Tomonari Inamura

Composition dependent crystallography of α″-martensite in Ti–Nb-based β-titanium alloy

Composition dependence of crystallographic features of α″-martensite in Ti–Nb-based alloy was systematically examined in Ti–Xmol%Nb–3mol%Al alloy, where X is 10–30. One of the lattice deformation strains, η3, became 0 at the critical Nb concentration of 23 mol% and the sign of η3 changed across the composition. The phenomenological theory of martensite crystallography (PTMC) predicted that the lattice invariant shear (LIS) would vanish at the critical composition. Transmission electron microscopy revealed that non-twinned martensite was formed with a habit plane of {755}b when η3 = 0, and that martensite with an internal twin of {111}o type I was formed with a {443}b habit plane for η3 = +0.008. Subscript b and o indicate β and α″ lattice, respectively. The crystallography of these two α″-martensites were well explained by the PTMC. On the other hand, non-twinned martensite with {111}b–{775}b habit plane was formed even when η3 was not zero. It was found that there is a critical value of |η3| for the formation of twinned martensite and that class B transformation, in which LIS is a {011}o compound twin, hardly occurs in β-titanium alloys.

Material design and shape memory properties of smart composites composed of polymer and ferromagnetic shape memory alloy particles

Abstract Ferromagnetic shape memory alloys (FSMAs) such as NiMnGa are expected to be new practical actuator materials with high driving frequency by magnetic field and large strain due to the shape memory effect (SME). However, the brittleness and poor workability of FSMAs, especially at a polycrystalline state, are serious problems and should be improved for a practical use. From this viewpoint a smart composite has been designed by a combination of a polymer matrix and FSMA particles (FSMAP), and a systematic investigation has been done for a NiMnGa-FSMAP/epoxy smart composite. This paper summarizes the design concept and some experimental results of the smart composite. It is pointed out that the single-crystal NiMnGa-FSMAP are easily made by mechanical crush due to the brittleness of FSMAs, and microstructural control is also possible by applying magnetic field during curing. Experimental study revealed that the NiMnGa-FSMAP/epoxy smart composites exhibit both tensile ductility and SME, and that shape memory properties become improved by decreasing particle size of FSMAP. It is concluded that the FSMAP/polymer smart composite has a large potential to be a new practical actuator material.

Self-accommodation of B19′ martensite in Ti–Ni shape memory alloys – Part I. Morphological and crystallographic studies of the variant selection rule

The self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Twelve pairs of minimum units consisting of two habit plane variants (HPVs) with V-shaped morphology connected to a B19′ type I variant accommodation twin were observed. Three types of self-accommodation morphologies, based on the V-shaped minimum unit, developed around one of the {111}B2 traces, which were triangular, rhombic and hexangular and consisted of three, four and six HPVs, respectively. In addition, the variant selection rule and the number of possible HPV combinations in each of these self-accommodation morphologies are discussed.

Antiphase boundary-like stacking fault in α″-martensite of disordered crystal structure in β-titanium shape memory alloy

The ordering and antiphase boundary (APB)-like fault found in the α″-martensite of β-Ti shape memory alloys are studied. Long-range chemical ordering was not found, but APB-like faults were observed in every martensite plate studied by transmission electron microscopy. These faults have morphology similar to the APBs observed in ordered phases. The superlattice reflections observed in some previous works were a consequence of multiple diffractions. APB-like faults were not observed in the parent phase, leading to the conclusion that the faults were introduced by the martensite transformation. The fault took the form of a wavy tube running perpendicular to the habit plane. The fault was a ‘transformation-induced APB’ with an additional small displacement due to the pre-existing athermal ω phase. The displacement vector was determined to be [–3/50, −23/50, 1/2]. Geometrical aspects of the formation of APB-like faults are also discussed.

Self-accommodation of B19′ martensite in Ti–Ni shape memory alloys. Part III. Analysis of habit plane variant clusters by the geometrically nonlinear theory

Competition between the invariant plane (IP) condition at the habit plane, the twin orientation relation (OR) and the kinematic compatibility (KC) at the junction plane (JP) of self-accommodated B19′ martensite in Ti–Ni was investigated via the geometrically nonlinear theory to understand the habit plane variant (HPV) clusters presented in Parts I and II of this work. As the IP condition cannot be satisfied simultaneously with KC, an additional rotation Q is necessary to form compatible JPs for all HPV pairs. The rotation J necessary to form the exact twin OR between the major correspondence variants (CVs) in each HPV was also examined. The observed HPV cluster was not the cluster with the smallest Q but the one satisfying Q = J with a { 1}B19′ type I twin at JP. Both Q and J are crucial to understanding the various HPV clusters in realistic transformations. Finally, a scheme for the ideal HPV cluster composed of six HPVs is also proposed.

Self-accommodation of B19′ martensite in Ti–Ni shape memory alloys – Part II. Characteristic interface structures between habit plane variants

Four characteristic interface microstructures between habit plane variants (HPVs) in the self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by scanning transmission electron microscopy (STEM). The straight interface of a B19′ type I twin is present at interface I. The relaxation of the transformation strain at interface II is achieved by a volume reduction of the minor correspondence variants (CVs) in the relevant habit plane variants (HPVs). The relaxation of the transformation strain at interface III is mainly due to the formation of a B19′ type I twin between the two major CVs. Subsequently, local strain around the tips of the minor CVs perpendicular to the interface is released by the formation of micro-twins with the ⟨011⟩B19′ type II and/or B19′ type I relation. The major and minor CVs in each HPV are alternately connected through fine variants with the B19′ type I twin relation parallel to interface IV. The results are compared with macroscopic observations and the predictions of PTMC analysis.

Pseudoelastic Properties of Cold-Rolled TiNbAl Alloy

Microstructures and pseudo-elasticity of as-rolled Ti-24mol%Nb-3mol%Al, a newly developed functional biomedical β-Ti alloy (bcc), at room temperature were characterized. The material was homogenized at 1273K after arc-melting and quenched into water to obtain single phase of β. Cold-rolling of 99% reduction in thickness was carried out and then, microstructures and pseudo-elastic properties were examined. θ-2θ XRD measurement revealed that the as-rolled material was consisted with the parent β phase and martensite phase (α’’, C-center orthorhombic). X-ray pole figure measurements revealed that the rolling texture was a mixture of <110>β{001}β-type and <112>β{111}β-type textures. Shape recovery strain of 4% appeared along TD without any intermediate annealing after the cold-rolling.

Incompatibility and preferred morphology in the self-accommodation microstructure of β-titanium shape memory alloy

The frequency distribution of habit plane variant (HPV) clusters and the deviation from twin orientation relationships (ORs) at the junction plane (JP) are investigated by transmission electron microscopy together with theoretical evaluation of the kinematic compatibility (KC) at the JP in a β-titanium shape memory alloy. Even though there are more than 10 types of possible HPV clusters, only three types are formed. V-shaped couplings of HPVs by {111} type I twins (VI: 49%) and by ⟨211⟩ type II twins (VII: 42%) are the predominant types. A triangular morphology due to coupling of {111} type I twins is observed with a frequency of only 9%. These preferred morphologies are well explained by the degree of incompatibility (the rotation necessary for compatible connection of HPVs). The exact twin OR and KC are maintained at the JP in a VI cluster instead of KC at the habit plane (HP), whereas the JP in a VII cluster is incompatible and the ⟨211⟩ type II twin OR shows slight deviation at the JP by about 0.4°. The competition between KC at the JP and KC at the HP (invariant plane) is responsible for the frequency distribution of HPV clusters and the character of the interfaces in the self-accommodation microstructure.

Phase Stability and Mechanical Properties of Ti-Ni Shape Memory Alloys Containing Platinum Group Metals

In order to develop Ti-Ni base shape memory alloys (SMAs), the effects of ternary additions on phase constitution and mechanical properties were investiga ted for TiNi alloys containing some platinum group metals: Ir, Rh and Pt. All the al loys fabricated were made by Ar arc melting method using high purity elemental materials fol owed by hot-forging at 1173-1673K in Ar and furnace cooling. Then X-ray diffraction analysis a nd tensile tests were carried out at room temperature (RT). It was found that, whe n the amount of ternary is lower than 10mol%, all the additions reduce martensitic transf ormation temperature ( Ms) of TiNi and B2 phase becomes stable. Besides monoclinic, L1 0 and B19 phases appear for TiNi containing 40-50mol%Ir, 30-50mol%Rh and 20-50mol%Pt, respectively. The t ensile ductility at RT decreases with increasing the amount of terna ry additions, and the ductility becomes very limited when monoclinic (TiIr), L1 0 (TiRh) and B19 (TiPt) phases appear in the Ti-Ni-Ir, Ti-Ni-Rh and Ti-Ni-Pt systems, respectively . Strength strongly depends on Ms and crystal structure of the apparent phase. Small work hardening is recognized in all the alloy systems. Introduction Ti-Ni SMAs exhibit several smart functions represented by shap e memory effect and superelasticity. The applications of the Ti-Ni alloys are, how ever, still limited mainly because the martensitic transformation temperature ( Ms) is below 400K in the binary systems [1]. Then, in order to expand the applications related with SMAs, shape memory alloys actuated at higher temperature than binary Ti-Ni are required. Several investigations have been done for the improvement of actuation temperature of Ti-Ni by a lloying additives [2-4]. Most of additional elements such as Co, Fe, Mn, Cr and V reduce Ms. On the other hand, some refractory metals such as Hf and Zr and platinum group meta ls (PGMs) such as Pd and Materials Science Forum Online: 2003-08-15 ISSN: 1662-9752, Vols. 426-432, pp 2333-2338 doi:10.4028/www.scientific.net/MSF.426-432.2333

Phase Constitution and Mechanical Properties of Ti-(Cr, Mn)-Sn Biomedical Alloys

In order to produce new β (bcc) Ti alloys for medical applications, effects of Mn substitution for Cr on phase constitution and mechanical properties of Ti-Cr-Sn alloys were investigated. All the Ti-7mol%(Cr, Mn)-3mol%Sn alloys investigated by XRD analysis were identified as β (bcc) alloys, and athermal ω phase was also detected in Ti-7mol%Mn-3mol%Sn . The lattice parameter of β was slightly decreased by Mn substitution. Besides, the Mn substitution for Cr raised the hardness and the strength while reduced the ductility of the Ti-Cr-Sn alloys. The hardening by Mn substitution must be due to ω precipitation. The hardening is discussed from the viewpoint of electron atom ratio (e/a) in comparison with Ti-Cr binary alloys in the literature.