The Experts below are selected from a list of 219 Experts worldwide ranked by ideXlab platform

B V Cockeram - One of the best experts on this subject based on the ideXlab platform.

  • the role of stress state on the fracture toughness and toughening mechanisms of wrought Molybdenum and Molybdenum Alloys
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2010
    Co-Authors: B V Cockeram
    Abstract:

    Rolling unalloyed Molybdenum, Molybdenum Alloys, and Oxide Dispersion Strengthened (ODS) Molybdenum into sheet produces microstructures with elongated, pancake shaped grains that can result in anisotropic mechanical properties. In this work, unalloyed Molybdenum, Molybdenum Alloys, and ODS Molybdenum are rolled to thinner sheet and then subjected to tensile and fracture toughness testing and examination of the toughening mechanism. The ductile laminate toughening mechanism observed for wrought Molybdenum results from a lower toughness in the short-transverse orientation that leads to separation of the layers of sheet-like grains of the microstructure along the grain boundaries in the regions of stress concentration. This splitting of the microstructure results in the formation of ligaments of grains, or non-constrained laminates, that are stretched to failure under a plane stress-state with large amounts of plastic deformation. The thinner specimens exhibit higher fracture toughness values and lower Ductile to Brittle Transition Temperature (DBTT) values than for thicker specimens machined from thicker starting material from the same alloy. The lower constraint of the thinner specimens tested in this work results in higher toughness and lower DBTT values. The finer grain size, finer precipitate size, and state of plane stress achieved for the thinner sheet specimens appears to enhance the ductile laminate toughening to result in higher fracture toughness and lower DBTT values. The detrimental effect of crack initiation from brittle carbides, oxides, and second phases is also observed to be diminished under a stress-state of plane stress.

  • the mechanical properties and fracture mechanisms of wrought low carbon arc cast lcac Molybdenum 0 5pct titanium 0 1pct zirconium tzm and oxide dispersion strengthened ods Molybdenum flat products
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006
    Co-Authors: B V Cockeram
    Abstract:

    Abstract Molybdenum Alloys such as low carbon arc cast (LCAC) unalloyed Molybdenum, oxide dispersion strengthened (ODS) Molybdenum, and Molybdenum–0.5pct titanium–0.1pct zirconium (TZM) Molybdenum are of interest for structural applications at high temperatures, but these Alloys are poorly characterized with respect to fracture toughness and the ductile to brittle transition temperature (DBTT) in the presence of a notch. Both tensile and fracture toughness testing of these flat rolled Molybdenum Alloys at temperatures above the DBTT are shown to produce a ductile laminate fracture mechanism, where cracks initiate along grain boundaries in the region of triaxial stresses to leave ligaments of sheet-like grains that are stretched to failure with a high degree of plasticity. The DBTT determined from toughness testing is 50–200 °C higher than determined from tensile testing, which shows the constraining effect of the notch. Use of the J -integral test method provided a more consistent and accurate measure of fracture toughness values at temperatures above the DBTT where large amounts of plasticity are observed. A transition was observed from toughness values between 5.8 and 29.6 MPa√m at temperatures below the DBTT to toughness values between 45 and 175 MPa√m for LCAC, 40–215 MPa√m for TZM, and 53–205 MPa√m for ODS. The variation in fracture toughness values at temperatures > DBTT is shown to correlate with size and number density of the ductile laminate features, where high fracture toughness values result from a fine laminate spacing. Since a finer grain size results in a smaller laminate size, the lower DBTT observed for fine grained ODS Molybdenum can be understood in terms of the ductile laminate failure mode.

G P Mishra - One of the best experts on this subject based on the ideXlab platform.

  • development preparation and characterization of uranium Molybdenum Alloys for dispersion fuel application
    Journal of Alloys and Compounds, 2009
    Co-Authors: V P Sinha, G J Prasad, P V Hegde, R Keswani, C B Basak, G P Mishra
    Abstract:

    Abstract Most of the research and test reactors worldwide have undergone core conversion from high enriched uranium base fuel to low enriched uranium base fuel under the Reduced Enrichment for Research and Test Reactor (RERTR) program, which was launched in the late 1970s to reduce the risk of nuclear proliferation. To realize this goal, high density uranium compounds and γ-stabilized uranium alloy powder were identified. In Metallic Fuels Division of BARC, R&D efforts are on to develop these high density uranium base Alloys. This paper describes the preparation flow sheet for different compositions of Uranium and Molybdenum Alloys by an innovative powder processing route with uranium and Molybdenum metal powders as starting materials. The same composition of U–Mo Alloys were also fabricated by conventional method i.e. ingot metallurgy route. The U–Mo Alloys prepared by both the methods were then characterized by XRD for phase analysis. The photomicrographs of Alloys with different compositions prepared by powder metallurgy and ingot metallurgy routes are also included in the paper. The paper also covers the comparison of properties of the Alloys prepared by powder metallurgy and ingot metallurgy routes.

Ulrich Martin - One of the best experts on this subject based on the ideXlab platform.

  • Investigation of solid solution hardening in Molybdenum Alloys
    International Journal of Refractory Metals & Hard Materials, 2010
    Co-Authors: I. Wesemann, Tobias Mrotzek, Andreas Hoffmann, Ulrich Martin
    Abstract:

    Mechanical properties of pure Molybdenum like yield strength, hardness and creep strength can be modified by alloying elements. The alteration of properties can be described as a result of different mechanisms like solid solution hardening (SSH), particle as well as grain boundary hardening. The aim of this work is to quantify the solid solution hardening effect of different solutes. Therefore the influences of other strength affecting mechanisms like grain boundary hardening or particle hardening have been excluded. Based on the powder metallurgical manufacturing route different Molybdenum Alloys with Cr, Re, Ta, Ti or W additions were prepared. Vickers hardness measurements at room temperature and tensile tests at 500 °C were performed to quantify the hardening effect. Additionally the modulus of rigidity was measured by ultrasonic inspection and the dynamical resonance method. Lattice parameters were measured by means of X-ray diffraction experiments. Applying existing theories of solid solution hardening an attempt was made to separate and quantify dielastic and parelastic effects.

  • hardening mechanisms and recrystallization behaviour of several Molybdenum Alloys
    International Journal of Refractory Metals & Hard Materials, 2006
    Co-Authors: Tobias Mrotzek, Andreas Hoffmann, Ulrich Martin
    Abstract:

    Abstract The present study describes the effect of thermomechanical treatments on the microstructure of Molybdenum–titanium and the Molybdenum alloy TZM (0.5 wt.% Ti, 0.08 wt.% Zr). These Alloys were processed by a powder metallurgical route including extensive rolling at temperatures above 1200 °C achieving deformation degrees of 2.87 (logarithmic). The Alloys exhibit a distinct increase in yield stress without decreasing elongation at fracture. To study the recrystallization behaviour, several samples were annealed at temperatures between 1100 °C and 1600 °C for 15 min until 6 h. The microstructure was examined by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). Strain rate controlled tensile tests and Vickers hardness measurements are used to determine the mechanical properties. In both Alloys a distinct substructure containing subgrains is formed during deformation. A kind of (sub)grain refinement was considered to be responsible for hardening. Present particles are able to raise the recrystallization temperature and thus sustain the grain size effects even at working temperatures above 1200 °C.

Guo Jun Zhang - One of the best experts on this subject based on the ideXlab platform.

  • Microstructure and Mechanical Properties of Molybdenum Alloys Doped with ZrB2 Particles
    Advanced Materials Research, 2010
    Co-Authors: Guo Jun Zhang, Jun Zhuo
    Abstract:

    The Molybdenum Alloys doped with various mass fraction zirconium diboride (ZrB2) particles were successfully prepared by using the powder metallurgy technology. The microstructure, hardness and the tensile properties of the pure Molybdenum and Molybdenum Alloys doped with ZrB2 particles were determined at room temperature. Results showed that the addition of zirconium diboride refined the Molybdenum grains obviously, meanwhile, Molybdenum grain sizes decrease with the increasing of zirconium diboride mass fraction. Measurements on mechanical properties showed that Molybdenum Alloys doped with ZrB2 particles had higher hardness and strength than pure Molybdenum. The analysis and discussion results show that the strengthening mechanisms of ZrB2-doped Molybdenum Alloys may be attributed to the fine-grain strengthening, particle dispersion strengthening, interfacial-bond strengthening and oxygen reduction strengthening.

  • microstructure and mechanical properties of multi components rare earth oxide doped Molybdenum Alloys
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
    Co-Authors: Guo Jun Zhang
    Abstract:

    Pure Molybdenum and Molybdenum Alloys doped with two- or three-components rare earth oxide particles were prepared by powder metallurgy. Both the tensile property and fracture toughness of the pure Molybdenum and multi-components rare earth oxide-doped Molybdenum Alloys were determined at room temperature. The multi-components rare earth oxide-doped Molybdenum Alloys are fine grained and contain a homogeneous distribution of fine particles in the submicron and nanometer size ranges, which is why the Molybdenum Alloys have higher strength and fracture toughness than pure Molybdenum. Quantitative analysis is used to explain the increase in yield strength with respect to grain size and second phase strengthening. Furthermore, the relationship between the tensile properties and microstructural parameters is quantitatively established.

  • Microstructure and Tensile Properties of Oxide Dispersion Strengthened Molybdenum Alloys Prepared by Different Doping Process
    Key Engineering Materials, 2007
    Co-Authors: Guo Jun Zhang
    Abstract:

    The oxide lanthanum dispersion strengthened Molybdenum Alloys were prepared by proprietary powder metallurgy technology, in which the Mo-La2O3 powders were prepared by liquid-liquid doping process which the oxide lanthanum was added to ammonium bi-molybdate solutions as aqueous solutions of La(NO3)3 and liquid-solid doping process which the oxide lanthanum was added to Molybdenum oxide solid particles as aqueous solutions of La(NO3)3, respectively. The microstructure and tensile properties of the Molybdenum Alloys were investigated at room temperature. The results show that the Molybdenum Alloys all have fine Molybdenum grains, and the Molybdenum alloy prepared by liquid-solid doping process mainly contain fine oxide lanthanum particles of submicron and nano-sized while the alloy prepared by liquid-liquid doping process mainly contain nano-sized fine oxide lanthanum particles. The Molybdenum Alloys prepared by liquid-liquid doping process have higher yield strength and ductility than yield-solid doping process. The results of strengthen mechanism analysis show that the high strength of the Molybdenum Alloys can be advisablely explained by the fine grain strengthening and particles dispersion strengthening mechanism through the Hall–Petch relationship and Orowan model.

Arthur J Freeman - One of the best experts on this subject based on the ideXlab platform.

  • electronic origin of solid solution softening in bcc Molybdenum Alloys
    Physical Review Letters, 2005
    Co-Authors: N I Medvedeva, Yu N Gornostyrev, Arthur J Freeman
    Abstract:

    : The intrinsic mechanism of solid solution softening in bcc Molybdenum Alloys due to 5d transition metal additions is investigated on the basis of ab initio electronic-structure calculations that model the effect of alloying elements on the generalized stacking fault (GSF) energies. We demonstrate that additions with an excess of electrons (Re, Os, Ir, and Pt) lead to a decrease in the GSF energy and those with a lack of electrons (Hf and Ta) to its sharp increase. Using the generalized Peierls-Nabarro model for a nonplanar core, we associate the local reduction of the GSF energy with an enhancement of double kink nucleation and an increase of the dislocation mobility, and we reveal the electronic reasons for the observed dependence of the solution softening on the atomic number of the addition.