Availableonlineatwww.sciencedirect.com SCIENCE Acta materialia ELSEVIER Acta Materialia 54(2006)1917-1925 www.actamat-journals.com on the fcc- monoclinic martensite transformation in a pu-1.7 at. Ga alloy J.P. Hirth J. N. Mitchell.Ds. schwartz T.E. mitchell Structure-Property Relations Group, MST-8, Mail Stop G755, Los Alamos ory, Los Alamos, NM 87545, US.A b Nuclear Materials Science Group, Los Alamos National Laboratory, Los Alamos, NM87545, USA Received 23 September 2005: received in revised form 12 December 2005: accepted 14 December 2005 Available online 28 February 2006 Abstract The face-centered cubic 8- monoclinic martensite transformation in a Pu-1.7 at Ga alloy is analyzed in terms of the defect- based topological model. Disconnections and terrace planes for the transformation are deduced. The predicted habit plane is in good agreement with experimental results Observed twinning is associated directly with the transformation strain. The lattice invariant defor- mation is connected with slip in the a plates. Implications for hysteresis in the transformation as observed by dilatometry and calorim etry are discussed o 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved Keywords: Plutonium; Crystal structure; Martensitic phase transformations; Interface defects: Dislocation boundaries 1. Introduction (010) planes are perfectly coplanar with no puckering. The stacking of these planes is ABAB; the arrangement of Face-centered cubic(fcc)8-phase plutonium is stabilized atoms in the A and B planes is the same but they are at room temperature by the addition of less than about I rotated by 180 with respect to each other. The near neigh at Ga. Stabilized plutonium alloys, such as the Pu-1.7 bors and their bond lengths for the eight types of atoms are at Ga alloy considered here, undergo a martensitic trans- given in Table 1. These were calculated using the atomic formation on cooling from the fcc 8 phase to the mono- positions determined by Zachariasen and Ellinger [3] for clinic o' phase at about -100C. The latter is called a' pure a-Pu, and the unit cell dimensions appropriate for rather than a because ois supersaturated with Ga and a Pu-1.7 at Ga published by Hecker [4]. There are short refers to pure plutonium. The presence of Ga in the a' bonds from about 2.6 to 2.8 A in length, and longer bonds phase has the effect of expanding the unit cell volume by of 3. 2-3. 7 A in length. Atoms I and 3 have 12 near neigh about 1.8% relative to the pure a phase, but does not bors while all the rest have 14; the number of short bonds change the crystal structure. Notable characteristics of this varies from 5 for atom I to 3 for atom 8. Atom 8 has a sig- transformation include a significant reversion hysteresis of nificantly longer average bond length than the others: 150-200C, a >20% volume contraction, and incom- 3. 28 A compared to approximately 3. 16 A pleteness of the transformation during cooling [1, 2]. The crystallography of the 8-a'transformation, and As shown in Fig. 1, the monoclinic unit cell (space group of twinning, is greatly facilitated by the replacement of P21/m)of Pu contains 16 atoms in 8 different positions with the monoclinic phase by a hexagonal close packed (hcp) pairs of atoms related by the screw diad. The atoms in the pseudostructure, an innovation introduced by Crocker [5]. As indicated in Fig. I and Table 1, most atoms in the a' phase in fact have 14 nearest neighbors, rather than 12 ing author.Tel:+15056670938;fax:+150566780 in the hep structure. The pairs of atoms in the(0 10) planes E-jmail address: temitchellalanl gov (T E. Mitchell). above and below that need to be ignored for the hep 1359-6454/$30.00 O 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:l0.1016 actant2005.12030On the fcc ! monoclinic martensite transformation in a Pu–1.7 at.% Ga alloy J.P. Hirth a , J.N. Mitchell b , D.S. Schwartz b , T.E. Mitchell a,* a Structure–Property Relations Group, MST-8, Mail Stop G755, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Nuclear Materials Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 23 September 2005; received in revised form 12 December 2005; accepted 14 December 2005 Available online 28 February 2006 Abstract The face-centered cubic d ! monoclinic a0 martensite transformation in a Pu–1.7 at.% Ga alloy is analyzed in terms of the defect￾based topological model. Disconnections and terrace planes for the transformation are deduced. The predicted habit plane is in good agreement with experimental results. Observed twinning is associated directly with the transformation strain. The lattice invariant defor￾mation is connected with slip in the a0 plates. Implications for hysteresis in the transformation as observed by dilatometry and calorim￾etry are discussed. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plutonium; Crystal structure; Martensitic phase transformations; Interface defects; Dislocation boundaries 1. Introduction Face-centered cubic (fcc) d-phase plutonium is stabilized at room temperature by the addition of less than about 1 at.% Ga. Stabilized plutonium alloys, such as the Pu–1.7 at.% Ga alloy considered here, undergo a martensitic trans￾formation on cooling from the fcc d phase to the mono￾clinic a0 phase at about 100 C. The latter is called a0 rather than a because a0 is supersaturated with Ga and a refers to pure plutonium. The presence of Ga in the a0 phase has the effect of expanding the unit cell volume by about 1.8% relative to the pure a phase, but does not change the crystal structure. Notable characteristics of this transformation include a significant reversion hysteresis of 150–200 C, a >20% volume contraction, and incom￾pleteness of the transformation during cooling [1,2]. As shown in Fig. 1, the monoclinic unit cell (space group P21/m) of Pu contains 16 atoms in 8 different positions with pairs of atoms related by the screw diad. The atoms in the (0 1 0) planes are perfectly coplanar with no puckering. The stacking of these planes is ABAB; the arrangement of atoms in the A and B planes is the same but they are rotated by 180 with respect to each other. The near neigh￾bors and their bond lengths for the eight types of atoms are given in Table 1. These were calculated using the atomic positions determined by Zachariasen and Ellinger [3] for pure a-Pu, and the unit cell dimensions appropriate for Pu–1.7 at.% Ga published by Hecker [4]. There are short bonds from about 2.6 to 2.8 A˚ in length, and longer bonds of 3.2–3.7 A˚ in length. Atoms 1 and 3 have 12 near neigh￾bors while all the rest have 14; the number of short bonds varies from 5 for atom 1 to 3 for atom 8. Atom 8 has a sig￾nificantly longer average bond length than the others: 3.28 A˚ compared to approximately 3.16 A˚ . The crystallography of the d ! a0 transformation, and of twinning, is greatly facilitated by the replacement of the monoclinic phase by a hexagonal close packed (hcp) pseudostructure, an innovation introduced by Crocker [5]. As indicated in Fig. 1 and Table 1, most atoms in the a0 phase in fact have 14 nearest neighbors, rather than 12 in the hcp structure. The pairs of atoms in the (0 1 0) planes above and below that need to be ignored for the hcp 1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.12.030 * Corresponding author. Tel.: +1 505 667 0938; fax: +1 505 667 8021. E-mail address: temitchell@lanl.gov (T.E. Mitchell). www.actamat-journals.com Acta Materialia 54 (2006) 1917–1925