Quantcast


CURATOR
A pinboard by
Sravya Tekumalla

Research Scholar , National University of Singapore

PINBOARD SUMMARY

A light weight material suiting the requirements of the aerospace and military applications

Reduction in the weight of automobiles, aircrafts and rockets in the transportation sector accounts for a pronounced increase in the fuel efficiency is an assertive route to restrict the emissions (constituting up to 14% in 2010 & 26% in 2014) . Replacement of the currently dominant heavier aluminum, titanium alloys and steels will turn out to be a game changer in this field, primarily in view of the coarsening climate change. Magnesium is one of the lightest available structural metals available on the earth's crust. Magnesium alloys have been trending of late in automobile, aerospace, defense, sports, electronic and biomedical sectors as they offer an advantage in light-weighting. In the aluminum, titanium and steel dominated aerospace and defense sectors, applications of Mg are banned/restricted until recently due to its perceived easy ignition and inability to self-extinguish, reason for usage of magnesium in pyro applications like fireworks. This easy ignition inhibits usage of magnesium rampantly. Further, mechanical strength is generally inversely related to ductility and ignition resistance, rendering it challenging to optimize all three concurrently. We address this challenge by designing a low density (~ 1.76 g/cc) Mg nanocomposite developed in-situ. This Mg nanocomposite is a result of a sequence of in-situ reactions that occur during the melt processing and extrusion. It exhibits a desirable combination of properties, outperforming the other traditional commercial materials. This renders magnesium to replace currently used heavier materials, thus contributing to the reduction of the green house gas emissions and thus enabling a cleaner earth.

31 ITEMS PINNED

Microwave Assisted Magnesium Phosphate Coating on AZ31 Magnesium Alloy.

Abstract: Due to the combination of many unique properties, magnesium alloys have been widely recognized as the suitable metallic material for degradable biomedical implants. However, extremely high degradation kinetics of magnesium alloys in physiological environment have hindered their clinical applications. This paper reports for the first time of using a novel microwave assisted coating process to deposit magnesium phosphate coatings on Mg alloy AZ31 and improve its the in vitro corrosion resistance. Results present that newberyite layer and trimagnesium phosphate hydrate (TMP) layer with distinct features were fabricated via various processing time and temperature. Afterwards, the corrosion resistance, degradation behavior, bioactivity and cytocompatibility of the magnesium phosphate coated AZ31 samples were investigated. The potentiodynamic polarization tests reveal that the corrosion current density of AZ31 magnesium alloy in SBF is significantly suppressed by the deposited magnesium phosphate coatings. Additionally, it is seen that magnesium phosphate coatings remarkably reduced the mass loss of AZ31 alloy after immersion in SBF for two weeks and promoted precipitation of apatite particles. The high viability of preosteoblast cells cultured with extracts of coated samples, indicates that the magnesium phosphate coatings can improve the cytocompatibility of AZ31 alloy. All of these attractive results suggest that magnesium phosphate coatings, serving as the protective and bioactive layer can enhance the corrosion resistance and biological response of magnesium alloy. &#13.

Pub.: 13 Jun '17, Pinned: 31 Aug '17

Analysis of the solidification and deformation behaviors of twin roll cast Mg-6Al-X alloys

Abstract: Abstract In this study, the solidification and deformation behaviors in twin roll cast (TRC) Mg-6Al-X alloys have been investigated. The TRC simulation results showed that the AX60 alloy tended to have lower segregation while the AZ60 had the highest segregation due to the different solidification behavior and thermal properties. Compared to the as-cast microstructure, the segregation area was well matched with the melt to roll nip distance predicted in simulation. Mg alloys with Ca or Sr elements showed weaker textures when compared to A6 alloys rolled at 350 °C. In addition, there was a significant change in (0002) pole figures from strong basal textures to random textures when the rolling temperature increased from 350 °C to 450 °C. This may be attributed to the non-basal slip system activity at high temperatures. The results of visco-plastic self-consistent simulation revealed that critical resolved shear stress of the tension twin increased with increasing rolling temperature. This led to tension twin suppression in compression, which were associated with enhancing the yield isotropy of Mg alloys. Furthermore, the relative activities of basal <a> slip in AX60 alloy were higher than the other Mg alloys. This means they were responsible for enhancing the formability and yield isotropy of Mg alloys.AbstractIn this study, the solidification and deformation behaviors in twin roll cast (TRC) Mg-6Al-X alloys have been investigated. The TRC simulation results showed that the AX60 alloy tended to have lower segregation while the AZ60 had the highest segregation due to the different solidification behavior and thermal properties. Compared to the as-cast microstructure, the segregation area was well matched with the melt to roll nip distance predicted in simulation. Mg alloys with Ca or Sr elements showed weaker textures when compared to A6 alloys rolled at 350 °C. In addition, there was a significant change in (0002) pole figures from strong basal textures to random textures when the rolling temperature increased from 350 °C to 450 °C. This may be attributed to the non-basal slip system activity at high temperatures. The results of visco-plastic self-consistent simulation revealed that critical resolved shear stress of the tension twin increased with increasing rolling temperature. This led to tension twin suppression in compression, which were associated with enhancing the yield isotropy of Mg alloys. Furthermore, the relative activities of basal <a> slip in AX60 alloy were higher than the other Mg alloys. This means they were responsible for enhancing the formability and yield isotropy of Mg alloys.

Pub.: 01 Nov '16, Pinned: 31 Aug '17

A measure of plastic anisotropy for hexagonal close packed metals: Application to alloying effects on the formability of Mg

Abstract: Mg is inherently plastically anisotropic and, over the years, alloying development efforts have sought to reduce the plastic anisotropy in order to enhance formability. To understand the relationship between alloy type and plastic anisotropy, we use a visco-plastic self-consistent (VPSC) polycrystal plasticity model to relate the macroscopic constitutive response to the underlying slip and twinning mechanisms in pure Mg and several Mg alloys. In the calculations, the influence of alloy type is represented by the differences in the CRSS values among the basal, prismatic, pyramidal slip and tensile twin systems. We show that for the same initial texture, this microscopic-level CRSS anisotropy can have a significant effect on the macroscopic indicators of formability, namely the anisotropy of the post-deformation polycrystal yield surface, tension-compression yield asymmetry, and Lankford coefficients. A plastic anisotropy (PA) measure is formulated to quantify the degree of single crystal plastic anisotropy acquired by the dissimilarities in the CRSS values of the slip and twinning modes for a given alloy. We demonstrate a strong correlation between the PA measure with the formability indicators mentioned above for multiple initial textures commonly enountered in processing. We find that alloys can be classified into two groups, those with a PA value below 2, which are more formable, less twinnable, and less sensitive to initial texture, where PA ∼2 for pure Mg, and those with a PA value above 2, which possess the opposite deformation response.

Pub.: 01 Nov '16, Pinned: 31 Aug '17

Prediction of the plastic anisotropy of magnesium alloys with synthetic textures and implications for the effect of texture on formability

Abstract: Crystallographic texture is a well-known microstructural feature influencing the formability of magnesium alloys. However, the effects of individual texture characteristics common after thermomechanical processing have not been isolated due to the experimental challenge associated with varying them independently. Similarly, the effect of the propensity for twinning on formability, which both accommodates deformation and reorients the crystal, have not been systematically studied. This study uses synthetic sheet textures in conjunction with a viscoplastic self-consistent (VPSC) polycrystal plasticity model to predict deformation and formability behavior. The VPSC model was first parameterized based on experimental mechanical data and textures from fine-grained thixomolded and thermomechanically processed AZ61L. Subsequently, synthetic textures were generated to examine the effects of basal peak intensity, prismatic plane distribution, and asymmetry of the basal pole figure peak. Of these texture characteristics, basal peak strength is the most important predictor of forming behavior, with prismatic plane distribution and c-axis anisotropy resulting in comparatively weak effects. In the second part of the study, the effective critical resolved shear stress for twinning was varied, resulting in poorer forming behavior with easier twin activation. In both cases, increasing prismatic slip activity was deleterious to the predicted forming behavior.

Pub.: 22 Aug '16, Pinned: 31 Aug '17

Cellular Magnesium Matrix Foam Composites for Mechanical Damping Applications

Abstract: The damping characteristics of metal alloys and metal matrix composites are relevant to the automotive, aerospace, and marine structures. Use of lightweight materials can help in increasing payload capacity and in decreasing fuel consumption. Lightweight composite materials possessing high damping capabilities that can be designed as structural members can greatly benefit in addressing these needs. In this context, the damping properties of lightweight metals such as aluminum and magnesium and their respective composites have been studied in the existing literature. This review focuses on analyzing the damping properties of aluminum and magnesium alloys and their cellular composites. The damping properties of various lightweight alloys and composites are compared on the basis of their density to understand the potential for weight saving in structural applications. Magnesium alloys are observed to possess better damping properties in comparison to aluminum. However, aluminum matrix syntactic foams reinforced with silicon carbide hollow particles possess a damping capacity and density comparable to magnesium alloy. By using the data presented in the study, composites with specific compositions and properties can be selected for a given application. In addition, the comparison of the results helps in identifying the areas where attention needs to be focused to address the future needs.

Pub.: 23 Oct '15, Pinned: 31 Aug '17

Study on the mutual effect of La and Gd on microstructure and mechanical properties of Mg-Al-Zn extruded alloy

Abstract: The effects of rare earth elements (La, Gd) addition on the microstructure and the mechanical properties of as-extruded Mg-Al-Zn magnesium alloy under conditions of a medium temperature (230 °C) and a slow ram speed (0.1 mm/s) have been throughly investigated. The results show that the as-extruded bar presents smooth surface from the macroscopic view due to the fact that many non-basal <a> dislocations and few pyramidal 〈c + a〉 dislocations could be activated to enhance the formability in the extrusion process. A bimodal grain structure composed of fine dynamic recrystallized (DRXed) grains with globular Mg17Al12 precipitates located at grain boundaries and non-recrystallized (non-DRXed) coarse grains with thin continuous precipitation laths inside is generated in the alloy. More dislocation pile-ups and subgrain boundaries are exhibited in the DRXed region which shows a typical texture of extruded alloys containing rare earth elements. The modified extruded alloy shows much higher strength than that of as-extruded AZ80 under either tension or compression condition owing to grain refinement strengthening, precipitation strengthening, solid-solution strengthening, dislocation strengthening and subgrain strengthening. The finer DRXed grain size and the fine precipitates in the modified extruded alloy bring about its lower yield anisotropy. In addition, large and fragile Al2Gd and Al8Mn4Gd phases mainly give rise to the decrease in ductility of the modified extruded alloy.

Pub.: 06 Jul '16, Pinned: 31 Aug '17

Influence of Extrusion on the Microstructure and Mechanical Behavior of Mg-9Li-3Al-xSr Alloys

Abstract: Mg-9Li-3Al-xSr (LA93-xSr, x = 0, 1.5, 2.5, and 3.5 wt pct) alloys were cast and extruded at 533 K (260 °C) with an extrusion ratio of 28. The microstructure and mechanical response are reported and discussed paying particular attention to the influence of extrusion and Sr content on phase composition, strength, and ductility. The results of the current study show that LA93-xSr alloys contain both α-Mg (hcp) and β-Li (bcc) matrix phases. Moreover, the addition of Sr refines the grain size in the as-cast alloys and leads to the formation of the intermetallic compound (Al4Sr). Our results show significant grain refinement during extrusion and almost no influence of Sr content on the grain size of the extruded alloys. The microstructure evolution during extrusion is governed by continuous dynamic recrystallization (CDRX) in the α-Mg phase, whereas discontinuous dynamic recrystallization (DDRX) occurs in the β-Li phase. The mechanical behavior of the extruded LA93-xSr alloy is discussed in terms of grain refinement and dislocation strengthening. The tensile strength of the extruded alloys first increases and then decreases, whereas the elongation decreases monotonically with increasing Sr; in contrast, hardness increases for all Sr compositions studied herein. Specifically, when Sr content is 2.5 wt pct, the extruded Mg-9Li-3Al-2.5Sr (LAJ932) alloy exhibits a favorable combination of strength and ductility with an ultimate tensile strength of 235 MPa, yield strength of 221 MPa, and an elongation of 19.4 pct.

Pub.: 07 Nov '12, Pinned: 31 Aug '17

In vitro and in vivo corrosion, cytocompatibility and mechanical properties of biodegradable Mg-Y-Ca-Zr alloys as implant materials.

Abstract: This study introduces a class of biodegradable Mg-Y-Ca-Zr alloys novel to biological applications and presents evaluations for orthopedic and craniofacial implant applications. Mg-Y-Ca-Zr alloys were processed using conventional melting and casting techniques. The effects of increasing Y content from 1 to 4 wt.% as well as the effects of T4 solution treatment were assessed. Basic material phase characterization was conducted using X-ray diffraction, optical microscopy and scanning electron microscopy. Compressive and tensile tests allowed for the comparison of mechanical properties of the as-cast and T4-treated Mg-Y-Ca-Zr alloys to pure Mg and as-drawn AZ31. Potentiodynamic polarization tests and mass loss immersion tests were used to evaluate the corrosion behavior of the alloys. In vitro cytocompatibility tests on MC3T3-E1 pre-osteoblast cells were also conducted. Finally, alloy pellets were implanted into murine subcutaneous tissue to observe in vivo corrosion as well as local host response through H&E staining. SEM/EDS analysis showed that secondary phase intermetallics rich in yttrium were observed along the grain boundaries, with the T4 solution treatment diffusing the secondary phases into the matrix while increasing the grain size. The alloys demonstrated marked improvement in mechanical properties over pure Mg. Increasing the Y content contributed to improved corrosion resistance, while solution-treated alloys resulted in lower strength and compressive strain compared to as-cast alloys. The Mg-Y-Ca-Zr alloys demonstrated excellent in vitro cytocompatibility and normal in vivo host response. The mechanical, corrosion and biological evaluations performed in this study demonstrated that Mg-Y-Ca-Zr alloys, especially with the 4 wt.% Y content, would perform well as orthopedic and craniofacial implant biomaterials.

Pub.: 03 Jul '13, Pinned: 31 Aug '17

Atomic-scale investigation of creep behavior in nanocrystalline Mg and Mg-Y alloys

Abstract: Magnesium (Mg) and its alloys offer great potential for reducing vehicular mass and energy consumption due to their inherently low densities. Historically, widespread applicability has been limited by low strength properties compared to other structural Al-, Ti- and Fe-based alloys. However, recent studies have demonstrated high-specific-strength in a number of nanocrystalline Mg-alloys. Even so, applications of these alloys would be restricted to low-temperature automotive components due to microstructural instability under high temperature creep loading. Hence, this work aims to gain a better understanding of creep and associated deformation behavior of columnar nanocrystalline Mg and Mg-yttrium (Y) (up to 3at.%Y(10wt.%Y)) with a grain size of 5 nm and 10 nm. Using molecular dynamics (MD) simulations, nanocrystalline magnesium with and without local concentrations of yttrium is subjected to constant-stress loading ranging from 0 to 500 MPa at different initial temperatures ranging from 473 to 723 K. In pure Mg, the analyses of the diffusion coefficient and energy barrier reveal that at lower temperatures (i.e., T < ~423K) the contribution of grain boundary diffusion to the overall creep deformation is stronger that the contribution of lattice diffusion. However, at higher temperatures (T > ~423K) lattice diffusion dominates the overall creep behavior. Interestingly, for the first time, we have shown that the(101-1),(101-2),(101-3) and (101-6) boundary sliding energy is reduced with the addition of yttrium. This softening effect in the presence of yttrium suggests that the experimentally observed high temperature beneficial effects of yttrium addition is likely to be attributed to some combination of other reported creep strengthening mechanisms or phenomena such as formation of stable yttrium oxides at grain boundaries or increased forest dislocation-based hardening.

Pub.: 23 Jun '15, Pinned: 31 Aug '17

Development of non-flammable high strength AZ91 + Ca alloys via liquid forging and extrusion

Abstract: Non-flammable high strength magnesium alloys were developed from AZ91 alloy mainly for the weight reduction in industrial applications. Calcium of 2, 4 and 6 wt.% was added and the ignition point was tested to study the flammability. Liquid forging and hot extrusion were applied to these alloys in order to enhance the mechanical properties. The microstructure, hardness and tensile properties were investigated to study the effects of calcium additions and different manufacturing processes. The results showed that calcium additions significantly improved the non-flammability, but mechanical properties of the gravity cast samples were degraded due to large amounts of Al2Ca at grain boundaries. After liquid forging and extrusion process, an increase of the mechanical properties was observed in the non-flammable magnesium alloys. The enhancement of properties was mainly contributed by the refined microstructure, redistribution of second phase and less porosity defects. Among all the tested alloys, AZ91 + 2% Ca prepared by liquid forging + extrusion demonstrated the optimal balance of high ignition point and excellent strength and ductility. 4% Ca addition further improves the ignition resistance and tensile strength of AZ91 alloy. The advantages of the manufacturing process combining liquid forging and hot extrusion were demonstrated to achieve improved mechanical properties for non-flammable magnesium alloys.

Pub.: 04 Mar '16, Pinned: 31 Aug '17

Influence of Applied Pressure on Tensile Behaviour and Microstructure of Squeeze Cast Mg Alloy AM50 with Ca Addition

Abstract: The development of alternative manufacturing processes is essential for the success in applying Ca-containing magnesium alloys for automotive applications due to their relatively poor die castability. Squeeze casting with its inherent advantages has been demonstrated capable of minimizing the formation of casting defects in Mg-Al-Ca alloys. In this study, the effect of applied pressures on tensile behavior and microstructure of squeeze cast Mg-5wt.%Al-1%wt.%Ca alloy (AMX501) was investigated with the applied pressure varying from 3 to 90 MPa. The results of tensile testing indicate that the tensile properties of AMX501 alloy including ultimate tensile strength, yield strength, and elongation (Ef) increase from 153.7, 80 MPa and 3.26% to 183.7, 90.5, and 5.42% with increasing applied pressure levels from 3 to 90 MPa, respectively. The analysis of true stress versus strain curves shows that an increase in applied pressure levels result in high straining hardening rates during the plastic deformation of the alloy. Microstructural analysis and density measurements indicate that, as the applied pressure increases, the porosity levels of the alloy decrease considerably, despite of almost no significant reduction in grain sizes of the squeeze cast alloys due to their high aspect ratio of cylindrical castings. Hence, the improvement in tensile properties should be primarily attributed to casting densification resulting from applied pressures. The scanning electron microscopy observation on fractured surfaces reveals that the fracture modes of the squeeze cast alloys transit to ductile from brittle with increasing applied pressures.

Pub.: 11 Jan '11, Pinned: 31 Aug '17

Deformation Behaviors of Twin Roll Cast Mg-Zn-X-Ca Alloys for Enhanced Room-Temperature Formability

Abstract: Formability and viscoplastic self-consistent simulations of Mg-4Zn-X-Ca alloys were carried out in an effort to understand the relationship between deformation behaviors and room-temperature formability. The microstructural, textural and mechanical properties of Mg-4Zn-X-Ca alloys were also investigated. All of the alloys showed a sound twin roll cast microstructure without the occurrence of inverse segregation. Annealed Z4 and ZSX400 alloys exhibited strong basal textures; however, the other Mg alloys showed weaker basal textures with splitting in the transverse direction resulting from their different types of static recrystallization. Among the Mg-4Zn-X-Ca alloys, the ZAX400 alloy exhibited a high yield strength value of 189.3 MPa and excellent formability of 7.5mm with enhanced yield isotropy, comparable to those of Al alloys. The higher formability values of the Mg-4Zn-X-Ca alloys were closely related to the modified deformation behaviors resulting from the texture evolution. The ZX40, ZAX400, ZWX400 and ZCX400 alloys, which were more formable, had higher relative activity of the basal <a> slip upon compression deformation, thus improving the yield isotropy. However, the Z4 and ZSX400 alloys in this study, had relatively low activity of the basal <a> slip in compression modes, as caused by their different CRSS ratios (tension twin/basal <a> slip).

Pub.: 15 Oct '16, Pinned: 31 Aug '17