Browsing by Subject "Severe Plastic Deformation"
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Item Effect of Severe Plastic Deformation and Subsequent Heat Treatment on Hardness and Electrical Conductivity of Oxygen-Free High Conductivity (OFHC) Copper, Commercial Pure Copper, and Copper Chromium Alloy(2014-12-15) Kao, Yi-TangSamples of oxygen-free high conductivity (OFHC) copper (C101), commercially pure copper (C110), and copper chromium alloy (C182) were subjected to severe plastic deformation (SPD) using equal-channel angular extrusion (ECAE) to determine the effect of large amounts of plastic strain on the hardness and electrical conductivity for electrical conductor applications. Different levels of plastic strain and strain orientation combinations were applied by ECAE at room temperature. Heat treatments in the range 100?C to 500?C for times from 10 minutes to 4 days were applied to the materials after ECAE. The electrical conductivity and hardness were determined by four-point probe measurement and Vickers microhardness measurement. The hardness of all test materials increased significantly and the electrical conductivity decreased after ECAE, presumably because of the higher density of dislocations caused by the plastic strain. The properties changed most dramatically after a strain of ~2.3 and reached a near plateau after a strain of ~4. A post-strain heat treatment for temperatures at and above 250?C and for times of at least 1 hr. caused the conductivity and hardness to return to pre-strain levels (near 100 % IACS and VH 50) in C101 and C110, with the change occurring more rapidly for higher temperature annealing. For the case of C182, the post-strain heat treatment induced the highest hardness (VH 162) at 450?C for which the material had a conductivity of 76 %IACS. Copper 101 and 110 showed a plateau in hardness and conductivity after 3 hours heat treated at 150?C and higher; the hardness and conductivity of C182 did not reach stable values at 350?C and 450?C after 48 hours. SPD and post SPD heat treatment successfully improved the combination of hardness and electrical conductivity of the three Cu-based alloys studied for room temperature electrical conductor applications. The best combination of hardness and conductivity (99 %IACS and VH 137) occurred in C110 after two passes of ECAE (plastic strain of 2.3) and heat treated at 100 ?C for 1 hour, for which the hardness increased by 58% over the fully annealed condition. The results of this work can be applied to other metals such as aluminum and silver contemplated for electrical conductor applications.Item Fabrication of amorphous metal matrix composites by severe plastic deformation(Texas A&M University, 2006-10-30) Mathaudhu, Suveen NigelBulk metallic glasses (BMGs) have displayed impressive mechanical properties, but the use and dimensions of material have been limited due to critical cooling rate requirements and low ductility. The application of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of bulk amorphous metals (BAM) and amorphous metal matrix composites (AMMC) is investigated in this dissertation. The objectives of this research are a) to better understand processing parameters which promote bonding between particles and b) to determine by what mechanisms the plasticity is enhanced in bulk amorphous metal matrix composites consolidated by ECAE. To accomplish the objectives BAM and AMMCs were produced via ECAE consolidation of Vitreloy 106a (Zr58.5Nb2.8Cu15.6Ni12.8Al10.3-wt%), ARLloy #1 (Hf71.3Cu16.2Ni7.6Ti2.2Al2.6 -wt%), and both of these amorphous alloys blended with crystalline phases of W, Cu and Ni. Novel instrumented extrusions and a host of postprocessing material characterizations were used to evaluate processing conditions and material properties. The results show that ECAE consolidation at temperatures within the supercooled liquid region gives near fully dense (>99%) and well bonded millimeter scale BAM and AMMCs. The mechanical properties of the ECAE processed BMG are comparable to cast material: ????f = 1640 MPa, ????f = 2.3%, E = 80 GPa for consolidated Vitreloy 106a as compared to ????f = 1800 MPa, ????f = 2.5%, E = 85 GPa for cast Vitreloy 106, and ????f = 1660 MPa, ????f = 2.0%, E = 97 GPa for ARLloy #1 as compared to ????f = 2150 MPa, ????f < 2.5%, E = 102 GPa for Hf52Cu17.9Ni14.6Ti5Al10. The mechanical properties of AMMCs are substandard compared to those obtained from melt-infiltrated composites due to non-ideal particle bonding conditions such as surface oxides and crystalline phase morphology and chemistry. It is demonstrated that the addition of a dispersed crystalline phase to an amorphous matrix by ECAE powder consolidation increases the plasticity of the amorphous matrix by providing locations for generation and/or arrest of adiabatic shear bands. The ability of ECAE to consolidated BAM and AMMCs with improved plasticity opens the possibility of overcoming the size and plasticity limitations of the monolithic bulk metallic glasses.Item Investigation and modeling of processing-microstructure-property relations in ultra-fine grained hexagonal close packed materials under strain path changes(2009-05-15) Yapici, Guney GuvenUltra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.Item Shape memory response and microstructural evolution of a severe plastically deformed high temperature shape memory alloy (NiTiHf)(Texas A&M University, 2006-04-12) Simon, Anish AbrahamNiTiHf alloys have attracted considerable attention as potential high temperature Shape Memory Alloy (SMA) but the instability in transformation temperatures and significant irrecoverable strain during thermal cycling under constant stress remains a major concern. The main reason for irrecoverable strain and change in transformation temperatures as a function of thermal cycling can be attributed to dislocation formation due to relatively large volume change during transformation from austenite to martensite. The formation of dislocations decreases the elastic stored energy, and during back transformation a reduced amount of strain is recovered. All these observations can be attributed to relatively soft lattice that cannot accommodate volume change by other means. We have used Equal Channel Angular Extrusion (ECAE), hot rolling and marforming to strengthen the 49.8Ni-42.2Ti-8Hf (in at. %) material and to introduce desired texture to overcome these problems in NiTiHf alloys. ECAE offers the advantage of preserving billet cross-section and the application of various routes, which give us the possibility to introduce various texture components and grain morphologies. ECAE was performed using a die of 90?? tool angle and was performed at high temperatures from 500??C up to 650??C. All extrusions went well at these temperatures. Minor surface cracks were observed only in the material extruded at 500 ??C, possibly due to the non-isothermal nature of the extrusion. It is believed that these surface cracks can be eliminated during isothermal extrusion at this temperature. This result of improved formability of NiTiHf alloy using ECAE is significant because an earlier review of the formability of NiTiHf using 50% rolling reduction concluded that the minimum temperature for rolling NiTi12%Hf alloy without cracks is 700??C. The strain level imposed during one 90?? ECAE pass is equivalent to 69% rolling reduction. Subsequent to ECAE processing, a reduction in irrecoverable strain from 0.6% to 0.21% and an increase in transformation strain from 1.25% to 2.18% were observed at a load of 100 MPa as compared to the homogenized material. The present results show that the ECAE process permits the strengthening of the material by work hardening, grain size reduction, homogeneous distribution of fine precipitates, and the introduction of texture in the material. These four factors contribute in the increase of stability of the material. In this thesis I will be discussing the improvement of mechanical behavior and stability of the material achieved after various passes of ECAE.