Dissertations/Theses - Department of Materials & Metallurgical Engineering
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Item Design of bioresorbable magnesium alloy(Department of Materials and Metallurgical Engineering, 2021-11-13) Saha, Tilottoma; Gulshan, Dr. FahmidaIn recent times, Magnesium alloys have been attractive in medical science since they have compatible mechanical properties with the cortical bone and bioresorbable properties. Depending on the fabrication method, alloy element, and coating surface, the degradation rate in the physiological condition can be tailored. The present study investigated the Spark Plasma Sintering (SPS) effect on the entirely newly designed Mg alloy. All alloying elements (Zn, Er and Ce) were chosen based on the Mg matrix’s solubility and their mechanical properties and cytotoxic behaviour in the in-vivo condition. The sintering temperature on the microstructure, hardness and corrosion behaviour of a high-energy ball-milled Mg.67Zn.02Er.2Ce.01 was investigated. Along with that, the effect of alloying elements on the microstructure and radiographic properties was also examined. During the SPS, holding time and sintering pressure kept constant while varying the sintering temperature from 400ᵒC to 420ᵒC. The grain size and microstructure were investigated using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Corrosion behaviour was studied in the simulated body fluid using the milligram per decimeter per day (MDD) method, and the radiographic property was tested in the soft X-ray range. Vickers hardness and corrosion performance improved for the samples prepared at 420ᵒ C, whereas radiopacity under x-ray increased. A possible reason for strengthening and corrosion behaviour has been discussed in light of reduced porosity and grain size. Alloying bioserobale Mg with rare earth metal-Er can provide duel effects such as bioabsorbility and high radiopacity. So, a positive correlation between the opacity enhancement under the x-ray and the presence of Erbium is also under consideration. Suggestions for future design and processing of Mg alloys for bioresorbable implants are provided. Keywords: Magnesium alloys; Spark Plasma Sintering (SPS); Bioresorbable; Microstructure; Hardness; Corrosion; RadiopacityItem Effects of Gd on microstructure and deformation mechanisms of hot-rolled Mg-Zn-Y alloy(Department of Materials and Metallurgical Engineering (MME), BUET, 2023-09-24) Mallick, Bijoy; Al Rashed, Dr. Hossain Mohammad MamunThe microstructural evolution under various conditions (as-cast, homogenized, and hot rolled) and the deformation mechanisms at four distinct temperatures (250°C, 300°C, 350°C, and 400°C) and two strain rates (1x10-4 and 5x10-4 s-1) of the Mg-1Zn-1.5Y alloy were investigated with the addition of Gd (1%, 2%, and 6 wt%) to the alloy system. The chemical composition was analyzed using XRF, confirming the desired alloy composition with negligible deviation. Optical microscopy was employed to observe dendritic arm presence, measure grain size, assess precipitation conditions, and identify secondary phases within the matrix. Dendritic arms were evident in the as-cast condition, with density increasing due to Gd addition. Additionally, a trend of grain size reduction was observed with Gd inclusion in all conditions. Notably, hot rolling significantly reduced grain size through dynamic recrystallization (DRX). XRD, SEM, EDS, DSC, and CALPHAD analysis were utilized to determine different phases present in the alloy matrix, phase morphology, chemical composition, phase formation temperature, and phase evolution and volume fraction with temperature for the three alloys. The 14H-LPSO phase was consistently found within the α-Mg matrix of all compositions, with volume fraction increasing as Gd content rises. However, morphological variations were identified due to compositional changes, with lamellar and blocky LPSO phases forming in alloy A (1% Gd) and B (2% Gd), and blocky LPSO in alloy-C (6% Gd). Furthermore, in CALPHAD analysis Mg5(Gd,Y) was observed in alloy-A and C, while alloy-B contained the W-phase (Mg3Zn3(Y,Gd)2), which remained stable till the elevated temperatures around 600°C. However, above 450°C, the 14H-LPSO phase was found to start dissolving within the α-Mg matrix. Tensile testing at four different temperatures and two strain rates was conducted to assess changes in strength, ductility, and deformation mechanisms due to Gd addition. An optimal combination of strength and ductility was observed in alloy-B (2% Gd) at high temperature (400°C) and both strain rates, attributed to the presence of a bimodal 14H-LPSO phase (mix of lamellar and blocky LPSO) and the W-phase at elevated temperatures. Conversely, alloy-C demonstrated promising performance below 350°C, possibly due to the maximum volume fraction of the blocky LPSO phase. To determine the deformation mechanisms, activation energy, strain exponent (n) values, and strain rate sensitivity (m) of the alloys were calculated. Dispersed strengthening mechanism was predominant at lower temperatures (250°C and 300°C), transitioning to climb-controlled creep, glide-controlled creep, and grain-boundary sliding at higher temperatures (350°C and 400°C).Item Effects of Sn on microstructure and dynamic grain growth in a binary Mg-Ca alloy(Department of Materials and Metallurgical Engineering, 2020-03-14) Mesbah, Monira Binte; Al Rashed, Dr. H. M. MamunThe evolution of microstructure and dynamic grain growth of a binary Mg-1Ca alloy with different tin addition (1, 2 and 4 wt.%) was investigated under various conditions of temperatures and strain rates. Mg-1Ca, Mg-1Ca-1Sn, Mg-1Ca-2Sn and Mg-1Ca-4Sn samples were cast and homogenized. The homogenized samples were rolled at 4000C. Optical microstructures of both cast and rolled samples were taken for microstructural analysis. Scanning electron microscopy of four rolled samples was carried out for identifying phases present. Tensile test of rolled alloys was carried out at temperatures 300, 350, 400 and 450°C, and at two different strain rates 5x〖10〗^(-4) s^(-1) and 1x〖10〗^(-4) s^(-1). Optical microstructures of tensile tested samples were acquired for observing dynamic grain growth. Samples were taken from six different location of failed (tensile loaded) samples. For understanding deformation mechanisms during grain growth and total deformation, activation energy, strain hardening exponent and strain rate sensitivity index were calculated. It was found that tin addition increased dendrite formation in as-cast alloys and continuously reduced grain sizes of rolled alloys. Addition of small amount of Sn increased both strength and elongation to failure values. However, higher amount of Sn addition lowered the properties to some extent due to formation of excessive primary intermetallic particles. At lower temperatures, work hardening rate is higher leading to higher strength and lower elongation to failure values. Conversely, at elevated temperatures, strength values were reduced due to inevitable softening and greater grain growth was observed as well. At slower strain rate (1x〖10〗^(-4) s^(-1)), higher dynamic grain growth was observed than higher strain rate (5x〖10〗^(-4) s^(-1)). At slower strain rate, value of strain rate sensitivities became higher (m>0.33), which indicated superplastic behavior, in addition, deformation mechanisms of grain boundary sliding and diffusional creep.Item Evaluating interactions of second phases with chemical conversion coating on Mg-Zn-RE (Gd, Y) alloy for surface protection(Department of Materials and Metallurgical Engineering (MME), BUET, 2025-05-27) Tasnuva Zahan Liza; Mamun Al Rashed, Dr. Hossain MohammadMagnesium alloys are highly suitable for lightweight manufacturing and applications due to their impressive mechanical properties and high strength-to-weight ratio. However, their vulnerability to corrosion restricts their practical applications. Using a chemical conversion process, a calcium-phosphate-vanadium (Ca-P-V) composite coating was fabricated on Mg-Zn-RE (Gd, Y) alloy effectively in this research. SEM and EDX analysis of the base alloy and coated sample were carried out, and the coated surface showed different phases from different concentrations of vanadium. XRD analysis of the base alloy and coated samples were carried out, phases and crystal structure were determined. XPS analysis was performed to evaluate the interaction of coating materials on the surface of the sample and their binding energies. Ca3(PO4)2, CaCO3, and MgCO3, made up the majority of the composite coating which had a limited number of hydroxides of V(V) distributed into it. Electrochemical tests were also conducted to determine how the concentration of vanadium affected the substrate's ability to resist corrosion. Both electrochemical tests and scratch immersion tests in a 3.5 wt % NaCl solution were utilized to examine the corrosion behavior of the conversion coating. The findings demonstrated that the coated samples could self-heal in addition to having superior corrosion resistance. A unique composite layer was formed on the scratched area as a result of the Ca, P, and V ions that are liberated from the coated surface migrating in the corrosion solution. Moreover, another Mg alloy sample with different alloying percentages was coated and characterized to evaluate and compare the interaction of second phases of base alloy and coated sample.Item Experimental study of microstructure and degradation properties of magnesium-zinc-neodymium-yttrium bioresorbable alloys(Department of Materials and Metallurgical Engineering (MME), BUET, 2024-09-09) Snigdha, Nusrat Jahan; Gulshan, Dr. FahmidaThe aim of this study was development of Mg-Zn-Nd-Y alloy for biomedical applications and to identify the effect of yttrium (Y) addition on the phase development and mechanical and corrosion behavior of the alloys in as-cast, rolled, and extruded conditions. The result showed that Mg-2Zn-1Nd alloy consists of α-Mg, Mg41Nd5, and some eutectic phases. The addition of a small amount (0.5 wt.%) of Y in the alloy led to the formation of the icosahedral quasi-crystalline I (Mg3YZn6) phase, and further addition of Y (1.0wt%) led to the formation of W-phase (Mg3Y2Zn3). The Y addition significantly refined grains in the extruded state with the grain size of 17μm when Y content is 1%. The precipitates were more fragmented in extrusion than in the rolling state. The secondary dendritic arm spacing was more refined in the as-cast state as a function of Y addition. The presence of the I-phase in the alloy with 0.5wt% Y increased hardness than the alloy containing 1wt% Y due to the formation of the W phase. The maximum hardness achieved is 78.15Hv in extruded state when the Y content is 0.5%. the refined grain structures with higher volume fraction of grain boundaries, act as barriers to dislocation motion increasing the material's hardness in extruded sample than the rolled one. The weight loss corrosion was performed for the corrosion rate calculation and the alloy with the I phase had the lowest corrosion rate, whereas the W phase containing alloy showed a moderate corrosion rate, and the alloy without Y content had the most corrosion rate. The extruded state exhibited the best corrosion resistance with the corrosion rate of 0.392mm/a when Y is 0.5% than the rolled and the as-cast state due to more grain refinement, and fragmentation of the second phases. The corrosion electrochemistry also ensured the corrosion resistance behavior of the extruded alloy.Item Study of Sn-Zn solder alloy as a conventional Sn-Pb solder replacement(Department of Materials and Metallurgical Engineering, BUET, 2008-10) Das, Sazol Kumar; Sharif, Dr. AhmedDue to the lElherenttoxicity of lead (PI'), environme"tal regulations around Ihe world have bCCT1:.rgeted to el1mlT,alCthe usage of Pb-beming solder, iTlelectronic ""emillies, rlus has tl'1ggcrodthe development of "Pb-free" solder, fOl the elec{ronic indust"e,. In ordcr to become a successful solder motenal, Ph-free soldeL'alloy, need to he reliubl. over a long tenn, Even though many Ph-free