Materials Science Forum Vols. 702-703

Abstract: Abstract In the present work, we summarized two calculation methods to determine some specific crystallographic elements based on electron diffraction orientation measurements performed by TEM. The first one is to determine the type and the Burgers vector of dislocations for a known crystal structure. The method calculates the orientation of the projections of all the possible dislocation line vectors in the TEM screen coordinate system using the determined crystallographic orientation of the grain and then compares them with the observed ones to identify the observed dislocations. The second is to characterize the surface crystalline planes and directions of faceted nano-particles. With the determination of the edge trace vectors and then the plane normal vectors in the screen coordinate system of the TEM, their Miller indices in the crystal coordinate system can be calculated through coordinate transformation. These methods are expected to facilitate the related studies.

Abstract: This work describes the development of texture during hot rolling of two alloys namely, Ti-12Nb and Ti-16Nb. The alloys have been unidirectionally hot rolled to 80 % reductions at 800°C and air cooled. Both the alloys show the presence of a² (orthorhombic) and small volume fraction of b (bcc) phases in hot rolled condition. The alloy Ti-12Nb exhibit moderate intensity texture while the alloy Ti-16Nb displays quite strong texture. The high overall intensity of texture in alloy Ti-16Nb in 80 % HR specimen can be attributed to the presence of large volume fraction of b phase in comparison to that of the alloy Ti-12Nb. This has been extended to study the textural changes after b solution treatment. This heat treatment consists of two types of phase transformations that are a² ® b ® a² and a² ® b ® a after water quenching and furnace cooling from β phase field.

Abstract: The development of preferred grain orientation has been investigated in the directionally solidified samples of Tb_0.3Dy_0.7Fe_1.95 as a function of pulling speed viz. 10, 40, 70 and 100 cm/h. The study indicates that at lower solidification rate (10 cm/h) growth of and texture components are preferred, whereas, texture component becomes dominant at higher pulling rate (100 cm/h). However, as the solidification progresses, growth of texture component is observed subduing the other components. Consequently, the magnetostriction improves from 1100 to 1350 micro-strains with higher pulling speed.

Abstract: The 9Cr-1Mo-0.1C steel was solutionzed (1323 K for 1hr) and step cooled (at 40 K/hr between Ms and Mf) to effect martensitic transformation at slower rates, and then subsequently tempered (1023 K for 2 hr). The microtexture of the tempered martensite structure was evaluated using SEM-Electron Back Scattered Diffraction (EBSD) technique, and the crystal orientation data of ferrite crystallites formed within a single prior-γ grain were analyzed in detail. The Kurdjumov-Sachs (K S) orientation relationship (111)γ // (011)α´ [-101]γ // [-1-11]α´ is commonly obeyed in low carbon steels, and a restitution methodology was used to back calculate the orientation of parent γ grain assuming the K-S relationship. The austenite was identified as the common solution of the data set, and its mean orientation has been calculated. The mean deviation from the K-S was calculated to be 3.75º.

Abstract: The Nickel-Titanium (Ni-Ti) alloys are the most attractive amongst shape memory alloys (SMA) due to their good functional properties coupled with high strength and good ductility. The transformation temperatures in Ni-Ti SMA can be altered by chemical composition and thermal and/or mechanical treatments adequate to obtain reversible martensitic transformation in one or more steps. The goal of the present work is to investigate the evolution of texture in Ni-Rich Ni-Ti (50.8at%Ni-Ti) SMA showing different phase transformation temperatures as a result of different thermal/mechanical history: straight-annealed (as-received condition) and subsequent thermal treatment at 500°C for 30 minutes in air. The microstructural and textural results were obtained by Electron Backscattering Diffraction on Scanning Electronic Microscopy (ESBD/SEM) and by X Ray Diffraction (XRD) at room temperature. Mechanical properties were measured by Vickers micro hardness tests at room temperature.

Abstract: The thermal cycling (quenching in liquid nitrogen and reverting back to room temperature: austenite martensite reversible transformation) response of Ni-Ti-Fe shape memory alloys has been investigated. It was clearly noted that residual deformation, estimated in terms of noticeable differences in austenite grain size, depend on the relative clustering of fine grains. During repeated thermal cycling, the residual deformation, in-grain misorientation developments and retained martensite content scaled together: bringing out a clear picture of microstructural irreversibility.

Abstract: Evolution Texture and microstructure has been investigated in a Fe-14Mn-6Si-9Cr-5Ni shape memory alloy during cold rolling and annealing. The starting solution-annealed material show a nearly random texture with microstructure composed of equi-axed austenite grains with some e martensite plates inside. Cold rolling induces a strong alloy type texture with Brass {011}<211> and Goss {011}<100> as major components. Annealing of the cold deformed material produces a nearly random texture. The microstructural investigation reveals that with increasing cold deformation the amount of stress induced e and a’ martensite volume fraction increases. The electron back scattered diffraction (EBSD) phase mapping shows that reversion of the e martensite starts only after commencement of recrystallization.

Abstract: NiMnCoIn alloys are new-type magnetic shape memory alloys (MSMAs) in which a reversible magnetic-field-induced phase transformation was observed. They are ideal candidates of materials used in actuators and sensors. The polycrystalline NiMnCoIn alloys are generally brittle so that they can not be easily deformed into the shape applicable to actuators and sensors until now. In the present paper, the influence of hot deformation on the microstructure of Ni₄₅Co₅ Mn_36.7In_13.3 alloy was studied. The experimental results showed that second phase was observed after deformation at high temperature between 800~900 °C and at strain rate lower than 4×10^-3s^-1. The content of Co of second phase was higher than the matrix alloy, while the content of In was lower than the matrix alloy. It was determined by TEM measurements as γ phase with fcc structure which was popular in NiFeGa and NiMnGaCo alloys. It is possible to improve the ductility of Ni₄₅Co₅Mn_36.7In_13.3 alloy by control of amount and distribution of γ phase by hot deformation.

Abstract: Single crystals of the alloy Ti-48%Ni-2%Fe, consisting of the phase B2, were rolled at 350°C with deformation degrees up to ~80% in 11 different initial orientations. Texture development in single crystals under rolling was analyzed to determine the active mechanisms of their plastic deformation. There are three stable initial orientations, which remain unchanged during rolling of Ti-Ni single crystals: {011}<011>, {111}<011> and {111}<112>. It was shown, that Ti-Ni single crystals deform by means of combined action of slip in systems {011}<001> and twinning in systems {114}<221> and {118}<441>. Variants of rolling texture formation in single crystals are very diverse and depend on their initial orientations and deformation degrees.

Abstract: NiTi based shape memory alloys (SMAs) have been identified as potential candidates for sensors and actuators in various industrial applications. During service life of actuators, SMAs are subjected to large number of stress/strain cycles through the complete transformation range, known as thermomechanical cycling (TMC), which introduces various defects in the material. In the present study, an attempt has been made to understand the microstructural changes taking place during thermo-mechanical cycling of NiTi wires using Transmission Electron Microscope (TEM). These microstructural observations have been discussed in conjunction with the cyclic shape memory behavior of the wires.