The effect of boron (0.1 weight%) on mechanical properties, martensitic transformation temperatures and aging behavior in Cu 14.0Zn-8.5Al shape memory alloy were studied by tensile and Rockwell hardness tests, electrical resistivity measurements, X-ray and TEM.
The boron-containing alloy showed a significant increase in tensile strengths and ductilities compared to the boron-free alloy. The ultimate tensile strength increased from 160 Mpa to 380 Mpa (about a 130% increase). In the meantime, the elongation increased from 6% to 9% (about 50% increase). Optical microscopical examinations were conducted for the tensile tested samples. The boron addition gave rise to a significant reduction in grain size, accompanied by the refined martensitic microstucture. The boron addition resulted in about 80% decrease in grain size from 2.0mm to 400μm. It is proposed that the significant increase of strengths and ductilities observed in the Cu-14.0 Zn-8.5 A1-0.1B shape memory alloy are associated with grain boundary strengthening coupled with the grain size refinement effects.
Rockwell hardness measurements were conducted before and after the aging (400℃ for 1h) for the solution-treated (850℃, 0.1h) and quenched samples. Prior to the aging, the Rockwell hardness ($R_c$) was about 4, and it increased to 42 after aging treatment. The increase of hardness by aging was caused by the precipitates formed at grain boundaries as well as in the matrix by decomposition of beta-prime and beta-1 phases.
The kinetics of the aging behavior at the temperature of 250℃, 300℃, 350℃ with varying aging times were studied by electrical resistivity measurements after the solution treatment (850℃, 0.5h) and quenching to room temperature. The apparent activation energies of the boron-free alloy and boron-containing alloy were found to be about 150 and 200 KJ/mol, respectively. The absolute rate of aging of the boron-containing alloy was faster than that of the boron-free alloy at the temperatures above about 270℃, while the rate was slower below about 270℃.
Variation of martensitic transformation temperatures ($M_s$) were studied with various solution-treatment conditions, intermediate step quenching conditions, and holding times at room temperature. The $M_s$ temperature increased with increasing solution-treatment temperatures from 550℃ to 900℃ for a constant solution-treatment time (50min.) and intermediate step quenchings (140℃, 2 days). For example, $M_s$ of the boron-containing alloy increased from 84℃ for the solution-treatment temperature of 550℃ to 96℃ for solution-treatment at 900℃. The $M_s$ temperature increased with both increasing solution-treatment temperature and time. The cause of increasing $M_s$ temperature is considered to be grain coarsening accompanying the solution-treatment. The variation of $M_s$ temperature with varying intermediate step quenching conditions after a constant solution-treatment (850℃, 50min.) has been studied. The intermediate step quenching imployed involved aging times at the two step quenching temperatures of 140℃, 180℃. There were three different ranges with varying aging times. The $M_s$ temperatures of the boroncontaining alloy maintained a constant temperature of about 90℃ in range I and II regardless of the aging temperature. However, the boron-free alloy experienced decreased of $M_s$ temperature with aging times in the range I, followed by maintenance of a stable $M_s$ temperature of about 105℃ in the range II. In the range III, stable $M_s$ temperatures were maintained for the step quenching temperature of 140℃. On the other hand, in range III, $M_s$ temperatures decreased with increasing aging time for step quenching temperature of 180℃. The difference between boroncontaining alloy and boron-free alloy occurred in the early stage of range I. There was no variation of $M_s$ temperature with holding time at room temperature after the constant solution-treatment (805℃, 50 min.) and the intermediate step quenching (140℃, 2 days).
The identification of the precipitates formed during aging was studied by means of X-ray and TEM. The specimens were solution-treated at 850℃ for 30min. And intermediate step quenched before aging treatment at 200℃ for 1 day. The precipitates formed in aged specimen analyzed from X-ray diffraction line patterns were found to be 18R ordered structure having internal stacking faults, which was similar to 18R thermoelastic martensite of unaged specimen. The morphology investigated by TEM showed that the precipitates which formed during isothermal aging by thermal activation process were isolated in the matrix.
And a new theoretical model of thermoelastic martensitic transformation which is based on the compatibility of Young's modulus $E_0$ with electric permittivity $ε_0$ and magnetic permeability $μ_0$ of vacuum is proposed.
