Scripta Materialia

Editors for Scripta Materialia

2010-05-01

Effect of shot-peening on the corrosion resistance of a Zr-based bulk metallic glass

A. Gebert, A. Concustell, A.L. Greer, L. Schultz, J. Eckert — 2010-01-11

Surfaces of bulk glassy Zr59Ti3Cu20Al10Ni8 samples were shot-peened for different times and their corrosion behaviour was tested in 0.01M Na2SO4+xM NaCl (x=0; 0.01; 0.1) and 6M HCl. Slight improvement of spontaneous passivity but a decrease of pitting resistance are detected with prolonged shot-peening duration. The corrosion damage evolution is governed by the nature of the mechanically generated defects and their surrounding stress fields.

Electron backscatter diffraction study of dislocation content of a macrozone in hot-rolled Ti–6Al–4V alloy

T. Ben Britton, Soran Birosca, Michael Preuss, Angus J. Wilkinson — 2010-01-11

We compare the dislocation substructure within macrozone and non-macrozone regions of hot-rolled Ti–6Al–4V. Hough-based and cross-correlation-based analysis of electron backscatter diffraction (EBSD) patterns are used to establish the grain orientations and intra-granular misorientations, respectively. The set of geometrically necessary dislocations (GNDs) that support measured lattice curvatures and minimize the total GND line energy are calculated. The GND content in the macrozone is approximately twice that in the non-macrozone region, and 〈a〉 GNDs are present at densities ∼10 times higher than 〈c+a〉 GNDs.

Strong orange–red light emissions from amorphous silicon nitride films grown at high pressures

2010-01-13

We observed very bright orange–red light emissions from amorphous silicon nitride prepared under high-pressure deposition conditions. By increasing the pressure from 300 to 700Pa, the photoluminescence (PL) intensity could be more than doubled. The introduction of N–H bonds is shown to play a key role in enhancing the PL intensity of the films. The improved PL intensity is attributed to the good hydrogen passivation of nonradiative defect states related to N under high growth pressures.

First-principles calculations of twin-boundary and stacking-fault energies in magnesium

Y. Wang, L.-Q. Chen, Z.-K. Liu, S.N. Mathaudhu — 2010-01-13

The interfacial energies of twin boundaries and stacking faults in metal magnesium have been calculated using first-principles supercell approach. Four types of twin boundaries and two types of stacking faults are investigated, namely, those due to the () mirror reflection, the () mirror glide, the () mirror reflection, the () mirror glide, the I1 stacking fault and the I2 stacking fault. The effects of supercell size on the calculated interfacial energies are examined.

Microstructure and texture in Ni50Mn29Ga21 deformed by high-pressure torsion

R. Chulist, W. Skrotzki, C.-G. Oertel, A. Böhm, T. Lippmann, E. Rybacki — 2010-01-18

The texture in a polycrystalline Ni50Mn29Ga21 magnetic shape-memory alloy deformed by high-pressure torsion (HPT) at 1173K was investigated with high-energy synchrotron radiation. HPT changed the extrusion texture to a predominant slightly rotated cube {001} 〈100〉 and a weak F {110}〈100〉 component. During HPT the grain structure coarsened. Phase transformations during cooling from the deformation temperature led to coarse and fine sets of martensitic twin variants. The results are discussed with respect to deformation mode, grain growth and phase transformations.

The rate-limiting step in the thermal oxidation of silicon carbide

2010-01-14

Using first-principles density-functional calculations of the total energy, we performed a systematic study of the diffusion activation energies of O2 and CO in SiO2 and Si1−xCxO2. Our results suggest that the dense Si1−xCxO2 (e.g., Si2CO6) layer may play a critical role in the SiC thermal oxidation process. The out-diffusion of CO through SiO2 or Si2CO6 is the controlling step of the SiC thermal oxidation. Known experimental data are explained well by our results.

The nature of grain boundaries in alumina fabricated by fast sintering

Jinyong Zhang, Fancheng Meng, Richard I. Todd, Zhengyi Fu — 2010-01-15

Fast sintering processes offer promising new routes for the fabrication of bulk ceramics but it is not clear that the microstructural details and properties resulting from a very short, rapid densification procedure are the same in all respects as those resulting from traditional sintering. Here, not fully relaxed grains and their weak mechanical properties of as-prepared nanoscale alumina are described. It was found that low temperature annealing did not cause grain growth but allowed normal boundaries to develop.

Green emission from ZnO nanorods: Role of defects and morphology

O.D. Jayakumar, V. Sudarsan, C. Sudakar, R. Naik, R.K. Vatsa, A.K. Tyagi — 2010-01-15

Highly luminescent and weak ferromagnetic ZnO nanorods were prepared by a facile solvothermal method. Luminescence studies carried out on as-synthesized ZnO nanorods, subjected to heat treatment in a variety of environments, established that Zn vacancies are responsible for the green emission. A significant change in morphology has been found on treating these as-synthesized nanorods in hydrogen, even at room temperature, which is attributed to the removal of oxygen from the already Zn-deficient ZnO lattice.

Role of initial {10−12} twin in the fatigue behavior of rolled Mg–3Al–1Zn alloy

Sung Hyuk Park, Seong-Gu Hong, Chong Soo Lee — 2010-01-15

Initial {10−12} twin influenced the plastic deformation mechanism activated during fatigue deformation, especially during compressive loading, by enhancing the activity of the detwinning mechanism. With the aid of initial {10−12} twin, compressive deformation could be fully accommodated by detwinning alone. This change in the predominant plastic deformation mechanism during compressive loading significantly decreased compressive flow stress, causing an increase in mean stress, and thus deteriorated the fatigue resistance of the material.

Substructures of martensite in Fe–1C–17Cr stainless steel

Hsin-Yi Lee, Hung-Wei Yen, Hsiao-Tzu Chang, Jer-Ren Yang — 2010-01-15

An Fe–1.0C–17Cr (wt.%) stainless steel was subjected to subzero treatment to investigate the structure of the martensite midrib. During the course of the isothermal holding in liquid nitrogen (−196°C), the thin-plate martensite formed first, and lenticular martensite later. The substructures of thin-plate martensites and lenticular martensite were examined using transmission electron microscopy, focusing on the details of the midrib. The results provide strong evidence to suggest that thin-plate martensite can be transformed into lenticular martensite.

Quantitative determination of free volume in Pd40Ni40P20 bulk metallic glass

Yuanli Xu, Jixiang Fang, Herbert Gleiter, Horst Hahn, Jiangong Li — 2010-01-18

For a long time, the determination of free volume has been a challenging problem in research on metallic glasses. An approach to determine quantitatively the free volume of metallic glasses from enthalpy measurements and calibration with the equilibrium free volume was developed and validated for as-cast and annealed Pd40Ni40P20 bulk metallic glasses. The free volume change with annealing time is in good agreement with that calculated theoretically from the free volume annihilation kinetics and that deduced from the density measurement results.

Improved plasticity of bulk metallic glasses upon cold rolling

M.H. Lee, K.S. Lee, J. Das, J. Thomas, U. Kühn, J. Eckert — 2010-01-18

The intrinsic plasticity of Zr44Ti11Cu9.8Ni10.2Be25 and Zr55Ti5Al10Cu20 Ni10 bulk metallic glasses (BMGs) are improved from 0.5% up to 15% plastic strain due to the introduction of microstructural inhomogeneities upon cold rolling at room temperature. This approach shows an easy way to overcome the intrinsic brittleness of the BMGs by modifying their physical properties, which enables easy nucleation and branching of multiple shear bands upon unconstrained loading during the compression test.

Glass formation enhanced by oxygen in binary Zr–Cu system

Y.X. Wang, H. Yang, G. Lim, Y. Li — 2010-01-18

We have found that a better glass former can be obtained in the Zr–Cu binary alloys with high oxygen content. However, the best glass former is shifted to different composition as oxygen content changes. Our findings provide an important insight into the mechanism of the oxygen on glass formation, which is a useful guideline for designing new BMGs with low-grade materials and low processing cost.

Low temperature coefficient of resistivity induced by magnetic transition and lattice contraction in Mn3NiN compound

Ying Sun, Cong Wang, Lihua Chu, Yongchun Wen, Man Nie, Fusheng Liu — 2010-01-18

We report the low temperature coefficient of resistivity (TCR) of antiperovskite Mn3NiN compound for the first time. Interestingly, the resistivity remained almost constant with temperature from 250K. The dρ/dT value and the TCR were determined to be about 7.17×10−8ΩcmK−1 and 12.3×10−5K−1, respectively. It was found that the low TCR behavior was related to the magnetic transition and large lattice contraction. The origin of low TCR behavior was further analyzed based on the Labbé–Jardin tight-binding approximation model.

Synthesis of hollow metallic particles via ultrasonic treatment of a metal emulsion

Dierk Raabe, Dennis Hessling — 2010-01-18

We introduce a synthesis method in which liquid metals in an emulsion mix and solidify under the influence of ultrasound radiation. We use it to produce two types of metallic nano- and microparticles. The first group, formed by rapid droplet solidification, is characterized by a nearly perfect spherical shape. The second group consists of hollow particles which were probably formed by near-adiabatic cooling of the metal inside an expanding cavitation bubble.

Influence of stacking fault energy and short-range ordering on dynamic recovery and work hardening behavior of copper alloys

Farzad Hamdi, Sirous Asgari — 2010-01-21

True stress vs. true strain responses of Cu–6wt.% Al and Cu–12wt.% Mn alloys are presented. While Cu–6wt.% Al alloy shows the typical mechanical response of low stacking fault energy alloys, the Cu–12wt.% Mn alloy behaved similarly to medium to high stacking fault energy alloys. These findings clearly show that while short-range ordering triggers slip planarity, it has a minor effect on total dynamic recovery of these copper alloys.

Competing processes in reactions between an edge dislocation and dislocation loops in a body-centred cubic metal

D. Terentyev, Yu.N. Osetsky, D.J. Bacon — 2010-01-25

Molecular dynamics simulation was used to investigate reactions of a edge dislocation with interstitial dislocation loops of and 〈100〉 type in a model of iron. Whether loops are strong or weak obstacles depends not only on loop size and type, but also on temperature and dislocation velocity. These parameters determine whether a loop is absorbed on the dislocation or left behind as it glides away. Absorption requires glide of a reaction segment over the loop surface and cross-slip of dipole dislocation arms attached to the ends of the segment: these mechanisms depend on temperature and strain rate, as discussed here.

In situ transmission electron microscopy study on microstructural changes in NbF5-doped MgH2 during dehydrogenation

2010-01-25

Microstructural changes in NbF5-doped MgH2 during the dehydrogenation reaction (MgH2→Mg+H2) have been investigated by in situ heating transmission electron microscopy. Nanocrystalline MgH2 shows fast hydrogen sorption kinetics, as well as a high density of defects induced by high-energy ball milling. Since the network-structured Nb layer covering Mg grains acts both as an impediment to grain growth of Mg and a gateway for hydrogen diffusion by forming metastable NbH1−x, the improved kinetics can be maintained during hydrogen cycling.

The diffraction patterns from β″ precipitates in 12 orientations in Al–Mg–Si alloy

Wenchao Yang, Mingpu Wang, Ruirong Zhang, Qian Zhang, Xiaofei Sheng — 2010-01-25

Various orientations and diffraction patterns of metastable β″ precipitates during artificial aging are investigated by means of high-resolution electron microscopy and electron diffraction in Al–Mg–Si alloys. Twelve orientations of β″ precipitates with C-centered monoclinic crystal structure are analyzed systematically. The orientation relationships can be expressed as: (010)β″//{100}Al; [001]β″//<310>Al; [100]β″//<230>Al. In addition, a new diffraction patterns model from β″ precipitates in 12 orientations is established under the [001]Al zone axis.

The effect of residual hydrogen on hydrogenation behavior of titanium thin films

E. Tal-Gutelmacher, A. Pundt, R. Kirchheim — 2010-01-25

The effect of residual hydrogen, sorbed during the deposition process, on the hydrogenation behavior of ion-beam sputtered titanium thin films was investigated. Electromotive force and in situ stress measurements were conducted to study hydrogen absorption, phase boundaries and hydrogen-induced stress development in the Ti–H thin film system. Tests were conducted on both as-sputtered and previously discharged films; the effect of residual hydrogen is significantly manifested in the thermodynamic isotherms and stress–concentration curves.

Effect of pre-strain at elevated temperature on strain hardening of twinning-induced plasticity steels

O. Bouaziz, S. Allain, Y. Estrin — 2010-01-25

The effect of different tensile pre-strains at 400°C on subsequent room-temperature deformation behaviour of a Fe–Mn–C twinning-induced plasticity steel has been investigated. We report a surprising finding that the strain hardening at a constant strain rate and ambient temperature depends solely on the flow stress irrespective of the strain history. An important consequence of this observation is that the rate of generation of twins is controlled by the flow stress and is not an explicit function of strain.

Numerical solution and comparison to experiment of solute drag models for binary alloy solidification with a planar phase interface

Shu Li, Jiong Zhang, Ping Wu — 2010-02-01

A generalized solute drag model for binary alloy solidification with a planar phase interface was proposed as an extension of Hillert–Sundman model. Using a new thermodynamic parameter set of the Si–As system, the present model can, with the introduction of three new adjustable parameters, fit the available experimental data of Si–9 at.% As alloy. In addition, non-solute drag models with the new set of thermodynamic parameters cannot reproduce the experimental data.

Comments on “Modelling differential scanning calorimetry curves of precipitation in Al–Cu–Mg”

M.J. Starink, Shuncai Wang — 2010-01-13

With several fitted model parameters, the Kampmann–Wagner (KW) model can be made to fit a differential scanning calorimetry curve of an Al–Cu–Mg alloy fairly well. However, this procedure will generally not be able to provide predictions of the coarsening kinetics of the precipitates. Past work has shown that a KW model can only work if the interfacial energy in the coarsening stage is much higher than that in the nucleation and growth stage. The KW model fails to accurately predict the impingement stage of the precipitation reaction.

Reply to “Comments on ‘Modelling differential scanning calorimetry curves of precipitation in Al–Cu–Mg’” by M.J. Starink and S. Wang

E. Hersent, J.H. Driver, D. Piot — 2010-01-13

We recently published a short paper on modelling DSC curves of AA 2024-T351 to which Starink and Wang have made a number of critical comments. We recognize that some features of our paper were not fully developed initially and we now appreciate the opportunity of clarifying these points.

Corrigendum to “Inducement of bainite and carbide transformation from retained austenite based on a high strain rate” [Scr. Mater. 62 (2010) 372–375]

2010-02-16

The author regrets that the captions for Figs. 3 and 4 on page 3 are incorrect. Please find the correct captions listed below: Fig. 3. (a) Microstructure of upper bainite (b) in which the carbide M7C3 precipitates with zone axis after impact at a strain rate of 1×103s−1.