PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
Recently, Robertson et al. [139] have conducted a study to seek further understanding
of how processing and heat treatment can affect the texture in Ni-Ti bulk material, specifically
by quantifying the texture of a variety of raw material forms, and to determine if annealing
heat treatments can be utilized to change the strength and orientation of the texture in order to
vary the mechanical response. They have shown that Ni-Ti does exhibit crystallographic
orientation resulting from its forming and processing, which cannot be eliminated by
annealing at high homologous temperatures up to ≈ 0.85 T m (with T m the melting
temperature).
Thus, due to the importance of the present topic, several researchers have studied the
influence of cold work, annealing and precipitation kinetics during aging, on the martensitic
transformation in Ni-Ti alloys.
Ni-Ti SMAs have a bcc superlattice structure in the B2 phase and, for that reason,
common bcc textures are presented in Tab. 1.2.
Fibre Fibre axis Relevant components associated with the fibre
α-fibre I ||RD {001} - {112} - {111}
α-fibre II ||RD {111}-{110}
γ-fibre ||ND {111}-{111}
η-fibre ||RD {001}-{011}
Tab. 1.2: Typical fibre textures observed in bcc metals and alloys (RD - rolling
direction, ND - normal direction). The α-fibre II is not common in general but
is observed in SMAs [138].
The calculated lattice correspondence between parent phase and martensite was
proposed by Miyazaki et al., [140]:
• (110)[1 ⎺1 0] parent phase variant gives rise to {111} and {002}
martensite variants,
• (111)[1 ⎺1 0] parent phase variant gives rise to {210} and {210}
martensite variants.
It has been shown that in the case of textured SMA rolled sheet, there is a considerable
difference in plateau-strain and shape recovery strain when tested along different directions.
For a rolled sheet, the direction where the highest shape recovery strain is obtained
corresponds to the direction where the longest stress-plateau is obtained in the stress-strain
curve during the martensite deformation. Least shape recovery is obtained for the test
direction where the martensite stress-strain curve has no stress-plateau [141]. The α-fibre I is
Chapter 1 - Background and Literature Review 70