S. Deville et al Acta Materialia 52(2004)5709-5721 The ABCI correspondence is the more favorable from an energetic point of view. grains likely to have their c, axis close to the surface normal are the first ones to transform [17] 1 For this correspondence, three different modes of var wants growth have been identified, all of which leading to a final fourfold symmetry. In particular, the partial 4 transformation formed by four opposed variants is a stable configuration, since all the transformation strains are accommodated on long range. In this case, the transformation is proceeding progressively to its completion Variants can propagate into the volume by the exter 3 nal growth mode until an obstacle (i.e. grain bound ary)is encountered. Variants then grow from the inside(internal growth). All the variants are thus joining themselves at a grain boundary, at a common origin If isolated single variants are formed (i.e. without a symmetric variant), transformation strains are not Fig. 10. Observation of transformation induced relief in a large portion of the surface. The relief features allow the approximate accommodated, so that very high level of transforma determination of the location of grain boundaries. Deduced grains are tion induced stresses can be obtained locally. Consid numbered, and grain pop-out tions are indicated. Grains 1.4 ering the very large energy disequilibrium, the and 5 present a very low crysta nic disorientation. Dashed lines transformation is brought to its completion quasi-in represent the expected grain bo stantaneously when stimulated by an external(extra grain)event. In this case, the variants growth speed is at least larger of one range of order than for stable of grain 4. The misfit is consequently more important internal or external growth than between grains I and 4 for which crystallographic orientations are very similar The formation of micro- As far as the nucleation of variants leading to the for- cracks that could later grow in size and potentially lead mation of partially transformed grains(Fig. 1(a)is con to critical defects and components failure is here under- cerned, two mechanisms can be considered: stood at a microscopic scale, in terms of crystallographic arguments. A phenomenon occurring at the scale of the Each one of variants is formed after the other (i.e grain has macroscopic consequences. The presence of needle growth mode), by nucleation at the surface microcracks in the transformed zone has previously and propagation into the volume as the variant been demonstrated [21, 22], though the underlying criti grows. As the magnitude of transformation induced cal factors for their formation have never been under stresses increases with the growth of the variants, stood so straightforwardly these stresses can act to trigger the transformation of neighboring complementary variants. Hence, it will very unlikely lead to the partial transformation 4. Discussion configuration, considering the energy disequilibrium and the above mentioned remarks 4 Variants nucleation The four variants composing the final arrangement are formed at the same time so that no intermediate As far as the nucleation of the variants is concerned situation with non-accommodated transformation it is worth resuming the main features of the experimen strains is encountered. The arrangement is stable tal observations at this point from its beginning, so that further growth of the var wants can proceed slowly and at equilibrium An incubation stage seems to be present during the aging induced transformation. No relief is observed Taking into account the partial conclusions pre- during the first 60 h of the aging treatment. Since sented the second mechanism is the more favorable can he AFM scans size is obviously limited, it cannot didate able to explain the experimental observations. In however be ascertained that no variants at all are this case. the variants must be formed all at once and formed at the surface of the sample during this nucleat ate at a common origin point. This analysis stage. draws several consequences concerning the chemicalof grain 4. The misfit is consequently more important than between grains 1 and 4 for which crystallographic orientations are very similar. The formation of micro￾cracks that could later grow in size and potentially lead to critical defects and components failure is here under￾stood at a microscopic scale, in terms of crystallographic arguments. A phenomenon occurring at the scale of the grain has macroscopic consequences. The presence of microcracks in the transformed zone has previously been demonstrated [21,22], though the underlying criti￾cal factors for their formation have never been under￾stood so straightforwardly. 4. Discussion 4.1. Variants nucleation As far as the nucleation of the variants is concerned, it is worth resuming the main features of the experimen￾tal observations at this point. An incubation stage seems to be present during the aging induced transformation. No relief is observed during the first 60 h of the aging treatment. Since the AFM scans size is obviously limited, it cannot however be ascertained that no variants at all are formed at the surface of the sample during this stage. The ABC1 correspondence is the more favorable from an energetic point of view. Grains likely to have their ct axis close to the surface normal are the first ones to transform [17]. For this correspondence, three different modes of var￾iants growth have been identified, all of which leading to a final fourfold symmetry. In particular, the partial transformation formed by four opposed variants is a stable configuration, since all the transformation strains are accommodated on long range. In this case, the transformation is proceeding progressively to its completion. Variants can propagate into the volume by the exter￾nal growth mode until an obstacle (i.e. grain bound￾ary) is encountered. Variants then grow from the inside (internal growth). All the variants are thus joining themselves at a grain boundary, at a common origin. If isolated single variants are formed (i.e. without a symmetric variant), transformation strains are not accommodated, so that very high level of transforma￾tion induced stresses can be obtained locally. Consid￾ering the very large energy disequilibrium, the transformation is brought to its completion quasi-in￾stantaneously when stimulated by an external (extra￾grain) event. In this case, the variants growth speed is at least larger of one range of order than for stable internal or external growth. As far as the nucleation of variants leading to the for￾mation of partially transformed grains (Fig. 1(a)) is con￾cerned, two mechanisms can be considered: Each one of variants is formed after the other (i.e. needle growth mode), by nucleation at the surface and propagation into the volume as the variant grows. As the magnitude of transformation induced stresses increases with the growth of the variants, these stresses can act to trigger the transformation of neighboring complementary variants. Hence, it will very unlikely lead to the partial transformation configuration, considering the energy disequilibrium and the above mentioned remarks. The four variants composing the final arrangement are formed at the same time, so that no intermediate situation with non-accommodated transformation strains is encountered. The arrangement is stable from its beginning, so that further growth of the var￾iants can proceed slowly and at equilibrium. Taking into account the partial conclusions pre￾sented, the second mechanism is the more favorable can￾didate able to explain the experimental observations. In this case, the variants must be formed all at once and nucleate at a common origin point. This analysis draws several consequences concerning the chemical Fig. 10. Observation of transformation induced relief in a large portion of the surface. The relief features allow the approximate determination of the location of grain boundaries. Deduced grains are numbered, and grain pop-out (gp) locations are indicated. Grains 1, 4 and 5 present a very low crystallographic disorientation. Dashed lines represent the expected grain boundaries. S. Deville et al. / Acta Materialia 52 (2004) 5709–5721 5717