Mihriban Pekguleryuz

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.

Effects of lithium, indium, and zinc on the lattice parameters of magnesium

The lattice parameters of magnesium solid-solution alloys with lithium, indium, and/or zinc have been determined via x-ray diffraction (XRD). Li decreased the axial ratio (c/a) of Mg from 1.624 to 1.6068 within 0–16 at.% Li. Indium increased the c/a of Mg to 1.6261 with increasing In toward 3.3 at.% while Zn showed no effect on c/a in the 0.2–0.7 at.% range. The effects were explained by electron overlap through the first Brillouin zone and by Vegard’s Law. A relationship was determined between electron concentration (e/a) and c/a as c/a −15.6(e/a) + 60(e/a) − 55.8.

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).

Magnesium Die Casting Alloy AJ62x with Superior Creep Resistance, Ductility and Die Castability

Magnesium die casting alloys for elevated temperature applications are coming of age. Several research centers and companies have been working on alloy systems based on alkaline earth and rare earth alloying additions to push the limits for the creep performance of Mg-based die casting alloys. Norandas Mg-Al-Sr based alloys have shown superior creep performance and high-temperature performance at temperatures as high as 150-175C and stress levels of 50MPa - 70MPa. The most recent alloy formulation AJ62x (Mg-6Al-2Sr) has in addition shown excellent castability, and superior hot-tear resistance. Based on these attributes AJ62x is positioned well for applications such as transmission cases and oil pans. In this paper, the mechanical properties (creep and tensile) of AJ62x are presented. The high ductility of the AJ62Lx version is an added advantage for this alloy. Lab-scale evaluation of AJ62x shows that the alloy is more resistant to hot-tearing and cracking than all other magnesium alloys and the A380 aluminum alloy and industrial trials indicate that the alloy is highly castable.

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.

Hot tear susceptibility of aluminium–silicon binary alloys

Abstract Hot tear susceptibility (HTS) of Al–Si alloys in the 0–5–3·0 wt-%Si has been determined using a constrained rod casting test and a quantitative index. Hot tear susceptibility was seen to decrease with increasing silicon content. Hot tear severity varied directly with solidification shrinkageβ, the non-equilibrium freezing range ΔT′, dendrite arm spacing and the effective grain size d eff. The highest HTS was seen in the Al–0·5%Si alloy with the largest ΔT′, the highest β, the largest dendrite arm spacing, the lowest amount of eutectic phase f eu, and, hence, the longest local freezing time t f. Correlation was observed between HTS and the theoretical hot tear criteria Clyne–Davies CSC, Feurer and Campbell CSC b indices, with Campbells criterion showing a very strong correlation as HTS c≈0·03CSCb + 6 with R2=0·93, where HTS c is HTS of rod C.

Surface characterization, in vitro and in vivo biocompatibility of Mg-0.3Sr-0.3Ca for temporary cardiovascular implant.

Magnesium-based alloys are attractive candidate materials for medical applications. Our earlier work showed that the ternary Mg-0.3Sr-0.3Ca alloy exhibits slower degradation rates than both binary Mg-Sr and Mg-Ca alloys. The ternary alloy immersed in simulated body fluid (SBF) forms a compact surface layer of corrosion products that we hypothesized to be a Sr-substituted hydroxyapatite (HA). The main objectives of the current work are to understand the bio-degradation mechanism of Mg-0.3Sr-0.3Ca, to identify the exact nature of its protective layer and to evaluate the in vitro and in vivo biocompatibility of the alloy for cardiovascular applications. To better simulate the physiological environment, the alloy was immersed in SBF which was daily refreshed. Raman spectroscopy and X-Ray photoelectron spectroscopy (XPS) confirmed the formation of a thin, Sr-substituted HA layer at the interface between the alloy and the corrosion products. In vitro biocompatibility evaluated via indirect cytotoxicity assays using HUVECs showed no toxicity effect and ions extracted from Mg-0.3Sr-0.3Ca in fact increased the viability of HUVECs after one week. In vivo tests were performed by implanting a tubular Mg-0.3Sr-0.3Ca stent along with a WE43 control stent into the right and left femoral artery of a dog. Post implantation and histological analyses showed no thrombosis in the artery with Mg-0.3Sr-0.3Ca stent after 5weeks of implantation while the artery implanted with WE43 stent was extensively occluded and thrombosed. Microscopic observation of the Mg-0.3Sr-0.3Ca implant-tissue interface confirmed the in situ formation of Sr-substituted HA on the surface during in vivo test. These results show that the interfacial layer protects the surface of the Mg-0.3Sr-0.3Ca alloy both in vitro and in vivo, and is the key factor in the bio-corrosion resistance of the alloy.

Magnesium Diecasting Alloys for High Temperature Applications

New growth area for automotive use of magnesium is powertrain applications such as the transmission case and engine block. These applications see service conditions in the temperature range of 150–200C under 50–70 MPa of tensile and compressive loads. In addition, metallurgical stability, fatigue resistance, corrosion resistance and castability requirements need to be met. A decade of research and development has resulted in a number of creep- resistant magnesium alloys that are potential candidates for elevated-temperature automotive applications. These alloys are mostly based on rare-earth and alkaline earth element additions to magnesium. This paper gives an overview of the various magnesium alloy systems for use in elevated-temperature applications.

Effect of Sb on wetting behavior of near eutectic Sn-Cu solder micro-alloyed with Ni and Ge

Purpose The purpose of this paper is to investigate the effect of Sb (0, 0.2 and 2 wt.%) on wetting performance of lead-free solder of near eutectic Sn-Cu micro-alloyed with Ni and Ge. Design/methodology/approach The melting characteristic of the lead-free alloys was studied using differential scanning calorimetry. Wettability was examined using wetting balance test for two liquid fluxes, water based and alcohol based in two temperatures 265°C and 277°C. Also, contact angle was measured using sessile drop test. Findings It is shown that 0.2 wt.% Sb reduces the melting temperature and pasty range. Moreover, the addition of 0.2 wt.% Sb improves wetting behavior for alcohol-based flux. It is also demonstrated that the effect of Sb on meniscus height in wetting balance test and contact angle in sessile drop test follows the trend of wetting performance. Originality/value It is found that adding 0.2 wt.% Sb improves the wettability of Ni-Ge micro-alloyed Sn-Cu solder; however, higher concentration of Sb does not benefit the alloy.

Development of Creep Resistant Mg‐Al‐Sr Alloys

There have been attempts since in the 70’s to develop creep resistant magnesium diecasting alloys for automotive applications such as automatic-transmission case and engine components. The earliest die casting alloys developed as a result of these activities were the Mg-Al- RE and Mg-Al-Si systems (AE and AS alloys). The shortcomings of these two alloy systems related to high cost or borderline properties have led to renewed activity in the 90’s in the development of magnesium alloys with improved elevaied-temperature properties. This paper presents the development of a new family of creep-resistant Mg alloys based on the Mg-Al-Sr system. Creep resistance, the tensile yield strength and the bolt-load-retention of these alloys at 150°C and 175°C show improvement over Mg-Al-RE and Mg-Al-Si system. The microstructure of the alloys is characterized by Al-Sr-(Mg) containing intermetallic second phases. The absence of the Mg17Al12 phase in the microstructure, either creep-induced or as-cast, is one of the factors that contribute to improved creep-resistance of these alloys over the Mg-Al based diecasting alloys. Furthermore, the alloys exhibit better salt-spray corrosion resistance (0.09-0.15mg/cm2/day) than other commercial magnesium diecasting alloys such as AM60B, AS41, AE42 and the aluminum diecasting alloy A380.