at ~920 °C sample temperature. Top left: Overlay of phase colour over the band contrast, α-Ti in blue, β-Ti in red. Top right: Pole figures showing β-Ti orientations. Bottom row: IPF orientation maps of β-Ti, showing grain boundaries >10° in black and twin boun

Fig 3. Measurement results at ambient conditions after cooling down from 920 °C sample temperature. Top left: Overlay of phase colour over the band contrast, β-Ti in red, α-Ti in blue. Top right: Pole figures showing α-Ti orientations. Bottom row: IPF orientation maps of α-Ti, sho

Fig 4. Reconstructed parent microstructure from ambient measurements. Top left: Pole figures showing the Burgers orientation relationship between measured α-Ti and reconstructed β-Ti in one parent grain region. Top right: Pole figures showing reconstructed β-Ti orientations. Bottom row: IPF orientation maps of reconstructed β-Ti, showing grain boundaries >10° in black and twin boundarie

Summary

This application note demonstrates the effectiveness of the parent grain reconstruction tool in AZtecCrystal for the reliable reconstruction of high temperature parent microstructures using only the measured orientations of child phases at ambient conditions. The approach was successfully verified by performing high temperature in-situ EBSD analyses on a Ti sample using a new generation of interference filter to remove the effect of infra-red radiation on the EBSD detector. The results showed excellent agreement between the true parent microstructure of β-Ti measured at elevated temperatures and a reconstruction of the β-Ti microstructure derived solely from ambient analyses of the child α-Ti microstructure.

This parent grain reconstruction capability has multiple applications in the research of the effect of any displacive phase transformations, with particular importance in the additive manufacturing of Ti-alloys and the processing of martensitic steels.

References

• C.-Y. Huang, H.-C. Ni and H.-W. Yen, New protocol for orientation reconstruction from martensite to austenite in steels, Materialia 9 (2020) 100554