Intrinsic and Extrinsic Factors Influencing the Glass‐Forming Ability of Alloys

Abstract

Bulk glassy alloys (also called bulk metallic glasses, BMGs) currently attract significant attention in the field of materials science. Research on glassy alloys started after the formation of the first Au-Si sample with an amorphous structure in 1960 [1] at a very high cooling rate of 10 6 K/s. The formation of a glassy (amorphous) phase from a melt takes place through the glass-transition phenomenon. [2,3] Metallic glasses are metastable at room temperature and devitrify/crystallize on heating [4,5] above the crystallization temperature T x (b) (b is the heating or cooling rate) for kinetic reasons. [6] A large number of BMGs, defined as 3-dimentional massive glassy articles with a size not less than 1 mm in any dimension, have been produced during the last decade. [7][8][9] BMGs can be obtained at cooling rates of the order of 100, 10, 1 K/s and even less. [10,11] The largest number of metallic glass applications takes advantage of their exceptional and highly tailorable magnetic properties. [12] Bulk glassy alloys exhibit not only high strength, hardness, wear resistance and large elastic deformation, but high corrosion resistance as well. Moreover, the fatigue-endurance limits of Zr-based alloys are comparable with those of high-strength structural alloys. [13] Pt-based [14] and Zr-based [15] bulk metallic glasses were recently reported to exhibit high room-temperature plasticity. Although some explanations have been given, [16,17] the mechanism of plasticity is not fully established.The combination of exceptional structural and functional properties and the ability to produce bulk product forms has led to a significant number of important applications. However, further discovery, development and application of metallic glasses is limited by an understanding of the fundamental features that control glass-forming ability (GFA) and thermal stability. Bulk glassy alloys possess three common features summarized by Inoue, [4,6] i.e., belong to multicomponent systems with 3 or more constituents, have significant mismatch in atomic radii of greater than 12 %, and exhibit large, negative mixing enthalpies (DH) among the constituent elements. A large number of physical-mathematical parameters (determining characteristics) and factors (features contributing to a particular result) have been proposed so far to indicate the GFA of BMGs. These include a number of derived thermal parameters that depend on the glass transition temperature (T g ), the crystallization temperature (T x ) and the liquidus temperature (T l ). [18] Other factors include compositional proximity to a deep eutectic trough [19] and particular distributions in atomic sizes. [20]. However, none of these guidelines have been established to be sufficiently robust and predictive to be considered as necessary and sufficient for bulk glass formation.In the present paper we consider solidification of a metallic liquid phase by casting and will discuss kinetic, [21,22] thermodynamic, processing and other factors [23,24] as well as semi-empirical parameters ...

Influence of Casting Parameters on the Critical Casting Size of Bulk Metallic Glass

“…Figure 2(b) shows cooling rate data (three repeats) as a function of the mold diameter using an injection temperature of 420°C for the Ca 65 Mg 15 Zn 20 alloy. Although the critical cooling rate of a glass-forming alloy is accurately defined by the nose of the continuous cooling transformation curve normalized by the casting temperature, [28] a useful linear approximation often used to determine the critical cooling rate of a glass-forming alloy can be found using the time interval (Dt) between T l and T g , as indicated in Figure 3, and the following relation, in which [10] dT dt ¼ T l À T g Dt ½1 Figure 4(a) shows cooling rate as a function of the mold thickness during cooling between T l and T g for the Mg 65 Cu 25 Y 10 alloy. The best fit of the data gives dT/dt = 184/Z (°C/s) where Z is the casting thickness (mm).…”

“…Because the thermal conductivity of amorphous alloys is generally much lower than that of the alloys' crystalline counterparts, [33,34] it is important that this parameter is taken into consideration with respect to the possible generation of thermal gradients within a BMG casting. The Biot number (Bi) is often used to identify the rate-limiting factor for the heat-transfer process (cooling) within a given system and is defined as [28,34] Bi ¼ hr=k ½2…”

“…A value close to unity also indicates that thermal conductivity is a significant parameter involved in rapid cooling and, hence, cannot be ignored. [28,34] In the following heat-flow analysis, a cylindrical casting of the diameter D and constant length L ) D will be examined. Assuming that the mold is much larger in extent than the casting and has a much higher thermal conductivity than the casting (and hence remains at a constant equilibrium temperature T o ), the cooling rate in the central zone of a casting is governed principally by thermal conduction in the casting and heat flow through the mold/metal interface.…”

Role of different factors in the glass-forming ability of binary alloys

“…[30] and separated for intrinsic and extrinsic factors. For example, the addition of Zr or Sc substituting for Y reduces the effective DEN values among the constituent elements and increases DT x of an Al-Y-Ni-Co alloy [31].…”

Unusual solidification behavior of a Zr–Cu–Ni–Al bulk glassy alloy made from low-purity Zr

“…These include the reduced glass-transition temperature, T rg ¼ T g /T l [11] where T g is the glass-transition temperature and T l is the liquidus temperature (though the overall validity of this criterion has been questioned recently [12,13]); the width of the supercooled liquid region (DT x ) defined as T x À T g where T x is the crystallization onset temperature [14]; and the g ¼ T x /(T g þ T l ) parameter [15], which somehow combines both DT x and T rg criteria into a single parameter [16] and many other criteria [17,18]. It was shown that purely extrinsic factors also have a significant influence on the GFA [19].…”