Deformation and strength of Ti-6Al-4V alloyed with B at cryogenic temperatures

Plastic deformation and strength of Ti-6Al-4V (Ti64) alloyed with minor additions of B at cryogenic temperatures were investigated through unnotched and notched tensile tests at 20 and 77 K Marked microstructural refinement that occurs with the trace addition of B to Ti64 was exploited for examining the role of microstructural length scales on the cryogenic plastic deformation. The tensile tests were complemented with detailed microstructural characterisation using transmission electron microscopy and electron back scattered diffraction imaging of the deformed specimens. Experimental results show that the addition of 0.30 wt% and above of B to Ti64 reduces ductility, and in turn enhances the notch sensitivity to the extent that those alloys become unsuitable for low temperature applications. However, the addition of similar to 0.10 wt% B is beneficial in enhancing the low temperature strength. An examination of the yield strength variation at various temperatures reveals that at 77 K, the colony size determines the yield strength of the alloy, just as it does at room temperature; implying dislocation-mediated plasticity continues to dominate up to 77 K At 20 K however, twinning dominates the flow response, with the activation of {11 (2) over bar1} and {5 (6) over bar1 (3) over bar} twinning in addition to {10 (1) over bar2} in the base alloy resulting in enhanced ductility of it as compared to either B-modified alloys at 20 K or the base alloy itself at 77 K The observation of a reasonable correlation between the lath aspect ratio, given by the colony-to-lath thickness ratios, and yield strength variation at 20 K suggests that coarse colony size in the base alloy allows for the activation of additional twinning mechanisms. (C) 2014 Elsevier B.V. All rights reserved.

Remarkable enhancement in hydrogen storage on free-standing Ti3B and BC3 supported Ti-3 clusters

Hydrogen storage capacity of Tin-1B (n = 3-7) clusters is studied and compared with that of the pristine Ti-n (n = 3-7), using density functional theory (DFT) based calculations. Among these clusters, Ti3B shows the most significant enhancement in the storage capacity by adsorbing 12 H-2, out of which three are dissociated and the other nine are stored as dihydrogen via Kubas-interaction. The best storage in Ti3B is owed to a large charge transfer from Ti to B along with the largest distance of Ti empty d-states above the Fermi level, which is a distinct feature of this particular cluster. Furthermore, the effect of substrates on the storage capacity of Ti3B was assessed by calculating the number of adsorbed H-2 on Ti-3 cluster anchored onto B atoms in the B-doped graphene, BC3, and BN substrates. Similar to free-standing Ti3B, Ti-3 anchored onto boron atom in BC3, stores nine di-hydrogen via Kubas interaction, at the same time eliminating the total number of non-useful dissociated hydrogen. Gibbs energy of adsorption as a function of H-2 partial pressure, indicated that at 250 K and 300 K the di-hydrogens on Ti-3@BC3 adsorb and desorb at ambient pressures. Importantly, Ti-3@BC3 avoids the clustering, hence meeting the criteria for efficient and reversible hydrogen storage media. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

High-Temperature Deformation Processing Map Approach for Obtaining the Desired Microstructure in a Multi-component (Ni-Ti-Cu-Fe) Alloy

An equiatomic NiTiCuFe multi-component alloy with simple body-centered cubic (bcc) and face-centered cubic solid-solution phases in the microstructure was processed by vacuum induction melting furnace under dynamic Ar atmosphere. High-temperature uniaxial compression experiments were conducted on it in the temperature range of 1073 K to 1303 K (800 degrees C to 1030 degrees C) and strain rate range of 10(-3) to 10(-1) s(-1). The data generated were analyzed with the aid of the dynamic materials model through which power dissipation efficiency and instability maps were generated so as to identify the governing deformation mechanisms that are operative in different temperature-strain rate regimes with the aid of complementary microstructural analysis of the deformed specimens. Results indicate that the stable domain for the high temperature deformation of the multi-component alloy occurs in the temperature range of 1173 K to 1303 K (900 degrees C to 1030 degrees C) and (epsilon) over dot range of 10(-3) to 10(-1.2) s(-1), and the deformation is unstable at T = 1073 K to 1153 K (800 degrees C to 880 degrees C) and (epsilon) over dot = 10(-3) to 10(-1.4) s(-1) as well as T = 1223 K to 1293 K (950 degrees C to 1020 degrees C) and (epsilon) over dot = 10(-1.4) to 10(-1) s(-1), with adiabatic shear banding, localized plastic flow, or cracking being the unstable mechanisms. A constitutive equation that describes the flow stress of NiTiCuFe multi-component alloy as a function of strain rate and deformation temperature was also determined. (C) The Minerals, Metals & Materials Society and ASM International 2015

Enhancement in Strain Hardening on Boron Addition in As-Cast Ti-6Al-4V Alloy

It has been previously reported that the addition of boron to Ti-6Al-4V results in significant refinement of the as-cast microstructure and enhancement in the strain hardening. However, the mechanism for the latter effect has not been adequately studied. The aim of this study was to understand the reasons for the enhancement in room temperature strain hardening on addition of boron to as cast Ti-6Al-4V alloy. A study was conducted on slip transmission using SEM, TEM, optical profilometry and four point probe resistivity measurements on un-deformed and deformed samples of Ti-6Al-4V-xB with five levels of boron. Optical profilometry was used to quantify the magnitude of offsets on slip traces which in turn provided information about the extent of planar or multiple slip. Studies on deformed samples reveal that while lath boundaries appear to easily permit dislocation slip transmission, colony boundaries are potent barriers to slip. From TEM studies it was also observed that while alloys containing lower boron underwent planar slip, deformation was more homogeneous in higher boron alloys due to multiple slip resulting from large number of colony boundaries. Multiple slip is also proposed to be the prime cause of the enhanced strain hardening.

On the contributions of different micromechanisms for enhancement in the strength of TI-6Al-4V alloy upon B addition: A nanomechanical analysis

The addition of small amount of boron to Ti and it alloys refines the as-cast microstructure and enhances the mechanical properties. In this paper, we employ nanoindentation on each of the constituent phases in the microstructure and `rule-of-mixture' type analysis to examine their relative contributions to the strength enhancement in a Ti-6Al-4V alloy modified with 0.3 wt% B. Our results indicate to two main contributors to the relatively higher flow strength of B-modified alloy vis-a-vis the base alloy: (a) strengthening of alpha phase due to the reduction in the effective slip length that occurs as a result of the microstructural refinement that occurs upon B addition, and (b) composite strengthening caused by the TiB whiskers present in the alloy. (C) 2015 Elsevier B.V. All rights reserved.

Relaxor ferroelectricity and electric-field-driven structural transformation in the giant lead-free piezoelectric (Ba,Ca)(Ti, Zr)O-3

There is great interest in lead-free (Ba0.85Ca0.15)(Ti0.90Zr0.10)O-3 (15/10BCTZ) because of its exceptionally large piezoelectric response Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this paper, we have analyzed the nature of: (i) crystallographic phase coexistence at room temperature, (ii) temperature-and field-induced phase transformation to throw light on the atomistic mechanisms associated with the large piezoelectric response of this system. A detailed temperature-dependent dielectric and lattice thermal expansion study proved that the system exhibits a weak dielectric relaxation, characteristic of a relaxor ferroelectric material on the verge of exhibiting a normal ferroelectric-paraelectric transformation. Careful structural analysis revealed that a ferroelectric state at room temperature is composed of three phase coexistences, tetragonal (P4mm)+ orthorhombic (Amm2) + rhombohedral (R3m). We also demonstrate that the giant piezoresponse is associated with a significant fraction of the tetragonal phase transforming to rhombohedral. It is argued that the polar nanoregions associated with relaxor ferroelectricity amplify the piezoresponse by providing an additional degree of intrinsic structural inhomogeneity to the system.

Microstructure Formation in Dissimilar Metal Welds: Electron Beam Welding of Ti/Ni

We present results for electron beam welding of a binary Ti/Ni dissimilar metal couple. The difference in physical properties of the base metals and metallurgical features (thermodynamics and kinetics) of the system influence both macroscopic transport and microstructure development in the weld. Microstructures near the fusion interfaces are markedly different from those inside the weld region. At the Ti side, Ti2Ni dendrites are observed to grow toward the fusion interface, while in the Ni side, layered growth of gamma-Ni, Ni3Ti, and Ni3Ti + NiTi eutectic is observed. Different morphologies of the latter eutectic constitute the predominant microstructure inside the weld metal region. These results are compared and contrasted with those from laser welding of the same binary couple, and a scheme of solidification is proposed to explain the observations. This highlights notable departures from welding of similar and other dissimilar metals such as a significant asymmetry in heat transport that governs progress of solidification from each side of the couple, and a lack of unique liquidus isotherm characterizing the liquid-solid front.

Resputtering Effect on Nanocrystalline Ni-Ti Alloy Films

We report on the effect of resputtering on the properties of nanocrystalline Ni-Ti alloy thin films deposited using co-sputtering of Ni and Ti targets. In order to facilitate the formation of nanocrystalline phases, films were deposited at room temperature and 573 K (300 A degrees C) with substrate bias voltage of -100 V. The influence of substrate material on the composition, surface topography microstructure, and phase formations of nanocrystalline Ni-Ti thin films was also systematically investigated. The preferential resputtering of Ti adatoms was lesser for Ni-Ti films deposited on quartz substrate owing to high surface roughness of 4.87 nm compared to roughness value of 1.27 nm for Si(100) substrate.

A nano-scale instability in the beta phase of dilute Ti-Mo alloys

A nano-scale instability in the beta phase resulting in the formation of the disordered orthorhombic O' phase has been discovered in a fairly dilute binary Ti-Mo alloy, using selected area electron diffraction and high resolution scanning transmission electron microscopy. The O' phase informed in the alloy when the Mo content exceeds a critical value. The instability occurs in beta-solutionized samples that have been quenched to room temperature and is found to co-exist with athermal omega to phase. Interestingly, this nano-scale instability, involving the {110}<1<(1)over bar>0> soft-phonon shuffle, occurs in the beta phase without deliberate additions of either interstitial or substitutional solutes. (C) 2016 Elsevier Ltd. All rights reserved.