A Study on the Interrelationship between the Microstructural Features and the Elevated Temperature Strength of Multicomponent Al-Si-Cu-Ni Casting Alloys

The effect of Cu/Mg solute combined with ultrasonic melt treatment (UST) on the microstructure and mechanical properties of a hypoeutectic Al-7Si alloy was investigated. Cu up to 4 wt pct and Mg to 1 wt pct were added to the alloy and UST was performed at about 100 °C above the liquidus temperature. UST at such a high temperature was ineffective in refining grain sizes but enhanced the microstructural homogeneity of the alloys with the refinement of α-Al secondary dendrite arm spacing (SDAS). It was found that both the grain size and the SDAS were solute sensitive: In the Al-7Si alloy with UST they insignificantly changed with Cu but decreased with an increase in Mg content. Along with the structural refinement, UST was likely to affect the solute redistribution and the subsequent formation of secondary phases (e.g., eutectic Si and Cu/Mg-rich intermetallic compounds (IMCs)). Quantitative analysis indicated that UST had significantly minimized microsegregation of Cu and Mg solutes, decreasing the amount of the IMCs finely and uniformly distributed. The underlying reason was attributed to the reduction in the size of the SDAS, which enabled the solid-state diffusion to occur more efficiently upon solidification. The most significant increase in the tensile properties was achieved for the Al-7Si-2Cu-1Mg alloy with UST where the SDAS was well refined and the grain size was the smallest among the alloys investigated. The important role of UST on the casting structure refinement and the solute distribution is discussed and the resulting mechanical properties are further explored.

Chemical grain refining of high-silicon-content aluminium alloys, such as 355 alloy, is impaired by the high silicon content. One solution is the use of ultrasonic melt treatment (UST). This study sought to determine the duration of UST and type of horn (steel or Ti) required to achieve optimal grain refining in these alloys. Samples produced by conventional casting underwent UST for different times with steel and Ti horns and were compared with as-cast samples. For all the conditions studied, analysis of the samples showed that UST is an effective grain refining technique and yields satisfactory values of average grain size and primary and secondary dendrite arm spacing (λ1, λ2) as well as low porosity. The chemical composition of the samples was analysed by SEM–EDS mapping and point analysis to identify the intermetallic phases before and after UST. Best results were achieved after only 20 s of UST with a steel horn. UST for this length of time with a steel horn produced a homogeneous microstructure and possible homogeneous mechanical properties. Grain size was 160 µm; primary dendrite arm spacing, or dendrite cell size, was 130 µm; secondary arm spacing was approximately 18 µm; and Vickers hardness was approximately 95 HV.

By using the finite element method (FEM), the stress-strain curves of multiphase steels was simulated based on the stress-strain curves of single-phase ferrite, bainite, and martensite steels; then the measured result was compared with the simulated one. Effective factors such as the different distribution of microstructure, the volume fraction of hard phase and the yield stress ratio between single-phase hard phase steel with single-phase ferrite steel in multiphase steel are discussed in this work. The results show that the simulated result closely fits the measured one, which proves that this FEM built in this work is correct. The coarser the microstructure, the higher the drag effects of bulk structure, and the larger deformation degree of ferrite phase. With the increase of bainite and martensite volume fraction, the maximum stress rise gradually and the maximum strain decreases gradually. Meanwhile, the effects of volume fraction of hard phase on the stress-strain curve of multiphase steels are larger than that of yield stress ratio between single-phase hard phase steels and single phase ferrite steel.

Microstructure, tensile and fatigue properties of the Al–10%Si–2%Cu alloy with different Fe and Mn content cast under controlled conditions

Co-based alloy tools having the γ/γ’ microstructure with distribution of hard phases hardly experience large deformation and severe wear during friction stir welding (FSW) of ferritic steels at the welding temperature lower than the γ’-solvus. Further examinations implied that the tool wear were suppressed more effectively on Co-based alloy tool with larger volume fraction of hard phases with average size of 2 to 5 μm. In this study, Co-based alloy tools with different volume fractions of hard phases were designed, and effect of hard phases on tool wear was examined in FSW of 0.45% carbon steel. FSW trial showed that the tool wear decreased with increasing volume fraction of hard phases. At the volume fraction of 36%, the wear volume was successfully reduced to two third of that of the tool exhibiting the good wear resistance in a previous study. This study showed a possibility that the tool performance is effectively enhanced by further microstructure optimization of the tool material.

The mechanical properties of Al-Si alloys are affected by several microstructural features such as secondary dendrite arm spacing (SDAS), size and shape of eutectic Si-particles, presence of intermetallics as well as by porosity. In the current study, Al-Si-Cu alloy A380 was prepared by a unique directional solidification method to produce samples with two different SDAS of 9 μm and 27 μm. The lower solidification rate resulted in larger SDAS, larger grain size, larger eutectic Si and larger intermetallics including Fe-rich β phase. The microstructure with higher solidification rate was found to be finer and more homogeneous with smaller eutectic Si and intermetallics. The specimen with larger SDAS exhibited stronger texture than the one with smaller SDAS. The specimen with smaller SDAS showed improved mechanical properties including YS, UTS and ductility.

We synthesized series of shape memory polyurethanes with amorphous reversible phase (Tg‐SMPUs) and systematically studied their microphase structure and shape memory properties. The Tg‐SMPUs having no or less hard phase showed lower shape recovery. When the volume fraction of hard phase was in the range of 20–30%, the Tg‐SMPUs exhibited the highest shape recovery. As the fraction of hard phase increased further the shape recovery decreased, because more polymer components with higher glass transition temperatures (Tgs) would participate in strain storage. For the Tg‐SMPUs having similar Tgs, those polymers having higher volume fraction of hard phase exhibited higher shape fixity, broader shape recovery region, and larger recovery stress. Increasing deformation strain could raise shape fixity and recovery stress but broaden shape recovery region. The highest recovery stress of a material could be achieved when the deformation occurred at its glass transition temperature below which decreasing deformation temperature could not increase recovery stress further. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

Fracture initiation and propagation in two phase alloys containing fairly large volume fractions of nonplastically deforming inclusions have been analyzed. The Argon, Im and Safoglu treatment of fracture initiation of elastic inclusions as a result of back stresses resulting from strain accommodation of the phases has been extended to relatively large volume fractions of second phase. This allows calculation of the distribution of fractured particles as a function of alloy strain provided the fracture stress of the elastically deforming phase is known. The analysis has been applied to the Co-CoAl two phase alloy for volume fractions of CoAl ranging from about two to twenty-five pct. Quantitative metallographic analysis of fractured specimens indicates very good agreement between the measured fraction of fractured particles and those predicted from the theory without recourse to any adjustable parameters. Critical crack propagation in alloys of this type can also be analyzed on the basis of a fracture mechanics approach of Rice which was modified to consider that the crack spacing decreases with increasing strain due to cumulative hard phase cracking. The tensile strengths of the alloys can then be predicted with recourse to one adjustable parameter which varies with hard phase volume fraction. The deduced variation of this parameter with hard phase volume fraction, however, is as expected.

Phase distribution, grain size and coercivity in nanocomposite permanent materials

In this study, the modification effects and mechanism of manganese (Mn) and ultrasonic vibration (USV) on the needle-like Fe-containing intermetallic compounds of Al-20Si-xFe-2.0Cu-0.4Mg-1.0Ni (x=1, 2 wt.%) alloy have been studied respectively. The effect of Fe-containing phases on volume fraction of hard phases is also investigated. The results show that the mechanism and effect of Fe-containing intermetallic compounds improved by Mn are in close relationship with Fe content. Mn can promote to form less harmful α-Al15(Fe,Mn)3Si2 phase, or replace some Fe atoms of β-Al5FeSi and δ-Al4FeSi2 according to different Fe content. When USV was applied to this alloy containing 2%Fe near liquidus temperature, most of the acicular β phases formed in traditional process are substituted by fine plate δ phases. With the combined effects of 0.5%Mn and USV, the acicular β phases are almost repressed and the Fe-containing phases exist in form of fine Al4(Fe,Mn)Si2 and Al5(Fe,Mn)Si particles about 20~30μm. Consequently, the total volume fraction of hard phases which are composed of primary silicon particles and Fe-containing phases increases significantly.

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pct and Nb concentrations between 22 and 38 at. pct has been investigated for temperatures between 25 °C and 650 °C. Several microstructural features were evaluated in an attempt to identify microstructure-property relationships. In particular, the effects of the phase volume fraction, composition, morphology, and grain size were examined. In addition, the constitutive properties were evaluated using single-phase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O)+body-centered-cubic (bcc) microstructures. The disordered fully-bcc (β) Ti-12Al-38Nb microstructure, produced through heat treatment above the β-transus, exhibited a room-temperature (RT) elongation of more than 27 pct and the lowest yield strength (YS-553 MPa) of all the alloys studied. The ordered fully-bcc (B2) microstructures, produced through supertransus heat treatment of near-Ti2AlNb alloys, exhibited fracture strengths up to 672 MPa and low elongations-to-failure (e f≤0.6 pct). Thus, increasing the Al content, which favors ordering of the bcc structure, significantly reduces the ductility of the bcc phase. Similar to the ordered B2 microstructure, the ordered fully-O Ti2AlNb microstructures exhibited intermediate RT strength (≤704 MPa) and e f (≤1 pct). The O+bcc microstructures tended to exhibit strengths greater than both the fully-O and fully-bcc microstructures, and this was attributed to the finer grain sizes in the two-phase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O+bcc microstructures containing greater bcc-phase volume fractions tended to exhibit greater elongations yet poorer elevated-temperature strengths. A higher Al content typically resulted in larger elevated-temperature strengths. For the Ti-12Al-38Nb bcc-dominated microstructures, fine O platelets, which precipitated during aging, provided significant strengthening and a reduction in e f for the Ti-12Al-38Nb alloy. However, large RT elongations (e f>12 pct) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 vol pct O phase. Morphology did not appear to play a dominant role, as fully-lath and fully-equiaxed two-phase microstructures containing the same phase volume fractions exhibited similar RT tensile properties. The slip and cracking observations provided evidence for the ductile and brittle characteristics of the single-phase microstructures, and the slip compatibility exhibited between the two phases is an important part of why O+bcc microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.

Correlation between ultimate tensile strength and solidification microstructure for the sand cast A357 aluminium alloy

It is well documented that ultrasonic melt treatment (USMT) can refine dendritic and eutectic microstructures during solidification, but much less attention has been paid to the effect of USMT on macro-segregation and intermetallic transformations. In this research, macro-segregation and primary Fe-containing intermetallic peritectic transformations in an Al-19 wt pct Si-4 wt pct Fe alloy were investigated without and with USMT. Macrostructural examination showed that in the absence of USMT the ingot revealed considerable non-uniform distribution of both the primary Fe-containing intermetallic and primary Si particles, whereas the ingot with USMT exhibited near homogeneous distribution of both primary phases, i.e., reduced macro-segregation. The beneficial effect of USMT on relieving macro-segregation was further examined using quantitative microstructural metallography and the results indicated that the area fraction, number density, and size distribution of both primary phases became essentially uniform across the ingot after USMT. USMT further exerted a significant impact on the constitution of the primary Fe-containing intermetallics, where complex particles of δ-Al3FeSi2/β-Al5FeSi were prominent without USMT, while few δ-Al3FeSi2 particles were observed after USMT and the primary Fe-containing intermetallics existed mostly as the single-phase β-Al5FeSi. The underlying reason was attributed to the reduction in the size of the primary δ-Al3FeSi2 particles which ensures the complete transformation of most primary δ-Al3FeSi2 particles to the peritectic β-Al5FeSi phase.

The microstructure and mechanical properties of Mg–6Zn–1Y and Mg–6Zn–3Y (wt%) alloys under different cooling rates were investigated. The results show that the second dendrite arm spacing (SDAS) of Mg–6Zn–1Y and Mg–6Zn–3Y is reduced by 32 and 30% with increasing cooling rates (R c) from 10.2 to 23 K/s, which can be predicted using a empirical model of \( {\text{SDAS}} = 68R_{\text{c}}^{ - 0.45} \) and \( {\text{SDAS}} = 73R_{\text{c}}^{ - 0.45} \), respectively. The compressive strength of both alloys increases with increasing the cooling rate, which is attributed to the increase of volume fraction (V f) of secondary phases under high cooling rate. The interaction of the cooling rate and component with SDAS has been theoretically analyzed using interdependence theory.

Improved mechanical properties of near-eutectic Al-Si piston alloy through ultrasonic melt treatment