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

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

Xiaopeng. Zhu^a[1][1], M. K. Lei^b, Hisayuki Suematsu^a, Weihua Jiang^a, Kiyoshi Yatsui^a

^aExtreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka,Niigata 940-2188, Japan ^bSurface Engineering Laboratory, Department of Materials Engineering, Dalian University of Technology, Dalian 116024, China

Abstract. Metallic materials have been irradiated with intense pulsed ion beam (IPIB), for studying effects of the ion-beam irradiation on the materials. The surface morphology and phase microstructures of metallic materials are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Formation of new phase due to beam-materials interaction significantly contributes to the modification of material properties (e. g. micro-hardness).

Introduction

Intense pulsed ion beam (IPIB) technique has been developed as a tool for materials surface modification and synthesis in recent years [1–3]. This technique utilizes a high-energy density ion beam (~10² J/cm²) delivered by a short pulse (<1 ms) to irradiate onto target materials, the followed two processes are significant for material applications as a result of beam-target interaction: rapid melting and re-solidification of surface layer at ultra-fast heating and cooling rate typical of 10⁸–10¹¹ K/s; and ablation of target materials producing a high-density plasma. The former could be applied to modify the surfaces of materials (surface treatment), and the latter to synthesize films/powders from condensation of the ablated plasma on adjacent substrates (deposition). Due to the characteristics of this method, surface morphology, composition and microstructures of target materials may undergo tremendous changes with a single or several shots of ion beam, which could noticeably affect the material’s properties during the subsequent modification and/or syntheses. Consequently, investigations on alternation of materials’ properties during IPIB irradiation/ablation are one of crucial concerns to understand beam-material interaction processes, which may be helpful to make this technique more effectively and efficiently in materials applications. For this purpose, surface morphology and phase transformation of pure Ti and TiAl alloys have been studied recently under IPIB irradiation with a medium energy density up to 5 J/cm² [4, 5]. In this work, an IPIB with a much higher energy density up to 100 J/cm² is used to irradiate the two kinds of materials, where the study mainly focus on the phase transformations of the materials.

Experimental

The irradiation of pure Ti and TiAl alloys was conducted in an intense pulsed ion-beam apparatus, ETIGO-II [6]. A pulsed ion beam (~1 MV, ~60 kA, ~70 ns, >75 % proton with balance C ions) was irradiated onto the target surface. An energy fluence of 50 J/cm² or 100 J/cm² was used in this study. The samples with dimensions of 15×15×2 mm² are mechanically polished down to 800-grit SiC paper. The irradiation was performed with 1~10 shots for pure Ti and 1 shot for TiAl alloy in a vacuum of ~10^–2 Pa, respectively. Four groups of Ti samples (refered as A, B, C and D) were positioned in an alignment from the center to edge of ion-beam spot to compare the irradiation effects by various energy densities. It is a simple and effective way to study ablation behavior of IPIB irradiation due to an energy distribution along radial direction of ion- beam spot [7]. The surface morphologies and crystalline structures are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and hardness measurement was performed for Ti samples, respectively.

Results and discussion

The surface morphologies of Ti and TiAl samples at 100 J/cm², 1 shot are shown in Figure 1. Surface roughening with waviness is clearly seen on both the materials, especially for TiAl, as a result of ultra-fast re-solidification from disturbance state of molten surfaces during ablation [4]. It is noted that flat surface was formed in the center region of Ti sample (group A), according with the speculation of Ti smoothing under higher power of IPIB irradiation [4]. Surface smoothing is expected from a uniform ablation in the center even under severer ablation. However, the adjacent area to the center are roughening, presenting a feature of ripples [Figure 1b] whose «oriented»shape implies great interference of molten layer from a more intense ablation in the center of sample. Numerous cracks occurred on the irradiated TiAl alloys along with huge waviness [Figure 1c]. Since the thermal and mechanical properties of TiAl alloys obviously different from pure Ti, e. g., low thermal conductivity and high brittleness, this material is liable to cracking under IPIB ablation even in a smaller energy density [8].