G.P. Chaudhari

An assessment of ultra fine grained 316L stainless steel for implant applications.

UNLABELLED Ultra fine-grained metals obtained by severe plastic deformation exhibit higher specific strength that is useful for many applications and show promise for use as body implants. This work studied the microstructural evolution, mechanical and sliding wear behavior and corrosion behavior of 316L stainless steel warm multi axially forged at 600°C. Microstructural evolution studied using electron backscatter diffraction technique and transmission electron microscopy confirmed the formation of ultra fine-grained structure. Average grain size reduced from 30μm to 0.86μm after nine strain steps. A combination of Hall-Petch strengthening and strain hardening increased the hardness. Improved sliding wear resistance is attributed to a transition from micro cutting to wedge-forming mode of abrasive wear. Load-bearing orthopedic implants often fail from pitting initiated corrosion fatigue. Potentiodynamic tests, cyclic polarization, and FeCl3 immersion tests revealed enhanced pitting resistance of forged steel that is confirmed by Mott-Schottky analysis. This is ascribed to an increase in the grain boundary volume, and homogenization of pit inducing impurities and non-metallic phases due to severe deformation, which influenced the passive film properties. These model studies on 316L steel demonstrate that severely deformed ultra fine-grained metals have potential to deliver improved implant performance. STATEMENT OF SIGNIFICANCE This model study on 316L steel demonstrates that severely deformed ultra fine-grained (UFG) metals have potential to deliver improved load-bearing implant performance. It is as interesting as is unclear as to how such severely deformed UFG material behaves electrochemically in the corrosive body fluids. This work is on studying the inter-relationship between structure, and mechanical, wear, and corrosion behavior of warm multiaxially forged (MAFed) UFG 316L stainless steel. Warm MAF is a bulk processing method capable of yielding large volume of UFG material and is an easily readily adaptable technique in industry. It can be a promising alternative to the expensive metallic alloys available for implant applications.

Slurry Erosion Response of Heat Treated 13Cr-4Ni Martensitic Stainless Steel

In the present work, effect of various heat treatments on slurry erosion behavior of 13Cr-4Ni martensitic stainless steel (MSS) at different impingement angles has been studied. The as-received cast bars of MSS were given various heat treatments. These heat treatments involved the austenitization of cast steel at temperatures of 950° C, 1000° C and 1050° C for different soaking durations of 2, 4 and 6 h at each temperature. This was followed by oil quenching then tempering for 1 h at a 600° C air cooled. Heat treated MSS samples were characterized for microstructure and mechanical properties viz. hardness, ductility (% elongation), tensile strength (UTS), and toughness. For wear characterization, slurry erosion tests were conducted at different impingement angles for as-received cast and heat treated samples. The heat treated MSS shows approximately 34% lesser weight loss as compared to as-received cast MSS. Increase in toughness of heat treated samples is found to be responsible for the improved slurry erosion resistance. Impingement angle close to 0° showed least wear loss.

Effect of Ultrasonic Treatment on the Grain Refinement and Mechanical Properties of AZ91 Magnesium Alloy

The present study examines the microstructural evolution and mechanical properties of AZ91 magnesium alloy solidified under high intensity ultrasonic treatment (UST). High intensity ultrasonic vibrations were introduced isothermally below the liquidus temperature into the solidifying AZ91 alloy for the refinement of primary α-Mg grains and β-Mg17Al12 intermetallic phase. Various microstructures were produced using different intensity of ultrasonic vibration at a constant temperature and fixed duration of UST. Without any ultrasonic treatment, the structure contains dendrites of primary α-Mg phase which are coarser and non-uniform in size. Nearly uniform, equiaxed grains with continuous and uniform network of intermetallic phase segregated along the grain boundaries were obtained in alloy subjected to high intensity ultrasonic vibration. The average grain size in this case decreased drastically from 300 μm for (without UST) to 17 μm (with UST at intensity of 4 kW/cm2). Vickers hardness also increased steadily with increase in ultrasonic intensity. The mechanisms for microstructural refinement are discussed and it is concluded that the fine uniform grain structure achieved under ultrasonic vibrations is attributed to the cavitation and the acoustic flow induced by ultrasonic vibrations.

Cavitation and Slurry Erosion of Aluminum in the Slurry Pot Tester

In harnessing clean and renewable energy sources water turbines represent a significant portion of the power generation worldwide. Because of erosion, repair and maintenance of hydraulic turbines is a difficult problem. Material removal in hydraulic turbine components may occur either by particle erosion or cavitation erosion or by their combined action. Many ASTM standard and non standard test rigs are aimed at specific tests, like solid particle erosion, cavitation erosion. To simulate the real conditions in a laboratory setup, a novel method is employed to combine the effect of cavitation erosion and slurry erosion in the slurry pot tester. Triangular prismatic cavitation inducers are used in the conventional slurry pot tester. The aluminum test specimens are investigated in the slurry pot tester. A wide variation in material loss was noted under different exposure conditions. The maximum material loss is ascribed to combined effects of solid particle erosion and cavitation erosion.

Grain Growth Kinetics of Accumulative Roll Bonded AZ61 Alloy

A detailed study was performed on the grain growth kinetics of ultrafine-grained AZ61 magnesium alloy produced by accumulative roll bonding by carrying out isothermal annealing treatments on the roll bonded samples. Annealing treatments were carried out in the temperature range 423 to 573K for 2 to 120 minutes. As the annealing time and temperature increased, the grain size increased. The effect of annealing temperature and time, on the grain growth can be well explained by the kinetic equation and Arrhenius equation. Based on the experimental results of grain growth during annealing treatments, the grain growth exponent and the activation energy for grain growth were determined. The grain growth kinetic parameters were compared with other magnesium alloys processed by various methods.

Ultrasonic Processing and Microstructural Analysis of AZ91 Magnesium Alloy

In the present study ultrasonic vibrations are used for the grain refinement of AZ91 alloy during its solidification. The microstructures and mechanical properties of the treated AZ91 alloy are characterized. It is found that due to the acoustic cavitation and flows induced by ultrasonic vibration during the nucleation stage, the formation of the fine uniform globular grains are attributed to the combination of the enhanced heterogeneous nucleation and dendrite fragmentation. The average grain size decreases with increase in ultrasonic intensity but the largest reduction occurs mainly from 1.34KW/cm2 to 2.69K W/cm2; the benefits of further increase in the ultrasonic intensity from 2.69KW/cm2 to 4.031KW/cm2 are small.

Corrosion of nanostructured and ultrafine-grained metallic implant materials

Nanostructured metallic materials are increasingly investigated for numerous biomedical implant applications. They offer benefits of high strength with enough ductility and often possess superior biocompatibility. Several materials processing routes involving severe plastic deformation of candidate materials lead to bulk nanostructured and surface nanostructured metals. Literature is replete with studies claiming their superior mechanical behaviour and biocompatibility in terms of wettability, cell adhesion and proliferation. In spite of their metallic properties, permanent metallic implants with an expected life span of 15 years or more, often break down prematurely and corrosion of some form is a common cause or trigger. Therefore, in order to ensure reliable performance of newly developed nanostructured metal implants in service, it is necessary to understand their electrochemical degradation behaviour. Concerns stem from adverse reactions from corrosion products, desired longevity of long term implants, and biodegradability of temporary implants. Present review covers the developments in this domain.

Metallography of Fe–P–C and Fe–P–C–Si–N Alloys

The microstructures of Fe–P–C and Fe–P–C–Si–N alloys have been studied to understand the phase distributions and phase transformations during heat treatment. Iron–phosphorus alloys show ghosting features when etched with Nital, since it attacks iron with lower phosphorus content preferentially. The ghostlines mark the boundaries of phosphorus content as it existed at higher temperatures in the austenite–ferrite duplex phase region. Oberhoffer’s reagent reveals the dual-phase microstructure by depositing copper on the low-phosphorus region. The ferrite-to-austenite and reverse transformations were studied and found to occur by nucleation and growth along with diffusion of phosphorus. The morphologies of the phases conform to the Dube system. The phase distribution in dual-phase microstructures was studied using quantitative metallography and compared with the phase fractions obtained from phase diagram calculated using a thermodynamic software. Silicon and nitrogen were added to improve the toughness of the base alloy. Providing homogenization treatment in the upper ferrite region will take less time.

Effect of Deposition Parameters & Molybdenum Percentage on Nickel-Molybdenum Alloy Coatings

The use of Potassium-phosphate baths for the alloy deposition of Nickel–Molybdenum is recently proved equally good as Sodium-citrate baths. The coatings so obtained from Potassium-phosphate baths were examined for atomic weight percentage of Molybdenum. The variation in weight percentage of Molybdenum was obtained by varying different plating parameters like - molar ratio, current density range, and potential range. The observed co-deposited Molybdenum Atomic percentage variation in alloy was investigated for grain size, porosity in structure and surface roughness. The results revel that Molybdenum Atomic percentage in the Nickel–Molybdenum alloy has effect on porosity and surface roughness. It was also found that root mean square value of surface roughness was not only affected by the Molybdenum Atomic percentage but also by potential at relaxation time (TOFF) potential.

Role of activation energies of individual phases in two-phase range on constitutive equation of Zr–2.5Nb–0.5Cu alloy

Abstract Dominant phase during hot deformation in the two-phase region of Zr–2.5Nb–0.5Cu (ZNC) alloy was studied using activation energy calculation of individual phases. Thermo-mechanical compression tests were performed on a two-phase ZNC alloy in the temperature range of 700–925 °C and strain rate range of 10 −2 –10 s −1 . Flow stress data of the single phase were extrapolated in the two-phase range to calculate flow stress data of individual phases. Activation energies of individual phases were then calculated using calculated flow stress data in the two-phase range. Comparison of activation energies revealed that α phase is the dominant phase (deformation controlling phase) in the two-phase range. Constitutive equations were also developed on the basis of the deformation temperature range (or according to phases present) using a sine-hyperbolic type constitutive equation. The statistical analysis revealed that the constitutive equation developed for a particular phase showed good agreement with the experimental results in terms of correlation coefficient ( R ) and average absolute relative error (AARE).