Weight Loss with Magnesium Alloys Pollock, Tresa M.
Science (American Association for the Advancement of Science),
05/2010, Letnik:
328, Številka:
5981
Journal Article
Recenzirano
New design and processing approaches for magnesium-based alloys are offering a variety of opportunities in lightweight and energy-efficient applications.
The compelling need for lightweight, ...energy-efficient, environmentally benign engineering systems is driving the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges motivate wider spread use of magnesium—the eighth most common element in the earth's crust and also extractable from seawater. In addition, the ease of recycling, compared with polymers, makes magnesium alloys environmentally attractive. Importantly, with a density of 1.74 g/cm
3
—about 30% less than aluminum, one-quarter that of steel, and nearly the same as many polymers—magnesium is attractive for lightweight structural systems and, most notably, automotive systems. A typical car weighing 1525 kg currently contains about 975 kg of steel, 127 kg of Al, 114 kg of polymeric materials, and 5 to 6 kg of magnesium (
1
). It is estimated that 22.5 kg of mass reduction would improve fuel efficiency by around 1%; thus, automotive manufacturers worldwide have goals to increase the Mg content of automobiles to between 45 and 160 kg (
1
,
2
).
L12-Strengthened Cobalt-Base Superalloys Suzuki, Akane; Inui, Haruyuki; Pollock, Tresa M
Annual review of materials research,
07/2015, Letnik:
45
Journal Article
Recenzirano
The discovery of the γ′-Co
3
(Al,W) phase with an L1
2
structure provided Co-base alloys with a new strengthening mechanism, enabling a new class of high-temperature material: Co-base superalloys. ...This review discusses the current understanding of the phase stability, deformation, and oxidation behaviors of γ′ single-phase and γ + γ′ two-phase alloys in comparison with Ni-base γ′-L1
2
phase and γ + γ′ superalloys. Relatively low stacking fault energies and phase stability of the γ′ phase compared with those in Ni-base alloys are responsible for the unique deformation behaviors observed in Co-base γ′ and γ + γ′ alloys. Controlling energies of planar defects, such as stacking faults and antiphase boundaries, by alloying is critical for alloy development. Experimental and density functional theory studies indicate that additions of Ta, Ti, Nb, Hf, and Ni are effective in simultaneously increasing the phase stability and stacking fault energy of γ′-Co
3
(Al,W), thus improving the high-temperature strength of Co-base γ′ phase and γ + γ′ two-phase superalloys.
Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive ...industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing. We selected the nucleants on the basis of crystallographic information and assembled them onto 7075 and 6061 series aluminium alloy powders. After functionalization with the nucleants, we found that these high-strength aluminium alloys, which were previously incompatible with additive manufacturing, could be processed successfully using selective laser melting. Crack-free, equiaxed (that is, with grains roughly equal in length, width and height), fine-grained microstructures were achieved, resulting in material strengths comparable to that of wrought material. Our approach to metal-based additive manufacturing is applicable to a wide range of alloys and can be implemented using a range of additive machines. It thus provides a foundation for broad industrial applicability, including where electron-beam melting or directed-energy-deposition techniques are used instead of selective laser melting, and will enable additive manufacturing of other alloy systems, such as non-weldable nickel superalloys and intermetallics. Furthermore, this technology could be used in conventional processing such as in joining, casting and injection moulding, in which solidification cracking and hot tearing are also common issues.
The high-temperature strength and deformation behavior of γ/γ′ two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and ...higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873
K due to slip of pairs of 1/2〈1
1
0〉 superpartial dislocations on {0
0
1} planes, in addition to {1
1
1} planes, in the γ′ precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073
K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/2〈1
1
0〉 dislocations bypass the γ′ precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/3〈1
1
2〉 partial dislocation slip that introduces a high density of stacking faults in the γ′ precipitates.
Pathways for ductility
Alloys containing multiple elements can be very strong but often suffer from poor ductility. F. Wang
et al.
found that different mechanisms accommodated plasticity in a ...molybdenum-niobium-titanium multiprincipal element alloy (see the Perspective by Cairney). Instead of so-called “screw” dislocations, deformation is accommodated by multiple pathways that include “edge” dislocations and activation of crystallographic slip planes. These results offer a design paradigm for developing new high-strength alloys.
Science
, this issue p.
95
; see also p.
37
A combination of pathways allows high-strength, multiple-element alloys to be ductile.
Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum.
Precipitate shearing mechanisms during single-crystal tensile creep of L12-containing Co- and CoNi-base alloys have been investigated. Interrupted creep tests were conducted at 900°C under vacuum at ...constant load, and stresses varied between 275 and 345MPa. Transmission electron microscopy diffraction contrast analysis revealed superlattice intrinsic and extrinsic stacking faults in the γ′ precipitates in the Co-base alloys. However, in alloys containing significant additions of nickel (CoNi-base alloys), antiphase boundaries formed by shearing of a/2〈110〉 dislocations on octahedral planes. The shearing mechanisms in the Co- and CoNi-base γ′ phase are compared to commercial Ni-base alloys, and the relative fault energies inferred from these observations are discussed.
High resolution scanning electron microscope digital image correlation (SEM DIC) was performed in situ during uniaxial loading on Ti-6Al-4V rolled titanium plate to determine the dependence of strain ...localization on microstructure and microtexture. Individual grains with preferred orientation for basal slip exhibited plastic localization along basal planes before macroscopic yielding. With additional strain, but still below macroscopic yielding, pyramidal and prismatic plastic activity was observed as slip bands transmitting across many grains and entire microtextured regions (MTRs). The localization of long range plastic strain occurred within MTRs that allowed for slip transmission across grains with low angle boundaries. The rolled titanium plate material having strong 0001 and101¯0texture components showed pyramidal and prismatic type slip extending across entire MTRs at strains well below macroscopic yielding. These strain localization processes occur much earlier in straining and over lengthscales much longer than observed with conventional slip offset imaging. The implications for properties are discussed.
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The fatigue behavior of the nickel-base superalloy René 88 DT has been investigated at room temperature with fully reversed loading in an ultrasonic fatigue apparatus operating at a frequency close ...to 20kHz. A characterization protocol based on the electron backscatter diffraction technique has been developed to identify the combination of microstructural features within crack initiation sites and surrounding neighborhoods that leads to the transition from initiation to early small crack growth. Surface grains that were more than three times the average grain size, that were favorably oriented for cyclic slip localization and that also contained Σ3 twin boundaries inclined to the loading axis were most favorable for fatigue crack initiation. Fatigue cracks subsequently grew in grain clusters within which grains are misoriented by less than 20° relative to the initiation grains. More highly misoriented neighboring grains resulted in crack arrest. The material characteristics that promote crack initiation and small crack growth exist only at the extreme tails of the microstructural distributions. The implications for modeling of fatigue life and fatigue life variability are discussed.
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