niti

Grain growth during sintering of nanomaterial is one of the main drawback is this process. Since the nanomaterials maintain their unique performance as long as their size dos not exceed a specific size range, various strategies, including the addition of oxide particles, are adopted to prevent this phenomenon and to slow down grain growth. In this research, titanium oxide particles were used to prevent the growth of NiTi nanostructured alloy grains during sintering operations due to their suitable thermal stability with amount of 10 wt.%. The effect of titanium oxide particles was investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. For this purpose, two groups of samples including with and without presence of titanium oxide were prepared and studied. The results of kinetic analysis on both sample groups showed that grain growth occurred after the process, in the presence of this oxide, up to twice the initial size and before sintering. But the growth rate in the absence of titanium oxide increased up to five times. The reason for this is the amount of activation energy required for grain growth. This energy was 31.85 kJ in the sample without titanium oxide and 47.96 kJ in the sample with the presence of 10% of this oxide. Also, based on DSC data, the barrier energy were calculated as 9.93 and 7.75 kJ, and the grain growth activation energy were calculated as 13.4 and 14.4 kJ, for the samples with out and with titanium oxide respectively.

In this research similar joining of NiTi shape memory alloy was studied. For this purpose, NiTi alloy in the form of wires with circular cross section possessing martensitic phase structure at room temperature was used. By utilizing Nd:YAG pulsed laser welding method followed by optimizing its technical parameters, a defectless joint in terms of appearance and metallurgical properties was obtained. In the next step, the effect of various pulsed laser duration time on properties of the obtained similar joint of NiTi was investigated. Moreover, the resultant microstructures were studied using optical microscope (OP) and Scanning Electron Microscope (SEM) equipped with chemical analysis of EDS. Furthermore, the samples prepared under different pulsed laser duration time conditions were characterized by using tensile and micro-hardness tests. Investigating the results of the performed evaluations revealed that higher levels of heat input has resulted in grain growth, dissolution of precipitations as well as reduction in hardness and ultimate tensile strength of the samples in the joint zone.