Precipitation of Heusler phase (Ni2TiAl) from B2-TiNi in Ni-Ti-Al and Ni-Ti-Al-X (X = Hf, Zr) alloys

J. Jung, G. Ghosh*, D. Isheim, G. B. Olson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

31 Scopus citations

Abstract

The precipitation of Heusler phase (L21: Ni2TiAl) from a supersaturated B2 (TiNi-based) matrix at 600°C and 800°C is studied using transmission electron microscopy (TEM), analytical electron microscopy (AEM), and three-dimensional atom-probe (3DAP) microscopy in Ni-Ti-Al and Ni-Ti-Al-X (X = Hf and Zr) alloys. The B2/L21 two-phase system, with ordered structures based on the bcc lattice, is chosen for its microstructural and analogy to the classical γ/γ′ system with an fcc lattice. Knowledge of the temperature-dependent partitioning of alloying elements and their atomic volumes in the B2-TiNi and L21 phases is desired to support design of high-performance shape-memory alloys (SMAs) with controlled misfit strain and transformation temperatures. After aging at 600°C for up to 2000 hours, the L21 precipitates remain fully coherent at a particle diameter of ∼20 nm. The observed effects of a misfit strain of -1.9 pct on the microstructure of the B2/L21 system are similar to those theoretically predicted and experimentally observed for the γ/γ′ system. The similarities are demonstrated in terms of the precipitate shape, spatial distribution, and minimum distance of separation between L21, precipitates. However, all these effects disappear after aging the alloys at 800°C for 1000 hours, when the L21 precipitates become semicoherent at particle diameters above ∼400 nm. A simple analysis of the size evolution of L21, precipitates after an isochronal aging (100 hours) experiment suggests that they follow coarsening kinetics at 600°C and growth kinetics at 800°C, consistent with the Langer-Schwartz theory of precipitation kinetics, which predicts that a high supersaturation suppresses the growth regime. Microanalysis using AEM and 3DAP microscopy define the TiNi-Ni2TiAl phase boundaries at 800°C and 600°C. At 800°C, Hf and Zr partition to the B2-TiNi, while at 600°C, they partition slightly to the L21 phase, reducing the lattice misfit to -1.7 and -0.011 pct, respectively, and partition strongly to the metastable phase Ti2Ni3. To describe the composition dependence of the lattice parameter of multicomponent B2 and L21 phases, the atomic volumes of Al, Hf, Ni, Ti, and Zr in the B2-TiNi and L21 phases are determined. A simple model is proposed to predict the lattice parameters of these phases in multicomponent systems.

Original languageEnglish (US)
Pages (from-to)1221-1235
Number of pages15
JournalMetallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
Volume34 A
Issue number6
DOIs
StatePublished - Jun 2003

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Metals and Alloys

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