Study On Bulk Glass-Forming Ability Of Binary Alloys

Amorphous solid can be manufactured by various methods from crystalline materials with the major bonding types such as ionic, covalent, van der Waals, hydrogen, and metallic bonding. The type of amorphous alloy with the mainaly metallic bonding is called metallic glass (MG). Since the first MG made by Pol Duwez at California Institute Technology (Caltech) of USA in 1960 via rapid quenching techniques at very high rate of about 106 K/s. MGs have been drawing increasing attention due to their scientific and engineering significance. Among the ways by which the MGs can be achieved, the rapid quenching techniques are widely used. In the early 1970's and 1980's, the researches of MGs are focused on several alloy systems, of which the thickness or diameter is in the scale of the millimeters. Chen and Tturnbull reported the first BMG in 1974, and they obtained a critical casting thickness of 1mm in Pd-T-P (T=Ni, Co, Fe) alloys at a significantly lower cooling rate of 10³ K/s. Since 1980's, Inoue'group at Tohoku University of Japan and Johnson's group of Caltch have discovered strongly glass-forming multi-component La-, Mg-, Zr-, Pd-, Fe-, Cu-, and Ti-based alloys with large undercoolingand and the casting thicknesses in several centimeters at low critical cooling rates of 1K/s to 100K/s, which is similar to oxide glasses. Up to this century, many attentions were focussed on the development investigation of multi-component BMGs and excellent multi- component BMGs, such as Pd40Cu30Ni10P20 (the maximum thickness D_max of 10cm and the critical cooling rate R_c of 0.10K/s) and Zr41Ti14Cu12.5Ni10Be22.5 (D_max=5¹0 cm, R_c=1 K/s).Generally, the development of a new BMG with good glass-forming ability (GFA) can be predicted by a criterion for determining the GFA. In 1949 year, Turnbull augured that the reduced glass transition temperature Trg=T_g/T_m can predict the GFA of an alloy and then, in 1969 year, defined the famous"T_g/T_m=2/3 law"as the criterion for glass formation. Up to now after Turnbull, the various modifications of Trg criterion, such as the undercooled liquid regionΔT (=T_x-T_g), parameters K (=(T_x-T_g)/(T_l-T_x)) andγ(=T_x/(T_g+T_l)), have been referred to the determining for GFA. There also suggested the criterions in light of the thermodynamic and thermochemical data, such as parametersγ*(=ΔH^amor/(ΔH^int-ΔH^amor)) andε(= -ΔS^mix/ΔH^mix), whereΔH^amor,ΔH^int,ΔS_mix, andΔH_mix are the formation enthalpies for glasses and intermetallic compounds, mixing entropy and mixing enthalpy, respectively. Although above mentioned parameters are used as indicators of the GFA for metallic glasses, there are some difference in their sense of GFA when the alloy system changes from one to other or alloy composition changes for a given alloy system.On the other hand, in light of the historical viewpoint of the development of metallic glasses, most of the MGs were mainly binary alloys before the end of the 1980's though the first BMG was made in 1974 year, while the BMGs were mostly muti-component alloys, found at the end of the 1980's and the early of the 1990's. According to Greer's"confusion principle"and Inoue's"three component rule", containing at least three elements has long been considered a"must"rule for the bulk glass formation. Since the bulk glass formation was solely the privilege of mlti-conponent alloys, the binary alloy systems have received dramatically reduced attention. Therefore, there bring forward questions of the reason why binary alloys should distinctly differ from muti-component alloys in terms of GFA but nobody has answered for the question so far. Inoue and his colleagues, in their recent paper, concluded the following;"it is well known that no bulk glassy alloy is formed in Cu-Zr binary alloys by the copper mold casting method". In 2004-2005 years, however, Xu and Johnson at Caltech successfully prepared the binary bulk metallic glasses (BBMGs) with a critical casting thickness up to 2 mm in Cu-Zr and Cu-Hf stystems, by using the copper mold casting method. Also in China at the same time, by W.H.Wang (Cu₅₀Zr₅₀ BBMG), D.Wang (Cu_64.5Zr_35.5 BBMG), K.F.Yao (Pd₈₁Si₁₉ BBMG), L.Xia (Ni₆₂Nb₃₈ BBMG) et al., many BBMGs with the maximum thickness from 2 mm (typicaly, Ni₆₂Nb₃₈) to 6 mm (Pd₈₁Si₁₉) are developed. These results indicate directly that the previously well-accepted"confusion principle"and"three component rule"for BMG formation are actually wrong and there may not be a strict boundary between the GFAs of binary alloys and of multi-component alloys. The BBMG should be an interesting subject for theoritical studies on the fundamental problem of glass formation as it possesses both the simplicity of binary alloys and the good GFA like multi-component alloys. Moreover, from an engineering point of view, such a BBMG might provide a important guidance for developing a new BMG with extremely good GFA and might improve the current alloy developing efficiency considerably. However, because the natures of the BBMG formation still remain in the unknown problems although there are some models explaining them mentioned above, the answer to the question of the reason why binary alloys could form into bulk glassy states like muti-component alloys is very important. Recently, by our research team, the correlation between the GFA of BMG and the average valence electron concentration per atom e/a concerning withΔH including mixing entropyΔS are established in light of the Gibbs free energy differenceΔG. Although the results obtained from the multi-component BMGs and used to design the component number, one should adopt for predicting the GFA of binary BMGs. In this thesis, the study is focused on binary metallic glasses, and in light of the corelation between GFA and the mixing entropy differenceΔS, thermodynamic qualifications are suggested. Moreover, by using thhermodynamic data known previously, a parameter as new criterion for predicting the GFA of BBMG is proposed. The present study chatacteristic is that all results referred to the analysis of the equilibrium phase diagram.The detailed contents are listed as follows:1. The thermodynamic qualification for BBMG formation is interpreted in light of the valance concentration per atom, e/a, and the mixing entropy difference between liquid and crystals,ΔS, which are concerned with Gibbs free energy difference,ΔG. In general, the GFA of a glass former is concerned with its Gibbs free energy difference between liquid and crystals,ΔG, and the smallerΔG, the better GFA is the glass formation. SinceΔG=ΔH-TΔS, ifΔS reaches the maximum at a certainΔH(or e/a), the liquid has the smallestΔG. There are suggested that for the best GFA of multi-component glass-forming liquid, the e/a must be 3.5 andΔS has the certain value of about 5 J mol-1K-1 in our early model. Using of the model is introduced to interpret the GFA of several binary glass-forming alloys. Usually, the average valent electron concentration per atom is calculated as e/ a=∑_iⁿ x_ie_i, when total amount of an alloy consisting of n-components is∑_iⁿ x_i=1 in atomic fraction (or molar fraction), where xi is component amount and ei is the valence, used from the latest for e values, of i-th component, respectevly. On the other hand, the the mixing entropy difference,ΔS(= Sl–Sc), between the liquid (denote l ) and the corresponding crystal (denote c), can be estimated after firstly making certain crystalline phase (or metastable compound phase), which competes with the glass formation. In light of the previous results reported for the mode of primary crystallization (MPC) of some BBMGs and conventional binary metallic glasses in literatures, there have selected the a metastable eutectic systems (MES), which is consisted of a metastable compound (to be major competitor with glass formation) and a terminal solid solution on binary equilirium phase diagram. And then, according to our structure model, the competing crystallin phases with AmBn-type structure have the artifical redistribution structure (ARS) into a forms of (A_αB_β1)(B_β2) or (A_α1)(B_βA_α2), which is necessary for calculatiing S_c. The choice of the MES is based on two major assumptions, as follows: (1) the metastable compound which its melting point is the highest among all compound phases in given the alloy sytstem; (2) the metastable compound which its liquidus is linked directly with the lowest eutectic line, except for peritectic. Of course, the composition of considering alloy should be in the metastable eutectic system between the metastable compound and terminal solid solution. In present work, the some binary metallic glass alloy, such as Cu-Zr, Cu-Hf, Ni-Nb, Fe-B, Pd-Si and Ni-P introduced in previous studies so far, have been deal with the metastable eutectic system points of view. Else, some of Zr-based binary conventional metallic glasses (BCMGs) were considerated in a single metastable phase formation points of view. The results show that for BBMGs such as Cu-Zr(or -Hf) and Ni-Nb, their GFA are well interpreted in terms of our early criteria of e/a andΔS. Although their e/a values have a little departure from the criteria e/a=3.5,ΔS≈5 J·g-atom^-1K^-1 for all BBMGs. On the other hand,ΔS≤4.5 J·g-atom^-1K^-1 for the conventional binary metallic glasses (be not bulk glassy state). These results imply that, under a certain e/a value condition, the mixing entropy difference between liquid phase and corresponding crystalline phase, competing with liquid phase, dominates the GFA for BBMG formation andΔS should be one among important thermodynamic parameters for interpreting the GFA of a binary glassy alloy. Moreover, if for an alloy, its mode of primary crystallization (MPC) is known, the GFA of the alloy can be estimated preparedly byΔS criteria though can not predict the composition of an alloy with good GFA.2. Above mentioned indicators of GFA for BMGs are widely used up to now although there are some difference in their sense of GFA. Those parameters can be determined only after a thermal analysis, i.e., only after BMGs have been fabricated. Thus, these parameters can hardly supply a composition-design principle to find an alloy having the best GFA. Therefore, deepening premise to a more general thermodynamic basis is necessary for the composition design of BMG. In our study, a parameter ? as a new criterion for glass-forming ability (GFA) of the binary conventional and bulk metallic glasses (BCMGs and BBMGs) has proposed by using the thermodynamic data and the equilibrium phase diagram. Theφparameter is defined byφ=1- [R(T_m^mix-Te)] /ΔH_m^mix, considering the relative thermodynamic stability of the liquid phase. The parameterφreflects the relative decreasing degree of the drive force for crystallization nearby eutectic composition, the largerφ, the better will be the GFA in different binary alloy systems. The precent results show thatφhas a good correlation with GFA whereφ∝1/log Rc. Moreover, the result suggests thatφ≥0.73 for bulk metallic glasses. On the other hand, ? is a sensitive parameter of the alloy composition. For example, the composition difference of Pd82Si18 and Pd81Si19 alloys is only 1 at. %, the correspondingφvalues however are 0.73 and 0.76, respectively. In our knowledge, no any criterion for GFA up to now could show a so large change with this small composition change. The relationship between theφand the GFA can be expressed as R_c=7.73×10²³.exp(-65.26φ), where R_c is critical cooling rate for glassy state. Since ? parameter can be calculated simply using thermodynamic and temperature data on equilibrium phase diagram, the GFA for a binary glass former can be easily estimated using this parameter without any thermal analysis for a metallic glass. The use of this parameter is simple relatively in the compare with the previous criterion, because using the well-known thermodynamic data and the equilibrium phase diagram, without any analytical experiment, can determinate it. Since all of the above consideration has not limited the component numbers of an alloy system although the present results concerned with binary systems, it could be extended to multi-component systems if the corresponding eutectic temperatures of the alloys are known.