G. Dirras

Microstructure and mechanical behavior of ultrafine-grained Ni processed by different powder metallurgy methods

Ultrafine-grained samples were produced from a Ni nanopowder by hot isostatic pressing (HIP) and spark plasma sintering (SPS). The microstructure and mechanical behavior of the two specimens were compared. The grain coarsening observed during the SPS procedure was moderated due to a reduced temperature and time of consolidation compared with HIP processing. The smaller grain-size and higher nickel-oxide content in the SPS-processed sample resulted in a higher yield strength. Compression experiments showed that the specimen produced by SPS reached a maximal flow stress at a small strain, which was followed by a long steady-state softening while the HIP-processed sample hardened until failure. It was revealed that the softening of the SPS-processed sample resulted from microcracking along the grain boundaries.

Elastic properties of β-SiC films by Brillouin light scattering

Brillouin light scattering has been used to investigate elastic properties of a monocrystalline and of 〈111〉 textured polycrystalline 3C polytype silicon carbide films that have been deposited on silicon substrate by chemical vapor deposition. Taking advantage from the detection of different acoustic modes, a complete elastic characterization of the films has been achieved. The three unknown elastic constants of the monocrystalline 3C-SiC, namely, c11=395 GPa, (c11−c12)/2=136 GPa, and c44=236 GPa have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes traveling parallelly to the film surface. These determinations are in agreement with the frequency of the observed Rayleigh surface mode, of the pseudosurface mode, and of the bulk waves propagating at different angles from the normal of the single crystal film plane and consistent with existing theoretical calculations of β-SiC elastic constants. Finally, the calculated Voigt average valu...

Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity

ABSTRACT A new bcc Ti-rich high-entropy alloy (HEA) of composition Ti35Zr27.5Hf27.5Nb5Ta5 was designed using the ‘d-electron alloy design’ approach. The tensile behavior displays a marked transformation-induced plasticity effect resulting in a high normalized work-hardening rate of 0.103 without loss of ductility when compared to the reference composition Ti20Zr20Hf20Nb20Ta20. In this paper, a detailed microstructural analysis was performed to understand the deformation process, revealing architectural-type microstructures and a high volume fraction (65%) of internally twinned stress-induced martensite α″ after mechanical testing. This study opens the way to mechanical properties optimization and enhancement of titanium-based HEAs by combining multiple alloying designs. IMPACT STATEMENT For the first time, proof is given that transformation-induced plasticity was triggered in a bcc refractory high-entropy alloy, leading to a twofold increase in the normalized work-hardening rate. GRAPHICAL ABSTRACT

An Approach of Precipitate/Dislocation Interaction in Age-Hardened Al-Mg-Si Alloys: Measurement of the Strain Field around Precipitates and Related Simulation of the Dislocation Propagation

TEM study of deformed samples complemented by a new approach combining image analysis and simulation of the dislocation motion have been carried out to study the precipitate /dislocation interaction in Al-Mg-Si alloys (AA-6XXX). The analysis of HRTEM images allows a direct measurement of the strain field around precipitates and is further introduced in the simulation of dislocation propagation. In the case studied here, the simulation indicates that for an applied stress close to the yield stress, dislocations’ motion in the matrix occurs by both the by-pass of precipitates through the activation of the Orowan mechanism and the shearing of precipitates. This is in agreement with TEM observations on deformed samples showing numerous dislocations loops along with laths shearing

Macroscopic behaviour versus dislocation substructures development under cyclic shear tests on the aluminium–3004 alloy

Abstract Cyclic shear tests of various amplitudes have been carried out on aluminium–3004 alloys. The macroscopic behaviour shows the occurrence of either cyclic softening or cyclic hardening, depending on the initial state of the alloy (recrystallised, recovered or extra-hardened). This macroscopic behaviour is discussed with the help of both isotropic and kinematic concepts and in connection with the texture, the development of dislocation substructures and its evolution upon straining.

High performance Ti-6Al-4V alloy by creation of harmonic structure design

Ti-6Al-4V alloy is an advanced structural material having applications in a wide range of areas spanning from biomedical to aerospace sectors due to the excellent combination of mechanical and chemical properties. In the present work, a new tailored heterogeneous microstructural design with a specific topological distribution of fine and coarse grained areas, called harmonic structure, has been proposed for the strengthening of Ti-6Al-4V alloy to achieve improved performance of the components in service. It has been demonstrated that Ti-6Al-4V alloy with harmonic structure can be successfully prepared via a powder metallurgy route consisting of controlled severe plastic deformation of pre-alloyed powders via mechanical milling followed by their consolidation. The Ti-6Al-4V compacts with harmonic structure design exhibited significantly better strength and ductility, under quasi-static as well as rapid loading conditions, as compared to their homogeneous fine and coarse grained counterparts. It was found that the harmonic structure design has the ability to promote the uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding localized plastic instability.

Bulk Ultrafine-Grained Nickel Consolidated from Nanopowders

Ultrafine-grained samples were consolidated from Ni nanopowders with the nominal particle size of 50 and 100 nm by Hot Isostatic Pressing (HIP) and Spark Plasma Sintering (SPS). The higher nickel-oxide content and the smaller grain size of SPS-processed samples result in a higher yield strength at room temperature compared with HIP-processed specimen. It is found that during compression the dislocation density increases while the twins decay in both samples, indicating that the deformation is mediated mainly by dislocations. As a consequence of the higher oxide content, the flow stress of the SPS-processed samples saturates at small strain values while the HIP-processed specimen shows strain hardening even at the strain value of 0.35. After saturation of the flow stress for SPS-processed samples the deformation is most probably mediated rather by grain rotation or grain boundary-related mechanisms such as sliding and/or decohesion instead of dislocation motion.

Influence of Triaxial Stress State on Ductile Fracture Strength of Polycrystalline Nickel

In this contribution, the effect associated with stress triaxiality on ductile damage evolution in high purity nickel has been investigated from both experimental and theoretical points of view. Tensile tests on smooth and notched round bar specimens were performed to calibrate the fracture strain in a wide range of stress triaxiality. The capability of the Gurson model to reproduce and predict physical failure behaviour was examined. It was shown that stress triaxiality played a major role on damage evolution as demonstrated by the progressive reduction of material ductility under increasing triaxial states of stress.

Microstructure and Mechanical Properties of Commercial Purity HIPed and Crushed Aluminum

Ultrafine-grained aluminum microstructures were processed from commercial purity powder by combining hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2µm. The material was then subsequently impacted, using a falling weight at a strain rate of 300s-1. The resulting material showed a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture parallel to the impact direction. The mechanical properties of the impacted material were subsequently characterized under compressive tests at room temperature at a strain rate of 10-4s-1. The effect of the change of the deformation path on the mechanical response parallel (DN) and perpendicular (DT) to the impact direction was also investigated. These results are here discussed in relation with microstructure and texture evolution.

Data on processing of Ti-25Nb-25Zr β-titanium alloys via powder metallurgy route: Methodology, microstructure and mechanical properties

The data presented in this article are related to the research article entitled “Cyclic Shear behavior of conventional and harmonic structure-designed Ti-25Nb-25Zr β-titanium alloy: Back-stress hardening and twinning inhibition” (Dirras et al., 2017) [1]. The datasheet describes the methods used to fabricate two β-titanium alloys having conventional microstructure and so-called harmonic structure (HS) design via a powder metallurgy route, namely the spark plasma sintering (SPS) route. The data show the as-processed unconsolidated powder microstructures as well as the post-SPS ones. The data illustrate the mechanical response under cyclic shear loading of consolidated alloy specimens. The data show how electron back scattering diffraction(EBSD) method is used to clearly identify induced deformation features in the case of the conventional alloy.