Papers by Keyword: Intermetallic Particles

Abstract: Rolled 7xxx-series Al alloys exhibit pronounced microstructural anisotropy (pancake grains and particle stringers) that can strongly affect fatigue initiation and-crack growth. In the current study, an AA7075-T6 plate was examined in three orthogonal machining orientations—L–S (longitudinal), L–T (long-transverse) and T–S (short-transverse)—using high-cycle three-point bending fatigue at room temperature (R = 0, f = 25 Hz, σ_max = 360–400 MPa, i.e., ~0.79–0.88σ_y). Optical/SEM observations reveal elongated grains and a grain-density gradient through thickness, accompanied by orientation-dependent distributions of intermetallic particles. Despite only small differences in monotonic response, fatigue performance is strongly orientation-dependent: the T–S specimens exhibit the longest lives in the S–N curves. Fractography and striation-based kinetics show the lowest Paris-regime crack-growth rate for T–S (da/dN ≈ 1.85×10⁻⁷ m/cycle at ΔK ≈ 10.5 MPa√m), while L–S shows the fastest growth (da/dN ≈ 4.3×10⁻⁷ m/cycle at ΔK ≈ 13.0 MPa√m). The improved T–S fatigue resistance is discussed in terms of crack-path interaction with grain boundaries and particle populations (coherent/penetrable vs non-coherent/coarse particles), which can either deflect/retard cracks or act as initiation sites. The results provide a compact microstructure–mechanics map for rolling-induced anisotropy in AA7075-T6 under bending fatigue.

Abstract: Grain growth of Aluminium alloys at high temperature reduces their strength significantly. Therefore it is essential to control the grain growth by suitable techniques. The effect of Cr on refining the grain structure of Al alloys is investigated in the present work. Cr is added into Al-Si-Mg alloy by stir casting techniques. Subsequently the alloys were subjected to annealing at 500oC for 1 hour to study the grain growth behavior of these alloys. An optical microscope is used to analyze the grain size and microstructure. The mechanical properties of these alloys were studied using a universal testing machine. The Cr additions were varied between 0.5 to 4 weight percent. The Cr addition had shown a significant effect in refining the grain size of Al-Si-Mg alloys. The average grain diameter after annealing was found to be about 151 μm in unalloyed samples while, the 2 wt % Cr added samples showed about 92 μm. The second phase particles, Al₇Cr found precipitated along the grain boundaries apart from Al₂Si and Mg₂Si. However Cr additions more than 2% does not show significant role in refining the grain size. Cr also improved the mechanical properties such as yield strength, ultimate tensile strength and fracture toughness moderately.

Abstract: This work is a study of the segregation of alloying elements and salts to the surface of Direct Chilled-cast (DC-cast) ingots of alloy: (i) AA5083; and (ii) AA5182. The ingots only have slightly different chemical compositions. Alloy AA5182 contains Cu, while alloy AA5083 is grain refined, contains Cr, and is lower in Si and Fe compared to AA5182. The ingot microstructures were investigated using Optical Microscopy, Scanning Electron Microscope (SEM) and Secondary Ion Mass Spectrometry (SIMS). The results showed that the surfaces contained aluminum dendrites, large intermetallic crystals (Al₆(MnFe) and Al₃(Fe,Mn)) and impurities, and small crystals between dendrite arms. Between the dendrites small salt crystals containing Na, K, Ca, Mg, F and/or Cl were found. The conclusion is that salt and minority impurities are mainly located to grain boundaries in the segregation zone.

Abstract: The mechanisms of fatigue crack initiation due to second phase particles are studied in 2050-T8 and 7050-T74 plate material. The particles in the specimens gauge lengths are imaged using SEM at the initial state. In 7050-T74, Mg₂Si particles are very often cracked before any loading, whereas Al₇Cu₂Fe particles are not. In 2050-T8, the fraction of (Al, Cu, Fe, Mn) particles initially cracked is larger than that of Al₇Cu₂Fe in 7050-alloy, but lower than that of Mg₂Si particles for similar sizes. For (Al, Cu, Fe, Mn) particles, the proportion of cracked particles increases when the modified shape ratio (aspect ratio including orientation versus rolling direction) increases. This effect is present but less pronounced for Mg₂Si particles in 7050-T74. Fatigue cracks initiate at cracked (Al, Cu, Fe, Mn) particles in 2050-T8 alloy, and at both Al₇Cu₂Fe (cracked during cycling) and Mg₂Si in 7050-T74 alloy.

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: The effect of manganese on grain refinement of a commercial AZ31 alloy has been investigated using an Al-60%Mn master alloy splatter as an alloying additive at 730 °C in aluminium titanite crucibles. It is shown that grain refinement by manganese is readily achievable in AZ31. Electron microprobe analyses reveal that prior to the addition of extra manganese the majority of the intermetallic particles found in AZ31 are of the Al8Mn5 type. However, after the addition of extra manganese in the range from 0.1% to 0.8%, the predominant group of intermetallic particles changes to the metastable AlMn type. This leads to a hypothesis that the metastable AlMn intermetallic particles are more effective than Al8Mn5 as nucleation sites for magnesium grains. The hypothesis is supported by the observation that a long period of holding at 730 °C leads to an increase in grain size, due probably to the transformation of the metastable AlMn to the stable Al8Mn5. The hypothesis has also been used to understand the mechanism of grain refinement by superheating.

Abstract: Using transmission electron microscopy, x – ray diffraction analysis, tensile testing and fractography the changes were analyzed that occur in microstructure and tensile properties of E635 alloy annealed after quenching from b – region of phase diagram. The interrelation is demonstrated between microstructure and tensile properties depending on deformation/heat treatment conditions.