Experimental observations elucidating the mechanisms of structural bcc-hcp transformations in {beta}-Ti alloys

Journal Article · Sun Oct 01 00:00:00 EDT 2006 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PHYSREVB.74.134114· OSTI ID:20853737

Sietsma, J ^[1]; Zwaag, S van der ^[2]

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1. Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft (Netherlands)
2. Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft (Netherlands)

The formation mechanisms of two hcp {alpha} phase morphologies in Ti-4.5Fe-6.8Mo-1.5Al have been investigated by optical microscopy (OM), atomic force microscopy (AFM), electron probe microanalysis (EPMA) and dilatometry. At relatively high temperatures primary {alpha} forms predominantly on prior bcc {beta} grain boundaries, whereas at lower temperatures so-called bainitic {alpha} plates nucleate both at grain boundaries and intragranularly. This morphological transition with decreasing temperature is associated with a change in transformation mechanism. The combined results of EPMA, OM and dilatometry show that the growth of these bainitic {alpha} plates is partitionless, and not accompanied by a volume change. Subsequently, a post-transformation redistribution of Fe takes place, which causes a dilatation that can be modelled based on the diffusion of Fe and the interface-area density. This mechanism as well as the formed microstructure are similar to bainite in steel, and therefore we chose to denote this transformation product as bainitic {alpha}. In addition, the AFM observations on bainitic {alpha} plates show an invariant plane strain surface relief with tilt angles that are consistent with the Burgers' transformation model based on shear. In contrast, the AFM results show that the formation of primary {alpha} is accompanied by an irregular dip on a free surface, which is in agreement with the volume decrease measured using dilatometry. Furthermore, the EPMA results show that primary {alpha} is formed by a partitioning transformation. The change in transformation mechanism with decreasing temperature is supported by the observed trend in both the dilatation and the volume fraction {alpha} as a function of temperature.

• OSTI ID: 20853737
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 74, Issue 13; Other Information: DOI: 10.1103/PhysRevB.74.134114; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ALUMINIUM ALLOYS
ATOMIC FORCE MICROSCOPY
BAINITE
BCC LATTICES
DIFFUSION
DILATOMETRY
ELECTRON MICROPROBE ANALYSIS
ELECTRON PROBES
GRAIN BOUNDARIES
HCP LATTICES
INTERFACES
MOLYBDENUM ALLOYS
MORPHOLOGY
NUCLEATION
OPTICAL MICROSCOPY
PHASE TRANSFORMATIONS
PLATES
SHEAR
SOLIDS
STRAINS
SURFACES
TEMPERATURE DEPENDENCE
TITANIUM ALLOYS