Mert Celikin

Creep resistance in magnesium alloys

Abstract Mg creep resistant alloys have seen increased interest in the 1980s and 90s due to the weight reduction objectives of automotive companies. Development of Mg-Al based alloys with rare earth (RE) and alkaline earth element additions has led to the automotive application of Mg-6Al-2Sr (AJ62) alloy in the BMW engine block and the Mg-4Al-4RE (AE44) alloy in the engine cradle of the Corvette in 2002-2004. Most creep resistant Mg alloy development activities during this period emphasised the creation of stable grain boundary intermetallic phases in the cast microstructure. This elevated the creep performance of automotive Mg alloys to higher temperature and stress combinations (175°C, 70 MPa). Further improvement in creep performance can only arise from an in depth understanding of the creep mechanisms and the related microstructural interactions in Mg alloy systems. This paper gives an in depth review of creep mechanisms in Mg alloys and provides insight into alloy design principles for further development of creep performance in Mg.

Thermal exposure effects on the in vitro degradation and mechanical properties of Mg-Sr and Mg-Ca-Sr biodegradable implant alloys and the role of the microstructure.

Magnesium is an attractive biodegradable material for medical applications due to its non-toxicity, low density and good mechanical properties. The fast degradation rate of magnesium can be tailored using alloy design. The combined addition of Sr and Ca results in a good combination of mechanical and corrosion properties; the alloy compositions with the best performance are Mg-0.5Sr and Mg-0.3Sr-0.3Ca. In this study, we investigated an important effect, namely thermal treatment (at 400 °C), on alloy properties. The bio-corrosion of the alloys was analyzed via in vitro corrosion tests in simulated body fluid (SBF); the mechanical properties were studied through tensile, compression and three-point bending tests in two alloy conditions, as-cast and heat-treated. We showed that 8h of heat treatment increases the corrosion rate of Mg-0.5Sr very rapidly and decreases its mechanical strength. The same treatment does not significantly change the properties of Mg-0.3Sr-0.3Ca. An in-depth microstructural investigation via transmission electron microscopy, scanning electron microscopy, electron probe micro-analysis and X-ray diffraction elucidated the effects of the thermal exposure. Microstructural characterization revealed that Mg-0.3Sr-0.3Ca has a new intermetallic phase that is stable after 8h of thermal treatment. Longer thermal exposure (24h) leads to the dissolution of this phase and to its gradual transformation to the equilibrium phase Mg17Sr2, as well as to a loss of mechanical and corrosion properties. The ternary alloy shows better thermal stability than the binary alloy, but the manufacturing processes should aim to not exceed exposure to high temperatures (400 °C) for prolonged periods (over 24 h).

Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

Compressive Creep Behaviour of Cast Magnesium Under Stresses Above the Yield Strength and The Resultant Texture Evolution

Abstract Creep behaviour of commercial purity cast magnesium with preferred orientation was examined in compression at 100, 125 and 150°C and 35 and 50 MPa. The as-cast structure is composed of large columnar grains in which prismatic poles are parallel to the longitudinal direction. Cylindrical compression specimens were prepared from the cast material in the way that columnar grains were almost perpendicular to the compressive loading direction. Steady-state creep rate increased from -2 xl x 1 0-7/s to 5 x 10-7/s at 35 MPa and from 5.7 x 1O--7/s to 2 x 10- 6/s at 50 MPa with increasing temperature from 100 to 150 °C. The apparent activation energy, Qc, was determined to be 34 kJ/mol at 50 MPa and 29kJ/mol at 35 MPa. The stresses used were above the yield strength of the cast Mg and instantaneous plastic deformation occurred at creep loading. It was noticed that creep under the aforementioned conditions changed the initial grain orientation. Such evolution of orientation was stronger at higher temperatures. Some evidence of grain boundary serrations was also observed after creep and these were attributed to creep-deformation due to strain-induced grain boundary migration.

The nature of synthetic basic ferric arsenate sulfate (Fe(AsO4)1−x(SO4)x(OH)x) and basic ferric sulfate (FeOHSO4): their crystallographic, molecular and electronic structure with applications in the environment and energy

In this study we investigate a new family of arsenate-bearing phases belonging to one set of structures (Basic Ferric Sulfate-BFS: monoclinic–orthorhombic FeOHSO4) that are significant as an industrial arsenic control material in the environment or a cathodic material in rechargeable Li-ion battery cells. We determine for the first time (after two decades of its known existence in the processing industry) the average crystallographic structure of Basic Ferric Arsenate Sulfate-BFAS: Fe(AsO4)1−x(SO4)x(OH)x·(1−x)H2O) that is a member of this family of phases and how it relates to its parent BFS structure. Moreover, we demonstrate how the substitution of AsO4 ↔ SO4 affects the crystallographic structure of these phases, the phase(s) that are formed and their material properties as environmentally stable arsenic controls or Li-ion battery cathodes.

The effect of copper on the precipitation of scorodite (FeAsO4·2H2O) under hydrothermal conditions: Evidence for a hydrated copper containing ferric arsenate sulfate-short lived intermediate

The effect of copper sulfate on scorodite precipitation and its mechanism of formation at 150 °C was investigated. Scorodite was determined to be the dominant phase formed under all conditions explored (0.61 < Fe(III)/As(V) < 1.87, 0.27-0.30 M Fe(SO(4))(1.5), 0-0.3 M CuSO(4), 0-0.3 M MgSO(4), at 2.5 h and 150 °C). The produced scorodite was found to incorporate up to 5% SO(4) and ≤1% Cu or Mg in its structure. The precipitation of scorodite was stoichiometric, i.e. the Fe/As molar ratio in the solids was equal to one independent of the starting Fe/As ratio in the solution. The presence of excess ferric sulfate in the initial solution (Fe/As>1) was found to slow down the ordering of the H-bond structure in scorodite. Precipitation under equimolar concentrations (As = Fe = Cu = 0.3 M), short times and lower temperatures (30-70 min and 90-130 °C) revealed the formation of a Cu-Fe-AsO(4)-SO(4)-H(2)O short lived gelatinous intermediate that closely resembled the basic ferric arsenate sulfate (BFAS) type of phase, before ultimately converting fully to the most stable scorodite phase (96 min and 138 °C). This phase transition has been traced throughout the reaction via elemental (ICP-AES, XPS), structural (PXRD, TEM) and molecular (ATR-IR, Raman) analysis. ATR-IR investigation of an arsenic containing industrial residue produced during pressure leaching of a copper concentrate (1 h and 150 °C) found evidence of the formation of an arsenate mineral form resembling the intermediate basic ferric arsenate sulfate phase.

Development of regenerated fiber Bragg grating sensors with long-term stability

The effect of annealing cycle on regeneration efficiency was investigated through isothermal treatments between 700 and 1000°C. We determined an inverse relationship between the recovery rate of the peak reflectivity and temperature. A regeneration efficiency of 85.2% and long-term stability at 1000°C for 500 hours were achieved via a slow regeneration process. Thermal sensors developed by isothermal regeneration were determined to be reliable up to 1000°C (±2 °C). Experimental findings suggest the involvement of both diffusion related phenomena and stress variation through densification of the fiber core in type-I FBG during the thermal regeneration process.

Creep Resistant Mg–Mn Based Alloys for Automotive Powertrain Applications

Quaternary Mg–Sr–Mn–Ce alloys were developed for automotive powertrain applications. The design strategy was based on previous studies of the authors studies on the creep behaviour of subsystems (Mg–Mn, Mg–Ce–Mn, Mg–Sr–Mn) and the role of Mn in the dynamic precipitation of fine nano-scale dispersoids. In the present work, the creep resistance and the microstructural evolution of the selected quaternary Mg–Sr–Mn–Ce compositions were investigated. The final creep strain in the quaternary alloys was seen to be four times lower than the ternary Mg–Sr–Mn alloys creep tested at 200 °C under 50 MPa stress. The creep strengthening was attributed mainly to the dynamic co-precipitation of Mg12Ce and α-Mn phases.

The Effect of Ce Addition on the Creep Resistant Mg-Sr-Mn Alloys

Recent studies on Mg-Sr-Mn system exhibited the importance of α-Mn dynamic precipitation during creep on creep strengthening. In the present work, the effect of trace levels of Ce addition on the creep resistance of Mg-Sr-Mn system was investigated. The creep deformation in the quaternary alloys (Mg-Sr-Mn-Ce) were four times lower than the deformation seen in ternary alloys (Mg-Sr-Mn). Dynamic co-precipitation of Mg12Ce and α-Mn was mainly responsible for creep strengthening. Additionally, the presence of a trace amount of Sr in the Mg matrix affected the growth of the creep-induced Mg12Ce precipitates which were found to be lying parallel to the []Mg direction. This growth orientation is different from the orientation of Mg12Ce precipitates typically observed in Mg-Ce binary and Mg-Ce-Mn ternary alloys where plate-like precipitates lie along []Mg.