Deformation mechanisms in commercial Ti (0.5 at. pct oineq) at intermediate and high temperatures (0.3 - 0.6 tinm)

The plastic flow of the commercial titanium material Ti-50A (0.5 at. pct O_eq) of 22 μm grain size was investigated over the temperature range of 600 to 1150 structure) and strain rates of 3 x 10^-5 to 3 x 10^-2 per s employing both constant strain rate and strain rate cycling tests. Dynamic strain aging occurred in the temperature range of 600 to 850 (0.31 to 0.44T_m) with an activation energy of 50 kcal per mole derived from the start of serrations in the stress-strain curves, maxima in strain hardening and minima in ductility. This value is in accord with that for the diffusion of oxygen in titanium. At temperatures above 850 (0.46 to 0.59T_m) the data were very well represented by Weertman’s glide and climb high temperature creep mechanism, giving ε_skT/Dμb= 1.1 x 10⁶ (σ/μ)^4.55 withD = 1.0 x exp (- 57,800/RT). The value of 57.8 kcal per mole is in accord with available self-diffusion data for titanium.     

Temperature dependence of yield stress in fe-1.67 at. pct Cu and fe-0.5 at. pct Au

A supersaturated solid solution of 1.67 at. pct Cu in Fe has a lower temperature dependence of the yield stress (prestrained 0.2 pct at 368°K) than pure iron over the temperature range 77° to 298°K. No change in temperature dependence is observed on aging to form copper clusters (1 hr at 475°C) but after precipitation of ε the temperature dependence is the same as in pure iron. The activation volume at 77°K is unaffected by treatment in the alloy and the amount of deformation. The value is essentially the same as in iron. It was suggested that copper clusters present after aging 1 hr as well as dissolved copper assist formation of double kinks at 77°K reducing the stress required to surmount the Peierls barrier. Addition of 0.5 at. pct Au also reduced the temperature dependence of iron. Formerly Graduate Student, Department of Materials Science and Materials Research Center, The Technological Institute, Northwestern University, Evanston, Ill.     

Deformation mechanisms in commercial TI-5Al-2.5 Sn (0.5 At. pct Oeq) alloy at intermediate and high temperatures (0.3-0.6 tm)

The plastic flow of the commercial α-alloy Ti 5A1-2.5 Sn (0.5 at. pct Oeq) of 11 to 19 μm grain size was investigated in tension over the temperature range of 600 to 117.3 K (CPH structure) and strain rates of 3x10^-5 to 3 per s employing both constant strain rate and strain rate cycling tests. Dynamic strain aging (due to substitutional solutes) occurred in the temperature range of 600 to 850 K (0.31 to 0.44T _m)with an activation energy of 22.5 KcalJmole (94.1 KjJmol) derived from the start of serrations in the stress-strain curves. At temperatures above 850 K (0.46 to 0.60T _m), the deformation could be described by Dorn’s general equation for diffusion controlled mechanisms with an activation energy of 50 KcalJmole (209 KJJmol).     

Microstructural Evolution During the Hot Deformation of Ti-55Ni (at. pct) Intermetallic Alloy

The hot deformation behavior of Ti-55Ni (at. pct) alloy was studied using compression testing at 1173 K (900 °C) to 1323 K (1050 °C) and at the strain rates of 0.001 to 0.35 s⁻¹. The microstructure evolution was characterized using optical and scanning electron microscopy (SEM). The influences of hot-working parameters on the flow stress and microstructural features of this alloy were then analyzed. The results indicate that, depending on the temperature and strain rate, the dynamic recrystallization (DRX) is the dominate mechanism. Besides, the particle-stimulated nucleation (PSN) mechanism could partially recrystallize the structure. The PSN phenomenon is of significant importance for the Ti-55Ni (at. pct) that suffers from insufficient workability because of its high content of intermetallic phases. It is of interest that the discontinuous yielding phenomenon has been observed when the specimens were deformed at 1173 K (900 °C). Finally, the optimum parameters for hot working of Ti-55Ni (at. pct) alloy are determined as well.     

Deformation mechanisms in commercial TI-5Al-2.5 Sn (0.5 At. pct Oeq) alloy at intermediate and high temperatures (0.3-0.6 tm)

The plastic flow of the commercial α-alloy Ti 5A1-2.5 Sn (0.5 at. pct Oeq) of 11 to 19 μm grain size was investigated in tension over the temperature range of 600 to 117.3 K (CPH structure) and strain rates of 3x10^-5 to 3 per s employing both constant strain rate and strain rate cycling tests. Dynamic strain aging (due to substitutional solutes) occurred in the temperature range of 600 to 850 K (0.31 to 0.44T _m)with an activation energy of 22.5 KcalJmole (94.1 KjJmol) derived from the start of serrations in the stress-strain curves. At temperatures above 850 K (0.46 to 0.60T _m), the deformation could be described by Dorn’s general equation for diffusion controlled mechanisms with an activation energy of 50 KcalJmole (209 KJJmol). M. D?NER was formerly with Department of Metallurgical Engineering and Materials Science, University of Kentucky, Lexington, Ky. 40506     

Effect of undercooling on the microstructure of Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloys

Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloy specimens have been solidified from various levels of undercooling in the differential thermal analysis (DTA) and the electromagnetic levitation (EML) units in a pyrex/vycor bed. The evolution of the microstructure in the solidified specimens has been examined in terms of the degree of undercooling, the nature of the first phase to nucleate from the melt, and the specimen cooling rate. The melt has been observed to undercool more in the presence of intermetallic NiMo (β) phase as compared to that in the presence of nickel-rich solid solution (γ). The “anomalous eutectic” type of microstructure has been shown to result from the initial formation of the dendritic skeleton of either of the two phases, its segmentation due to convection and ripening, and the subsequent nucleation of the other phase in the interdendritic liquid regions. The recalescence behavior has been examined as a function of undercooling and the nature of the phase nucleating first in the melt.     

Tensile flow and fracture behavior of DO3 Fe-25 at. pct Al and Fe-31 at. pct al alloys

The tensile properties, fracture modes, and deformation mechanisms of two DO₃ alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600°C. The first alloy was produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated formation of antiphase-boundary (APB) bands, while in the Fe-31 Al alloy, plasticity occurred by the motion of superlattice dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage fracture modes. The variation of tensile properties and fracture modes with temperature is presented. H. A. LIPSITT, formerly with the Materials Laboratory of the Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, OH 45433-6533     

The effect of temperature and carbon content on the cavitation behavior of a 1.5 pct Cr-0.5 pct V steel

Constant strain rate tests at 10^-6 s^-1 have been carried out at 823 K and 923 K on a vacuum melted 1 1/2 pct Cr 1/2 pct V ferritic steel containing 3 different carbon contents. After straining to various elongation values specimens were unloaded, cooled and fractured at 77 K. This gave fracture surfaces consisting almost entirely of intergranular facets, enabling a quantitative study to be made of the different stages of cavity nucleation and growth. It was found that cavity growth rates were independent of carbon content but were higher at 923 K than at 823 K. Subsequent grain boundary sliding measurements, using a surface offset technique showed that sliding increased with increasing carbon content and that cavity nucleation occurred selectively at large grain boundary carbides. Formerly of the Department of Metallurgy, University of Manchester.     

The diffusivity of lattice atoms in dilute interstitial solid solutions

The enhancement of the diffusivity D of host atoms in a lattice containing dissolved interstitial atoms has been discussed in terms of a model in which the enhancement is due to complexes formed due to vacancy-(interstitial atom) interactions and a model which considers that the enhancement is primarily due to the lattice dilation caused by the presence of the interstitial atoms. The validity of these two concepts is discussed in terms of experimental data.     

The effect of environment and grain size on the creep behavior of a ni-6 pct w solid solution

The effects of environment and grain size on the steady-state creep and creep rupture properties of a Ni-6 pct W solid solution are examined by testing in vacuum and commercial purity argon at 5000 psi and 900°C. The steady-state creep rate is found to decrease with increasing grain size at small grain sizes, both in vacuum and argon, owing to the effects of grain boundary sliding. At large grain sizes the creep rate is independent of grain size in vacuum and increases with grain size in argon. It is suggested that the increase in creep rate with increasing grain size is associated with fact that large-grained samples tested in argon do not reach steady-state before rupture occurs. Formerly Graduate Student, Stanford University, Stanford, Calif.     

On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain size

The role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method has been investigated, using a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests. It is found that the addition of 0.5 wt pct C to Fe-24 wt pct Ni alloy leads to retention of austenite to a temperature close to -196 °C, when the alloy is quenched from the melt. The austenite grain size varies from ∼0.2 μm to ∼2 μm on going from the wheel to the gas side. The cooling rate, accordingly, changes from 5 × 10⁷ to 4 × 10⁴ Ks^-1. The changes in the mechanical properties have been correlated with the accompanying changes in the ribbon microstructure. The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct Fe 319-type alloys

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the β-Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like β-Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the β-Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energydispersive X-ray (EDX) analysis.     

Deformation kinetics of commercial Ti-50A (0.5 At. Pct Oeq) at low temperatures (T<0.3T m )

The effects of strain rate and temperature on the flow stress of commercial Ti-50A titanium (0.5 at. pct Oeq) wire of 2 and 22 μm grain size were investigated over the range of 4.2 to 700 K. Employing the thermally activated plastic flow concept, it was concluded that the rate-controlling mechanism in Ti-50A at low temperatures (T<≈600 K) is the thermally activated overcoming of interstitial solute atom obstacles (mainly oxygen) by dislocations moving on the first-order prism planes. The Gibbs free energy of activation for the process is ≈1.5 ev(≈0.2 μb³), the maximum force of interaction ≈8 X 10^-5 dynes (≈0.2 smb²), and the activation distance at which the force first rises rapidly ≈ lb. The derived deformation kinetics parameters are in reasonable accord with the Fleischer-Friedel model of solid solution strengthening, the interaction energy being given by the combined effect of the tetragonal distortion and the modulus mismatch produced by the interstitials. Qualitative accord is also obtained for the breaking of chemical bonds between the interstitial solutes and the surrounding titanium atoms by the passage of dislocations.