Mechanical properties of AlZnMg alloys

The yield stress and the microstructural parameters of GP zones and η′ precipitates as well as the temperature dependence of the yield stress were investigated in an Al-4.7 wt.% Zn-1.2 wt.% Mg alloy and in a series of the same base alloys with additional elements of Fe, Si, Cr and Zr. It was found that the GP zones formed either during RT or 50°C ageing after quenching are sheared by the moving dislocations upon deformation. By correlating the theoretical model of this cutting mechanism the particle strength, K, and the specific surface energy, Γ, necessary to cut through a unit area of a particle were determined. According to our results Γ is a linear, whereas K is a quadratic function of the average zone radius. The investigations have shown that in the case of η′ precipitates formed during ageing at 160°C the Orowan mechanism is effective in the dislocation movement. It was found that the largest yield stress increment can be expected by producing particles with an average radius of about 3 nm.The influence of strain rate and temperature on the yield stress of samples aged for different periods of time at RT was also investigated. The analysis of the experimental data has shown that the temperature dependent part of the yield stress, τ_T, decreases linearly with temperature up to a certain critical temperature, T_c which was found to be 260 K. The experiments have shown that the activation volume is linear whereas the activation enthalpy is a quadratic function of temperature. The largest values obtained for these parameters were 3,16 × 10⁻²⁷m³ and 0.58 eV. The thermodynamic analysis of the results have revealed that the interaction between dislocations and GP zones can be characterized by a square potential profile.     

Mechanical properties and microstructures of Al-Mg-Sc alloys

The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated. Room-temperature tensile and toughness properties were found to reflect a superposition of the properties of Al-Mg and Al-0.5Sc alloys and are quite competitive with high-performance Al alloys. A combination of substructure refinement by Mg and stabilization by Al₃Sc precipitates produces exceptional superplasticity as exemplified by superplastic forming (SPF) elongations in excess of 1000 pct at a strain rate of 0.01 s^-1. Overall, these alloys demonstrate an extremely attractive combination of strength, toughness, density, and SPF fabricability.     

Transitions between stress-induced martensites in Cu Al Ni alloys

Stress-induced martensitic transformations have been studied in β₁ Cu Al Ni single crystals at temperatures aboveM _s . Close toM _s γ′ martensite is formed, well aboveM _s β ₁ ′ martensite forms, whilst in an intermediate temperature range β₁′ martensite initially forms and then transforms to γ′ on continued stressing and particularly on unloading, γ′ martensite is also formed when the stress-induced β₁′ is cooled below a critical temperature. The γ′ martensite has a (101) twinned structure. The morphological and crystallographic aspects of the γ₁′→ γ′ transition are discussed in detail. The two twin variants have different habit planes with respect to the β₁′ phase, one being (201)γ′ and the other (001)γ′. A thermodynamic argument is presented to explain the γ₁′→ γ′ transition, taking into account the relative stabilities of the β₁′ and γ′ phases with respect to the β₁, and the relative value for the critical driving force to nucleate the stress-induced β₁′ and γ′ structures from the β₁ phase Formerly Post Doctoral Fellow, Department of Metallurgy, University of British Columbia, Vancouver, Canada     

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.     

Mechanical properties of AlYNi amorphous alloys

AlYNi amorphous alloys with high mechanical strength and good bending ductility have been produced by rapid solidification technique. Vickers hardness (H_v), Young’s modulus (E) and tensile fracture strength (σ_f) for the amorphous single phase are in the range 365–385 DPN, 71.5–82.2 GPa and 920–1150 MPA, respectively. The crystallized structure of the amorphous alloys consists of coexistent amorphous and fcc-Al phase with a particle size of 5–25 nm. The hardness and tensile fracture strength of these amorphous alloys containing the nanoscale Al particles increase to 550 DPN and 1450 MPA respectively, annealed at 400–550 K for 30 min. This increase in (H_v) and σ_f is considered because the defect-free nanoscale fcc-Al particles homogeneously dispersed in the amorphous matrix effectively resist against shear deformation of the amorphous matrix. This paper describes in detail the effects of annealing temperature on microstructure and mechanical properties of AlYNi amorphous alloys.     

The thermodynamics of stress-induced martensites in Cu Al Ni alloys

Stress-induced martensitic transformations have been studied in Β₁ Cu Al Ni single crystals in which two martensite crystal structures can form, Β _i ^′ and γ′. By straining specimens at one temperature and releasing the strain at either the same temperature or a different temperature, stresses corresponding to the transitions Β₁ ⇌ Β _i ^′ ,Β ₁ ⇌ γ′,Β _i ^′ ⇌ γ′ could all be measured. This enabled a quantitative stress-temperature diagram to be drawn, giving the stability ranges of the Β₁,Β _i ^′ and γ′ phases. The slope of the stress-temperature lines separating the different phases enabled the value of the entropy changes for the transformations to be calculated. This was very small for theΒ _i ^′ → γ′ transformation (0.08 J/mole K) and much larger for the Β₁ →Β _i ^′ and Β₁ → γ′ transformations (-1.21 and -1.4 J/mole K, respectively). The hysteresis between the forward and reverse transformations enabled evaluation of the critical free energy for transformation. This was small for the Β₁ → Β _i ^′ transformation (-2.9 J/mole), and large for the Β₁ → γ′ and Β _i ^′ → γ′ transformations (-28 and -29 J/mole respectively). Formerly Post Doctoral Fellow, Department of Metallurgy, University of British Columbia     

The thermodynamics of stress-induced martensites in Cu Al Ni alloys

Stress-induced martensitic transformations have been studied in Β₁ Cu Al Ni single crystals in which two martensite crystal structures can form, Β _i ^′ and γ′. By straining specimens at one temperature and releasing the strain at either the same temperature or a different temperature, stresses corresponding to the transitions Β₁ ? Β _i ^′ ,Β ₁ ? γ′,Β _i ^′ ? γ′ could all be measured. This enabled a quantitative stress-temperature diagram to be drawn, giving the stability ranges of the Β₁,Β _i ^′ and γ′ phases. The slope of the stress-temperature lines separating the different phases enabled the value of the entropy changes for the transformations to be calculated. This was very small for theΒ _i ^′ → γ′ transformation (0.08 J/mole K) and much larger for the Β₁ →Β _i ^′ and Β₁ → γ′ transformations (-1.21 and -1.4 J/mole K, respectively). The hysteresis between the forward and reverse transformations enabled evaluation of the critical free energy for transformation. This was small for the Β₁ → Β _i ^′ transformation (-2.9 J/mole), and large for the Β₁ → γ′ and Β _i ^′ → γ′ transformations (-28 and -29 J/mole respectively).     

Reordering kinetics and magnetic properties of mechanically disordered nanocrystalline Ll2-type Ni3Al + Fe alloys

The paramagnetic disordered high temperature states of Ll₂ or γ′-Type Ni₃AlFe alloys become ferromagnetic at low temperatures if retained in metastable disordered (γ) state. Using mechanical attrition (milling) which also transforms the material into nanocrystalline form, various Ni₃AlFe alloys have been prepared in metaastable-γ form and their magnetisations M(γ) and M(γ′) and Curie temperatures T_c(γ) and T_c(γ′) measured. Fe atoms possess a giant magnetic moment in these alloys and polarise neighbouring Ni atoms but this polarisation effect is lower in the disordered state due to a lower number of FeNi nearest neighbours resulting in M(γ) < M(γ′) and T_c(γ) < T_c(γ′). Currently available expressions for magnetic ordering energies based on Bragg-Williams nearest neighbour interactions are found to be inadequate for these giant-moment-type alloys. Isothermal ordering kinetics of the metastable-γ-state which is also nanocrystalline indicate that the ordering reaction cannot propagate across the disordered nanograin boundaries and each grain must obtain its own γ′ nucleus. This results in a Johnson-Mehl-Avrami exponent n ≈ 1.     

Mechanical properties and structure of age-hardened Ti-Cu alloys

The room temperature tensile properties of age-hardened Ti-Cu alloys, 0.9 and 4 at. pct Cu, were investigated. The results were correlated to the microstructure and to the observed interaction mechanism between moving dislocations and precipitated particles. Two types of precipitates were observed upon aging between 400° and 500°C. One type was identified as a metastable, ordered precipitate coherent with the matrix. The other type was the stable Ti₃Cu phase which was partially coherent with the matrix. Both types of particles were precipitated from a martensitic microstructure which resulted from the β → α transformation. Although the martensitic microstructure contributed to the high flow stress, the elongation to fracture was principally determined by the dislocation-particle interaction mechanism. The maximum elongation to fracture was obtained by inducing a dislocation by-pass mechanism in a structure containing homogeneously distributed, partially coherent Ti₃Cu particles. The tendency of the Ti₃Cu particles to precipitate preferentially on boundaries was minimized by low temperature aging. Formerly Assistant Professor Materials Research Laboratory, Rutgers University, New Brunswick, N.J.     

Mechanical properties and microstructure of sintered titanium alloys

Conclusions A study was made of the mechanical properties and microstructure of titanium alloys in the sintered state. The influence of alloying elements was evolved. It was established that for alloys used in the sintered state it is favorable to use vanadium, aluminum, and molybdenum as alloying elements.The titanium alloy containing 3% Al and 3% V, made by the method of combined reduction of oxides of TiO₂, Al₂O₃, V₂O₅ by calcium hydride (_B-774.2 MN/m², -15%, -26%, a_k-25.4 joules/cm²) and also the alloy Ti+3% Al+3% V+2% Mo (_B-857.5 MN/m², -14.3%,-27%, a_k-24.5 joules/cm²) can be recommended for the manufacture of components by methods of powder metallurgy.     

Stress-induced martensite formation in Cu−Al−Ni alloys

A study has been made of superelasticity and the strain-memory effect in Cu?Al?Ni alloys in the composition range 14 wt pct Al and 2 to 3 wt pct Ni. These alloys have a bcc structure on quenching and show a low temperature martensitic transformation which is responsible for both the superelastic and strain-memory effects. Tests on both single and polycrystalline specimens showed that the maximum superelasticity occurred close toA _s. At higher temperatures the effect gradually decreased, whilst at lower temperatures it decreased very quickly. The magnitude of the effect was large in single crystal specimens (>5.8 pct), but small in polycrystal specimens (<1.5 pct). The superelastic effect was caused by stress-induced martensite (SIM). Two types of SIM were observed; thin plates of thermoelastic martensite which were always reversible, and wide plates of burst-type martensite. This burst-type martensite was responsible for the major portion of SIM, and whether it was reversible or not on removal of the stress controlled the amount of superelasticity observed. The strain-memory effect occurred on deformation either in the martensitic state (temperature <M _f) or in the temperature range where the martensite once formed was stable (temperature close toM _s). Deformation caused reorientation of the martensite plates and when the specimen was heated, the martensite disappeared and the specimen reverted back to its original shape. This effect was explained on the basis of development of martensite plates of favorable orientation on stressing.