In-situ deformation characterization of Nd-containing Mg alloys

An in-situ characterization technique combining mechanical testing inside a scanning electron microscope (SEM) with electron backscatter diffraction (EBSD) analysis was employed to study the tensile deformation behavior of Nd-containing Mg alloys over the temperature range of 50‑250°C. EBSD data were collected before and after the experiments. The active slip and twinning systems were identified from the secondary electron SEM images and EBSD orientation data using a MATLAB code with the input of grain Euler angles [1]. The critical resolved shear stress (CRSS) ratios between the different deformation systems were estimated based on the distribution of identified deformation modes and texture according to the methodology described in Ref. [2]. Details of this testing technique and the results obtained on Mg‑1Mn‑xNd (wt.%) [0 \u3c x \u3c 1] alloys in the as-cast and extruded conditions will be presented. Rare-earth additions to Mg alloys tend to reduce the strong basal texture exhibited by conventional wrought Mg alloys [3, 4]. Compared with other RE elements, neodymium (Nd) has proven to be a stronger texture modifier [4]. The alloys were gravity cast to produce billets of 125 mm diameter and then machined to a diameter of 93 mm. The billets were homogenized at 350°C for 15 hours prior to extrusion. Indirect extrusion was carried out to produce round bars of 17 mm diameter. The billets were extruded at temperatures between 275°C and 300°C and between 5.6 mm/s and 8.3 mm/s. The tensile strength of the as-cast materials was lower than that of the extruded materials. For the material containing 1wt.%Nd, both the as-cast and extruded materials exhibited superior high temperature strength retention compared with conventional alloys [5]. This is expected to be the result of the Nd addition. In the case of the extruded materials, basal slip, prismatic slip, and pyramidal slip were active at all temperatures. Extension twinning was also observed at all temperatures for the 275°C extruded Mg‑1Mn‑1Nd(wt.%) alloy; however, it was not observed at 250°C for the same alloy extruded at 300°C. The extent of twinning decreased with increasing temperature and basal slip was the major deformation mode at 150°C and 250°C. Basal slip was associated with high Schmid factors in all cases. Extension twinning was distributed over the entire Schmid factor range suggesting that extension twinning does not follow Schmid law. The estimated CRSS ratio of extension twinning with respect to basal slip was \u3c1, suggesting that the addition of Nd results in an increase in the CRSS of basal slip. In the case of the as-cast Mg‑1Mn‑1Nd (wt.%) alloy, basal slip and extension twinning were observed at 50°C. Nonbasal slip activity was observed at 150°C. REFERENCES [1] Boehlert, C., et al. Phil. Mag. 2013, 93 (6), 598–617. [2] Li H., et. al. Acta Mater. 2013, 61(20), 7555–7567. [3] Ulacia I., et. al. Acta Mater. 2010, 58(8), 2988–2998. [4] Bohlen J., et. al. Mater. Sci. Eng. A. 2010, 527(26), 7092–7098. [5] Boehlert C., et. al. Acta Mater. 2012, 60(4), 1889–1904

Current status and perspectives of zinc-based absorbable alloys for biomedical applications

Absorbable metals have the potential to serve as the next generation of temporary medical implant devices by safely dissolving in the human body upon vascular tissue healing and bone regeneration. Their implementation in the market could greatly reduce the need of costly and risky additional surgeries for either implant replacement or removal, often required in current permanent implants. Despite the extensive research done over the last two decades on magnesium (Mg) and iron (Fe) based alloys, they have not generally shown a satisfactory combination of mechanical properties, biocompatibility and controlled degradation rate in the physiological environment. Consequently, zinc (Zn) based alloys were introduced in the last few years as alternative materials to overcome the limitations of Fe and Mg-based alloys. The blend of different alloying elements and processing conditions have led to a wide variety of Zn-based alloys having tunable mechanical properties and corrosion rates. This review provides the most recent progress in the development of absorbable Zn-based alloys for biomedical implant applications, primarily for cardiovascular and orthopedic devices. Their biocompatibility, processability and metallurgical aspects, as well as their mechanical behavior and corrosion properties are presented and discussed, including their opportunities, limitations and future research directions

On the Controversy About the Presence of Grain Boundary Sliding in Mg AZ31

Highly-textured, rolled AZ31 sheet material shows a significant drop in the plastic anisotropy (r-value; r=εw/εt) in tension between 25°C and 200°C. This behavior was initially explained as a result of the increased activity of non-basal slip with increased temperature. Other authors suggested, however, that the mechanism resp onsible for this phenomenon was the activation of grain boundary sliding (GBS). Here, in-situ ten sile tests have been carried out in an SEM at various temperatures in order to obtain further evi dence of the role of GBS during moderate to high temperature deformation of Mg alloys, which remains highly controversial

Analysis of crystallographic slip and grain boundary sliding in a Ti-45Al-2Nb-2Mn (at%)-0.8 vol%TiB2 alloy by high temperature in situ mechanical testing

This work aims to contribute to a further understanding of the fundamentals of crystallographic slip and grain boundary sliding in the gamma-TiAl Ti-45Al-2Nb-2Mn (at%)-0.8 vol%TiB2 intermetallic alloy, by means of in situ high-temperature tensile testing combined with electron backscatter diffraction (EBSD). Several microstructures, containing different fractions and sizes of lamellar colonies and equiaxed gamma-grains, were fabricated by either centrifugal casting or powder metallurgy, followed by heat treatment at 1300 °C and furnace cooling. in situ tensile and tensile-creep experiments were performed in a scanning electron microscope (SEM) at temperatures ranging from 580 °C to 700 °C. EBSD was carried out in selected regions before and after straining. Our results suggest that, during constant strain rate tests, true twin gamma/gamma interfaces are the weakest barriers to dislocations and, thus, that the relevant length scale might be influenced by the distance between non-true twin boundaries. Under creep conditions both grain/colony boundary sliding (G/CBS) and crystallographic slip are observed to contribute to deformation. The incidence of boundary sliding is particularly high in gamma grains of duplex microstructures. The slip activity during creep deformation in different microstructures was evaluated by trace analysis. Special emphasis was placed in distinguishing the compliance of different slip events with the Schmid law with respect to the applied stress.Funding from the Spanish Ministry of Science and Innovation through projects (MAT2009-14547-C02-01 and MAT2009-14547-C02-02) is acknowledged. The Madrid Regional Government partially supported this project through the ESTRUMAT grant (P2009/MAT-1585). We also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through grant MAT2012-31889

In situ analysis of the tensile deformation mechanisms in extruded Mg–1Mn–1Nd (wt%)

An extruded Mg–1Mn–1Nd (wt%) (MN11) alloy was tested in tension in an SEM at temperatures of 323K (50°C), 423 K (150°C), and 523 K (250°C) to analyse the local deformation mechanisms through in situ observations. Electron backscatter diffraction was performed before and after the deformation. It was found that the tensile strength decreased with increasing temperature, and the relative activity of different twinning and slip systems was quantified. At 323K (50C), extension twinning, basal, prismatic (a) and pyramidal (c+a) slip were active. Much less extension twinning was observed at 423K (150ºC) while basal slip and prismatic (a) slip were dominant and presented similar activities. At 523K (250ºC), twinning was not observed, and basal slip controlled the deformation

In Situ Observations of the Deformation Behavior and Fracture Mechanisms of Ti-45Al-2Nb-2Mn+0.8 vol pct TiB₂

The deformation and fracture mechanisms of a nearly lamellar Ti-45Al-2Nb-2Mn (at. pct) + 0.8 vol pct TiB₂ intermetallic, processed into an actual low-pressure turbine blade, were examined by means of in situ tensile and tensile-creep experiments performed inside a scanning electron microscope (SEM). Low elongation-to-failure and brittle fracture were observed at room temperature, while the larger elongations-to-failure at high temperature facilitated the observation of the onset and propagation of damage. It was found that the dominant damage mechanisms at high temperature depended on the applied stress level. Interlamellar cracking was observed only above 390 MPa, which suggests that there is a threshold below which this mechanism is inhibited. Failure during creep tests at 250 MPa was controlled by intercolony cracking. The in situ observations demonstrated that the colony boundaries are damage nucleation and propagation sites during tensile creep, and they seem to be the weakest link in the microstructure for the tertiary creep stage. Therefore, it is proposed that interlamellar areas are critical zones for fracture at higher stresses, whereas lower stress, high-temperature creep conditions lead to intercolony cracking and fracture.The authors are grateful to Industria de Turbo Propulsores, S.A. for supplying the intermetallic blades. Funding from the Spanish Ministry of Science and Innovation through projects MAT2009-14547-C02-01 and MAT2009-14547-C02-02 is acknowledged. The Madrid Regional Government supported this project partially through the ESTRUMAT grant P2009/MAT-1585. C.J.B. acknowledges the support from Grant SAB2009-0045 from the Spanish Ministry of Education for his sabbatical stage in Madrid.Publicad

Effect of stress level on the high temperature deformation and fracture mechanisms of Ti-45Al-2Nb-2Mn-0.8 vol. pct TiB²: an 'In Situ' experimental study

The effect of the applied stress on the deformation and crack nucleation and propagation mechanisms of a gamma-TiAl intermetallic alloy (Ti-45Al-2Nb-2Mn (at. pct)-0.8 vol. pct TiB2) was examined by means of in situ tensile (constant strain rate) and tensile-creep (constant load) experiments performed at 973 K (700 °C) using a scanning electron microscope. Colony boundary cracking developed during the secondary stage in creep tests at 300 and 400 MPa and during the tertiary stage of the creep tests performed at higher stresses. Colony boundary cracking was also observed in the constant strain rate tensile test. Interlamellar ledges were only found during the tensile-creep tests at high stresses (sigma > 400 MPa) and during the constant strain rate tensile test. Quantitative measurements of the nature of the crack propagation path along secondary cracks and along the primary crack indicated that colony boundaries were preferential sites for crack propagation under all the conditions investigated. The frequency of interlamellar cracking increased with stress, but this fracture mechanism was always of secondary importance. Translamellar cracking was only observed along the primary crack.Funding from the Spanish Ministry of Science and Innovation through projects (MAT2009-14547-C02-01 and MAT2009-14547-C02-02) is acknowledged. The Madrid Regional Government partially supported this project through the ESTRUMAT grant (P2009/MAT-1585). CJB acknowledges the support from the Spanish Ministry of Education for his sabbatical stay in Madrid (SAB2009-0045).Publicad

In-situ observations of recrystallization and microstructural evolution in cerium-containing rolled magnesium alloys

The recrystallization and microstructural evolution of rolled Mg-2Zn-xCe (x = 0.2 and 0.6 wt%) were studied by combining annealing with in-situ electron backscatter diffraction (EBSD) analysis. Sequential EBSD orientation maps of the same microstructural patch were obtained at increasing temperatures, 298 K, 423 K, and from 473 K to 598 K in 25 K increments. In both alloys, the nucleation of new grains was first observed at 473 K, and recrystallization was complete by 573 K. It was found that greater Ce additions led to a higher fraction of recrystallized grains and an overall finer grain size. New grains with a misorientation angle-axis relationship of 60° about , 56° about , and 86° about were observed. Although the twin evolution during heating was not captured, the misorientation axis relationships along , , were characterized for the recrystallized grains. The misorientation angles were uniformly distributed between 30° and 90° in both alloys. The results are discussed and compared with Mg-3Al-1Zn (wt%). Keywords: Recrystallization, Microstructure, Magnesium alloys, Texture, In-situ microscop