Ablation and Dynamic Effects of Intense Pulsed Ion Beam on Metallic Materials, страница 2

Figure 1. Typical ablated surfaces: Ti, 100 J/cm², a — ion beam center; b — 5 mm from the center; TiAl, 50 J/cm²; c — ion beam center

A phenomenon of new phase formation is found for the first time for pure Ti irradiation, i. e., Ti-N compounds formed on the irradiated surfaces. The effect is more pronounced in group C with increasing shot number, whereas no Ti-N phases could be detected in groups A and B. Figure 2 shows the XRD patterns of Ti samples (group C), at 1, 2 and 10 shots, respectively. The diffraction peaks from Ti₂N and TiN phases are clearly observed after 2 shots, and became more obvious after 10 shots. Note that a better vacuum condition was used (~10^-3 Pa) in our previous study on TEMP-6 [2, 9]. Hence, the N is possibly from residual gas in vacuum chamber, solved into molten surface at the repetitive melting and re-solidifying, as inferred from the N enhancement (content of Ti-N compounds increase) with increasing shot number. The low energy density is critical during the formation process, without obvious mass removal on the irradiated surfaces. Moreover, surface smoothing has also been observed in groups C and D, in accordance with the results in the case of low-energy density [4, 8], repetitive melting with negligible ablation leading to the smoothing.

Figure 2. Formation of Ti-N compounds during IPIB irradiation, resulting in sample hardening: a — XRD patterns; b — micro-hardness. [JCPDS: TiN (#38-1420), Ti₂N (#17-0386), a-TiN_0.30 (#41-1352)]

Furthermore, microhardness measurement has been performed on the group C [Figure 2b]. It is unambiguously confirmed that surface hardening with increasing shot number, corresponding to the formation of Ti-N compounds.

Figure 3 presents the XRD patterns of irradiated TiAl alloys. The original samples are mainly composed of g-TiAl phase. A transformation into Ti₃Al phase took place irradiated at 50 J/cm², 1 shot, where original g-TiAl phase almost disappeared, implying formation of a thick Ti₃Al top layer of ~10 mm. The transformation of TiAl to Ti₃Al, possibly with a minor intermediate Ti₂Al layer, was firstly found in a lower energy range and a concept of selective ablation in TiAl has been proposed [5]. Nonetheless, the Ti₃Al layer seems to be much thinner (hundreds of nm) due to a low energy of ion beam. Subsequently, we abraded the top layer of sample by mechanical polishing to study the layer structure of modified samples, where all the Ti₃Al was nearly removed as indicated from XRD patterns with Ti₂Al peaks remained and re-occurrence of peaks from underlying TiAl substrate [Figure 3(b)]. Therefore, Ti₂Al phase virtually exists as an intermediate layer between Ti₃Al and TiAl phases, providing an evidence for selective ablation in IPIB-irradiated TiAl alloys.

Figure 3. XRD patterns of TiAl alloys: a — original; b — irradiated at 50 J/cm², 1 shot. [JCPDS: g-TiAl (#05-0678), Ti₃Al (#14-0451), Ti₂Al (#47-1410), Ti_3.3Al (#16-0867)]

Figure 4. XRD pattern of irradiated TiAl after removal of top layer using mechanical polishing

Acknowledgments

The authors are grateful to great help by Drs. S. M. Miao, Z. H. Dong, and Z. C. Xu from Surface Engineering Laboratory, Dalian University of Technology, China. One of author, X. P. Zhu, would like to thank financial support of 21^st Century COE Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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