CHARACTERISTICS OF FATIGUE SURFACE MICROCRACK GROWTH IN VICINAL INCLUSION FOR POWDER METALLURGY ALLOYS

Inclusion flaw is one of the worst flaws of powder metallurgy.The inclusion flaw plays an important role in the failure of high temperature turbine materials in aircraft components and automotive parts,especially fatigue failure.In this paper,an experimental investigation of fatigue microcrack propagation in the vicinal inclusion were carried out by the servo-hydraulic fatigue test system with scanning electron microscope(SEM).It has been found from the SEM images that the fatigue surface microcrack occurs in the matrix and inclusion.According to the SEM images,the characteristics of fatigue crack initiation and growth in vicinal inclusion for powder metallurgy alloys are analyzed in detail.The effect of the geometrical shape and material type of surface inclusions on the cracking is also discussed with the finite element method(FEM).

Improvement of Ductility of Powder Metallurgy Titanium Alloys by Addition of Rare Earth Element

Ti-4.5Al-6.0Mo-1.5Fe, Ti-6Al-1Mo-1Fe and Ti-6Al-4V alloys were prepared by blended elemental powder metallurgy （PM） process, and the effects of Nd on the microstructures and mechanical properties were investigated by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）. It was found out that the addition of Nd increased the density of sintered titanium alloys slightly by a maximum increment of 1% because small amount of liquid phase occurred during sintering. The addition of Nd shows little effect on the improvement of tensile strength, while the elongation is significantly improved. For example, the elongation of Ti-4.SAl-6.0Mo-1.5Fe can be increased from 1% without addition of Nd to 13% at a Nd content of 1.2 wt pct.

Processing TiAl-Based Alloy by Elemental Powder Metallurgy

TiAI-based alloys with various compositions (including Ti-48Al, Ti-47Al-2Cr-2Nb, Ti-47Al-2Cr-2Nb-0.2B and Ti-47Al-3Cr, in mole fraction) had been prepared by elemental powder metallurgy (EPM). The results have shown that the density of the prepared Ti-48AI alloy increases with increasing hot pressing temperature up to 1300℃. The Ti-48AI alloy microstructure mainly consisted of island-like Ti3Al phase and TiAl matrix at hot pressing temperature below 1300℃, however, coarse α2/γ lamellar colonies and γ grains appeared at 1400℃. It has also indicated that the additions of elemental Cr and B can refine the alloy microstructure. The main microstructural inhomogeneity in EPM TiAI-based alloys was the island-like α2 phase or the aggregate of α2/γ lamellar colony, and such island-like structure will be inherited during subsequent heat treatment in (α＋γ) field. Only after heat treatment in a field would this structure be eliminated. The mechanical properties of EPM TiAl-based alloys with various compositions were tested, and the effect of alloy elements on the mechanical properties was closely related to that of alloy elements on the alloy microstructures. Based on the above results, TiAI-based alloy exhaust valves were fabricated by elemental powder metallurgy and diffusion joining. The automobile engine test had demonstrated that the performance of the manufactured valves was very promising for engine service.

SOME DEVELOPMENTS IN RAPIDLY SOLIDIFIED ALUMINUM ALLOYS FOR ELEVATED TEMPERATURE APPLICATIONS

Therecent developments in elevated temperature ( ET) aluminum alloys prepared by therapidsolidification / powder metallurgy ( RS P/ M) process were reviewed briefly. TheRS P/ METaluminum alloyscan beclassified as(a) the aluminum transition metaltype, such as Al Fe, Al Cr, Al Ti, Al Zrsystem alloys,etc.,and (b) thealuminum rareearth elementtype,such as Al Y, Al Nd system alloys,etc. Among them ,the Al Fe and Al Ti system alloysarethe most attractive, which possessthe potentialto replacethetitanium alloy partson aircraft,engines,etc.,fortheuseattemperaturesrangingfrom 200 315℃. Theproblemsin applicationsfor RS P/ M ETaluminum alloys werealso discussed .

Superior Properties of Mg–4Y–3RE–Zr Alloy Prepared by Powder Metallurgy

Magnesium alloys are important materials for application in the automotive and aviation industries. During the last few years, the number of possible applications as biodegradable implants in medicine has grown. Mg-RE（rare earth） alloys belong to the most advanced group of products, offering the best combination of mechanical properties and corrosion resistance. Among these materials, WE43（Mg-Y-Nd）is a very well-known commercial alloy that has been extensively studied for applications at increased temperatures and also in organisms. Although this material has been described, there are still possibilities to improve its properties and subsequently expand its applicability. Powder metallurgy has already been used for the preparation of magnesium alloys with superior mechanical properties and occasionally superior corrosion properties. Therefore, the present paper is oriented toward the preparation of Mg-4Y-3RE-Zr（WE43） alloy by the powder metallurgy technique（WE43-PM） and comparison of the final properties with the product of extrusion of as-cast ingot（WE43-IM）. Our processing leads to a partial improvement in the mechanical properties and superior corrosion resistance of WE43-PM. The texture strength of WE43-PM was low, and therefore, anisotropy of mechanical properties was suppressed.

Hot Deformation Mechanism and Ring Rolling Behavior of Powder Metallurgy Ti_2AlNb Intermetallics

Powder metallurgy（PM） Ti–22Al–24Nb–0.5Mo（at.%） alloys were prepared by hot isostatic pressing. In order to study the feasibility of PM ＋ ring rolling combined process for preparing Ti_2AlNb rings, thermal mechanical simulation tests of PM Ti_2AlNb alloys were conducted and two rectangular PM rings（150 mm in height, 75 mm in thickness,350 mm in external diameter） were rolled as a validation experiment. Experimental results show that the flow stress of Ti_2AlNb alloys exhibited a significant drop at the very beginning of the deformation（true strain／0.1）, and became stable with the increase in strain. Stress instability phenomenon of PM Ti_2AlNb alloys was more obvious than that of wrought alloy. Flow stress fluctuation at the initial stage of deformation is related to phase transition of Ti_2AlNb alloys which strongly depends on heat treatment and thermal mechanical deformation process. Processing windows during initial stage of ring rolling process is very crucial. A sound PM Ti_2AlNb rectangular ring blank（height = 150 mm, thickness = 30 mm, external diameter = 750 mm） was successfully rolled in two passes by using the improved heat preservation method and optimized rolling parameters. Tensile properties of PM Ti_2AlNb alloy were improved, and the porosity was reduced after ring rolling.

Characterization of Mo–Si–B Nanocomposite Powders Produced Using Mechanical Alloying and Powder Heat Treatment

Mo-Si-B nanocomposite powders with a composition of Mo-12Si-8.5B （in at.%） were processed using mechanical alloying under milling conditions for different milling time and powder-to-ball ratios. The Mo-12Si-8.5B alloy, which consists of α-Mo and intermetallic Mo3Si and T2 phases, was also synthesized by hot-pressed sintering the mechanically alloyed powders under a pressure of 50 MPa at 1600 ℃. The results demonstrated that the sizes and morphologies of the powder particles became gradually refined and uniform by both increasing the milling time and decreasing the powder-to-ball ratio. After 15 h of milling, the powders were completely homogenized at the 1：10 and the 1 ： 15 powder-to-ball weight ratios, and the homogenization was accelerated to rapidly stabilize the milling process because of their high milling energy. Annealing the Mo-Si-B milled powders could promote the growth of the intermetallic Mo3Si and the T2 phases, which formed even after low-temperature annealing at 900 ℃. Increasing the annealing temperature only improved the crystallinity of various phases. When the milled and annealed powders were hot-pressed sintered, the Mo-Si-B alloy exhibited a fine-grained microstructure, where the intermetallics Mo3Si and T2 were distributed in a continuous α-Mo matrix.

Effect of Nano-sized B4C Addition on the Mechanical Properties of ZA27 Composites

In order to understand the influence of nano-sized B4C additive on ZA27 alloy, mechanical and physical properties of ZA27-B4C nanocomposites were investigated in terms of B4C content. While physical properties were determined in terms of microstructural studies, density and porosity tests, mechanical properties were determined in terms of ultimate tensile strength（UTS） and hardness experiments. Morphological and microstructural studies were carried out with scanning electron microscopy（SEM）. The experimental results indicate that nano-sized B4C can be used to enhance the mechanical properties of ZA27 alloy effectively. The highest mechanical performance can be obtained at ZA27-0.5% B4C（in weight） nanocomposite with values of tensile strength（247 MPa） and hardness（141,18 BH） and low partial porosity（0.5%）. After a pick point, increasing B4C ratio may cause the formation of agglomeration in grain boundaries, that＇s why density, tensile strength, and hardness values are declined.

In Situ Aluminum Alloy Coating on Magnesium by Hot Pressing

In the present study, pure magnesium was in situ coated with pre-alloyed Al–Cu–Mg alloy through hot pressing. The produced samples were characterized by means of hardness, wear properties and microstructure characterization. A ball-on-disk test was used to determine the dry sliding wear characteristics of the compacts. The results showed that the hot pressing technique has been successfully applied for producing magnesium parts with compatible wear resistance and hardness to aluminum. The in situ coating of Al on Mg by hot pressing resulted in an increase in hardness of about 30% compared with the pure Mg substrate. The wear rate and friction coefficient of the samples decreased with Al coating and increased with an increase in the applied load during the wear tests, compared with the uncoated material.