Influence of Annealing on Creep Indentation , Surface Properties and Electrochemical Corrosion Behavior of Ni-Cr Based Dental Alloy

This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT Influence of Annealing on Creep Indentation, Surface Properties and Electrochemical Corrosion Behavior of NiCr Based Dental Alloy Abu Bakr El-Bediwi, Eman Kashita, Salah.M .M. Salman Physics Department, Faculty of Science, Mansoura University, Egypt Qassim University, Ministry of High Education, Kingdom of Saudi Arabia Physics Department, Faculty of Science, Helwan University, Ain-Helwan, Egypt


Introduction
Due to economic reasons, non-precious dental alloys are used more frequently.These are mostly Co-Cr and Ni-Cr alloys which are considerably cheaper than gold.They have higher tensile strength, their modulus of elasticity is higher and they have less density.Cobalt-chromium alloys today are widely used in prosthetic dentistry for fabrication of removable partial dentures and also for fabrication of some fixed prosthetic appliances (Huang et al., 2005).Nickel alloys are harder than precious metal alloys; they provide the required rigid support for porcelain and prevent fracture.They have been the preferred choice in long-span bridge restorations, or when strength is of main concern (Craig and Peyton, 1975;Smith et al., 1986).Like all nonprecious alloys, nickel alloy are subjected to corrosion products,  (Roitt and Lehner 1980;Ary´kan, 1992).The microhardness of Ni-Cr alloys used in fixed prosthodontics after casting under different conditions and the effect of casting technique on surface roughness and consequent mass loss after polishing of Ni-Cr and Co-Cr base metal alloys was investigated (Bauer et al., 2006;Bezzon et al., 2004).Influence of solute atoms on the surface roughness, microstructure, and mechanical properties (tensile strength and hardness) of Ni-Cr alloy was studied and correlated (Das et al., 2010).Also alloying with other elements is required to ensure the achievement of mechanical and corrosion resistance, castability and porcelain bonding (Bezzon et al., 1998;Baran, 1983).The aim of this research was to study the effect of annealing at 700, 800 and 900 C for two hours on creep process, corrosion parameters and

Materials and Method
The specimens used in the present work are Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy manufactured in Protechno-Vilamalla, Girona, Spain.These samples annealed at 700, 800 and 900C for two hours in furnace and stay in until cooled.For all tests as structure, roughness, Vickers microhardness and corrosion behavior, the samples were prepared in convenient shape.Microstructure of used specimens was performed on the flat surface of all specimens using an Shimadzu X-ray Diffractometer (Dx-30, Japan), [Metallurgy institute, El-Tebbin, Cairo], of Cu-K radiation with =1.54056Å at 4.5 Kv and 35 mA and Ni-filter in the angular range 2 ranging from 0 to 100° in continuous mode with a scan speed 5 deg/min.Microhardness test of used specimens were conducted using a digital Vickers microhardness tester, (Model FM-7, Tokyo, Japan), applying a load of 100 gf for 5 seconds via a Vickers diamond pyramid.Ten measurements were recorded for each sample and then the mean value of all measurements was used.The roughness of used samples were measured by using surface roughness measurements device (surface test S.J 201.P).
Data are measured numerically and get it from computer program, the program is calculating roughness parameters then plot the result to give roughness profile and different roughness parameters, after that data saved to be analyzed, by calculating the average surface roughness parameter Ra along the total sliding distance.The polarization studies were performed using Gamry Potentiostat/Galvanostat with a Gamry framework system based on ESA 300.Gamry applications include software DC105 for corrosion measurements, and Echem Analyst version 5.5 software packages for data fitting.
Stress exponent was determined using the Mulheam-Tabor method.

X-ray Analysis
Fig. 1 (a-d) shows x-ray diffraction patterns of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing at 700, 800 and 900 C for two hours.The x-ray diffraction patterns showed formed Cr23C6 (400), Cr23C6 (331), Cr23C6 (620) and Cr23C6 (642) crystalline phases after annealing with changing matrix structure.X-ray analysis for Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy shows that, intensity (which related to cystallinity) and broadness (which related to crystal size) of diffraction peaks are increased but position of diffraction peaks (which related to orientation) changed after annealing for different temperature at the same time.In addition, intermetallic compound appeared with formed a solid solution during dissolved different atoms in matrix alloy such as Mo, Mn, Si and C. Crystal size of nickel in Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy was calculated using by Scherer formula (Timoshenko and Goodier, 1970) then listed in Table 1.The result shows that, it increased after annealing at 700, 800 and 900C for two hours.

Creep Behavior
Creep behavior was investigated by indentation method performed at room temperature.The indentation creep data, Fig. 2a, where the indentation length is plotted versus the indentation time at constant loads of 100, 200 and 300 g.These figures show that, the indentation length increases with the loading time and the applied load.Also the curves consist of two stages similar to an ordinary creep curve.The first stage of the curve records an increase in the indentation length with loading time, with a decreasing rate, followed by a steady-state region where indentation sizes increase linearly with time.As the hardness test is actually a compression test, fracture of the specimen dose not occurs and hence it is obviously not possible to record a third stage of the curve as opposed to what happens in an ordinary creep test.
In the Mulheam-Tabor method, Fig. 2b, Vickers hardness number of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing for different temperature at two hours is plotted versus indentation time on log-log scale for the indentation data.There exists a linear relationship between indentation time and hardness for all conditions.The slope of the resultant lines according Mulheam-Tabor method is where n is the stress exponent.The stress exponent values of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing for different temperature at two hours are given in Table 2.These exponent values are in the range of 18.02 to 12.49 depending on annealing temperature of used alloy.The change in stress exponent values are attributable to microstructural features, (changing in  matrix such as change in the lattice parameters, solid solution, size and distribution of strengthening phases, intermetallic phases).
With increasing the grain size, the grain boundary decreased which on dislocation movement affecting on stress exponent values.

Vickers Hardness
Measured Vickers hardness and calculated maximum shear stress of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy decreased after annealing at 700, 800 and 900 °C for two hours as seen in Table 3.That is because the annealing changed crystal size with disturbed a solid solution, (dissolved Mo, Mn, Si and C atoms) and formed more phases such as Cr-C intermetallic compound.

Roughness
The roughness profiles of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing at 700, 800 and 900 °C for two hours.isshown in Fig. 3.The average surface roughness parameter Ra along the total sliding distance increased after annealing as shown in Table 4. Also other roughness parameters changed after annealing.Electrochemical Behavior and Parameters Electrochemical polarization curves of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing at 700, 800 and 900 °C for two hours in 0.25 M HCl are shown in Fig. 4. The corrosion potential of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing at 700, 800 and 900 °C for two hours exhibited a negative potential.Also the cathodic and the anodic polarization curves exhibited similar corrosion trends.The corrosion current (ICorr), corrosion potential (ECorr) and corrosion rate (C.R) of Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing at 700, 800 and 900 °C for two hours in 0.25 M HCl are presented in Table 5. Corrosion rate Ni63Cr24.6Mo10.8Si1.5Mn0.06C0.04alloy before and after annealing increased after annealing.

Fig.
Fig. 1a: x-ray diffraction patterns of based sample