Formation And Crystallization Behaviors Of Al-TM-RE Based Amorphous Alloys

Al-based amorphous alloys containing Al over 85at.% possess high strength and good ductility. The strength can be further increased to above 1500 MPa when nanometer-sized fcc-Al precipitates dispersedly in the amorphous matrix by partial crystallization. Such composites exhibit the highest specific strength among all the metallic materials developed so far. However, limited by low glass-forming ability (GFA), the existing Al-based amorphous alloys can only be fabricated in ribbons, powders or foils by rapid solidification, which considerably confines their practical applications. Therefore, to improve the GFA of Al-based amorphous alloys and further understand their crystallization behaviors becomes an urgent subject to be solved in materials science field.In this dissertation, the effects of alloy composition, replacement between the elements, addition of Zr and trace Ti/B on the formation and crystallization behaviors of Al-Ni-La amorphous alloys were investigated by X-ray diffractometer (XRD) and differential scanning calorimetry (DSC). The main results are as follows:For Al-Ni-La alloys, La plays a greater role in improving the GFA than Ni. The GFA remains almost unchanged when Ce or Y partially replaces La or minor Zr replaces RE in Al₈₅Ni₆RE₉ alloys. The GFA decreases when excessive Cu or Co replaces Ni or trace Ti/B (especially Ti) is added.There exist four types of primary phases forming during crystallization of Al-Ni-La amorphous alloys, i.e. single fcc-Al, fcc-Al + bcc-(AlNi)11La3-like, fcc-Al + bcc-(AlNi)11La3-like + MP1 (an unknown metastable phase) and single MP1. The maximum La content resulting in the precipitation of single fcc-Al as primary phase in the alloys does not exceed 5%. A relatively high content of solutes, especially that of La, promotes the precipitation of metastable phase(s). Replacing Ni by Cu or addition of Zr promotes the precipitation of fcc-Al, while replacing Ni by Co or increasing the atomic radius and content of RE promotes the precipitation of metastable phase(s). Replacing La by partial Ce or Y or addition of trace Ti/B almost does not change the primary phase. It is found that the main factor affecting the type of primary phase is the diffusion coefficient of atoms for the element with larger atomic radius than Al or the maximum solid solubility S_max of atoms in Al at equilibrium for the element with smaller atomic radius than Al during the process of changing the content or replacing the elements. The rapider the diffusion rate of atom is, or the larger the S_max is, the easier the precipitation of fcc-Al is. Otherwise, metastable phase(s) tends to precipitate.A relatively high content of solutes, especially that of La, improves the thermal stability of Al-Ni-La amorphous alloys. The thermal stability is evidently enhanced when Co replaces Ni, or slightly improved when Ce or Y partially replaces La or trace Ti/B (especially Ti) is added. However, it becomes worsened remarkably when Cu replaces Ni. The thermal stability is significantly improved when RE in Al₈₅Ni₆RE₉ alloys is replaced by Zr together with Co. In the four series of the investigated alloys i.e. Al₈₇Ni₆RE7, Al₈₅Ni₆RE₉, Al₈₅Ni₆RE7Co₂ and Al₈₅Ni₆RE5Co₂Zr2, the thermal stability first remains almost unchanged when RE changes from La to Ce, and then gradually decreases when RE further changes through Nd to Y. The thermal stability of Al-based amorphous alloys can be interpreted by melting point Tm of the Al-rich intermetallic compound with minimum solute concentration combined with heat of mixingΔH^mix between atoms. The larger the Tm is or the more negative theΔH^mix is, the higher the thermal stability is. A parameterδis defined by , where C_i ' and T_m,i are the relative atomic percent of the i-th solute and melting point of the abovementioned Al-rich intermetallic compound respectively, C A'l and Tm,Al are the residual atomic percent and melting point of Al solvent. The crystallization onset temperature Tx1 of Al-TM-RE and Al-Ni-Cu(Co)-La alloys exhibits a good straight line relationship withδ. Al-Ni-La amorphous alloys exhibit lower thermal stabilities at given heating rates compared with those of multicomponent bulk metallic glasses, but their long-term thermal stabilities (LTTS) are better. Replacing Ni by an appropriate content of Cu or Co and replacing La by partial Ce or Y improve the LTTS. The effects of addition of Zr, type and content of RE on the LTTS of Al-Ni-RE amorphous alloys are complicated. Addition of trace Ti/B does not improve the LTTS.A relatively high content of solutes, especially that of La, promotes appearance of glass transition and stabilizes the supercooled liquid region of Al-Ni-La amorphous alloys. Glass transition is intensified when Co replaces Ni or Y partially replaces La, while it becomes weakened when Cu replaces Ni or remains almost unchanged when Ce partially replaces La. Replacing RE by Zr together with Co in Al₈₅Ni₆RE₉ alloys enhances the glass transition evidently. In the four series of the investigated alloys, the supercooled liquid regionΔTx of Al₈₅Ni₆RE₉ and Al₈₅Ni₆RE5Co₂Zr2 alloys gradually increases in the order of La, Ce, Nd and Y and the increasing tendency is more obvious for the latter one. The glass transition of Al-based amorphous alloys can be interpreted by difference in electronegativityΔEN between the solute and solvent atoms combined with heat of mixingΔH^mix between atoms. The smaller theΔEN is, or the more negative theΔH^mix is, the larger theΔTx is. Another parameterφis defined by , where Ci andΔENi are the atomic percent and difference in electronegativity of the i-th solute. Al-Ni-La amorphous alloys can be divided into two groups according to the values ofφ, formation enthalpyΔH^Form andδ, respectively. That is to say, the alloys with glass transition correspond toφ≥0.0516,ΔH^Form≤-10.23 kJ/mol andδ≤566.5, respectively.Apparent activation energy Ea1 for the first crystallization reaction in Al-Ni-La amorphous alloys is related to the type of primary phase, that is, Ea1 is greater for eutectic crystallization than that for primary crystallization, and reaches the highest when the number of products is the most. Ea1 first increases and then remains almost unchanged when Ce or Y partially replaces La. Ea1 remains unchanged by addition of trace Ti/B, but decreases or first increases and then decreases when Cu or Co replaces Ni. After replacing RE by Zr in Al₈₅Ni₆RE₉ alloys, the change in the Ea1 depends on the type of RE. Among the four series of the investigated alloys, Ea1 of Al₈₇Ni₆RE₇, Al₈₅Ni₆RE₉ and Al₈₅Ni₆RE7Co₂ alloys first increases when RE changes from La to Ce, and then gradually decreases when RE further changes through Nd to Y, while that of Al₈₅Ni₆RE5Co₂Zr₂ alloys remains unchanged with various RE. With increasing the La content, the first crystallization reaction in Al_94-xNi₆La_x alloys proceeds as the direct growth of quenched nuclei when x=4 and 5, but it does as the diffusion-controlled growth for x=6-9. Meanwhile, the nucleation changes from a decreasing to increasing rate. For Al₈₇Ni₆RE₇ and Al₈₅Ni₆RE₉ alloys, the first crystallization reaction is the diffusion-controlled growth with nucleation rates depending on the type and content of RE, except for the Al₈₅Ni₆Nd₉ alloy whose growth is interface-controlled. The first crystallization reaction with direct growth of quenched nuclei generally occurs in the Al-Ni-RE alloys with low contents or small atomic radius of RE.