L10 nanotwinned gold-rich Au–Cu–Pt

Ternary Au-Cu-Pt alloys, of unequal gold and copper atomic contents and containing 2.5 wt.% Pt, are ordered isothermally at 250 or 400 degrees C after annealing at 650 degrees C and quenching in water. The alloys are characterized by transmission electron microscopy, X-ray diffraction, hardness and tensile testing. With 75 and 76.5 wt.% Au, the microstructural evolution depends on the ordering temperature. At 400 degrees C, a classical nanotwinned "polytwin" structure with, locally, two L1(0) crystal orientation variants is formed; this evolves into a two-phase, L1(0) + Al, checkerboard-like microstructure after 10(6) s. At 250 degrees C, ordering develops a stable (up to 10(5) s) structure locally containing all three L1(0) crystal variants arranged in a network of {1 1 0} twins roughly 30 nm wide. At both ordering temperatures the 78 wt.% Au alloy develops a similar three-variant nanotwinned structure that also remains stable up to 10(5) s. The 75 and 76.5 wt.% Au alloys display a peak in hardness after roughly 120 s at both 400 degrees C and 250 degrees C; with 78 wt.% Au, peak hardness is not reached at 10(5) s. With all three alloys, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250 degrees C compared with the more classical polytwin structure that develops at 400 degrees C. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Microstructure and Mechanical Behaviour of Ordered Au-Cu-Pt Alloys

Edwina Klay

This thesis investigates nine new compositions of the ternary Au-Cu-Pt system, containing from 75 to 78 wt% Au with 0 to 15 wt% Pt, by means of in-situ synchrotron radiation X-ray diffraction, differential scanning calorimetry, transmission electron microscopy and mechanical tests, including hardness, tensile and repeated stress-relaxation testing. X-ray diffraction data show that the alloys can be separated into three groups according to their stable low-temperature ordered phase(s), namely L10, L12, or the two combined. Platinum increases transformation temperatures in comparison with binary AuCu, two-phase ordered alloys showing the highest transformation temperatures. Details of the evolution of the peak structures upon heating and cooling point to significant differences between mechanisms of disordering and ordering: whereas ordering visibly proceeds at significant undercooling by nucleation and growth, disordering appears, from the visible shifts in peak position, to progress in more homogeneous fashion within the alloy. Alloys containing 2.5 wt% Pt show different microstructural evolutions when ordered at 250 or 400 °C after annealing at 650 °C and a water quench. With 75 and 76.5 wt% Au, the microstructural evolution depends on the ordering temperature. At 400 °C, a classical polytwin structure with two L10 crystal orientation variants is formed; this evolves into a two-phase, L10 + A1, checkerboard-like microstructure after 106 s. At 250 °C, ordering develops a stable (up to 105 s) structure containing all three L10 crystal variants and a network of perpendicular {110} twins roughly 30 nm wide. The 78 wt %Au alloy develops at both ordering temperatures a similar three-variant nanotwinned structure that also remains stable up to 105 s. With all three alloys containing 2.5 wt% Pt, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250 °C compared with the more classical polytwin structure that develops at 400 °C. Activation volumes characteristic of room-temperature compressive plastic deformation in Au- 21 wt% Cu- 2.5 at% Pt, are measured using the repeated stress-relaxation method on the classical polytwin and the three-variant structures. All ordered nanotwinned microstructures obtained have an initial effective activation volume Veff of 100 to 120 b3; this is values typical of cross-slip or twin boundary bypass in FCC metals. The two structures differ strongly as they work-harden: the two-variant polytwin structure yields a straight line on a Haasen plot, suggesting that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a constant activation volume. This suggests the presence of a mechanism by which obstacles that accumulate during deformation of the two-variant structure can be removed when three variants are present; a possible mechanism is debris elimination by intraboundary glide coupled with triple line leakage or loop annihilation. Alloys containing 7.5 or 15 wt% Pt have microstructures in which the L12 phase is found, either alone or in combination with L10. Whereas fully L12 alloys (of higher Pt content) are relatively soft, two-phase alloys are nearly as hard and ductile as fully L10 alloys, and feature relatively stable structures, which TEM observation suggests consist in several variants of the L10 phase embedded in a matrix of L12.

EPFL2010

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008

Evaluation of the mechanical properties of the heat treated FeCrCuMnNi high entropy alloy

Roland Logé, Ali Shabani

As cast FeCrCuMnNi high entropy alloys were heat treated at different temperatures and times. Structure and mechanical properties of the heat treated alloy were studied using X-Ray diffraction, tensile test, hardness test and scanning electron microscopy. It has been found that the FeCrCuMnNi has thermal stability in mechanical properties almost up to 600 °C. Nevertheless, the alloy revealed an unusual behavior after heat treating at higher temperatures. A sharp drop of the tensile strength, strain to fracture and work hardening capability occurred at temperatures between 700 °C and 800 °C due to formation of a hard σ phase in the alloy. The hardness follows a plateau in the 700–800 °C range and then softening continues, together with a decrease in strength and increase in ductility, due to dissolution of the hard σ phase and decreased amount of BCC phase.