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Monday, September 22, 2008
Porous NiTi alloy were tested to define the thermal diffusivity dependence on temperature in the range from 300 up to 1300 K.
Samples were produced by combustion synthesis technique (diameter 8 mm) and characterized by different porosity values (30-68 %).
An experimental-numerical approach, adopted for this scope, is described and the obtained results have been compared with the ones relevant a full dense NiTi alloy.
The experimental work is based on the capability to heat one side of the NiTi cylindrical sample by means of laser radiation and to record the temperature history at different positions along the sample.
Tests were featured by different heating rates, ranging from 2 to 120 K/s, and maximum temperature values of1 300 K at the heated side of the NiTi sample.
The experimental curves have been reproduced by numerical code assuming an uniform heating of one of the cylinder bases while the opposite end is supposed in contact with a massive ceramic holder. This makes reasonable the assumption of quasi one-dimensional heat transfer along the sample axis and reduced heat loss through the sample holder.
The mathematical model used is thus based on the 1D unsteady heat diffusion equation with additional terms accounting for energy losses by radiation and convection. The energy input is defined by the time-dependent surface temperature (experimentally detected) provided as a boundary condition at the heated surface.
The numerical computations was observed to be in very good agreement with the experimental results and the obtained thermal diffusivity value showed a significant change during the phase transition that occurred during the martensitic transformation.
Samples were produced by combustion synthesis technique (diameter 8 mm) and characterized by different porosity values (30-68 %).
An experimental-numerical approach, adopted for this scope, is described and the obtained results have been compared with the ones relevant a full dense NiTi alloy.
The experimental work is based on the capability to heat one side of the NiTi cylindrical sample by means of laser radiation and to record the temperature history at different positions along the sample.
Tests were featured by different heating rates, ranging from 2 to 120 K/s, and maximum temperature values of1 300 K at the heated side of the NiTi sample.
The experimental curves have been reproduced by numerical code assuming an uniform heating of one of the cylinder bases while the opposite end is supposed in contact with a massive ceramic holder. This makes reasonable the assumption of quasi one-dimensional heat transfer along the sample axis and reduced heat loss through the sample holder.
The mathematical model used is thus based on the 1D unsteady heat diffusion equation with additional terms accounting for energy losses by radiation and convection. The energy input is defined by the time-dependent surface temperature (experimentally detected) provided as a boundary condition at the heated surface.
The numerical computations was observed to be in very good agreement with the experimental results and the obtained thermal diffusivity value showed a significant change during the phase transition that occurred during the martensitic transformation.