S. Deville et al Acta Materialia 52(2004)5709-5721 AFM experiments were carried out with a D3100 how obtaining a stack of four variants of corre nanoscope from Digital Instruments Inc, using oxide spondence ABCI was possible and energetically sta sharpened silicon nitride probes(Nanosensor, CONT-R ble, since all interfaces (inside and outside of the model)in contact mode, with an average scanning speed arrangement) between tetragonal and monoclinic of 10 um s. Since the I-m phase transformation is phases were habit planes of the same type. All the accompanied by a large strain, surface relief is modified transformation strain is accommodated by this con by the formation of monoclinic phase. The vertical res- figuration. Fig. 1(a) provides such an observation, olution of AFM allows following very precisely the with its evolution as a function of the aging treat- transformation induced relief. ment time. Treatments steps of 20 h at 413 K in autoclave were performed between each image. The progressive transformation of the inner tetragonal 3. Experimental results part of the arrangement is clearly observed. Thi behavior is interpreted in Fig. 1(b), with the progres 3. Variants growth modes sive growth of variants, toward the inside of the grain. The potential future habit planes are symbol Three different modes of variants growth are experi ized by the dashed line, the transformation strain mentally observed for the correspondence ABCl, lead- being constant along these planes. Hence, the vari- ing to a fourfold symmetry final arrangement. These ants can grow until the transformation is completed three modes will be, respectively, referred to as internal without being restrained by any transformation in growth, external growth and needle growth duced stresses. The way the primary variants can grow to form the initial fourfold symmetry arrange- 3.1.1. Internal growth ment will be discussed later, in regards of the other This first mode of transformation is related to the variants growth mode. This mode will be called"in partial transformation, reported in [17]. It was shown ternal growth, in regards of its peculiar features Fig. 1.(a) Observation of partial transform and internal growth. Aging steps of 20 h. Horizo tal scale: I um/div, vertical scale: 250 nm/div (b) nterpretation of the arrangement observed in(a). The vertical scale is not respected for clarity. The dashed lines represent the location of the inner habit planes before transformation.AFM experiments were carried out with a D3100 nanoscope from Digital Instruments Inc., using oxide sharpened silicon nitride probes (Nanosensor, CONT-R model) in contact mode, with an average scanning speed of 10 lm s
1 . Since the t–m phase transformation is accompanied by a large strain, surface relief is modified by the formation of monoclinic phase. The vertical res￾olution of AFM allows following very precisely the transformation induced relief. 3. Experimental results 3.1. Variants growth modes Three different modes of variants growth are experi￾mentally observed for the correspondence ABC1, lead￾ing to a fourfold symmetry final arrangement. These three modes will be, respectively, referred to as internal growth, external growth and needle growth. 3.1.1. Internal growth This first mode of transformation is related to the partial transformation, reported in [17]. It was shown how obtaining a stack of four variants of corre￾spondence ABC1 was possible and energetically sta￾ble, since all interfaces (inside and outside of the arrangement) between tetragonal and monoclinic phases were habit planes of the same type. All the transformation strain is accommodated by this con- figuration. Fig. 1(a) provides such an observation, with its evolution as a function of the aging treat￾ment time. Treatments steps of 20 h at 413 K in autoclave were performed between each image. The progressive transformation of the inner tetragonal part of the arrangement is clearly observed. This behavior is interpreted in Fig. 1(b), with the progres￾sive growth of variants, toward the inside of the grain. The potential future habit planes are symbol￾ized by the dashed line, the transformation strain being constant along these planes. Hence, the vari￾ants can grow until the transformation is completed, without being restrained by any transformation in￾duced stresses. The way the primary variants can grow to form the initial fourfold symmetry arrange￾ment will be discussed later, in regards of the other variants growth mode. This mode will be called ‘‘in￾ternal growth’’, in regards of its peculiar features. Fig. 1. (a) Observation of partial transformation and internal growth. Aging steps of 20 h. Horizontal scale: 1 lm/div, vertical scale: 250 nm/div. (b) Interpretation of the arrangement observed in (a). The vertical scale is not respected for clarity. The dashed lines represent the location of the inner habit planes before transformation. S. Deville et al. / Acta Materialia 52 (2004) 5709–5721 5711