C.L. Mendis

Microstructure, mechanical and corrosion properties of Mg-Dy-Gd-Zr alloys for medical applications.

In previous investigations, a Mg-10Dy (wt.%) alloy with a good combination of corrosion resistance and cytocompatibility showed great potential for use as a biodegradable implant material. However, the mechanical properties of Mg-10Dy alloy are not satisfactory. In order to allow the tailoring of mechanical properties required for various medical applications, four Mg-10(Dy+Gd)-0.2Zr (wt.%) alloys were investigated with respect to microstructure, mechanical and corrosion properties. With the increase in Gd content, the number of second-phase particles increased in the as-cast alloys, and the age-hardening response increased at 200°C. The yield strength increased, while the ductility reduced, especially for peak-aged alloys with the addition of Gd. Additionally, with increasing Gd content, the corrosion rate increased in the as-cast condition owing to the galvanic effect, but all the alloys had a similar corrosion rate (~0.5 mm year(-1)) in solution-treated and aged condition.

Microalloying Effect on the Precipitation Processes of Mg-Ca Alloys

The Mg-Ca binary alloys in the Mg-Mg2Ca two-phase region show little precipitation hardening by aging. However, the Mg-Ca alloys microalloyed with Al and Zn result in notable age hardening because of the formation of metastable, internally ordered, plate-like Guinier–Preston (GP) zones on the basal plane. To enhance the age-hardening response, we explored microalloying elements that can alter the habit plane from basal to prismatic. We found that an indium addition causes the homogeneous precipitation of thin plates on prismatic planes, resulting in a pronounced age-hardening response. Based on transmission electron microscopy and atom probe analysis results, we discuss the structure of the GP zones and the possible origin of the habit plane alternation.

Effect of laser power and specimen temperature on atom probe analyses of magnesium alloys

The influence of laser power, wave length, and specimen temperature on laser assisted atom probe analyses for Mg alloys was investigated. Higher laser power and lower specimen temperature led to improved mass and spatial resolutions. Background noise and mass resolutions were degraded with lower laser power and higher specimen temperature. By adjusting the conditions for laser assisted atom probe analyses, atom probe results with atomic layer resolutions were obtained from all the Mg alloys so far investigated. Laser assisted atom probe investigations revealed detailed chemical information on Guinier-Preston zones in Mg alloys.

Hierarchically organized Li–Al-LDH nano-flakes: a low-temperature approach to seal porous anodic oxide on aluminum alloys

This work suggests a low-temperature sealing approach for tartaric–sulfuric acid (TSA) anodized AA2024 based on hierarchically organized Li–Al-layered double hydroxide (LDH) structures. The new proposed sealing is expected to be directly competitive to the standard hot water sealing (HWS) approaches because of its reduced treatment temperature and high protection efficiency. A hierarchical organization of in situ formed LDH nano-flakes across the depth length of the TSA pores, from the macrodown to the nano-size range, was observed with transmission electron microscopy (TEM). Electrochemical impedance spectroscopy (EIS) studies showed that the densely packed LDH arrangement at the porous oxide layer is directly related to the drastically improved barrier properties of TSA. Moreover, LDH flake-like structures worked as “smart” reservoirs for corrosion inhibiting vanadium species (VOx) that are released on demand upon the onset of corrosion. This was confirmed using a scanning vibrating electrode technique (SVET), giving relevant insights into the time-resolved release activity of VOx and the formation of the passivation layer on cathodic intermetallics, corroborated with EDX and analytical Raman spectroscopy. Passive and active corrosion protection was imparted to the anodic layer via new Li–Al-LDH structures with long-term protection exceeding that of standard HWS procedures.

Melt Conditioning of Light Metals by Application of High Shear for Improved Microstructure and Defect Control

Casting is the first step toward the production of majority of metal products whether the final processing step is casting or other thermomechanical processes such as extrusion or forging. The high shear melt conditioning provides an easily adopted pathway to producing castings with a more uniform fine-grained microstructure along with a more uniform distribution of the chemical composition leading to fewer defects as a result of reduced shrinkage porosities and the presence of large oxide films through the microstructure. The effectiveness of high shear melt conditioning in improving the microstructure of processes used in industry illustrates the versatility of the high shear melt conditioning technology. The application of high shear process to direct chill and twin roll casting process is demonstrated with examples from magnesium melts.

Influence of Nd in Extruded Mg10Gd Base Alloys on Fatigue Strength

Magnesium alloys containing Rare Earth elements have proven to be suitable candidates for uses at high temperatures due to their good creep resistance as well as for use in biodegradable implants due to their adequate corrosion rate and biocompatibility. This work investigates the fatigue strength and cyclic deformation behavior of an extruded Mg10Gd1Nd in comparison to Mg10Gd and possible benchmark alloys WE43 and AZ31. The influence of the alloying element Nd is remarkable. The finite life fatigue strengths of Mg10Gd1Nd in the SN-diagram (Wöhler curve) are strongly improved compared to Mg10Gd and almost reach the strength values of WE43. Fracture surface morphology and crack propagation are discussed with attention given to low and high cycle fatigue. The very fine grain size, as the result of dynamic recrystallization during extrusion, offers high elongation at fracture. Therefore the residual fracture surface, where rapid failure occurs, is rather small in the high cycle fatigue samples. The size of the slow crack growth area has been determined by the appearance of benchmark ridges and fatigue striations and is discussed in correlation to stress and number of cycles. Scatter behavior of fatigue life was investigated by optical microcopy. The microstructure consists of second phase alignments in the extrusion direction, which differs in length, precipitate size and distance. Crack branching appears depending on microstructure and the load applied.

Corrosion and Creep Resistance of Thixomolded® Magnesium Alloys

Process optimization is one pathway to maximizing strength of a given alloy. Thixomolding® is a semi-solid casting process that combines pores reduction with a typical bimodal grain size distribution that can lead to enhanced strength. AZ91D and AZX911 were processed via Thixomolding® using two different processing conditions to change fraction solid of primary particles at the point of injection into the mould. The tensile properties, creep resistance and corrosion behaviour of the alloys were investigated. The creep resistance was measured in the range of 135–150 °C for stresses of 50–85 MPa. The corrosion behaviour was measured via hydrogen evolution for the two alloys and was smaller than that for die-cast AZ91. The AZX911 alloy showed improved creep resistance compared to the AZ91D. The differences in the property profile of the chosen alloys are correlated with their chemical compositions as well as with different microstructures obtained through the different processing conditions.

3D Microstructural Evolution on Solidifying Mg–5Nd–5Zn Alloy Observed via In Situ Synchrotron Tomography

In situ synchrotron tomography is a unique technique to study 3D microstructure evolution during solidification due to the high brilliance of the beam and the short acquisition time of the detector systems. In this work, in situ synchrotron tomographic observations were performed during the solidification of Mg–5Nd–5Zn (wt%) alloy with a cooling rate of 5 °C/min. The experiment was performed at the TOMCAT beamline of the Swiss Light Source (Paul Scherrer Institute (PSI), Villigen, Switzerland). The sample was melted using a laser-based heating system and then cooled until completely solidified. 3D tomograms were acquired during solidification. The microstructural analysis starts after the coherency point until the end of solidification. A differential thermal analysis (DTA) experiment was performed to estimate the liquidus and solidus temperature of the alloy. These values were used to correct the measured temperature from the in situ solidification experiment. Different microstructural parameters such as the volume fractions of the phases, i.e. α-Mg dendrites, interdendritics and pores, as well as the interconnectivity and skeletonization results are discussed.

As Solidified Microstructure Investigation of Mg15Y and MgxYyGd (x+y=15 wt.%) Ternary Alloys

MgxYyGd (x+y=15 wt.%) alloys were produced via permanent mould casting to investigate the microstructure evolution during solidification of the ternary system. The microstructure of the assolidified samples was characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In situ synchrotron radiation diffraction experiments were performed during the solidification of the alloys at the P07 (HEMS) Beamline of PETRA III at DESY. The phase evolution observed during controlled cooling at 20 and 100 K/min and the resultant microstructures were compared with the as-cast conditions. The experimental results were correlated with the calculations from the Pandat thermodynamic software. In the case of the ternary alloys the equilibrium phase diagram suggests the formation of the Mg24Y5 phase at elevated temperatures followed by the formation of the Mg5Gd phase at eutectic temperatures. However, the experiment shows only the formation of Mg24Y5 phase at eutectic temperatures even with a cooling rate (CR) of 100 K/min.

Hot tearing susceptibility of Mg-5Nd-xZn alloys

Magnesium-neodynium-zinc (Mg-Nd-Zn) alloys are promising candidates as creep resistant alloys. Further, Nd is a rare earth (RE) addition with lower solid solubility and a relatively lower cost. Hence, the use of such alloys may result in a feasible and cost effective alternative for enhancing Mg alloy use in high temperature applications. Nevertheless, studies on the castability of Mg-Nd-Zn alloys are lacking. As such, the aim of this research was to investigate the hot tearing susceptibility of Mg-5Nd-xZn (x = 0, 3, 5, 7 wt%) alloys during permanent mold casting. Specifically, a constrained-rod casting mold equipped with a load cell was used to characterize hot tearing severity and determine the onset temperature of hot tearing. The onset solid fraction of hot tearing was subsequently determined via thermodynamic software. The results suggest that hot tearing severity increased initially with addition of Zn (up to 5 wt%), but then decreased with further addition to 7 wt%. This was likely attributed to both the low onset solid fraction of hot tearing (i.e. 0.5) recorded for this alloy, which enabled enhanced feeding and opportunity to heal developing hot tears, as well as the divorced eutectic structure observed which may have facilitated late stage feeding of eutectic liquid and hence limit the alloy’s susceptibility to hot tearing.