Modification in Cu-Zn Alloy Properties by 2 MeV Ni + Ions Irradiation

We investigate the effects of 2 MeV Ni ion beam irradiation with various fluence ranging from 15 × 10 to 60 × 10 ions/cm on the surface, structural and mechanical properties of Cu-Zn alloy. The modification in target properties after irradiation is confirmed by using various characterization techniques viz. SEM, XRD, UTM and Vickers micro-hardness tester. The SEM results illustrate the formation of nano sized craters with different diameters. Their average diameter decreases from 190 nm to 90 nm by increasing ion fluence. The XRD analysis of irradiated targets reveals that Ni ion irradiation enhances the growth of (111) phase and its peak position varies due to ion induced tensile stresses in target matrix. Tensile and Vickers micro-hardness tests verify the mechanical properties of Cu-Zn alloy reduce monotonically upon irradiation. Various mechanisms such as generation, recombination, augmentation and annihilation of ion induced defects are responsible for this reduction. Understanding the relationships between various modified properties of irradiated target is essential for growing new advanced material by irradiation.


Materials Sciences and Applications
of ion-induced vacancy and interstitial concentrations takes place due to elastic and inelastic collisions. Frenkel defects are generated from the collisions between incident high-energetic ions and host atoms of lattice systems [2]. These defects can be lost either through recombination of vacancies and interstitials or by reaction with a defect sink (void, dislocation, dislocation loop, grain boundary or precipitate). The formation, growth and dissolution of defect aggregates such as dislocation, dislocation loops, voids and other types of lattice defects depend on the diffusion of point defects and their reaction with the defect aggregates [3] [4]. The increase in diffusion or atom mobility enhancement in an irradiated surface is due to enhanced concentration of the defects and production of new defects. Consequently, the probability for their mutually recombination is much higher leading to a partial defect annealing during ion irradiation. These ion induced defects can modify the surface, structural and mechanical properties of irradiated target.
Cu alloy (Cu and Zn) is commonly utilized in various industries due to of its good formability, excellent strength to weight ratio, high corrosion resistance, appropriate hardness and ductility. Auto industries are always in need of strong and light automotive parts. Therefore, it is probable to manufacture several parts and products for automobile applications using the Cu-Zn alloy [5]. The properties of Cu-Zn alloy can be further modified by ion irradiation.
The modification of material properties of metals and their alloys by ion irradiation has been probed for several years. Zuo et al. [6] irradiated Ti-6Al-4V alloy by H 2 and N 2 ion beams. The results reveal that the mechanical properties (tensile and fatigue) of irradiated target reduce significantly after irradiation with H 2 and N 2 ions. Wang et al. [7] irradiated Zr-45Ti-5Al-3V alloy by 84 MeV C ions with various ion doses. The XRD results show no new phase is formed whereas position and intensity of diffraction peak change with C irradiation dose. Zelaya et al. [8] studied the effects of 300 KeV Cu + ions irradiation on properties of Cu-Zn-Al. The size, shape and density distribution of irradiation induced cavities as a function of different conditions of irradiation. Yu et al. [9] reported that the yield strength of the Cu nanowires decreases after ion irradiation by increasing irradiation energy. Hu et al. [10] studied the effect of Cl 4+ and C 4+ ions irradiation with 25 MeV energy on the mechanical properties of bulk metallic glass. The results revealed that the hardness of irradiated target decreased after Cl 4+ ion irradiation.
The objective of this research work to correlate the mechanical modification with surface and structural variation after ion irradiation. The surface morphological growth after ion irradiation is examined by Scanning Electron Microscope (SEM). X-Ray Diffractrometer (XRD) is utilized to analyze the crystallographic structure of irradiated Cu-Zn alloy.

Experimental Details
The polycrystalline Cu-Zn alloy sheet (70 wt% Cu and 30 wt% Zn) with dimen- After this these prepared targets are enclosed in pyrex glass tube evacuated up to a base pressure of 10 −6 Torr using a rotary pump followed by a diffusion pump.
In order to relieve internal stresses and defects, these vacuum sealed pyrex glass tube containing targets are placed in a high temperature furnace (Nabertherm-LHT-02/18, Germany) and annealed at 774 K under vacuum condition  craters are formed due to micro explosions [12]. The formation of surface crater is dependent on the intrinsic properties of target and energy of incident ions.

Surface Morphology
More favorable conditions for the production of surface crater after ion irradiation are lower material density, low melting temperature of target material, low binding energy and smaller atomic displacements. The shape of the craters is dependent on the incident angle. Typically round in shape craters are formed at normal incidence. On the other hand when the angle is increased or decreased with respect to normal incidence it leads to produce alteration in shape of craters. At the oblique impact (60˚ angle) the shape of crater is usually elliptical [13]. Our targets were exposed at angle of 90˚ with respect to its surface. This angle was kept constant for irradiation. But a change in shape of nano craters is observed which is attributable to multiple scattering events. The increase in diameter of craters is attributed to coalescence process. The ion induced collision cascade develops into thermal spike regions where the hosts atoms of irradiated material are performed violent motion. Therefore the deposited energy of incident photons converts into heat energy. This heat energy cause to raise the surface temperature up to thousand degree Celsius. As a result molten zone is generated on the irradiated surface [14]. The ion induced shock liquefied and intense melting material refills the craters and basis reasons to reduce in their size [15]. The change in diameter of surface craters is due to ion induced imperfections in the lattice site of target surface such as vacancy, interstitial defects, small pits, dislocation loops and other heterogeneities. The non-uniform energy absorption, displacement spike, thermal spike, pressure spike and recrystallization are responsible for these imperfections [4].

XRD Analysis
XRD spectra of unirradiated and Ni ion irradiated target sample are shown in The average crystallite size is evaluated by using Sherrer's formula [17].
where D is crystallite size, λ is the wavelength of X-rays (1.542 Å), FWHM is full width at half maximum, and θ is the angle of diffraction.
The residual strain variations are evaluated by using following relation [17].

( )
where ε is the induced strain, d is the observed and d 0 is the standard plane spacing.
Induced stresses σ can be calculated by relation given below [17].

( )
where σ is induced stresses, ε is the induced and E is the young's modulus, for Cu-Zn its value is 102 GPa [18].
It is found that the intensity of Cu-Zn (111) peak increases by increasing ion fluence ( Figure 2). No new phase is formed after the ion irradiation. Wang et al. [7] also reported that no new phase is observed upon 84 MeV C ion irradiation.
The close examination of Figure 3 and it is observed that the peak intensity of irradiated target surface for the plane of (111) increases monotonically by increasing ion fluence up to a maximum value of 60 × 10 14 ions/cm 2 is attributed to reduction in density of generated defects by annihilation process after ion material interaction [19]. This improvement in peak intensity is due to the atomic diffusion of target matrix atom across the grain boundaries. Therefore, new alignment of host atom is formed after ion irradiation and this process is called crystal growth. The grains of the material consist of large number of crys-  [20]. From Figure 3 it is notable that diffraction peak of the plane (111) shifts to smaller angle which shows that lattice constant parameters (d-spacing and FWHM) are changed. Upon irradiation the inter-planar spacing (d-spacing) between the lattices is increased. As a result, the diffraction peak corresponding to (111) plane shifted towards lower value of angle [21]. After ion material interaction, Figure 4 reveals graphically that a noticeable increment in crystallite size occurs from 29 nm to 47 nm with an increase in ion fluence from  coupling and is called thermal spikes [22]. This energy leads to minimize the strain field between the grains of irradiated target. Therefore, the small grains emerge into large grain. As a result further improvement occurs in crystallite quality of material upon irradiation. The ion irradiation activates the annihilation process of defects which is one of the major reasons for the enhancement in the crystallite size [23]. Liu et al. [1] reported that the average grain size increases by increasing ion dose. The effect of ion fluence on the dislocation line density and residual stress is illustrated in Figure 5 and Figure 6 respectively for predominant peak of (111) plane.  subsurface. As a result annihilation process is activated [24]. Although more vacancies are produced by increasing ion fluence but at the same time annihilation rate of these induced vacancies also increase. Therefore sink volume increases by increasing irradiation fluence. Because the surface damage by Ni ion irradiation is tremendously high and the surface recovery is impossible [25].

Tensile Testing
The stress strain curves of unirradiated and irradiated targets for various fluences are depicted in Figure 7. The mechanical properties of irradiated targets such as Yield Stress (YS) and Ultimate Tensile Strength (UTS) decreases monotonically by increasing ion fluence and is graphically represented in Figure 8 and     Figure 4 and Figure 5 respectively. The increase in crystallite size is main reason to decrease the hardness. During ion metal interactions, the energy absorbed is used to increase the mobility of existing and ion induced dislocation defects. As a result the rate of annihilation is dominant than generation of vacancies and interstitial defects [29]. Another vital reason for the reduction in S. Ahmad et al. hardness of target surface after ion solid interaction is ion induced thermal sputtering [30]. The XRD analysis of irradiated targets no new phase is formed while growth improvement of (111) phase is clearly observed because of ion irradiation. The development in crystallite size and reduction in dislocation line density are observed by increasing irradiation fluence. It is attributed to the fact that during ion irradiation, vacancies and interstitials defects generated and intrinsic and extrinsic defects turned into collapses during self-annealing phenomenon. This ion induced self-annealing mechanism is responsible for diffraction peak shifting towards the lower angle. The change in YS, UTS and micro-hardness of target after ion material interaction is related with the improvement in crystalline size, a reduction in dislocation line density and ion induced thermal annealing.

Conclusions
Due to ion induced thermal spike, energy spike and displacement spike, lattice distortion is produced after irradiation. This causes production of Frenkelpairs defects and their mutual annihilation in the normal lattice site of crystal.

Novelty Statement
In this paper, the modification in properties of brass after 2 MeV Ni + ions irradiation has been explored. The surface modification has been correlated with the The variation in hardness as well as the strength of material is observed with increasing ion fluence.