Agnieszka Mielczarek

Mechanical and Fatigue Properties of Cu-Al-Mn Shape Memory Alloys with Influence of Mechanical Cycling on Amplitude Dependence of Internal Friction at Room Temperature

The mechanical and fatigue properties of Cu - Al - Mn shape memory alloys with different phase fractions at room temperature were investigated. The specimens with different chemical compositions (Al: 8.9 - 12.5 wt. % and Mn: 3.3 - 9.3 wt. %) were tensile loaded with 10-3 s-1 tensile strain rate. Austenitic specimens have the highest tensile strength and fracture strain. Yield strength, tensile strength and elongation of martensitic alloys were lower compared with austenitic alloys. Fracture strain of martensitic alloys depend only little on the chemical composition. Specimens of martensitic, austenitic and three different multiple phase specimens were tested in the high cycle fatigue range at room temperature. The Woehler curves for multiple specimens depend on the phase fraction at testing temperatures. Different elements as Co, Ni, Fe and Si were alloyed to CuAl11.6Mn5. All decreased the ductility of the specimens, and their fatigue properties. Maxima could be detected in the strain amplitude dependence of damping for multiple phase specimen. These maximum are shifted to lower damping and to higher strains with increasing number of mechanical cycles, compared to the as cast condition for not cycled specimen. The strain amplitude dependence of damping in martensitic and austenitic Cu – Al – Mn shape memory alloys does not change much during mechanical cycling.

Effects of grain growth on microhardness and coercivity in electrodeposited nanocrystalline nickel

Abstract Nickel specimens with various grain sizes were produced by electrochemical deposition. As-deposited pure nickel had a mean grain size of 300 nm. By dispersing nanoscaled SiO2- and Al2O3-particles in the electrolyte it was possible to reduce the grain size by incorporation of these particles into the growing nickel matrix. The grain sizes were 150 nm and 60 nm for the Al2O3- and SiO2-reinforced nanocomposite, respectively. Microhardness and coercivity of all samples were measured immediately after electrodeposition and after successive isochronal heat treatments with increasing annealing temperature. Grain growth of the samples could be observed by a decrease in hardness as well as coercivity with increasing annealing temperature.

Influence of Thermal Cycling and Equivalent Heat Treatment on Amplitude Dependence of Internal Friction in Cu-Al-Mn Shape Memory Alloys

The influence of thermal cycling between - 196 °C and 200 °C and equivalent heat treatment at 200 °C on the amplitude dependence of internal friction at room temperature has been studied in as cast Cu – Al - Mn shape memory alloys with different chemical compositions. Using X-ray diffraction one composition was found to be austenitic and two others martensitic with two martensite types (2H and 18R) at room temperature. All specimens were thermally cycled for 100 times. During one thermal cycle the specimen underwent altogether two phase transformations one in each direction. Thermal cycling causes microstructural changes in the specimens due to atomic reordering, thermal stresses, which are generated in the martensitic state due to the anisotropy of thermal expansion, or due to the nucleation and propagation of interphase cracks in parent phase. During repeated thermal cycling the transition peaks obtained in mechanical spectroscopy became narrower due to an enduring change of the microstructure and annealing effect at 200 °C. To compare between the effects of thermal cycling and heat treatment one martensitic specimen was annealed at 200 °C. For selected cycle numbers and heat treatment times the amplitude dependence of damping was measured at room temperature. The influence of thermal cycling of martensitic specimens on the damping level was found to be similar to the influence of heat treatment at 200 °C. It is most likely that the highest heat treatment temperature is more important for the amplitude dependence of damping than the temperature change during thermal cycling. Cracks due to thermal cycling were found in all cycled specimens. They have no significant effect on the amplitude dependence of damping of the martensitic samples, whereas some small influence could be observed in austenitic samples at room temperature.

Influence of Heat Treatment on Magnetic and Damping Properties of Fe-11 at.% Al Alloys

The influence of heat treatment on the amplitude dependence of internal friction in Fe - 11 at. % Al alloys with carbon contents in the range 0.005 - 0.2 at. % has been studied using an inverted torsion pendulum in the temperature range 300 – 950 K and a vibrating reed apparatus at room temperature. The specimens were annealed at 1273 K in vacuum and cooled down with different cooling rates in order to obtain different degrees of order. It was found that ordering is hardly avoidable in Fe - Al alloys with Al contents > 11 at. %. Ordered alloys are characterised by lower damping capacity due to higher coercivity caused by additional pinning of magnetic domain walls by antiphase boundaries. X-ray diffraction investigations indicate that water-cooling suppresses ordering in Fe - 11 at. % Al alloys while cooling in air or in furnace provokes D03–type ordering. Slowly cooled specimens are characterised by higher damping capacity due to lower coercivity than water cooled or plastically deformed specimens. The amplitude dependent magneto-mechanical damping was determined as the difference between amplitude dependent damping without and with saturating magnetic field (~ 20 kA/m). Magneto-mechanical damping was found to be proportional to the strain where the amplitude dependent damping is maximum and reciprocal to the coercivity and saturation polarisation. Cold rolling increases the coercivity and therefore decreases the magneto-mechanical damping. An increase of the grain size in the investigated samples by heat treatment leads to a qualitatively expected decrease of coercivity and therefore to an increase of magneto-mechanical damping.

Amplitude Dependent Internal Friction of CuAlMn Shape Memory Alloys

The strain amplitude dependent internal friction at room temperature and the transition temperatures of CuAlMn-shape memory alloys with Al contents from 8.9 wt.% to 12.7 wt.% and Mn contents from 4.7 wt.% to 9.3 wt.% were investigated. The investigated strain range was 10-6 - 10-3. Rods of various compositions were die cast and machined to single clamped damping bars. Their transition temperatures and amplitude dependent damping was determined in as cast and homogenized state. The damping in the investigated shape memory alloys was found to be generally much higher than in metals without martensitic transition. In as cast state some alloys exceeded the damping of a Sonoston type alloy measured in comparison for strains higher than 3 x 10-5. The influence of grain size on damping was investigated by additional sand casting and the use of Boron for grain refinement. It was found that only the material with the biggest grains had a noticeable higher damping over the whole measured strain range. Homogenization heat treatment can still extremely increase the damping of CuAlMn alloys. After homogenization this extremely high damping decreases slowly to medium values in the order of as cast alloys.

Effects of shot peening on internal friction in cp aluminum and aluminum alloy 6008

Abstract The strain-amplitude-dependent damping of bending beams of aluminum alloy 6008 and CP aluminum was measured at room temperature after different heat treatments and after shot peening. Shot peening led to an increase of damping in almost the whole measured amplitude strain range from 10 – 6 to 10 – 3 for CP aluminum. Strong ageing effects at room temperature were observed immediately after the shot peening process, namely an increase of the amplitude dependent part and a decrease of the amplitude-independent part of damping. After about 2700 h, ageing of the samples had saturated. For aluminum alloy 6008 much smaller ageing effects were found being due to compensating effects like formation of Cottrell clouds, precipitation of G.P. – zones, and the reduction of foreign atoms in solid solution. The found amplitude-dependent damping can be explained by the reversible movement of dislocations between strong pinning points like, e. g., precipitates and weak pinning points like solid solute atoms as proposed by the dislocation damping theory of Granato and Lücke. Using this model the found ageing effects can be explained by the diffusion of solid solute atoms to the dislocations.

Effect of Cold Rolling on the Damping of As Cast Cu-Al-Mn Shape Memory Alloys

The ductility of Cu – Al – Mn shape memory alloys at room temperature depends on the aluminium content. High aluminium contents make Cu – Al – Mn very brittle and unsuitable for plastic shaping. Two Cu – Al – Mn shape memory alloys were investigated. The ductile alloy CuAl7.8Mn9.5 (all contents in wt. %) could be easily cold rolled by 86 %. The alloy CuAl12Mn4.3 could be cold rolled by only 12 - 14 %. The amplitude dependence of damping of austenitic specimens increased with increasing degree of cold work, whereas the damping of martensiticaustenitic specimens decreased. These observations can be explained by the creation of stress induced martensite and therefore by new moveable interfaces like phase- and twin boundaries, which contribute to damping. Plastic deformation increases the dislocation density, too. Both the increase of dislocation density and the increase of martensite content can lead to a decrease of damping mainly for high deformation degrees. Same shape memory alloys have shown negligible hardness increase during cold rolling, too. This behaviour, untypical for metals, can be explained by the generation of new martensite and by the fact that the hardness of martensite is smaller than the hardness of austenite. Some aging effects of the specimen after cold rolling, which lead to decrease of damping, were detected. This can be explained by pinning of moveable interfaces by point defects and/or retransformation of martensite into austenite.

Anelastic Properties of Mg+3vol.%Gr Prepared by Ball Milling

Internal friction in ultra-fine grained Mg with 3vol% of Graphite was measured by forced vibration method at low frequencies of 0.1, 0.5, 1.0, and 2.0 Hz over a temperature range from room temperature to 753 K with continuous heating. The specimens were prepared by milling procedure in an inert atmosphere and subsequent compacted and hot extruded. Two developed peaks in the internal friction spectrum were obtained at temperatures ≈ 350 K and ≈ 550 K. While the position of the first peak is frequency dependent, the second peak position is stable, independent of measuring frequency. The activation energy of the low temperature peak was estimated. In the light of internal friction measurements, the high temperature internal friction peak is attributed to the generation and motion of dislocations produced by the difference in the coefficient of thermal expansion between the Mg matrix and Gr phase at the matrix–particle interfaces.