M. R. Nagy

Effect of Temperature and Stress on the Structure and Creep Parameters of Pb—2 at% Sb Alloy

The transient and steady state creep parameters of Pb-2 at% Sb alloy are determined in the temperature range from 443 to 503 K. The transient creep parameters β and n increase with working temperature and applied stress and have values changing from 43 x 10 -4 to 355 x 10 -4 and from 0.46 to 0.79, respectively. The strain rate sensitivity parameter m.changes from 0.12 to 0.2 which points to a cross slipping dislocation mechanism. The activation energies of both transient and steady state creep are calculated. In the low temperature range (443 to 473 K), the steady state activation energy is found to be stress dependent. The X-ray analysis supports that relaxation of the internal lattice strain takes place at the transformation temperature (473 K).

Effect of ≡ precipitates on creep deformation in Al2.5wt.%Cu

Abstract The change in the transient and steady state creep deformation of aluminium alloy containing 2.5 wt.% Cu was studied under various constant stresses ranging from 30 to 36 MPa in the temperature zone of ≡ precipitates. From the transient creep results, the peak values of the transient creep parameters β and n found in this temperature zone can be ascribed to dissolution of ≡ precipitates. The transient creep parameter β is related to the steady state creep rate ϵ st through the exponent γ. γ was found to range from 0.75 to 0.95. At the dissolution temperature (733 K) of ≡ precipitates, the steady state strain rate sensitivity parameter was 0.3 ± 0.01 at the steady state strain peaks which were characteristic of dislocation climb along ≡ grain boundaries. Microstructural analysis confirmed that the above-mentioned mechanism took place in the dissolution region of ≡ precipitates.

Control of the steady creep in Ag−2.7wt.%Al by dissolution-enhanced diffusion

The temperature dependence of the steady state creep rate of Ag−2.7wt.%Al was determined by measuring the creep rate for a period of 2 min at a series of temperatures, 10 °C apart. The temperature range covered was 180–280 °C to include the dissolution temperature (about 250 °C) of the μ phase. In the range 180–250 °C the material showed creep resistance due to an ordered μ phase in the form of a precipitate. Above 250 °C the creep resistance was affected by the dissolution of the precipitates. An activation energy for the creep rate above 250 °C was found to be 6.5 ± 1 kcal mol−1, much lower than the activation energy for self-diffusion (42 ± 1 kcal mol−1).

Study of stress-strain characteristics of SBR and blended NR/SBR rubber

The effect of temperature and carbon black (CB) on the mechanical characteristics of styrene-butadine rubber (SBR) and natural rubber (NR) was studied at various temperatures. The relation obtained between true stress and true strain for both types of rubber showed three regions at room temperature and two regions at high temperature. The optimum CB concentration was found to be 95 phr for the unblended samples as it increases the stiffness of the SBR rubber materials at a maximum value. It was also found that the addition of NR to SBR increases the elasticity in the plastic region. The activation energy at the fracture of SBR samples decreased from about 2.7×10−20 to 1.8×10−20 J while for the blended samples NR/SBR it increased from 8×10−20 to 10.1×10−20 J with increasing CB concentration.

Martensite-austenite transformation in maraging steel alloys during tensile deformation and thermal cycling

The effect of simultaneous additions of tungsten on the martensite (M) ⇌ austenite (γ) transformation, taking place during tensile deformation under different constant stresses and thermal cyclic rates for Fe-Ni-Co based maraging steel alloys was studied. The strain rate sensitivity parameterm was found to be 1.0 and 0.6 for the M →γ andγ → M transformations, respectively. The interpretation of deformation results implied a preponderantly diffusional mechanism in the M →γ transformation and a dislocation mechanism in theγ → M transformation. The increase of the lattice parameters of maraging steel alloys indicated that the hardening element, which is tungsten, was dissolved after tensile deformation.

The effect of thermal cycling across the eutectoid transformation temperature on superplasticity in ZnAl alloys

Abstract The effect of temperature cycling across the eutectoid transformation temperature (548 K) was investigated by the tensile deformation (dynamic deformation) of eutectoid and eutectic ZnAl alloys. Large tensile strains of about 220% were achieved by thermal cycling the samples under different constant stresses. About 70% of the deformation during transformation was attributed to creep by a dislocation slip process. The remaining 30% deformation could be due to an additional internal stress during transformation which causes excess diffusional creep.

Effect of transformation on the steady creep characteristics of Zn−Al alloys

Apparent steady state creep of eutectoid and eutectic Zn−Al alloys was studied under different constant stresses ranging from 0–20 MPa near the eutectoid transformation temperatures.The strain rate sensitivity parameterm amounted to 0.50−0.05 at the apparent maximum strain rate peaks. The energy activating the apparent steady state creep, determined in the vicinity of these peaks, amounted to (1.0±0.07) × 10−22 KJ/atom characterizing dislocation climb along the grain boundaries, thus causing boundary sliding and migration of the dissolved grains into new grains of an (Al)2 phase.Microstructural analysis confirmed that the afore-mentioned mechanism took place during apparent steady state transformation creep.

Effect of dissolution of Θ-phase on characteristic parameters of work-hardening in AL-2·5wt. % Cu alloy

Stress-strain curves of slowly cooled and quenched Al-2·5 wt. % Cu alloy were studied in the temperature range 693 K to 773 K. The linear work-hardening coefficient, the fracture time, the yield stress and the fracture stress of annealed and quenched samples decreased with increasing deformation temperature and exhibited a minimum at 733 K. The X-ray analysis of the slowly cooled and quenched samples showed that the lattice parametera of Al-matrix and the ratioc/a of the tetragonal Θ-phase reached a minimum and a maximum value, respectively, at the dissolution temperature.

Effect of μ phase precipitation on the transient creep in Ag10at.%Al alloy

Abstract The precipitation dependence of transient creep in Ag10at.%Al was studied on samples which had been given different heat treatments in the precipitation zone. The time-temperature dependence of the transient creep strain ϵ tr was described by ϵ tr = βt tr n where β is the transient creep parameter, t tr is the transient creep time and n is the creep time exponent. The pronounced decrease in the transient creep parameter β from 50 × 10 −3 to 1 × 10 −3 (min −1 ) n was attributed to the precipitation of ordered μ phase. The increase in the time exponent n from 0.1 to 0.9 could be due to the coarsening and increased interparticle spacing of the μ precipitates.

Vacancy enhancement of μ phase precipitation in Ag10at.%Al alloy

Abstract Locally prepared and homogenized Ag10at.%Al wires were quenched from 400°C to room temperature and were then given different aging treatments at temperatures in the precipitation zone to obtain samples with different amounts of μ phase precipitation. Room temperature steady state creep and microhardness measurements were made to assess the degree of precipitation in the samples. Increased precipitation of the μ phase was found to decrease the strain rate and to increase the hardness. The energy activating the process, which was deduced from the creep and hardening tests, was found to be on average about 3.5±0.5 kcal mol −1 . This low value for the activation energy characterizing the precipitation process was attributed to the possible enhancement of precipitation by the quenched vacancies. X-ray analysis after creep fracture showed a remarkable decrease in the line intensities, indicating the destruction of the ordered structure of the μ phase, i.e. the partial redissolution of this phase.