Papers by Keyword: R-Phase

Abstract: The pseudoelasticity (PE) and shape memory effect (SME) are the two main behaviors presented by the shape memory alloys (SMA's) and are associated respectively to mechanical and thermally induced martensitic transformations. The aim of this work is to investigate the effects of heat treatment temperature on the microstructure properties and phase transformation temperatures of a NiTi alloy with 57 w.t. % Ni. The X-ray diffraction (XRD) was carried out to obtain the phases present. The phase transformation temperatures were measured by differential scanning calorimetry (DSC). The alloy chemical composition and hardness were obtained by X-ray fluorescence (XRF), hardness (HRC) and microhardness (HV) tests, respectively. The analysis were performed in the state as received and after aging treatment at different temperatures between 350 °C and 600°C. The samples as received contained a fully austenitic microstructure at room temperature and the DSC analysis showed the presence of a phase transformation in multiple steps (B19'-R-B2). After aging at 350 °C the R phase was observed at room temperature with the austenite. With the aging treatment at 600 °C the R phase was solubilized and the alloy phase transformation occurred in a single step, ie, direct transformation from austenite to martensite and vice versa. The transformation temperatures Af, As, Ms and Mf changed with the aging treatment temperatures.

Abstract: Ti-50.7at.%Ni as one of Ni-rich TiNi shape memory alloy is very responsive to precipitation ageing treatment due to the formation of Ti₃Ni₄ precipitates. This study indicates that this alloy exhibits different transformation sequences under different ageing temperatures. Precipitation of Ti₃Ni₄ has introduced the formation of intermediate R-phase prior to martensitic transformation. The lack of 2-step R-phase transformation, B2→R1 and B2→R2 transformation in this alloy, suggested that Ti₃Ni₄ precipitation occurs homogeneously.

Abstract: The rhombohedral R-phase is an intermediate phase during the transformation from cubic austenite (B2) to monoclinic martensite (B19’). Compared to the martensitic transformation (B2 → B19’), the R-phase transformation (B2 → R) in NiTi based shape memory alloys (SMA) has a lot of advantages. The main advantages of the R-phase transformation are the very small temperature hysteresis (2-5K) and the high durability (> 100.000 cycles). The main disadvantage of actuators based on R-phase transformation is the small SMA effect (approx. 0,8%). Due to the small temperature hysteresis, the R-phase actuators are usable for temperature sensitive applications in heating technology or in air conditioning. The R-phase transformation doesn’t appear in each SMA alloy and needs a special thermo-mechanical treatment to be induced. Fuzzy logic is an approach to computing, based on degrees of truth, rather than the usual true or false (1 or 0) Boolean logic. It includes 0 and 1 as extreme cases of truth but also the various states of truth. The objective of this paper is to show the applicability of the approach of fuzzy logic in the field of SMA. To gain basic knowledge about influences, different binary NiTi alloys were investigated. The parameters annealing temperature, annealing time and Ni-concentration were varied and their influence on the transformation behavior of the R-phase was noted and added to the fuzzy logic knowledge base.

Abstract: We have found that deposited film can be crystallized without the post-annealing treatment but with the simultaneous ion-irradiation during sputter-deposition at very low substrate temperature. The present paper reviews the low temperature crystallized TiNi films deposited by the above technique. An RF magnetron sputtering apparatus equipped with separate confocal sources as well as with a heating and ion-irradiating system for substrates was used to make the films crystalline. Without using the ion-irradiating system, the films deposited on ambient-temperature substrate have been amorphous. However, crystallized film is deposited even at 353 K of substrate temperature with using the system. Appropriate ion-irradiation is considered to be help to crystallize the film at low substrate temperature. Broad and doublet X-ray diffraction profile of the film, which was diffracted from B19’ and/or R phase, was recorded between 42 degree to 45 degree in 2 theta. The crystallized film deposited on a polyimide sheet was cut into the shape of a double-beam cantilever and the ends of the two beams were connected to an electrical power supply. The cantilever shows a repeatable two-way motion by electrical cycle of 0.1 Hz at room temperature.

Abstract: The method of generating the most practicable shape recovery force in smart composite materials which embedded shape-memory alloy (SMA) fiber under the resin matrix is electric heating. However, because the calorie for the heating of the resin matrix increases in the low temperature environment, it is necessary to control the electric heating corresponding to an ambient temperature to obtaining a steady shape recovery force. Then, the method of self-detecting the temperature without using a special temperature sensor by measuring the electrical resistance of the SMA fiber in the composite material was examined.

Abstract: Change in the R-phase transformation behavior with the increase of Fe/Co content in Ti-(50-x)Ni-xFe and Ti-(50-x)Ni-xCo alloys has been investigated through thermal property measurements and microscope observations in order to clarify the relation among the stability of the B2-type structure, premartensitic phenomena, and the R-phase transformation. It was found that with increasing Fe/Co content, the R-phase transformation behavior approaches a second-order like one. The transformation product formed very close to the second-order is composed of nanoscale domains, and its Debye temperature is low compared with the R-phase formed by obvious first-order transformation.

Abstract: Precipitation behavior of intermetallic phases in ferrite matrix is investigated by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co (at%) alloys with and without 0.3at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873K. It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free alloy, the R-phase precipitates instead of I-phase at 873K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973K. The Laves phase formed at 973K transforms to the I-phase in the Si-added alloy but to the R-phase in the Si-free alloy during subsequent aging at 873K. The factors in controlling the phase stability of I-phase, R-phase and Laves phase precipitates in Fe-based alloys are discussed by the atomic size ratio and electron concentration factor (e/a).