Single-phase NiTi was fabricated through the thermal explosion mode of combustion synthesis of mechanically activated powders. Combustion and ignition temperatures of combustion synthesis were investigated in different milling times. In this process, equiatomic powder mixtures of nickel and titanium were activated by planetary ball mill and pressed into disk-shaped pellets then heated in a tube furnace, while temperature-time profile was recorded. X-ray diffraction analysis (XRD) was performed on milled powders as well as synthesized samples. Scanning electron microscopy (SEM) was also used to study the microstructural evolution during milling. The results showed that there was a threshold milling time to obtain single-phase NiTi. It was also seen that the ignition temperature and combustion temperature were reduced significantly by increasing milling time.
NiTi alloy combines the characteristics of shape memory effect and superelasticity with excellent corrosion resistance, wear resistance, mechanical properties, and good biocompatibility [
Conventionally, NiTi intermetallics are produced by arc or induction melting followed by hot working and forming [
Several researches have been done on fabricating NiTi via combustion synthesis [
Using mechanical activation before combustion synthesis in order to reduce secondary phases has been reported in Ti-Al [
The starting materials consisted of high-purity powders of nickel (99.5%, 10
Figure
XRD patterns of 50 Ni -50 Ti powder mixtures, after different milling times.
X-ray peak broadening with increasing the milling times is also observed in Figure
Variation of Ni and Ti crystallite size versus milling time.
It can be seen that the crystallite size of Ni and Ti is decreased with increasing the milling time. Displacement of Ni X-ray peaks towards lower angles is observed after 3 h as shown in Figure
Displacement of Ni peak with increasing the milling time.
According to the fact that the Ni lattice parameter increases by 0.1% per at.% of dissolved Ti [
Variation of Ni lattice parameter and at.% of dissolved Ti in Ni lattice versus milling time.
Ni-Ti binary system is considered to be a ductile-ductile system. Benjamin and Volin [
SEM images of the Ni-Ti powder mixture after milling for 1 h: (a) 1000X, (b) 15000X.
SEM images of the Ni-Ti powder mixture after milling for 5 h: (a) 1000X, (b) 5000X.
As it is obvious in Figure
Figure
Figure
Time-temperature profiles of powder mixtures milled in different milling times.
The XRD pattern of the products obtained after the first exothermic peak of the sample milled for 0 h is shown in Figure
The XRD pattern of the products obtained after the first exothermic peak of the sample milled for 0 h.
It could be concluded that the low temperature exotherm is due to the formation of little amounts of NiTi and Ni4Ti3 phases. The low temperature exotherm related to a precombustion synthesis is not observed in the time-temperature profiles of the samples milled for 5 and 10 h, which is due to the significant decrease in ignition temperature of the main combustion reaction. It is worth noting that the first exothermic peak of the sample milled for 10 h corresponded to the main combustion reaction since it is sharper and more exothermic than the peaks which were due to the precombustion reactions.
Figure
Variation of ignition and combustion temperature versus milling time.
The milling process, which causes the intimate mixing of particles on a nanoscale, leads to the reduction in the crystallite size and accumulation of defects in powder particles, which introduce an additional energy to the reactant system in the form of interfacial energy and strain energy. This effectively lowers the activation barriers for reactions. Moreover, the presence of a variety of crystal defects (i.e., dislocations, vacancies, staking faults, grain boundaries, etc.) enhances the diffusivity of elements [
Decrease in diffusion distances as well as activation barrier to NiTi formation reaction occurred during milling causes the ignition temperature to decrease significantly in postcombustion synthesis. According to Figure
In a nonadiabatic condition of a combustion synthesis reaction, the maximum temperature of products is called combustion temperature which is equivalent to the adiabatic temperature. Adiabatic temperature (
Figure
XRD patterns of powder mixtures milled in different milling times after combustion synthesis.
Moreover, as it can be seen from Figure
High chemical affinity of titanium with oxygen and high surface energy of the milled titanium powders which enhances the reactivity of Ti caused the formation of little amounts of TiO2, even in a high purity argon atmosphere.
In this paper mechanically activated powders were used to fabricate NiTi intermetallic compound through combustion synthesis method. The results showed that with premechanical activation at least for 3 hours, NiTi alloy can be fabricated without undesired Ni3Ti phase through thermal explosion mode of combustion synthesis. Moreover, decrease in diffusion distances caused by micrometric aggregation formation drastically lowers the ignition temperature and as a result lowers the combustion temperature.