Glass formation in la rich la al cu (ni) alloys

GLASS FORMATION IN La-RICH La-Al-Cu-(Ni) ALLOYS TAN HAO (B.Eng., HUST) (M.Sc., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MATERIALS SCIENCE NATIONAL UNIVERSITY OF SINGAPORE 2006 Acknowledgement I am thankful to my supervisor A/Prof. Li Yi for his invaluable guidance and advice throughout my entire candidature in the Department of Materials Science, National University of Singapore. I also appreciate the help offered by my cosupervisor A/Prof. Feng Yuan Ping, for his supervision and constructive advice. Special thanks are due to the technicians of the department: Mr. Chan Yew Weng, Mr. Chen Qun, Mr. Liu Bin Hai, Ms. Ying Hong, Ms. Serene, and Ms. Lim Mui Keow, for their kindly assistance. To the members of the non-equilibrium processing Lab, I extend my very sincere thanks. Their friendship and help during my study will always be wonderful memories on my mind. Words cannot express the debt of thanks I owe to my parents. Without their encouragement and support I would not have had the strength to finish this project. Last but not the least, I would like to acknowledge the support of the National University of Singapore for granting me a research scholarship. Apr, 2006 Singapore Hao TAN i Table of Contents Acknowledgement i Table of Contents ii Summary v List of Tables viii List of Figures x List of Publication xv Introduction Literature Review 2.1 BMG and BMG Matrix Composite . . . . . . 2.1.1 BMG . . . . . . . . . . . . . . . . . . . 2.1.2 Synthesis Methods for BMG . . . . . . 2.1.3 Emergence of BMG Matrix Composite 2.2 Glass Forming Ability . . . . . . . . . . . . . 2.2.1 The Role of Multiple Components . . . 2.2.2 Indicators for GFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 11 18 19 22 Experimental Procedure 27 3.1 Alloy Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 ii Table of Contents 3.2 3.3 3.4 3.5 3.6 Chill Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Suction Casting . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Copper Mould Casting . . . . . . . . . . . . . . . . . . Bridgman Solidification . . . . . . . . . . . . . . . . . . . . . . Melt Spinning . . . . . . . . . . . . . . . . . . . . . . . . . . . Microstructure Characterization Techniques . . . . . . . . . . 3.5.1 X-ray Diffractometry . . . . . . . . . . . . . . . . . . . 3.5.2 Optical Microscopy and Scanning Electron Microscopy Thermal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Differential Thermal Analysis . . . . . . . . . . . . . . 3.6.2 Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 . 28 . 28 . 29 . 30 . 31 . 31 . 31 . 32 . 32 . 32 Locating the Optimum Glass Former in La-Rich La-Al-Cu Ternary System 4.1 Eutectic Alloy near La55 Al25 Cu20 . . . . . . . . . . . 4.1.1 Melting Study . . . . . . . . . . . . . . . . . . 4.1.2 Microstructure Observation . . . . . . . . . . 4.2 GFA Study on La86−x Al14 Cux Alloys . . . . . . . . . 4.2.1 Results for As-spun Ribbons . . . . . . . . . . 4.2.2 Results for Bulk Samples . . . . . . . . . . . . 4.3 Correlation between Trg and GFA in La-Al-Cu Alloys 4.4 GFA Indicator Comparison . . . . . . . . . . . . . . . 4.4.1 Comparison Among the La86−x Al14 Cux Alloys 4.4.2 Comparison of La55 Al25 Cu20 and La66 Al14 Cu20 Phase Selection and Glass Formation 5.1 Criterion for Glass Formation . . . . . . . . . . . 5.2 Eutectic Coupled Zone and Glass Forming Zone . 5.2.1 Binary System . . . . . . . . . . . . . . . 5.2.2 Ternary System . . . . . . . . . . . . . . . 5.3 Parameters Governing GFA . . . . . . . . . . . . 5.3.1 The Effect of Te − Tg on GFA . . . . . . . 5.3.2 Factors Governing Ke . . . . . . . . . . . 5.4 Strategy to Pinpoint the Optimized Glass-forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 34 36 51 58 58 64 69 71 71 72 . . . . . . . . 73 74 76 77 86 90 90 91 94 Locating the Optimized Glass Former in La-Rich La-Al-(Cu,Ni) Quaternary System 97 6.1 Eutectic Alloy near La55 Al25 (Cu, Ni)20 . . . . . . . . . . . . . . . . 98 iii Table of Contents 6.2 6.3 6.4 GFA Study on La66 Al14 (Cu, Ni)20 . . . . . . . . . . . . . . . . 6.2.1 Chill Casting . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Bridgman Solidification . . . . . . . . . . . . . . . . . . 6.2.3 Critical Cooling Rate . . . . . . . . . . . . . . . . . . . Optimizing the GFA for La-Al-(Cu,Ni) . . . . . . . . . . . . . 6.3.1 Thermal Analysis . . . . . . . . . . . . . . . . . . . . . 6.3.2 Microstructural Evolution:Compositional Effect . . . . 6.3.3 GFA Analysis . . . . . . . . . . . . . . . . . . . . . . . 6.3.4 Microstructure Evolution: Effect of Cooling Rate . . . Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System 6.4.1 Microstructure Selection in La-Al-(Cu,Ni) . . . . . . . 6.4.2 Microstructure Selection Map in La-Al-(Cu,Ni) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 100 106 108 110 112 113 117 118 125 126 131 Conclusions 133 Bibliography 136 iv Summary Metallic glass formation was first discovered in the 1960s. Although many metallic glass and bulk metallic glass (BMG) forming systems have been reported, the factors governing glass forming ability (GFA) still remain unclear. There are many parameters for evaluation of GFA, but none of them can be regarded as universal and the predictions for GFA based on them sometimes even contradict each other. The recent developments of the in situ glass matrix composites, which have reinforcing crystalline phases formed in situ within the BMG matrix, have attracted more and more attention throughout the world. These composites demonstrated superior mechanical properties than the monolithic BMGs. However, there is no report on the formation mechanism of this novel material. In order to obtain a better understanding of these two issues, the current studies were carried out. The first part of the work was mainly focused on clarifying the correlation between the reduced glass transition temperature, Trg and GFA. Trg was one of the parameters for evaluation of GFA, but researchers used Tg /Tm and/or Tg /Tl (Tg , Tm and Tl are the glass transition temperature, solidus temperature and v Summary liquidus temperature of the alloy) for the calculation. Therefore, it is of practical importance to find out which definition is more closely related to GFA and the La-rich La-Al-Cu BMG system was selected as the modeling system. Firstly, the eutectic composition, La66 Al14 Cu20 was found by melting studies. Secondly, GFAs of alloys around La66 Al14 Cu20 were investigated. It was also found that there is a stronger correlation between Tg /Tl and GFA than that between Tg /Tm and GFA. Although Trg is a key indicator in determining GFA, it is based on the theory of avoiding detectable nucleation and the requirements may be too stringent for glass formation. Therefore, the concept of phase selection was introduced in glass formation. Microstructure selection maps for binary eutectic systems were constructed based on this concept, which showed clearly how to obtain in situ glass matrix composite, including controlling the microstructure. This concept was later extended to ternary/pseduo-ternary systems and the microstructure selection maps were also constructed. These maps revealed an important feature: the composite forming zone will surround the glass forming zone. Based on this principle, a strategy for pinpointing the best glass forming composition was established. In order to further improve the GFA of La66 Al14 Cu20 , the glass former with better GFA than those in literature, GFA studies were extended to La-rich La-Al(Cu,Ni) system by replacing half the amount of Cu with Ni. It was found that vi Summary La66 Al14 (Cu, Ni)20 is also a eutectic alloy. However, the addition of Ni did not improve the GFA. With the help of our pinpointing strategy, the optimized glass formation was finally achieved in an off-eutectic alloy, La62 Al15.7 (Cu, Ni)22.3 , which can form glassy rod with at least 10 mm in diameter. Further studies around this alloy also revealed the actual microstructure selection map in this system, which is consistent with our prediction. vii List of Tables 2.1 Critical size for glass formation in various alloys systems. . . . . . . 4.1 DTA results for La55 Al45−x Cux alloys. . . . . . . . . . . . . . . . . . 38 4.2 DTA results for La50 Al50−x Cux alloys. . . . . . . . . . . . . . . . . . 40 4.3 DTA results for La77−x Alx Cu23 alloys. . . . . . . . . . . . . . . . . . 42 4.4 DTA results for La65 Al35−x Cux alloys. . . . . . . . . . . . . . . . . . 45 4.5 DTA results for La88−x Al12 Cux alloys. . . . . . . . . . . . . . . . . . 47 4.6 DTA results for La86−x Al14 Cux alloys. . . . . . . . . . . . . . . . . . 49 4.7 DSC results of as-spun ribbon samples for various alloys. . . . . . . 63 4.8 DSC results for chill casting samples . . . . . . . . . . . . . . . . . 65 4.9 DSC results of Bridgman samples obtained under various growth velocities for La66 Al14 Cu20 . . . . . . . . . . . . . . . . . . . . . . . . 67 4.10 Comparison of Trg based on different definitions. . . . . . . . . . . . 69 4.11 Comparison of Trg , ∆Tx and γ. . . . . . . . . . . . . . . . . . . . . 71 5.1 Values for Te − Tg , mα , mβ and Rc for some typical glass-forming and non glass-forming alloy systems. . . . . . . . . . . . . . . . . . 91 6.1 Values of Tm and Tl for La66−x Al14 (Cu, Ni)x alloys. . . . . . . . . . . 99 6.2 Values of Tm and Tl for La100−x [Al0.412 (Cu, Ni)0.588 ]x alloys. . . . . . 100 6.3 DSC results for La66 Al14 (Cu, Ni)20 samples. . . . . . . . . . . . . . 105 viii List of Tables 6.4 DSC results for La66 Al14 (Cu, Ni)20 samples. . . . . . . . . . . . . . 106 6.5 Results of DSC analysis at a heating rate of 40 K/min and the critical diameter for glass formation for La100−x [Al0.412 (Cu, Ni)0.588 ]x alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 ix 6.4 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System 20 ✤m (a) 20 ✥ m (b) 50 ✦ m (c) 50 ✧m (d) Figure 6.21 Microstructure for in situ composites with variation in type and volume percentage of reinforcing crystalline phases: (a) La66 Al14 (Cu, Ni)20 , (b) La70 Al12.4 (Cu, Ni)17.6 , (c) La72 Al11.5 (Cu, Ni)16.5 and (d) La74 Al10.7 (Cu, Ni)15.3 . 127 6.4 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System 50 ★ m (e) Figure 6.21 Microstructure for in situ composites with variation in type and volume percentage of reinforcing crystalline phases (continued): (e) La60 Al10 (Cu, Ni)30 . 6.4.1.2 Glass Matrix with Primary Single Phase The graphs in Fig. 6.21 show composite microstructure with primary dendrites embedded in glassy matrix. This type of microstructure belongs to Region of Fig. 5.6. It can be found from Figs. 6.21(a) to 6.21(d) that the microstructural length scales (the dendrite tip radius, arm spacing etc.) and the volume fractions for the dendritic phase are different. Thus, it can be concluded that the microstructural length scales can be controlled. It is worthy to note that the crystalline phase in Fig. 6.21(e) is totally different from those in Figs. 6.21(a) to 6.21(d). Therefore, it confirms that the type of reinforcing phase can be varied. It follows obviously and accordingly that the mechanical behavior of the glass matrix composite can be adjusted or tailored as required. 128 6.4 6.4.1.3 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System Glass Matrix with Eutectic 20 ✩ m (a) 200 ✪ m (b) Figure 6.22 Microstructure for in situ composites with eutecticlike reinforcing phases: (a) La65 Al5 (Cu, Ni)30 and (b) La70 Al15 (Cu, Ni)15 . Figure 6.22 illustrates a glass matrix with eutectic-like structures distributed inside. This kind of microstructure is expected in Region of Fig. 5.6. As this type of microstructure differs greatly from those in Fig. 6.21, difference in mechanical properties can be expected. 129 6.4 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System 10 ✫m (a) 20 ✬m (b) Figure 6.23 Microstructure for in situ composites with new type of reinforcing primary phase microstructure: (a) La65 Al10 (Cu, Ni)25 and (b) La60 Al23.5 (Cu, Ni)16.5 . 6.4.1.4 Glass Matrix with Mixed Crystalline Phase Figure 6.23 reveals the microstructure of glass matrix with mixed crystalline phases. This type of microstructure is expected in Region of Fig. 5.6. The microstructure in Fig. 6.23a can be characterized as two kinds of dendrites with different colors distributed within the matrix. The amount of the black dendrite is much less than the other dendritic phase. There are basically two types of crystalline phase with different shapes in Fig. 6.23(b). One shape is plate/block like while the other is dendritic. Moreover, as the crystalline phases in Fig. 6.23(b) have different shapes from those in Figs. 6.21, 6.22 and 6.23(a), it strongly indicates that there is a new composite region (Region in Fig. 5.6) nearby. 130 6.4 6.4.2 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System Microstructure Selection Map in La-Al-(Cu,Ni) 40% 60% 50% Crystalline Composite 40% 80% 90% 30% 20% i) N u, 70% (C La 60% 100% 0% Glass 50% 10% 10% 20% 30% 40% 50% 60% 0% Al Figure 6.24 Microstructure selection map under a fixed cooling rate of around 160 K/s for the La-rich La-Al-(Cu,Ni) system . The numbers stand for the regions as shown in Fig. 5.6. Figure 6.24 shows the microstructure selection map of La-Al-(Cu,Ni) by combining the results from SEM. It clearly showed the existence of different microstructure selection zones in (pseudo-)ternary systems, as predicted by Fig. 5.6. Compared with Fig. 5.6, it can be clearly seen that the real microstructure selection map is more complex and each region becomes more irregular. It should be noted that the boundaries given in Fig. 6.24 is estimated based on selected number of alloys. Exact boundaries can be determined by studying more samples. Also, specific boundaries (including size and arm-spacing of the primary 131 6.4 Microstructure Selection Map in La-rich La-Al-(Cu,Ni) System phases, volume percentage, etc.) can be added, which can make the microstructure selection map more practicable for designing composite with desired properties. 132 Chapter Conclusions The present study revealed the existence of a eutectic composition in the Larich La-Al-Cu ternary system and La-Al-(Cu,Ni) quaternary system. Based on DTA and SEM results, La66 Al14 Cu20 and La66 Al14 (Cu, Ni)20 alloy are eutectic compositions. Chill casting results showed that La66 Al14 Cu20 can form glassy rod with mm in diameter, which is the largest size in La-Al-Cu system by convention processing methods. Bridgman experiments revealed that the critical cooling rate for La66 Al14 Cu20 is about 37.5 K/s. It was found that the reduced glass transition temperature, Trg , should be evaluated by Tg /Tl . There is a strong correlation between the reduced glass transition temperature defined by Tg /Tl and the glass-forming ability (GFA) in the 133 7. Conclusions La86−x Al14 Cux alloys. As the Cu content increased from 10 to 36%, there was a maximum for values of Tg /Tl for the La66 Al14 Cu20 alloy (as high as 0.56), which is consistent with the experimental fact that this alloy has the optimum GFA among the alloys. By treating glass as one of the competition phases during cooling of a glassforming liquid, a new glass formation criteria and the concept of glass/compositeforming zone were established. Microstructure selection maps based on this criteria revealed the formation mechanism for the in situ glass matrix composite. It also successfully explained why in some alloy systems, the optimum glass formation can only be achieved at off-eutectic compositions, which is due to the skewed glass-forming zone associated with such systems. The concept of glass-forming zone has one important feature: the composite-forming zone will surround the glass-forming zone. Thus, it allows one to pinpoint the alloy composition with the optimum GFA experimentally: the switch in morphology (from composite A to glass then to composite B) can nail down the alloy composition with optimized GFA. Based on these analysis, the La-rich La-Al-Cu system has a symmetric type of glass-forming zone. Our experimental results showed that the eutectic alloy, La66 Al14 (Cu, Ni)20 , can only form glassy rods with at most mm in diameter. DTA studies showed that the liquidus surface around the eutectic were not symmetric, suggesting a skewed eutectic coupled zone associated with this system. Pinpoint strategy was successfully applied to the pseudo-ternary La-Al-(Cu,Ni) system. With its help, the optimized glass formation was achieved at an off-eutectic composition, 134 7. Conclusions La62 Al15.65 (Cu, Ni)22.35 . A glassy rod of 10–12 mm in diameter was obtained at this off-eutectic composition. There is a strong dependence of GFA on the component element in La100−x [Al0.412 (Cu, Ni)0.588 ]x alloys. There is a narrow ”Λ–shape” for the GFA of these alloys, which means the optimized glass formation can be easily missed if the alloy compositions were not carefully designed. 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Phys., 43: 353, 1980. 142 [...]... of Cu content for La8 6−x Al1 4 Cux alloys 48 4.14 Compositional distribution of the alloys investigated by DTA 50 4.15 Liquidus surface constructed by Tl of the alloys investigated 50 4.16 Typical microstructure of La5 5 Al4 5−x Cux alloys: (a )La5 5 Al2 5 Cu2 0 and (b) La5 5 Al2 0 Cu2 5 52 4.17 Microstructure of La7 6−x Cu2 3 Alx alloys: (a) La6 9 Cu2 3 Al8 , (b) La6 6 Cu2 3 Al1 1... and Tl as a function of Cu content for La7 7−x Alx Cu2 3 alloys 42 4.8 DTA traces for La6 5 Al3 5−x Cux alloys 43 4.9 Tm and Tl as a function of Cu content for La6 5 Al3 5−x Cux alloys 44 4.10 DTA traces for La8 8−x Al1 2 Cux alloys 45 4.11 Tm and Tl as a function of Cu content for La8 8−x Al1 2 Cux alloys 46 4.12 DTA traces for La8 6−x Al1 4 Cux alloys (a) x=10, (b) x=14,... of Figures 4.1 Melting trace of La5 5 Al2 5 Cu2 0 alloy 35 4.2 DTA traces for La5 5 Al4 5−x Cux alloys 36 4.3 Tm and Tl as a function of Cu content for La5 5 Al4 5−x Cux alloys 37 4.4 DTA traces for La5 0 Al5 0−x Cux alloys 38 4.5 Tm and Tl as a function of Cu content for La5 0 Al5 0−x Cux alloys 39 4.6 DTA traces for La7 7−x Cu2 3 Alx alloys ... and test the validity of our phase selection model, GFA studies were extended to La- rich La- Al- (Cu, Ni) system by replacing half the amount of Cu with Ni It was found that La6 6 Al1 4 (Cu, Ni)20 is also a eutectic alloy However, the addition of Ni did not improve the GFA Following our pinpointing strategy, the optimized glass formation was finally achieved in an off-eutectic alloy, La6 2 Al1 5.7 (Cu, Ni)22.3... stringent for glass formation since the successive growth of those already formed nuclei may be suppressed and the remaining liquid can still form glass Thus, in principle, when analyzing GFA of alloys, not only avoiding nucleation, but also suppressing subsequent growth, should be contemplated Therefore, the concept of phase selection was introduced in glass formation and theoretical analysis based... Cu2 3 Al1 1 , (c) La6 3 Cu2 3 Al1 4 and (d) La6 0 Cu2 3 Al1 7 54 4.18 Microstructure of the La5 0 Al2 6 Cu2 4 alloy 55 4.19 Microstructure of La8 6−x Al1 4 Cux 57 4.20 XRD results of as-spun ribbon samples for La5 5 Al2 5 Cu2 0 and La8 6−x Al1 4 Cux alloys 59 4.21 DSC traces of the as-spun ribbon samples for La5 5 Al2 5 Cu2 0 alloy 61 4.22... correlation between Tg /Tm or Tg /Tl and GFA needs to be clarified in detail Based on Trg criterion, generally, glass forming alloys have compositions close to eutectics, i.e., the glass forming ranges by rapidly quenching are always around the eutectic compositions [6] In 1990, Inoue et al reported that La5 5 Al2 5 Cu2 0 can be made into glassy rods with 3 mm in diameter by high pressure die casting [7],... Subsequently, glassy rods of 4 mm and 7 mm in diameter were reported in Mg65 Cu2 5 Y10 (by metallic mold casting and high-pressure die casting respectively) [20, 21] In 1992, Li et al reported BMG formation with 3.5 mm in thickness in Mg65 Ni20 Nd15 by wedge casting [22] Meanwhile, BMGs were also found in Zr-based alloys Glassy rod with 7 mm in diameter was produced in Zr65 Al7 .5 Ni10 Cu1 7.5 by metallic mold... La7 0 Al1 2.4 (Cu, Ni)17.6 , (c) La7 2 Al1 1.5 (Cu, Ni)16.5 and (d) La7 4 Al1 0.7 (Cu, Ni)15.3 127 6.22 Microstructure for in situ composites with eutectic-like reinforcing phases: (a) La6 5 Al5 (Cu, Ni)30 and (b) La7 0 Al1 5 (Cu, Ni)15 129 6.23 Microstructure for in situ composites with new type of reinforcing primary phase microstructure: (a) La6 5 Al1 0 (Cu, Ni)25 and (b) La6 0... casting [23] About two years later, this diameter was increased to 16 mm by water quenching for the same alloy [24] Soon after that, a Be containing Zr based alloy, Zr41.2 Ti13.8 Cu1 2.5 Ni10.0 Be22.5 was reported to be able to form glassy rods at least 14 mm in diameter by water quenching [25] In 1996, large bulk glass formation up to 30 mm in diameter by suction casting was achieved in Zr55 Al1 0 Ni5 Cu3 0 . Strategy to Pinpoint the Optimized Glass- forming Alloys . . . . . . 94 6 Locating the Optimized Glass Former in La- Rich La- Al- (Cu, Ni) Quaternary System 97 6.1 Eutectic Alloy near La 55 Al 25 (Cu, Ni) 20 La 76−x Cu 23 Al x alloys: (a) La 69 Cu 23 Al 8 , (b) La 66 Cu 23 Al 11 , (c) La 63 Cu 23 Al 14 and (d) La 60 Cu 23 Al 17 . . . . . . . . . . . . . . . . . 54 4.18 Microstructure of the La 50 Al 26 Cu 24 alloy 136 iv Summary Metallic glass formation was first discovered in the 196 0s. Although many metallic glass and bulk metallic glass (BMG) forming systems have been r eported, the factors governing glass forming