24 research outputs found
Bimetallic bars with local control of composition by three-dimensional printing
Get PDFThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (p. 106-107).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Three Dimensional Printing (3DP) is a process that enables the fabrication of geometrically complex parts directly from computer-aided design (CAD) models. The success of 3DP as an alternative manufacturing technology to bulk machining of materials for complex parts has been demonstrated. By proof of concept, 3DP has demonstrated the ability to create parts with Local Control of the Composition (LCC). LCC allows tailoring the material properties in regions of a part for functional purposes. In this work, LCC was studied and demonstrated by fabricating bimetallic bars consisting of two layers of Fe-Ni alloys with different composition and, hence, different thermal expansion properties; the coefficient of thermal expansion (CTE) of Fe-Ni system is sensitive to its composition. Two types of the binder/dopant slurries were made for making the LCC bars. One type consisted of dispersions of Fe₂O₃ particles in water, and the other consisted of dispersion of NiO in water. The LCC bars were successfully made by printing the Fe₂O₃/NiO slurries into Fe-30Ni base powders. After heat treatment to impart strength to the printed bars, the bars were successfully retrieved from unbound powders. The bars, then, were annealed at 1400 ⁰C for 2 hours for sintering and homogenization. The final composition of the base powders were changed accordingly. In the layers on which an Fe₂O₃ slurry was printed, the Fe composition of the layers increased on average to 72wt%. Similarly, the Ni composition of the Ni-enriched layers of the bars increased on average to 33wt%. The densification and local homogenization resulting from reduction and sintering treatments were not satisfactory.(cont.) The major problem was presumably caused by the oxide residues. The presence of the oxide powders was evident from the microprobe measurement. The oxide residues caused the local compositions to be inhomogeneous. As a result, the compositional profiles showed considerable scatter. Moreover, the residues impeded the sintering rate of the bars; the sintering densities of the bars were as small as 78% of the theoretical density. The resulting bimetallic bars did exhibit bending deflection on uniform heating. However, the bending deflections were much smaller than expected. Evidently, the compositional profiles of the bars critically influence their thermal bending properties. The scatter in the compositional profiles resulted in local variations of CTE in the bars, which degraded the thermal bending properties. A linear elastic model that allows prediction of the deflection as a function of composition profile shows good agreement with the observed deflections in the bimetallic bars with LCC.by Ratchatee Techapiesancharoenkij.S.M
Understanding and development of combined acoustic and magnetic actuation of Ni₂MnGa single crystals
Get PDFThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 143-148).Ni-Mn-Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising new class of active materials capable of producing a large (several %) magnetic-field-induced strain (MFIS). FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 2 to 4 kOe must be overcome to initiate twin boundary motion and a larger field is required to achieve full actuation. The operating window of the stress output from FSMA actuators is narrow and limited to the range between 0.5 and 2 MPa. Outside the operating range, the strain output diminishes significantly. This thesis addresses these limitations and reports potential techniques to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The demagnetizing field due to magnetic poles on the surface of the sample is found to significantly influence the maximum field needed for full MFIS. The demagnetizing field decreases the effective internal field inside the FSMA sample; as a result, for a given external field, the magnetic driving force is reduced by the demagnetizing field. For a small demagnetization factor, full MFIS can be achieved at a field as low as 0.5 kOe. However, for a high demagnetization factor, full MFIS may require a field as high as 3.5 kOe. A phenomenological free energy model with an approximate magnetostatic term included properly describes this. The application of an acoustic assist from a 33-mode piezoelectric stack is shown to improve MFIS of Ni-Mn-Ga single crystals by reducing the required threshold field and twinning-yield stress. Threshold field reductions of up to 1 kOe are observed, and the twinning-yield stress is reduced by up to 0.5 MPa. The piezo assist on FSMA actuation can be understood as a form of time varying stress waves that facilitate twin boundary motion.(cont.)The stress-wave theory and FEM analyses, based on assumption of an elastic and isotropic material, are used to estimate the amplitude of stress waves. The stress values determined from the wave theory and FEM are comparable to the observed reduction in twinning-yield stress (0.6 to 2 MPa). The empirical stress wave amplitude is generally lower than the calculated one, because the actual stress waves generated in FSMA are limited by inelastic and anisotropic nature of the FSMA samples. For FSMA samples with twin planes oriented 45o to the elongation axis, longitudinal stress waves parallel to the elongation axis are the most effective type to facilitate twin boundary motion. Longitudinal stress waves impart uniaxial tensile/compressive stresses into the sample normal to its base, resulting in the maximum shear stresses along the 45o twin planes. On the contrary, the transverse stress waves are the least effective type, because the resulting shear stresses along the twin planes are equal to zero. This is confirmed by the comparison of the effectiveness of the piezo-assist using the longitudinal 33-mode and transverse 15-mode piezo stacks. The maximum reduction of twinning-yield stress of 0.5 MPa is observed with the longitudinal piezo assist, while a maximum twinning stress reduction of only 0.05 MPa can be achieved by the transverse piezo assist. For FSMA cyclic actuation, both operating stress and strain outputs of the FSMA actuation are significantly enhanced by the piezo-assisted effect. Without the piezoassistance, the maximum reversible strain of the sample used here is 3% and appears only in the limited external stress range between 0.7 and 1 MPa. With the piezo-assistance, the maximum reversible strain increases to 4.5% and appears in a broader range of stress output between 0.4 and 1.2 MPa.(cont.)The reduction in the twinning-yield stress due to the acoustic-assistance obviously improves the FSMA cyclic actuation performance; any magnetic energy not used to drive twin boundary motion can potentially be utilized to work against a larger external stress. The magnetic stresses for a given magnetic field, calculated from the free energy model, appear to be larger than the empirical ones. The demagnetization effect is shown to be responsible for much of the deviation. The demagnetization effect reduces the internal field needed to move twin boundary and do external work.by Ratchatee Techapiesancharoenkij.Ph.D
Frequency Response of Acoustic-Assisted Ni–Mn–Ga Ferromagnetic- Shape-Memory-Alloy Actuator
No full textA prototype of Ni–Mn–Ga based ferromagnetic-shape-memory-alloy (FSMA) actuator was designed and built; an acoustic-assist technique was applied to the actuator to enhance its performance. A piezoelectric stack actuator was attached to the Ni–Mn–Ga sample to generate acoustic energy to enhance twin-boundary mobility and, hence, reduce the magnetic threshold field required for activating twin-boundary motion. The dynamic response of the acoustic-assist FSMA actuator was measured up to 1 kHz actuation. The acoustic assistance improves the actuator performance by increasing the reversible magnetic-field-induced strain (MFIS) by up to 100% (increase from 0.017 to 0.03 at 10 Hz) for drive frequencies below 150 Hz. For frequencies above 150 Hz, the acoustic-assist effect becomes negligible and the resonant characteristic of the actuator takes over the actuator response. Even though the acoustic assist does not improve the actuation at high frequencies, the MFIS output of 5% can be obtained at the resonant frequency of 450 Hz without acoustic assistance. The FSMA actuator is shown to be ideal for applications that require large strain at a specific high frequency.United States. Office of Naval Research (Multi-University Research Initiative, Grant No. N0014-01-0758
Microstructural and Corrosion Characterizations of Nickel-Titanium Coatings Produced by Electrochemical Codeposition and Heat Treatment
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Pulsed-Current Electrochemical Codeposition and Heat Treatment of Ti-Dispersed Ni-Matrix Layers
No full textMicrostructural and Corrosion Characterizations of Nickel-Titanium Coatings Produced by Electrochemical Codeposition and Heat Treatment
No full textThe Thermal-Aging Effect on the Microstructure Evolution and Shear Strength of the Sn-Rich Au-Sn Soldering between Altic and Si Substrate in Microelectronics
No full textThe Au-Sn soldering alloys are commonly used in microsoldering process for microelectronic industry due to fluxless process and relatively low melting temperature with good eutectic microstructures. This study investigated the microstructures of Au-Sn soldering between AlTiC and Si substrates with Ti/Pt/Au under bump metallization (UBM). The microstructures of the solder samples under three conditions: before bonding, after bonding and after thermal-cycle aging, were investigated. The shear strength values of pre-aging and post-aging soldering were compared. The thermal-cycling temperatures were ranged from -40 to 125 °C for 300 cycles. The intermetallic compounds (IMCs) of the AuSn solders consist of AuSn, AuSn2, and AuSn4. After thermal-cycle aging, the bonding strength was increased due to the improved IMC bonding between solders and UBM; the shear surfaces were rougher due to the growth of AuSn and AuSn2.</jats:p
The effect of magnetic stress and stiffness modulus on resonant characteristics of Ni–Mn–Ga ferromagnetic shape memory alloy actuators
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