Advanced Materials Research Vols. 29-30

Abstract: Nitrogen implanted titanium alloys with enhanced wear resistance have been synthesized under the conditions of energy and N++N2 +ion dose in the range of 30keV to 120keV and 3×1017ions/cm2 to 1×1018ions/cm2, respectively. Auger electron spectroscopy and X-ray diffraction show that supersaturated titanium solid solution with the gradient nitrogen concentration and titanium nitride compounds are formed in the surface modified regions of the materials. Enhanced wear resistance of the nitrogen implanted titanium alloy at energy of 120keV and ion dose of 1×1018ions/cm2 has been showed and explained on the basis of observed microstructure including the formation of micropits on the wear track in the present study.

Abstract: Magnesium alloys are potential biodegradable implant materials. However, magnesium alloys normally corrode rapidly in the in-vivo fluid, resulting in subcutaneous gas bubbles and alkalisation of the in-vivo solution. The paper presents a new approach to control the degradation rate of magnesium in a simulated body fluid (SBF) through employing a recently developed anodising technique. It was found that the ceramic like anodised coating formed on the surface of magnesium can effectively slow down the biodegradation process and hence result in slow hydrogen evolution and solution alkalisation processes. The results imply that an anodised magnesium alloy may be successfully used as a biodegradable implant material.

Abstract: This study concerns the development of Pb free solder plating due to environmental concerns. The composition of the bath was 0.1 mol/dm3 SnY and 0.1 mol/dm3 BiY- in 2 mol/dm3 CH3COOH - 2 mol/dm3 CH3COONa buffer solution (pH 4.0). The bath used 100 cm3 of solution. Plating was carried out at a current density of 50 mA/cm2 using ultrasonic agitation (sonication) at 28 kHz (100 W). Various percentages of a Sn - Bi alloy could be plated. A 41wt.%Sn - 59 wt.%Bi alloy with a melting point of 415 K was obtained from a [BiY-]/[BiY-+SnY] = 0.3 bath. The surface morphologies of the plated films showed a striking difference in accordance with a change of composition.

Abstract: In this paper, a rapid solidification (RS) method was employed to produce yttrium-containing TiAl based alloy ribbons. The microstructure evolution was investigated in terms of yttrium addition and RS parameters. For comparison, the conventionally cast counterpart alloys were studied as well. It was found that the microstructure of as cast alloys is sensitive to the Y content. The alloys with addition of 0 to 1.0at.% Y were of lamellar microstructures, but the alloy samples with 1.5 and 2.0at.% Y additioin were of strip-like microstructure. The yttrium-free alloy exhibited full γ phase, while the Y-bearing alloys contain γ phase, a small amount of α2, and yttrium containing phases. With increasing Y content, the secondary dendritic arm spacing (SDAS) gradually reduced. In the case of the rapidly solidified alloys, the microstructure was refined evidently when compared with the as cast counterparts. The fine Y-rich precipitates were homogeneously distributed in the matrix with a particle size of several tens of nanometers. A bcc phase (a=0.314 nm) was found in the alloys containing more than 1.5at.% Y, which was attributed to the extension of the solubility of Y in the matrix by rapid solidification.

Abstract: Titanium (Ti) alloys have emerged to become valuable biomaterials for biomedical and orthopedic applications due to their high strength to weight ratio, excellent biocompatibility and corrosion resistance. In this study, the authors utilized Solid Freeform Fabrication (SFF), also commonly known as a rapid prototyping technology to investigate a new porous three-dimensional (3D) Ti alloy implant. Elemental powders for producing a Ti-Al-Fe-Zr alloy were mechanically alloyed and blended with water soluble binder material. The blended powders were manufactured by Three Dimensional Printer (3DP), followed by debinding and sintering in an inert environment. The effects of process parameters on structural and geometrical requirements were assessed. Results from these investigations demonstrated that Ti alloys are promising biomaterials for near net shape fabrication of porous 3D implants, which retained their interconnected pore network. As an illustration, complex geometries of porous 3D Ti alloy specimens were manufactured as a demonstration of 3D SFF System.

Abstract: In the present work binary Al-3Ti and Al-3B master alloys were prepared at different reaction temperatures in an induction furnace by the reaction of halide salts like potassium fluoborate and potassium titanium fluoride with liquid molten Al. The indigenously developed master alloys were used for grain refinement studies of Al-7Si alloy and evaluated for their grain refining ability by CACCA studies. The present results suggest that, the reaction temperature influences the size, size distribution and morphology of the intermetallic (Al3Ti in Al-3Ti, and AlB2/AlB12 in Al-3B) particles present in Al-3Ti and Al-3B master alloys. Grain refinement studies of Al-7Si alloy reveal that, Al-3Ti and Al-3B master alloys prepared at 8000C-60 min. have shown better grain refining efficiency on Al- 7Si alloy when compared to the master alloys prepared at 9000C-60 min and 10000C-60 min respectively. In addition, B-rich Al-3B master alloy shows efficient grain refinement than Ti rich Al- 3Ti master alloy.

Abstract: Recent research on the fatigue properties of nanostructured metals and alloys has shown that they generally possess superior high cycle fatigue performance due largely to improved resistance to crack initiation. However, this advantage is not consistent for all nanostructured metals, nor does it extend to low cycle fatigue. Since nanostructures are designed and controlled at the approximately the same size scale as the defects that influence crack initiation attention to preexisting nanoscale defects is critical for enhancing fatigue life. This paper builds on the state of knowledge of fatigue in nanostructured metals and proposes an approach to understand and improve fatigue life using existing experimental and computational methods for nanostructure design.

Abstract: The recrystallization behaviors of cold rolled aluminum alloys in electric field up to 400kV/mm and the phase transformation processes of proeutectoid steels under magnetic field up to 14 Tesla have been experimentally examined. It has been found that both the electric field and the magnetic field have influence on the evolution of texture and microstructure characteristics. During the recrystallization annealing under the electric field of the cold-rolled 3104 aluminum alloy sheets, the electric field postpones the recovery and recrystallization progress. First principle calculation was performed to study the electric structures of aluminum atoms and vacancies. It shows that vacancies that are helpful for recovery are electrically negative. As the sample worked as anode during electric field annealing, it was covered with positive surface charges that attract the electronegative vacancies in the vicinity of the free surface and annihilate them. In this way, the recovery and then the recrystallization are postponed. The magnetic field applied changes the precipitation sequence of transitional carbides during low temperature tempering that makes the relatively high-temperature monoclinic χ-Fe5C2 carbide precipitated without following the usual precipitation sequence, i.e. by skipping the precipitation of the usual orthorhombic η-Fe2C carbide. To reveal the working mechanism of this phenomenon, first principle calculations were performed to study the formation energies of the two iron-carbide systems and their electronic and magnetic structures and properties. Calculation results show that η-Fe2C has lower formation energy, which is proved by the formation sequence observed during the usual low temperature tempering process. However, χ-Fe5C2 has the higher magnetic moment, which enhances the stability under the magnetic field through magnetization. Therefore, under the magnetic field its precipitation tendency is increased.

Abstract: The range of commercial titanium alloys available is currently extremely restricted, with one alloy (Ti-6Al-4V), and derivatives of it, accounting for a very large proportion of all applications. High performance alloys are costly to fabricate and limited to low-volume applications that can sustain the cost. With the emergence of new processing technologies that promise to reduce significantly the cost of production of titanium metal, especially in powder form, there is an emerging imperative for cost-effective near net shape powder processing techniques to permit the benefit of reduced metal cost to be passed on to higher-volume applications. Equally, there is a need for the design and development of new alloys that are intrinsically low-cost and lend themselves to fabrication by novel cost-effective net shape processing. The approaches that might be used to select, design and process both conventional alloys and novel alloy systems will be reviewed, with a focus on innovation in design of low-cost alloys amenable to new processing paths and increasingly tolerant of variability in composition.

Abstract: Alumina-iron nanocomposite powders containing 5vol.% of iron were fabricated by high-energy ball milling with different ball-to-powder weight ratios (BPRs) as part of the study of ceramic-metal nanocomposite magnetic materials. The microstructure and morphology of the composite powders were characterized using the X-ray diffraction, optical microscopy and scanning electron microscopy. XRD analysis and SEM examination in combination with energy dispersive X-ray spectrometry confirmed that the nanocomposite structure of the powder particles formed only after 8 hours milling for both BPRs used. With a higher BPR of 16:1, Fe-Cr alloy material was broken from the stainless steel balls and incorporated into the nanocomposite powder. However, such a problem did not occur with a lower BPR of 5:1. The mechanism for formation of the alumina matrix nanocomposite powder is found to be dependent on BPR and milling time.