Novel metallic materials of increasing complexity are being developed in order to meet current demanding mechanical requirements. An intelligent combination of multiple phases with different mechanical characteristics can lead to a significant improvement of the mechanical response of the material [1]. Furthermore, the presence of a soft metastable phase in these complex multiphase microstructures, which can transform into a hard stable phase, opens the door to advanced materials with an extraordinary combination of strength and formability. Despite the beneficial effects of metastable phases for the macroscopic mechanical properties, limited knowledge is available on the conditions leading to their transformation. Previous experimental studies have aimed to link the phase fraction, average chemical composition and average grain size of the metastable phases to their stability. However, this average information has proved to be insufficient to tailor the stability for improved mechanical properties.

Our study investigates the stability of the metastable austenite phase present in the complex ferritic microstructure of low-alloy TRIP (Transformation Induced Plasticity) steels. In these high-strength materials, the outstanding elongation-strength combination stems mainly from the transformation of the soft metastable austenite phase into a hard martensite phase under applied stress [2]. Rather than studying the transformation as a function of stress, the equivalent transformation of the individual micrometre-sized austenite grains as a function of temperature was studied in situ by making use of the high-energy (80 keV) X-ray microbeam available at beamline ID11. Figure 27a displays the two-dimensional diffraction pattern of TRIP steel at room temperature. Each diffraction spot contains information on an individual grain within the material. The metastable austenite grains become unstable while cooling the sample from room temperature down to 100 K, owing to an increase in the chemical driving force for the transformation. The transformation behaviour of the individual austenite grains is assessed in situ by following the corresponding diffraction spot on the detector. In Figure 27b a single austenite diffraction spot is considered in detail. As shown in Figure 27c, this spot has vanished after cooling the sample to 100 K, indicating that the corresponding grain has transformed into martensite during cooling.

Fig. 27: X-ray diffraction pattern of TRIP steel. (a) The pattern at room temperature shows both ferrite and austenite reflections on separate diffraction rings. Single spots originating from individual grains appear within the austenite diffraction rings denoted in the figure as 200, 220 and 311, respectively. (b) A single austenite diffraction peak at room temperature before cooling. (c) The same region as in (b) after cooling the sample to 100 K and heating back to room temperature.


We have studied seventy individual austenite grains during the cooling of the TRIP-steel sample. The intensity of each diffraction spot is directly proportional to the volume of the grain from which it originates. Moreover, the lattice parameter of each individual grain, and hence the carbon content of that specific grain, is derived from the position of the corresponding diffraction spot. We have observed three forms of transformation behaviour for the austenite grains (Figure 28). Most grains transform completely into martensite in a single temperature step. However, a few grains show an incomplete or two-step transformation. Finally, a significant number of grains do not show any transformation down to 100 K. The occurrence of the martensitic transformation of the austenite grains at different temperatures reveals a stability distribution within the austenite phase. It is therefore clear that previously performed research on average characteristics is insufficient for optimal control of the material. The carbon content is found to be the dominant parameter that governs the stability of the large austenite grains. However, our results show that the stability of the austenite grains increases for decreasing grain volumes below 20 µm3.

Fig. 28: Observed transformation behaviour of individual austenite grains. (a) Grains that completely transform into martensite in a single temperature step. (b) Grains that partly transform into martensite. For some of these grains, a second transformation is observed at lower temperatures. (c) Grains that remain stable during cooling to 100 K.


These experiments provide the first in situ information about the phase transformation of individual metastable grains present in modern metallic materials of great complexity. The results reveal that the grain volume plays a key role in the stability of the grains when approaching the boundary between the micrometre and submicrometre scales.



[1] G.B. Olson, Science, 277, 1237-1242 (1997).
[2] P. Jacques, Q. Furnémont, A. Mertens, F. Delannay, Phil. Mag. A, 81, 1789-1812 (2001).

Principal publication and authors

E. Jimenez-Melero (a), N.H. van Dijk (a), L. Zhao (a), J. Sietsma (a), S.E. Offerman (a), J.P. Wright (b), S. van der Zwaag (a), Scripta Mater., 56, 421-424 (2007).
(a) Delft University of Technology (The Netherlands)
(b) ESRF