Materials Science Forum Vols. 638-642

Abstract: New thermo-mechanical processes (TMPs) to produce ultrafine-grained copper alloys utilizing continuous recrystallization (cRX) were proposed. These methods stand on our hypothesis that the evolution of ultrafine grains can be evolved by a mechanism of cRX even during severe plastic deformation at ambient temperature. A TMP of warm compression of 10 to 20 % of Cu-1.7mass%Fe alloy followed by annealing was cyclically repeated. The slight reduction, low-temperature annealing and pinning of grain boundaries by precipitates efficiently impeded occurrence of discontinuous recrystallization (dRX). The evolved substructures with nodes of the Fe precipitates gradually changed to new grains surrounded by low- and high-angle boundaries with increasing number of the repeated processes. Ultrafine grains with average size of 0.7 m were successfully evolved. However, the onset of dRX triggered extended grain coarsening accompanied by grain-boundary migration under conditions of insufficient annealing temperature and large pass stain. Another TMP cycles of cold rolling and annealing also induced fine-grained structure of about 0.6 m. The above results improved that ultrafine grain refinement can be realized simply by a mechanism of cRX even in the metallic materials with low stacking fault energy.

Abstract: The effect of equal-channel angular pressing (ECAP) on the pitting corrosion resistance of Al (AA1100) and Al-Mg (AA5052) alloy with anodization and without was investigated by means of polarization curves in solutions containing 300 ppm or 0.6 mol/L of Cl- and by surface analysis. In Al and Al-Mg alloy without anodization, the potentials for pitting corrosion were evidently shifted to the noble direction by ECAP process, indicating that this process improves resistance to pitting corrosion. SEM observation revealed that the pitting corrosion occurred near the impurity precipitates and the size of impurity precipitated decreased with ECAP process. The time-dependence of corrosion potential and the polarization resistance determined by AC impedance technique suggested that the formation rate of Al oxide films was increased with ECAP process. The improvement in pitting corrosion resistance of Al and Al-Mg by ECAP seems to be attributable to the decrease in the size of impurity precipitates and the increase in the formation rate of Al oxide films. On the other hand, the time required for initiating pitting corrosion of anodised Al was longer with ECAP than without, indicating improvement in the pitting corrosion resistance by application of ECAP. However, the pitting corrosion of anodized Al-Mg alloy occurred earlier with ECAP than without, showing that the corrosion resistance was worse with ECAP than without. In anodized Al-Mg alloy, cracks occurred in the anodic oxide films during initial corrosion and the cracks were larger with ECAP than without. It is assumed that the pitting corrosion was promoted by the cracks due to the higher internal stress resulting from ECAP.

Abstract: Polycrystalline iron was deformed by eight ECAE passes using the route Bc to a total strain of 9.2. After deformation the material was annealed at temperatures up to 800oC. Scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD) were used to characterize both deformed and annealed structures. In the as-deformed state, the mean grain size is 650 nm and the volume fraction of high angle boundaries (VHAB) is 56%. Upon annealing there is a pronounced softening above 300oC. At the beginning of recrystallization, at about 400oC, the VHAB increases to 71%. The results indicate that discontinuous recrystallization is the main softening mechanism in severely deformed iron.

Abstract: The interlayers from different materials were used in order to modify the interface structure/property and to improve the mechanical properties of NiAl composites reinforced by continuous single crystal Al2O3 fibers. It was found that the interface without interlayer had a good chemical stability during hot pressing (sample fabricating), resulting in high interface shear strength (about 250 MPa) at RT. But for the composites with interlayers, chemical reactions occurred in the interfacial area during diffusion bonding. The measured interface shear strength for the complete debonding was 70±30 MPa, 110±36 MPa and 32±5 MPa for the composites with hBN, Cr2AlC and V2AlC interlayers respectively. The possible influences by introducing the interlayers on mechanical performance of NiAl composites are discussed.

Abstract: The paper deals with the problem of the torque in the cantilever beam with annular cross section in the elasto-plastic domain, considering the material with elastic-linear hardening model. The cantilever beam is subjected to the alternating load of such frequency that the heat is generated in the beam. The quantity of the generated heat depends on the amplitude of the torque, frequency of the load and mechanical properties of the beam material. Equations which relate the value of the torque to the extent of the plastic region of the cross section and the shearing stresses of the beam in the elasto-plastic domain are given. Furthermore, equations for deformation work, transformation work, energy due to residual stresses and energy transformed into heat are presented. The article compares results concerning the temperature change at characteristic points along the beam, obtained via numerical analysis as well as experimentally.

Abstract: By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Abstract: As excellent scaffolds for cultivating bone cells, porous beads of bioactive ceramics such as HAp, TCP are considered to be promising. HAp and α-TCP are well known as non-toxic bioceramics to human cells, but their behavior in living body fluid are different. HAp is bioactive material which has both high strength and better tissue-adhesive properties, but that is not readily absorbed by the human body. On the contrary, α-TCP is highly bioabsorbable; it is quickly absorbed by the body, and, therefore, disappears before bone is completely replaced. To realize a composite which has suitable solubility in living body fluid, α-TCP/ HAp functionally graded porous beads were fabricated by the method of spheroidization in liquid nitrogen. This type of composite maintains the function of scaffold with sufficient strength up to growth of new bone, and after the growth, it is expected to absorbed completely in the body. In the present study, ceramic beads with α-TCP at the center were fabricated and coated with a functionally graded layer of HAp. By controlling the thickness of HAp layer, which could be realized by changing time of hydrolytic reaction, the absorption rate into the body would be easily controlled. In addition, to accelerate the formation of porous structure, some acid solutions were used to dissolve the beads surface and to penetrate pores toward inside of the beads. Observed surface and inner structure by SEM, also the measured change in absorption rate will be presented.

Abstract: Thin film stress is often seen as an unwanted effect in micro- and nanostructures. Since recent years, we could employ thin film stress as a useful tool to create nanowires. By creating stress at predetermined breaking points, e.g., in microstructured photo resist thin films, cracks occur on the nanoscale in a well defined and reproducible manner [ ]. By using those as a simple mask for thin film deposition, nanowires can be created. More recently this fabrication scheme could be improved by utilizing delamination of the thin film, in order to obtain suitable shadow masks for thin film deposition in vacuum [ ]. Now, these stress based nanowires can be integrated in microelectronic devices and used as field effect transistors or as hydrogen sensors [ ]. For the functional part of the sensor, it was proposed that thin film stress created by hydrogen adsorption in the nanowire is the driving force. In terms of function, thin films can be also applied on free standing nanoscale whiskers or wires to modify their mechanical features or adding additional functionality. As a second example for the utilization of thin film stress, recent experiments on a piezoelectric and magnetostrictive material combination will be presented. These piezoelectric-magnetostrictive nano-composites are potential candidates for novel magnetic field sensors [ ]. In these composites the magnetostriction will be transferred to the piezoelectric component, resulting in a polarization of the piezoelectric material, that can be used as the sensor signal. The results of two different composite layouts will be presented and discussed with a special focus on the comparison between classical macroscopic composites and the novel nanocomposites.

Abstract: This work presents a combined performance evaluation approach for SMAs as actuator material – an approach based on the combination of three testing modes: a) stress-free thermal recovery mode, b) fixed-support stress generation mode and c) constant (or variable) bias-stress recovery mode. Based on this testing, a so-called “design diagram” can be constructed. This diagram demonstrates the mechanical work generation potential of an SMA and therefore represents a must-have tool for the application engineer. Given that the working envelope depends on the material composition, microstructure and number of actuation cycles, this approach allows the selection of an appropriate material and the processing technique that will meet the functional requirements of a specific application. To illustrate the proposed performance evaluation technique, we show how it can be applied to the design of an SMA actuator for a morphing wing.

Abstract: This paper summarizes some of our recent results on crystal structure, microstructure, orientation relationship between martensitic variants and crystallographic features of martensitic transformation in Ni-Mn-Ga FSMAs. It was shown that Ni53Mn25Ga22 has a tetragonal I4/mmm martensitic structure at room temperature. The neighboring martensitic variants in Ni53Mn25Ga22 have a compound twinning relationship with the twinning elements K1={112}, K2={11-2}, η1=<11-1>, η2=<111>, P={1-10} and s=0.379. The ratio of the relative amounts of twins within the same initial austenite grain is ~1.70. The main orientation relationship between austenite and martensite is Kurdjumov-Sachs (K-S) relationship. Based on the crystallographic phenomenological theory, the calculated habit plane is {0.690 -0.102 0.716}A (5.95° from {101}A), and the magnitude, direction and shear angle of the macroscopic transformation shear are 0.121, <-0.709 0.105 0.698>A (6.04° from <-101>A) and 6.88°, respectively.