An amorphous alloy solidified from a metallic melt without crystallizing. With no dislocations, it has high strength and a large elastic strain limit. Thick pieces are called bulk metallic glasses.
Japanese name: 金属ガラス
Metallic glass appears in 4 practice questions across 1 topic, including Glasses and amorphous materials.
Metallic melts have low viscosity and their atoms rearrange quickly, so they normally crystallize as soon as they are cooled. Since an amorphous alloy was first made in 1960 by quenching an Au-Si alloy at about 10⁶ K/s, early metallic glasses were ribbons a few tens of µm thick made by melt spinning, in which the melt is jetted onto a rotating copper wheel, and similar methods. From around 1990, multicomponent alloys based on Zr, Pd, Mg, Fe and others brought critical cooling rates down to about 1–100 K/s, and bulk metallic glasses from several mm to centimeter scale could be made simply by casting into a copper mold.
Metallic glasses have no dislocations or grain boundaries, so their yield strength is much higher than that of crystalline alloys of the same elements: about 1.8 GPa for Zr-based glasses, and over 5 GPa for some Co-based ones. Their Young's modulus is about 30% lower than that of crystals, and their elastic strain limit of about 2% is several times that of most crystalline metals. At room temperature, however, deformation concentrates in thin shear bands and there is no work hardening, so in tension they fracture with almost no elongation. To offset this brittleness, composites with dispersed crystalline phases and similar approaches are being studied.
When a metallic glass is heated, it becomes a supercooled liquid of reduced viscosity above the glass transition temperature Tg and crystallizes at the higher crystallization temperature Tx. In the range between Tg and Tx it can deform greatly under small forces like a plastic, allowing precision forming that replicates fine mold features. Uses include springs, sporting goods and precision parts that exploit the large elastic energy, and Fe-based ribbons, which have no magnetocrystalline anisotropy and small coercivity, are used for low-loss transformer cores.
Answer: C. It has three or more elements, with main-element atomic radii differing by about 12% or more and a negative heat of mixing
Inoue's empirical rules are three: multiple components, an atomic radius difference of about 12% or more among the main elements, and a negative heat of mixing. When atoms of different sizes mix while attracting one another, the liquid takes a densely packed, disordered arrangement, and atomic diffusion and rearrangement slow down. In addition, forming a crystal from a liquid containing many elements requires extensive rearrangement of atoms, so crystal nucleation and growth become difficult. A lower liquidus temperature at compositions near a deep eutectic also helps. Combinations with equal atomic radii and positive heat of mixing tend to form solid solutions or phase-separate, and compositions close to a pure metal hardly become glass even with quenching at about 10⁶ K/s.
Answer: C. About 10 mm
The cooling rate can be written R = 1.0×10³×(1.0/d)² K/s (d in mm), so the thickness at which R = 10 K/s is d = 1.0×√(1.0×10³/10) = 10 mm. The time for heat to escape to the mold is proportional to the square of the distance heat must travel, so the cooling rate is inversely proportional to the square of the thickness. About 100 mm assumes the cooling rate is inversely proportional to the thickness itself, and about 0.10 mm inverts the ratio of cooling rates. Early metallic glasses had large critical cooling rates of about 10⁵–10⁶ K/s, so only ribbons a few tens of µm thick made by melt spinning could be obtained, but multicomponent alloys lowered the critical cooling rate and made centimeter-scale bulk metallic glasses possible.
They are thermodynamically less stable than crystals, but at room temperature atoms barely move, so for practical purposes they do not crystallize. Held long near Tg, however, they can become brittle through structural relaxation or crystallize, so the service temperature is kept well below Tg.
Both refer to alloys without crystalline periodicity and are used almost interchangeably. Those that show a clear glass transition on heating and have a supercooled-liquid range are sometimes distinguished specifically as metallic glasses.
When atoms of different sizes mix while attracting each other, the liquid takes a dense, disordered arrangement in which atoms move slowly, and forming crystals requires extensive rearrangement of atoms. A lower liquidus temperature, which raises the reduced glass transition temperature, also helps.
Early metallic glasses had large critical cooling rates of about 10⁵–10⁶ K/s, so only ribbons a few tens of µm thick made by melt spinning could be obtained, but multicomponent alloys lowered the critical cooling rate and made centimeter-scale bulk metallic glasses possible.
Sheet-shaped metallic glass is made by casting into a coppe…For steel (yield stress 0.5 GPa, Young's modulus 200 GPa) it is about 0.25%, and even for a high-strength titanium alloy (0.9 GPa, 110 GPa) about 0.8%, so the elastic strain limit of the metallic glass is much larger.
A Zr-based bulk metallic glass has a yield strength of 1.8 …In metallic glasses, shear stress makes small groups of atoms move cooperatively, locally increasing free volume and softening the material.
Which statement about plastic deformation of metallic glass…Dislocation Young's modulus Titanium alloy Work hardening Amorphous silicon Chemically strengthened glass Network former Network modifier Non-bridging oxygen Reduced glass transition temperature