Plastic instability and flow localization in shear at high rates of deformation

The occurrence of adiabatic shear bands in metals is analyzed using models based on

• 1.(i) load instability and
• 2.(ii) flow localization. In the former case, shear strain concentration is identified with the occurrence of a maximum in the shear stress-shear strain curve.

By contrast, the latter model treats localization as a process which begins to develop at a material imperfection at the onset of deformation. In both cases, the flow softening arising from adiabatic heating is taken to be the driving force behind the process. The predictive capabilities of the two types of model are compared using data on shear band formation during the explosive expansion of 4340 steel cylinders. It is shown that the flow localization model forecasts the occurrence of adiabatic shear bands more accurately and is thus more useful for the prediction of nonuniform flow at high rates of strain such as those which occur during impact loading and metalcutting.     

Prediction of steel flow stresses at high temperatures and strain rates

The flow behavior of steels during deformation in the roll gap was simulated by means of single hit compression tests performed in the temperature range 800 °C to 1200 °C. Strain rates of 0.2 to 50 s⁻¹ were employed on selected low-carbon steels containing various combinations of niobium, boron, and copper. The stress/strain curves determined at the higher strain rates were corrected for deformation heating so that constitutive equations pertaining to idealized isothermal conditions could be formulated. When dynamic recovery is the only softening mechanism, these involve a rate equation, consisting of a hyperbolic sine law, and an evolution equation with one internal variable, the latter being the dislocation density. When dynamic recrystallization takes place, the incorporation of the fractional softening by dynamic recrystallization in the evolution equation makes it possible to predict the flow stress after the peak. These expressions can be employed in computer models for on-line gage control during hot-rolling.     

Factors affecting the leachability of Ni/Co/Cu slags at high temperature

An oxidative pressure acid leaching process was investigated for the extraction of Ni/Co/Cu/Zn metal values from base metal smelter slags. The process is applicable to smelting furnace dump slags, as well as to mid-stream converter slags. In addition to the economical advantages, extracting the base metal values from the dump slags produces an environmentally benign residue. To produce high level of extraction for the metal values, it is necessary to have the slag in crystalline structure. This, in turn, requires cooling the molten smelter slags sufficiently slowly during solidification. It was shown that if the same slags are quenched in water thereby producing an amorphous structure, the resulting metal extractions are substantially lower. The role of oxygen in the leaching reactions was also tested. The presence of oxygen is necessary for successful leaching of the metal values. After fine grinding, the slags were subjected to pressure acid leaching at 250 °C with an oxygen overpressure of 520 kPa. Sulfuric acid at a typical 0.3 acid to slag ratio was employed. Extractions higher than 90% of each of the nickel, cobalt, copper and zinc were achieved. Operating at 250 °C ensures that the concentration of Fe and Al impurity metals in leached solution remains very low. It was finally found that for high concentrations of divalent metals (Ni + Co + Cu + Mg) in the slag, a much higher quantity of sulfuric acid than the stoichiometric value is required. The latter is due to the drop of hydrogen ion concentration due to bisulphate formation.     

The deformation of silicon at high temperatures and strain rates

Polycrystalline and 〈100〉 single crystalline semiconductor grade silicon samples have been subjected to uniaxial compression at strain rates from 10⁻⁵ to 12 s⁻¹ at temperatures ranging from 1100 to 1380 °C. Both intrinsic and p-type polycrystalline material and p-type single crystalline material were tested. Except at the highest temperature and lowest strain rate, no steady state deformation was observed for the polycrystalline material. In all other cases strain hardening was observed which increased with increasing strain rates. The polycrystalline material could be compressed by as much as 50 pct at 1380 °C and a strain rate of 7 s⁻¹ without cracking. An axial stress of approximately 170 MPa produces a strain rate of 5 s⁻¹ at 1380 °C. The stress necessary to produce a given strain rate increases rapidly with decreasing temperature while the ductility rapidly decreases. A preliminary forming limit diagram has also been determined for the polycrystalline material at 1380 °C. The deformation rate-controlling process in the polycrystalline material at high stresses could be the production of vacancies on jogged dislocations. Formerly with the Department of Materials Science and Engineering, University of Pennsylvania     

Gas permeability of powders solidified at high and ultrahigh cooling rates

Conclusions An expression has been derived for determining the duration of vacuum treatment of compacts from a powder of flake-lamellar particle shape, produced by solidification at ultrahigh cooling rates. The expression can be employed also for calculating the times of degassing of spherical powders. The coefficient of air permeability of a compact from a given fraction of a powder of flake-lamellar particle shape is independent of the height (thickness) of the compact. Air permeability varies as a function of compact density. An assembly of several compacts has, at any given relative density, the same resistance to evacuation as a single compact of the same thickness. (The height of the compacts investigated was smaller than their diameter, and their density therefore did not vary significantly along their vertical axes.)Translated from Poroshkovaya Metallurgiya, No. 6(222), pp. 22–27, June, 1981.     

On the structures formed in eutectic alloys at high growth rates

Abstract

It is well established that dendrite-free structures can be produced in off-eutectic alloys by solidification at high growth rates. It has also been demonstrated that solidification at high growth rate can lead to the formation of a layer of supercooled liquid at the solid-liquid interface. In the present work the relation between the presence of supercooled interface liquid and the formation of dendrite-free structures is examined. Observations on tin-rich lead-tin eutectic alloys have shown that wholly lamellar structures can be produced when a supercooled layer is present, but that such a supercooled layer is not a necessary condition for the formation of dendrite-free structures.

Résumé

Il est bien connu qu'il est possible de produire des structures libres de dendrites dans les alliages quasi-eutectiques à condition d'utiliser des taux de croissance élevés. De plus, on sait que des taux étevés de solidification donnent lieu à la formation d'une zone de liquide en surfusion à l'interface solide-liquide. Les auteurs étudient ici la relation entre la présence de cette zone de liquide en surfusion et la formation des structures libres de dendrites. Ils trouvent que des structures complètement lamellaires peuvent exister en présence d'une zone de liquide en surfusion mais que la présence de cette zone n'est pas une condition nécessaire pour la formation de structures libres de dendrites.     

Urinary flow rates in hypospadias

OBJECTIVES: To determine long-term results of patients who underwent primary ligament repair and delayed reconstruction for lateral ligament instability. DESIGN: Retrospective. SETTING: Outpatient clinic. PATIENTS/PARTICIPANTS: Patients who had undergone acute repair or delayed reconstruction at this institution between 1958 and 1977, excluding patients who were deceased or who could not be located. INTERVENTION: Forty-eight patients (fifty-three ankles) underwent twenty-two primary ligament repairs and thirty-one delayed reconstruction operations. MAIN OUTCOME MEASUREMENTS: Clinical results graded with clinical scale and radiologic results based on stress radiographs and plain film radiographs. RESULTS: At an average of twenty years after operation (range 12 to 33 years), patients were satisfied with forty-nine ankles, satisfied with reservations with two ankles, and dissatisfied with two ankles. Clinical results after repair were excellent in twenty ankles, good in one, fair in none, and poor in one. After reconstruction, the results were excellent in twenty-one ankles, good in six, fair in one, and poor in three. In the primary repair group, the mean talar tilt with stress testing improved from 20.7 +/- 10.7 degrees before operation to 2.8 +/- 3.0 degrees after operation. In the reconstruction group, the mean talar tilt improved from 20.7 +/- 8.4 degrees before operation to 2.8 +/- 3.5 degrees after operation. CONCLUSIONS: Clinical and radiologic results were similar in the repair and reconstruction groups. The majority of severe (Grade III) ankle sprains may be treated nonoperatively, but if residual instability occurs, late reconstruction should achieve satisfactory results.     

Increased effects of machining damage in beryllium observed at high strain rates

Metallurgical and Materials Transactions A -     

Contralateral and ipsilateral 'suppression' of evoked otoacoustic emissions at high stimulation rates

A previous study has shown that the amplitude of otoacoustic emissions decreases with increase in stimulus rate. Furthermore, these changes are similar to the changes initiated by contralateral suppression of the click-evoked otoacoustic emission. Further studies have been carried out which showed that contralateral suppression, due to a 60 dB SL white-noise suppressor, has a magnitude of about 2.7 dB for the normal subjects used in this study. The equivalent ipsilateral 'suppression' is approximately 8 dB at a stimulus rate of 2000 clicks/s. The two effects have an equivalent magnitude at a stimulus rate of approximately 300 clicks/s. In acoustic neuroma patients, where the nerve pathway is impeded by the tumour, the ipsilateral 'suppression' effect is virtually absent. These findings suggest that an ipsilateral 'suppression' similar in form but larger in magnitude than the more usual contralateral suppression can be created by increasing the stimulus rate using maximum length sequence (MLS) techniques.