One parameter model for strength properties of hardenable aluminium alloys

Models for strength properties are proposed for commercially aluminium alloys. The alloy group investigated are the hardenable alloys from the 2000 (Al–Cu and Al–Cu–Mg), 6000 (Al–Mg–Si) and 7000 (Al–Zn–Mg) series. The same model for solid solution hardening that has successfully been applied to non-hardenable alloys has been used. For precipitation hardening, particle cutting and the Orowan mechanism have been considered. The same basic model is used for all strength properties. It is demonstrated that with one fitting parameter for each property, a representation with reasonable accuracy can be obtained that is applicable to a wide range of alloys. Such models are useful in materials optimisation and selection.     

等径角挤压对Al-Cu-Mg-Ag合金组织性能的影响

为研究大塑形变形对耐热铝合金的作用,采用铸冶金工艺制备了新型的Al-Cu-Mg-Ag耐热铝合金,通过显微组织观察、差热分析及硬度测试等方法,研究了等径角挤压对耐热铝合金显微组织与力学性能的影响.结果表明:通过对挤压态的Al-Cu-Mg-Ag耐热铝合金在固溶淬火后时效前进行等径角挤压变形,可获得晶粒尺寸低于2μm的块体超...     

Hardening and phase stability in rapidly solidified Al–Fe–Ce alloys

Rapidly solidified (RS) Al–Fe–Ce alloys were prepared by melt spinning. The phases present and the thermal stability, at temperatures up to 500 °C, were then followed by X-ray analysis, chemistry, hardness and thermal analysis techniques. The results obtained indicated that the alloys studied have enhanced mechanical properties compared to commercial aluminium alloys and castings of the same alloy compositions, and the RS alloy also exhibit good stability up to about 300 °C; a result of stable second phase particles. It is suggested that these results indicate that there are two mechanisms responsible for the hardening and stability of the RS alloys: solid solution strengthening at lower temperatures, and semicoherent particles formed from supersaturated solid solution at higher temperature. The maximum hardness, after 2 h ageing occurred at about 300 °C. At higher temperatures the dispersed phase became incoherent with a dramatic loss in hardness.     

Strengthening of nickel -base superalloys for nuclear heat exchanger application

Strengthening mechanisms of nickel-base superalloys have been discussed with the background of the Japanese research and development activities in this field. As candidates for materials of intermediate heat exchangers which will be used for a future programme of nuclear steelmaking systems, two kinds of alloys have successfully been developed in Japan. The designs of these alloys have been reviewed from metallurgical aspects including their composition and creep properties. In addition to the conventional methods to strengthen these alloys, such as solid solution hardening or particle precipitation hardening, a grain-boundary precipitation strengthening due to tungsten-rich ₂ phase in the Ni-Cr-W system, would be expected as a further advanced method.     

Magnetism and solid solution effects in NiAl (40% Al) alloys

The solid solution effects of ternary additions of transition elements in intermetallic Ni–40% Al were investigated by both experimental studies and theoretical calculations. Co solute atoms when sitting at Ni sublattice sites do not affect the lattice parameter and hardening behavior of Ni–40Al. On the other hand, Fe, Mn, and Cr solutes, which are mainly on Al sublattice sites, substantially expand the lattice parameter and produce an unusual solid solution softening effect. First-principles calculations predict that these solute atoms with large unfilled d-band electrons develop large magnetic moments and effectively expand the lattice parameter when occupying Al sublattice sites. The theoretical predictions were verified by both electron loss-energy spectroscopy (EELS) analyses and magnetic susceptibility measurements. The observed softening behavior can be explained quantitatively by the replacement of Ni anti-site defects (potent hardeners) by Fe, Mn, and Cr anti-site defects with smaller atom size mismatch between solute and Al atoms. This study has led to the identification of magnetic interaction as an important physical parameter affecting the solid solution hardening in intermetallic alloys containing transition elements.     

Low-noble metal alloys: in vitro corrosion evaluation

The electrochemical behaviour and the resistance to corrosion and tarnishing of seven commercial low-noble metal alloys were investigated, in artificial saliva at two different pH values (3 and 6.7) and in 2% Na₂S solution. The alloys were tested as-received and after a hardening treatment suggested by the manufacturers: ageing 15 min in a temperature range between 360 and 450 °C. Scanning electron microscopy (SEM) examination of the alloys evidenced some slight microstructural modifications. The electrochemical characterization was performed by means of anodic polarization curves. Alloys with noble metal content > 30% show a good electrochemical behaviour and a satisfactory corrosion resistance against tarnishing, and the hardening treatment had no influence on the corrosion resistance.     

Microstructure and mechanical properties of Pt-added and Pd-added Zr-20Nb alloys and their metal release in 1 mass% lactic acid solution

The effects of Pt and Pd addition to a Zr-20Nb alloy on its microstructure and mechanical property, as well as the elution of metals from the alloys in lactic acid solution, were investigated. The microstructure was characterized with an X-ray diffractometer (XRD), an optical microscope (OM), and a transmission electron microscope (TEM). The mechanical properties were evaluated by a tensile test. The β phase is dominantly observed in the Zr-20Nb as well as in the Pt-added and Pd-added Zr-20Nb alloys. Needle-like microstructures are observed in equiaxed grains in all alloys. Pd addition to the Zr-20Nb alloy suppresses ω phase formation more than Pt addition does. The 0.2% offset yield strength and the ultimate tensile strength of the Pt-added and Pd-added Zr-20Nb alloys increase with the Pt and Pd concentrations. XRD analysis revealed that the lattice parameter of β-Zr in the Pt-added and Pd-added Zr-20Nb alloys decreases with the Pt and Pd concentrations. Pt and Pd solute in β-Zr as a substitutional element and contribute to the increase in the strength by solid solution hardening. The addition of 2Pt and 2Pd to the Zr-20Nb alloy also improves metal elution from the alloys in lactic acid solution.     

The influence of impurity level and tin addition on the ageing heat treatment of the 356 class alloy

The influence of the addition of 0.5 wt.% Sn to Al–7Si–0.3 Mg alloys (356 and A356) on their ageing behaviour and mechanical properties was evaluated. Adding Sn led to a reduction of the iron rich intermetallics volume fraction, and of hardness. During solution heat treatment, Mg went into the solid solution, and Sn particles grew by competitive growth, concentrating at phase boundaries and interfaces. During aging β″ and Si precipitated. In the alloys with Sn, the β″ precipitation was accelerated and its hardening effect was greater, whereas the Si precipitation did not changed significantly. The mechanical properties of the A356 alloy were compatible with the hardening achieved during the heat treatment and to the amount of defects (pores) present in the microstructure. The yield strength and elongation of the A356 + 0.5% Sn alloy decreased after solution heat treatment and with increasing ageing temperature. These detrimental effects were minimized by treating this alloy in the T5 condition at 150 °C.     

Solid solution hardening of copper crystals

The effect of Mg, Si, In and Cd solute atoms on the critical resolved shear stress of Cu single crystals has been studied in the “plateau” region in order to complete data on the solid solution hardening of Cu base alloys. Labusch's equation is obeyed both for the concentration dependences and for the dependence of dτ_p/dc^2/3 onε ^4/3. Edge dislocation/ solute atoms interaction dominates in Cu as in Au and Ag base alloys.     

Microstructure and mechanical properties of two-phase Cr–Cr2Nb, Cr–Cr2Zr and Cr–Cr2(Nb,Zr) alloys

The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below, or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr–Cr₂X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr₂X Laves phase in the Cr–Nb and Cr–Nb–Zr alloys, but not in the Cr–Zr alloys. The binary Cr₂Nb phase consisted of an extensively twinned ({111}<112> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr₂X Laves phase present. Age hardening due to solid-state precipitation of Cr₂X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr₂X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.     

The effect of small additions of scandium on the properties of aluminium alloys

Additions of up to 1 wt% scandium have been made to Al, Al-Mg, Al-Mg-Ag and Al-Zn-Mg alloys and the effects on age-hardening and mechanical properties studied. Scandium levels up to 1% could be retained in solution at solidification rates of about 300 K s^–1. The precipitation of Al₃Sc at ageing temperatures in the range 563–593 K (290–320 °C) gave significant additional hardening. The low solubility of Sc in the solid state makes it difficult to obtain optimum hardening from Sc and other precipitating elements because of difficulties in solution treatment. The effect of deformation prior to ageing and the temperature-dependent mechanical properties are described.     

Mechanical properties of the Sn-Zn eutectic alloys

The structure and mechanical properties of Sn-Zn unidirectionally frozen eutectic alloys have been examined over the growth range 5 to 4000mm h^–1. The structure is predominantly broken-lamellar below 750mm h^–1 but becomes increasingly fibrous at higher growth rates. The yield and ultimate strengths when tested in tension and compression were found to increase monotonically with growth rates up to 1000 mm h^–1 above which they assumed near constant values. This behaviour is attributed to some loss of axial growth at higher growth rates. The hardness measured on transverse sections increased over the entire growth rate range. Annealing at near eutectic temperatures followed by quenching increased the strength of alloys grown at less than 750 mm h^–1 and decreased that of those grown at higher rates. Similar behaviour was observed in selected Cd-Zn eutectic alloys. The increase in strength is attributed to solid solution hardening and the reduction to structural degradation during annealing. The Sn-rich matrix in this broken-lamellar eutectic appears to contribute significant strengthening to the composite.     

Creep deformation mechanism of magnesium-based alloys

Two heat-resistant magnesium alloys AJC421 and Mg-2Nd were prepared. Both as-cast Mg-2Nd and AJC421 alloys exhibited good creep resistance in comparison with commonly used magnesium alloys. The improvement in creep properties through Nd addition to pure magnesium is attributed to both solid solution and precipitation hardening. The stress exponents of 4.5–5.5 and activation energies of 70.0–96.0 kJ/mol obtained from the as-cast Mg-2Nd alloy at low temperatures and low stresses indicate the five power law can be used for predicting the creep mechanism. The additions of alkaline earth elements Sr and Ca into Mg–Al alloys suppress the discontinuous precipitation of Mg₁₇Al₁₂ and form thermal-stable intermediate phases at grain boundaries, leading to effective restriction to grain boundary sliding and migration. However, the mechanism responsible for creep deformation of Mg–Al based alloys with Ca and Sr additions is not consistent with the results of microstructure observations performed on the alloys before and after creep tests.     

新型高导电铜基电极合金的研究

对系列新型高导电Ｃｕ－Ｂｅ－Ｎｉ电极合金在不同变形量和时效后的组织、性能和强化机制进行了研究。结果表明，固溶处理后的冷变形促进了时效析出过程，使析出相更加弥散，同时使合金在时效过程中发生再结晶。两者交互作用使合金达到强度和导电性的良好配合。合金中加入Ｚｒ能显著细化晶粒，提高强度，并使电导率达５０～６０％ＩＡＣＳ。     

Room-temperature indentation creep of lead-free Sn–Bi solder alloys

Creep behavior of the lead-free Sn–Bi alloys with bismuth contents in the range of 1–5 wt.% was studied by long time Vickers indentation testing at room temperature. The materials were examined in the homogenized cast and wrought conditions. The stress exponents, determined through different indentation methods, were in good agreement. The exponents of 13.4–15.3 and 9.2–10.0, found respectively for the cast and wrought conditions, are close to those determined by room-temperature conventional creep testing of the same material reported in the literature. Due to the solid solution hardening effects of Bi in Sn, creep rate decreased and creep resistance increased with increasing Bi content of the materials. Cast alloys, with a rather coarser grain structure and some Bi particles at the grain boundaries, showed typically higher resistance to indentation creep compared to the wrought materials. These two factors have apparently resulted in a less tendency of the material for grain boundary accommodated deformation, which is considered as a process to decrease the creep resistance of soft materials.     

The effect of the thermal exposure on microstructure of MP159 alloy

The effect of the thermal exposure at different temperatures on the microstructure is investigated for MP159 alloy by optical microscopy and transmission electron microscopy (TEM). Thermal exposure at 660°C is related to the aging induced hardening mechanism, thermal exposure at the neighborhood of 920°C is associated with static recrystallization behavior, and thermal exposure at a temperature range of 920 to 1070°C for a short time is investigated with a view toward obtaining the longest permissible thermal exposure time during which microstructural reconstitution of the cold deformed microstructure would not occur at the corresponding temperature, which would provide guidance for determining reasonable hot forging parameters of fasteners. The results indicate that both solution heat treated (ST) and cold worked (CW) MP159 alloy could be hardened by aging. This aging induced hardening is attributable to the precipitation of a very finely dispersed fcc ordered solid solution. The precipitating process is so rapid that the air cooling (AC) or water cooling (WC) process of the sample exposed to elevated temperature higher than normal aging temperature could produce an appreciable hardening effect. The recrystallization temperature of the alloy is as high as 920°C, such a high recrystallization temperature is attributed to deformation twins and a variety of alloying elements due to their inhibition to the nucleation and growth of recrystallization grains, other than to the hcp-phase which used to be believed to explain high recrystallization temperature in MP alloys in some literature.     

The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys

A new in-house designed series of Ni based superalloys with stepwise increased Re and Ru additions has been investigated, to systematically determine the influence of Re and Ru on γ/γ′-microstructure and high temperature creep properties. Improved creep resistance and thus also a higher alloy temperature capability of up to 87 K/at.% was found for additions of Re. Additions of Ru revealed a lower temperature capability improvement of up to 38 K/at.% for low Re-containing second generation alloys. However, in third and fourth generation alloys with higher Re-contents, no significant influence of Ru on creep rupture strength was observed. The creep properties are discussed with respect to the γ′-volume fraction, γ′-size and γ′-coarsening rate, as well as the γ/γ′-lattice misfit and the γ/γ′ partitioning coefficient of the different Re and Ru containing alloys. The presented data shows, that these microstructure parameters are strongly influenced by additions of Re, but only marginally by additions of Ru. A further influence on creep rupture strength is given by the solid solution hardening of the γ-matrix, which is discussed based on solid solution hardener concentrations either experimentally derived or calculated from ThermoCalc data.     

Microstructure and microhardness of an AlFe alloy subjected to severe plastic deformation and aging

A nanocrystalline structure was produced in an Al-11 wt.% Fe alloy with the use of the novel technique of severe plastic deformahon of ingots by torsion under high imposed pressure. This technique allows a large departure of materials from equilibrium. The microstructure of the alloys was studied with the use of TEM and EDS. The severe plastic deformation led to solid solubility extension of iron in the aluminum matrix, dispersion and dissolution of second phase particles, grain size reduction into the nanometer range, and partial amorphization. Microhardness of the alloy increased substantially after the deformation due to the grain refinement and the solid solubility extension. Aging of the severe plastically deformed samples at 100 °C led to further increase of the microhardness due to the decomposition of the supersaturated solid solution and precipitation-induced hardening.