Development of New High Strength Alloy in Cu-Fe-Si System through Rapid Solidification
Copper based alloys play important role in high heat flux applications, particularly in rocket technology, the liner of the combustion chamber, and also in other heat transfer vessels. In these applications, one needs excellent high-temperature strength without sacrificing the thermal conductivity significantly. However, it is a challenging and difficult task to significantly improve the balance between strength and conductivities (electrical and thermal) of Cu-based alloys. In general, microstructural attributes, responsible for increasing mechanical strength of the alloy, also affect the transport properties by creating scattering centers. Hence, delicate optimization is needed for developing balanced alloy system for better performance. A substantial amount of research efforts has therefore been focused on devising methodologies to synthesize copper based alloys with a good combination of strength and conductivity. The present thesis deals with the development of a newer class of high strength high conductivity copper base alloy through tuning of phase transformation and careful additions of ternary and quaternary alloying elements and ultimately by microstructural engineering. In this thesis, we report the development of novel high strength high conductivity Cu-based alloy series in the Cu-Fe-Si system through rapid solidification process using suction casting apparatus. We have also optimized the alloys by altering and fine tuning the alloy compositions in order to achieve balanced and optimum properties. The strength of copper can be increased by various strengthening mechanisms. In general, precipitation hardening, dispersion strengthening and solid solution strengthening are the three most effective mechanisms for improving the strength of copper. Among these, solid solution strengthening has the most detrimental effect on the transport properties due to the presence of solute atoms which act as prominent scattering centres. Precipitation hardened copper alloys are often unable to retain strength at high temperatures, due to the coarsening of the precipitates. Currently, efforts are being made to develop newer dispersion strengthened copper alloys. These alloys contain a fine dispersion of nanometer sized oxides or other intermetallic compounds in the copper matrix. Dispersion strengthened copper alloys show impressive mechanical strength as well as thermal stability. In this thesis, we have explored the possibility of obtaining structurally ordered intermetallic dispersions through exploiting immiscibility of solutes in copper based alloys. The immiscibility promotes precipitation and decrease the solid solubility of solute elements in the matrix which in turn minimizes the scattering process and thus offers the possibility of improved transport properties. These ordered and coherent dispersion of intermetallic particles in the continuous copper matrix, dispersed during solidification, are believed to be the main contributor to the improvement of mechanical strength of the alloy. Crystallographically ordered structure and the coherency strain associated with the intermetallic particles in the copper matrix, together contribute to the mechanical strength through the mechanism of order hardening and coherency strengthening. These also, promote a low interfacial energy between precipitates and matrix in the alloy. This low interfacial energy reduces the driving force for coarsening process and thus helps in retaining the mechanical strength at elevated temperatures. Releasing of coherency strain at the precipitate-matrix interface with increasing temperature also yields a dramatic effect on the enhancement of thermal conductivity at high service temperatures. In the current study, we have selected three alloy compositions in the Cu-Fe-Si system at the higher end of copper. These are Cu-20Fe-5Si (at%), Cu-2.5Fe-2.5Si (at%) and Cu-1.0Fe-1.0Si (at%) respectively. We have systematically increased the concentration of copper, and altered the ratio of Fe and Si in order to achieve the better combination of properties (mechanical and transport) through fine tuning the microstructure. The present sets of alloys have been chill cast by the suction casting technique. This rapid solidification process, associated with moderate undercooling, is capable of accessing the submerged metastable miscibility gap of the Cu-Fe binary system. The higher quenching rate moves the system far away from equilibrium and hence, the solidification process occurs at the non-equilibrium regime. Rapid solidification of a copper rich Fe-Cu melt promotes the precipitation of the γFe from copper solid solution due to the immiscibility of Fe and Cu. In this scenario, the addition of a small quantity of silicon as a ternary element leads to its partition to both copper and iron rich phases. However, the larger chemical affinity between Fe and Si, leads to the formation of an ordered structure. However, the FCC crystal field of the copper matrix tends to promote an FCC based novel L12 ordered structure of the Fe3Si intermetallic particles instead of the ordered DO3 structure of Fe3Si composition normally observed in the bulk alloy. This nano meter sized L12 ordered particles maintain a cube-on-cube orientation relationship with the surrounding copper matrix and are associated with large coherency strain. A good lattice matching between these L12 ordered particles and copper matrix will promote a low interfacial energy and thus, a low driving force for particle coarsening. The present thesis is divided into eight chapters. The first chapter introduces the present work and the organization of the thesis. In the second chapter, current status in the development of the copper alloys and the general principle of alloy developments has been described. This includes both experimental and theoretical developments that can be used for developing high strength Cu based alloys. Chapter three, titled as „experimental procedure‟, describes the detailed description of materials and experimental techniques, adopted for the current studies. There are three chapters that deal with the main results of the thesis. Chapter eight, describes the suggestion for future work. The fourth chapter, titled as „Chill cast Cu75Fe20Si5 alloy: Microstructural Evolution and Properties‟, explores the detailed microstructural evolution of the Cu75Fe20Si5 alloy. This chapter also discusses the microstructure-property correlations. The microstructure of the alloy exhibits a multi-scale hierarchical structure during rapid solidification. The solidified microstructure contains Fe-rich globules with DO3 ordered structure, embedded in the continuous Cu-rich matrix. The continuous copper matrix also contains nanometer sized (average diameter 12 nm) coherent particles that exhibit Ashby-Brown strain contrast. Characterization of these phases has been carried out by a combination of X-ray diffraction, electron probe microanalysis and transmission electron microscopy coupled with energy dispersive spectroscopy. This multi-scale complex copper alloy (Cu75Fe20Si5 ) has achieved a remarkable yield and ultimate tensile strength at both room temperature and elevated temperatures in comparison to other copper based alloys. The yield strength and ultimate tensile strength at room temperature are 516±17 MPa and 635±14 MPa respectively whereas yield strength and ultimate tensile strength at 6000C turn out to be 95±11 MPa and 105±12 MPa respectively. In spite of achieving good mechanical strength, this alloy suffers from deterioration of electrical and thermal conductivity due to the presence of high volume fraction of the second phase and alloying elements. The room temperature electrical resistivity of this alloy shows that it is 10 times higher than that of pure copper (alloy resistivity = 1.70E-05 Ohm-cm at 250C and pure Copper- 1.68 × 10-6 Ohm-cm at 200C ). The thermal conductivity of this alloy turns out to be 88 W/m.K at 500C and 161 W/m.K at 6000C respectively which is much smaller in comparison to pure copper ( pure copper ≈ 401 W/m.K at 50 to 6000C). Attempts have been made to overcome the lowering of the transport properties by careful alteration of alloy compositions and fine tuning the microstructure. A new alloy with composition Cu-2.5Fe-2.5Si (at %) has been synthesized in order to achieve better transport properties without significantly sacrificing the mechanical strength. In this new alloy, we have reduced the volume fraction of the second phase (Fe-rich DO3 ordered globules) by lowering the addition of the alloying elements. We have also tried to alter the Fe to Si ratio in such a way that we can retain nanometer sized coherent particles in the matrix that provides strengthening. We arrived at a Fe and Si atom ratio of 1:1. The study of this alloy is presented in chapter five titled as „Chill cast Cu95Fe2.5Si2.5 alloy: Microstructural Evolution and Properties‟. Microstructural characterization indicates that the alloy contains only the nano meter sized coherent L12 ordered particles in the copper matrix. These particles show the Ashby-Brown strain contrast and are rich in iron and silicon. The absence of the high volume fraction of DO3 ordered Fe-rich globular phase and the smaller addition of the alloying elements ensure an improvement in the transport properties. The average resistivity value of this alloy at 250C is 3.5053 × 10-6 (Ohm-cm). This value represents a dramatic improvement in electrical properties in comparison to the Cu75Fe20Si5 alloy (Cu75Fe20Si5 alloy: 1.70E-05 Ohm-cm at 250C). The result is even better when we consider the temperature dependent thermal conductivity of the Cu95Fe2.5Si2.5 alloy. The thermal conductivity of this alloy turns out to be 236 W/m.K at 500C and 313 W/m.K at 6000C respectively. Though the thermal conductivity at room temperature is lower than pure copper, the gap reduces with increasing temperature (pure copper ≈ 401 W/m.K at 50 to 6000C and Cu75Fe20Si5 alloy: 88 W/m.K at 500C and 161 W/m.K at 6000C). This trend of temperature dependent thermal conductivity has made this alloy as one of the potential candidates for high-temperature applications. In situ heating experiment using transmission electron microscope (up to 4500C) and the heat treatment analysis at 6000C confirm that these L12 ordered particles are structurally stable at high temperatures and believed to be the main contributor to high mechanical strength in the alloy through the mechanism of order hardening and coherency strengthening. Coherent nature of the interface between the ordered particles and copper matrix also promotes low interfacial energy in the alloy and thus offers resistance to coarsening at elevated temperatures. Along with the attractive transport properties, this alloy also exhibits its success of retaining mechanical strength at both ambient and high temperatures as compared to the earlier alloy. The room temperature yield strength and ultimate tensile strength of this alloy are recorded as 580±18 MPa and 690±16 MPa respectively whereas the yield strength and ultimate tensile strength at 6000C of this alloy obtained as 128±8 MPa and 150±10 MPa respectively. Thus newly modified alloy exhibits an excellent balance between mechanical strength and conductivity (electrical and thermal) and can be regarded as a promising alloy for high strength high heat flux applications. The possibilities of the Cu95Fe2.5Si2.5 alloy as a potential candidate for high strength high conductivity application has provided the motivation for further optimization of the composition of this class of alloy. Mechanical strength and transport properties of a precipitation strengthened alloy always depends on the structure, shape, volume fractions and the number densities of the precipitate particles. Electrical and thermal conductivity are also sensitive to the presence of third elements and the number densities of the precipitates in the alloy. Thus, optimization of the volume fraction and the number density of the precipitates can yield a better alloy. With this objective, we have further increased the concentration of copper while keeping the Fe and Si atom ratio fixed at 1:1. Chapter six, titled as „Chill cast Cu98Fe1.0Si1.0 alloy: Microstructural Evolution and Properties‟ describes the microstructural evolution and microstructure-property correlation of this new alloy. Characterization analysis (X-ray diffraction, electron probe microanalysis and transmission electron microscopy) confirms that the microstructure of this alloy contains similar kind of nanometer sized L12 ordered particles with lower number density as compared to Cu95Fe2.5Si2.5 alloy (Relative planar number density of the particles: Cu98Fe1.0Si1.0 = 0.13 and Cu95Fe2.5Si2.5 = 0.20). This nano sized coherently ordered particles show the similar Ashby-Brown strain contrast and are rich in iron and silicon similar to the Cu95Fe2.5Si2.5 alloy. This dilute alloy exhibits slight improvement in transport properties in comparison to the earlier Cu95Fe2.5Si2.5 alloy. The electrical resistivity of this alloy at 250C is 3.438E-6 Ohm-cm (Cu95Fe2.5Si2.5 = 3.5053 × 10-6 Ohm-cm at 250C). The thermal conductivity values of this alloy are 243 W/m.K and 338 W/m.K at 500C and 6000C respectively (Cu95Fe2.5Si2.5 = 236 W/m.K at 500C and 313 W/m.K at 6000C). This increase in transport properties is associated with further compositional dilution and the presence of lower number density of the ordered particles in the copper matrix. The mechanism of strengthening is similar to the earlier alloys. The only difference lies in the fact that this present alloy contains lower number density of the L12 ordered particles in the copper matrix. This lower number density is responsible for the loss in mechanical strength of this alloy. The room temperature yield strength and the ultimate tensile strength of this present alloy are 467±16 MPa and 558±12 MPa whereas yield strength and ultimate tensile strength at 6000C are recorded as 102±13 MPa and 110±12 MPa respectively. Though the alloy exhibits some loss in mechanical strength, the values are still attractive in comparison to other commercially available copper based alloys. Both the alloy Cu98Fe1.0Si1.0 and Cu95Fe2.5Si2.5 demonstrate an excellent balance of mechanical strength and transport properties and have the potential to become a high strength and high conductivity materials for high temperature applications. Chapter seven is entitled as „Comparison between the alloy systems‟. In this chapter, we have presented a comparison of our new alloys with other commercially available Cu-base alloys. The thesis ends with a chapter titled as “Suggestions for future work”. We have included a descriptive note for possible future extension of our current work in this chapter.
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