Materials Science Forum Vols. 561-565

Abstract: The relationship between structure, ionic radius and electronegativity and solubility of the various rare-earth elements in Mg was studied. It is found that light RE(La-Sm, Eu, Yb) have more complicated phase relation with Mg but the heavy RE(Gd-Lu, Sc) have the similar crystal structure with magnesium. Also it is found that the less electronegativity difference between Mg and RE is, the more solubility limit of RE in Mg is. The fact of the RE solubility decreased in magnesium with lowering temperature suggests that there is a possibility of Mg supersaturated solid solution formation and it will decomposition during aging. According to the rule, an megnesium alloy with higher strength feature was developed. Their mechanical properties are UTS 347MPa, YTS 290MPa and elongation 12.5% at room temperature.

Abstract: The fracture mechanisms of E-A356 alloys (T5 treatment) have been investigated by means of in situ SEM under nonaxial tension loading. It is found that the crack initiated at the casting defects, such as at the gas or the shrinkage pores, because of the debonding of the silicon particles from the Al matrix. The crack propagation was mainly in the Al matrix and along the matrix/particle interface secondarily. The crack growth could be influenced by the eutectic silicon particles, when it encountered the Si particles, the growth direction would be changed and turned toward the weaker areas. The fracture resistance or the mechanical properties of the E-A356 alloys will be improved by means of reducing, eliminating gas or shrinkage pores and increasing the interface bound strength of the eutectic silicon particles with the Al matrix.

Abstract: The structure of intermetallic phases and planar defects in the as-cast and the solutiontreated microstructures of a Mg-8Y-2Zn-0.6Zr (wt%) alloy are characterized using transmission electron microscopy. The alloy was produced by permanent mould casting and solution treated at 500 °C. It is found that the intermetallic particles in the as-cast microstructure have a monoclinic structure. An appreciable amount of intermetallic particles is still retained along grain boundaries after solution treatments for up to 60 hrs. However, the structure of the retained intermetallic particles changes gradually from monoclinic to hexagonal during the solution treatments. Some planar defects are also detected in the as-cast and the solution-treated (1 hr) microstructures. These defects have characteristic features of stacking faults.

Abstract: A constitutive equation has been established to describe the effect of grain size on the deformation behavior of Ti-6.62Al-5.14Sn-1.82Zr alloy during the high temperature. In this paper, firstly a steady flow stress model is proposed, and a function relating to the grain size is introduced to modify the steady flow stress model. Meanwhile, a microstructure model established by the fuzzy neural network method is applied to calculate the grain size of prior α phase during the high temperature deformation of Ti-6.62Al-5.14Sn-1.82Zr alloy. The calculated flow stress using the present constitutive equation shows a good agreement with the experimental flow stress of the Ti-6.62Al-5.14Sn-1.82Zr alloy. The relative maximum error was not more than 15%.

Abstract: Ti6Al4V alloys are considered difficult materials to machine, especially at high cutting speeds due to their low thermal conductivity and specific heat which causes high localized cutting temperatures at the tool-workpiece interface. For these reasons machining titanium alloys usually results in excessive tool wear and its low modulus promotes chatter. This regenerative vibration or chatter is a significant problem. In the investigation reported here, vibration monitoring has been used to optimise machining processes by correlating machining process parameters with vibration severity. Machining experiments were carried out under wet and dry machining conditions while vibrations were measured and analysed. The results have demonstrated that the application of a vibration monitoring system can be an important tool to increase machining speed.

Abstract: Creep rupture tests were performed for a die-cast Mg-Al-Ca alloy AX52 (X representing calcium) at 29 kinds of creep conditions in the temperature range between 423 and 498 K. The creep curve for the alloy is characterized by a minimum in the creep rate followed by an accelerating stage. The minimum creep rate (ε& m) and the creep rupture life (trup) follow the phenomenological Monkman-Grant relationship; trup = C0 /ε& m m. It is found for the AX52 die-cast alloy that the exponent m is unity and the constant C0 is 2.0 x 10-2, independent of creep testing temperature. The values of m and C0 are compared with those for another die-cast magnesium alloys. The value m=1 is generally detected for die-cast magnesium alloys. On the contrary, the value of C0 sensitively depends on alloy composition, which is reduced with increasing the concentration of alloying elements such as Al, Zn and Ca.

Abstract: Assisted living instruments and medical implants, such as wheelchairs and joint prostheses are usually subjected to biaxial or three-axial stresses instead of uniaxial stress. So, authors already developed a servo biaxial fatigue-testing machine, and clarified about the performance evaluation. Moreover, closed-packed hexagonal lattice metal, such as magnesium and titanium, is frequently used for assisted living instruments or medical implants. In this research, fatigue crack propagation tests of magnesium alloy AZ31B and pure titanium TP340C were conducted under conditions of biaxial and uniaxial loading by using a cruciform specimen in a bi-axial fatigue machine, in order to investigate the effect of non-singular stress cycling on the fatigue crack growth properties ⊿K-da/dN. From these comprehensive experiments, in the magnesium alloy, the re-markable effect was found in the specific biaxial load stress ratio on ⊿K-da/dN relation. When biaxial load stress ratio was 0.5, it turned out that the fatigue crack propagation rate of a magnesium alloy becomes very slow. On the other hand, in the titanium, it was confirmed that there is a little influence of a biaxial load stress ratio on ⊿KⅠ -da/dN relation.

Abstract: A 10-T magnetic field was introduced to investigate precipitation behavior of Al-4wt.%Cu alloys during short time aging at 130°C. DSC, Vickers micro-hardness tests, EPMA and TEM were used to characterize aging process. The results show that high magnetic field improves the diffusion of Cu atom, lowers the stability of G.P.( I )zone and speeds up its solution, and lowers the precipitation temperature of the phases, thus accelerates the ageing course during the early stage at low temperature. Furthermore the high magnetic field has influence on the size of the θ″-phase. The micro-hardness of the field-treated specimen is always higher than the non-field ones, which indicates the high magnetic field strengthens the hardening effect.

Abstract: Severe plastic deformation (SPD) by equal channel angular extrusion (ECAE) was investigated as a way to produce ultra fine grain (UFG) microstructure in Ti-5Al-1Sn-1V-1Zr-0.76Mo. DEFORM 3D Finite Element Model (FEM) was used to predict loads and strain distribution associated with ECAE which were validated experimentally The effects of resulting ultra fine grain microstructure on room temperature mechanical properties and high temperature super plasticity were investigated. ECAE processing: resulted in the conversion of coarse Widmanstatten structure to fine equiaxed microstructure with an average grain size of 4 μm. The alloy with fine grained equiaxed microstructure has 20- 25% higher yield strength, 15% higher ultimate tensile strength, and 50-100% higher % elongation to fracture and % reduction in area compared to the as received alloy containing coarse Widmanstatten microstructure. The ECAE processed alloy exhibits attractive superplastic formability charactersistics at 900-950oC with m value >0.4, flow stresses < 20 MPa, and superplastic elongations > 200%

Abstract: Cross-roll rolling with different rolling routes (unidirectional rolling and reverse rolling) was carried out on a commercial Mg alloy sheets. In the cross-roll rolling, the roll axis was tilted by 7.5 degrees against the transverse direction. As a result of the Erichsen tests at 433 – 493 K, the stretch formability of the reverse cross-rolled specimen was higher than that of the unidirectional cross-rolled specimen. The reverse cross-rolled specimen showed lower average Lankford value compared with the unidirectional cross-rolled specimen. This is likely to be responsible for the enhanced stretch formability of the reverse cross-rolled specimen. The higher stretch formability was attributed to reduction of (0002) texture intensity, which was originated from random grain distribution.