Dislocation production in alpha Fe-C alloys by rapid heating

It is demonstrated that metastable carbide particles in α-Fe act as sources of dislocations when heated rapidly to a temperature where the particles are unstable. The type and arrangement of such dislocations are dependent on the rate of heating. At moderate heating rates, the particles produce well-defined groups of equally spaced interstitial dislocation loops lying on {100} planes, with Burgers vectors ofa<100>. Cementite (Fe₃C) particles of comparable size dissolve without producing dislocations. The original metastable particle sites retain carbon in the form of small globular particles which may be graphite. At faster heating rates, the dislocation arrangement becomes more disorderly with both b =a<100> and b =a/2<111> dislocations emanating from the same particle.     

Static strain-aging in commercial purity alpha titanium

Static strain-aging was investigated in commercial purity α-Ti, containing 0.41 at. pct of oxygen as the principal interstitial impurity. 17 μM grain size specimens, prestrained to 0.02 strain at a nominal strain-rate of 3.3×10⁻⁴S⁻¹, were agedin situ between 555 and 630 K for lengths of time between 150 and 2.55×10⁵ seconds. Both the upper and lower yield point increments were employed as measures of the aging progress. Under the stated conditions, the static strain-aging response of the titanium appeared to consist of four stages, of which only Stages 1 and 2 were considered in this work. Stage 1 corresponded to a Haasen and Kelly type of yield point, while Stage 2, after correcting for the time-independent yield point increment of Stage 1, conformed to Cotrell aging,i.e., the increase in flow stress after aging followed at ^2/3 behavior. The activation energy deduced for the process is in good agreement with that for the diffusion of oxygen in titanium.     

Kinetics of hydrogen absorption in alpha titanium

Hydrogen absorption and desorption often limits a material’s application. For titanium, hydrogen absorption kinetics determine its suitability for tritium storage, tritium gettering, and vacuum pump applications. This study examines the absolute rate theory energy surface which molecular hydrogen gas encounters as it is absorbed into alpha-phase titanium. This results in useful new predictions for hydrogen absorption rates, desorption rates, and surface coverages on titanium. The only energy surface which is consistent with observed activation and absorption enthalpies, while predicting all absorption/desorption rate data, is found to contain a new activation barrier. Accuracy is within a factor of 3.6 for two surface preparations and temperatures between 250 and 500 °C.     

Dynamic strain aging and hydrogen-induced softening in alpha titanium

Compression tests were carried out on samples of commercial-purity titanium charged with up to 4.7 at. pct hydrogen. Strain rates of 10⁻³ to 1 s⁻¹ were employed and testing was limited to the α phase field at temperatures of 773 to 973 K. The dependences of the flow stress on strain, strain rate, and temperature were determined. A plateau or bulge appeared in the temperature and strain-rate dependences of the flow stress, and the work-hardening rate also showed peaks. Serrations were observed on some of the stress-strain curves. All these features indicated that dynamic strain aging (DSA) was occurring. Analysis of the results (together with data from other authors) indicates that there are three ranges of DSA behavior in this material within the experimentally investigated temperature range; these appear to be associated with the diffusion of iron, carbon, and oxygen, respectively. Alloying with hydrogen decreases the magnitude of the DSA attributable to these elements and displaces the phenomenon to higher temperatures and/or to lower strain rates. The dependence on strain rate and temperature of the relative softening attributable to hydrogen addition was determined. The results indicate that hydrogen-induced softening is related to the occurrence of DSA in this temperature range. Possible explanations for this relationship are discussed. O.N. SENKOV, on leave from the Institute of Solid State Physics, Russian Academy of Sciences, Moscow Region 142432, Russia     

Determination of hydrogen pęrmeation parameters in alpha titanium using the mass spectrometer

The kinetic parameters of hydrogen permeation through α-Ti were determined from mass spectrographic measurements between 400° and 800°C and 0.004 and 2.0 torr H₂. The specimens were hollow cylinders and the hydrogen source was gaseous hydrogen. Hydrogen entered the specimen wall at the inside surface and emerged from the specimen at the outside surface. The permeated hydrogen flux is given byP = 1.6 × 10⁻⁵ p exp (-14,900 ± 856/RT) mole torr/s cm² where the hydrogen pressure isp and the permeation activation energy is 14,900 cal mole⁻¹ The surface dependent nature of hydrogen permeation in α-Ti is established in these experiments by the observations that 1) permeation is not proportional to the square root of input hydrogen pressure, 2) the concentration gradient calculated from experimental permeation data is lower than it would be if equilibrium solubility had developed, and 3) the diffusional frequency factor as determined under nonsteady-state conditions is low by a factor of three compared with values determined by others.     

Cathodic deoxygenation of the alpha case on titanium and alloys in molten calcium chloride

The oxygen-enriched alpha case on titanium and alloys was successfully deoxygenated to satisfactory levels by electrolysis in molten CaCl₂, in which the cathode was made from the metal to be refined. The oxygen distribution in the metal before and after electrolysis was characterized by microhardness tests, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The electrolysis has been carried out at voltages sufficiently below that for the decomposition of CaCl₂, and the results obtained suggest that the alpha case deoxygenation follows a simple oxygen ionization mechanism in which the oxygen in the metal is simply ionized at the cathode/electrolyte interface, dissolves in the molten salt, and then discharges at the anode. It is shown that by applying the electrochemical method, the alpha cases on both commercially pure titanium (CP Ti) and the Ti-6Al-4V alloy can be effectively deoxygenated. In particular, due to the removal of oxygen, the original alpha case (single phase) on the Ti-6Al-4V alloy has been converted back to the two-phase microstructure.     

Communication: Literature survey on diffusivities of oxygen,aluminum, and vanadium in alpha titanium,beta titanium,and in rutile

The online version of the original article can be found at     

Dislocation sources in silicon

Oxidation induced stacking faults have been observed to function as dislocation generating sources in silicon single crystals when the crystals are annealed at high temperatures. The presence of diffused boron in the silicon results in a large increase in the size of the dislocation colonies generated from the faults. The dislocations generated form three dimensional star shaped colonies with the colonies being decorated by a second phase in the form of fine precipitates or dislocation loops. Depending upon the initial oxidation temperature and the postoxidation annealing treatments both partial and whole dislocations are generated in the samples containing diffused boron. The partial dislocations are 1/3 «111» Frank dislocations and the whole dislocations have Burgers vectors 1/2 «110» and «100». The dislocation loops generated are interstitial and the stacking faults extrinsic in nature. A model is proposed to account for the growth of the colonies. The initial nucleation of the stacking fault is suggested to occur by the formation of an oxide of silicon. Diffusion of boron is postulated to result in a preferential accumulation of boron in the fault with the subsequent growth of the fault occurring when annealing forces in-diffusion of the boron concentrated in the faults.     

Dislocation dynamics in niobium-oxygen

The dislocation velocity-stress exponentm has been calculated from measurements of the strain rate sensitivity of the effective stress τ for single crystals of niobium-oxygen solid solutions which exhibit alloy softening. The values ofm evaluated at small plastic strains are found to be a sensitive function of temperature and oxygen concentration. Analysis of these results in terms of Peierls-Nabarro and dislocation-interstitial solute hardening mechanisms infers the latter is rate controlling. Furthermore, alloy softening is best interpreted in terms of the effects which solute interaction processes have on the thermally activated hardening caused by oxygen. Formerly Graduate Assistant, Division of Metallurgy Formerly Graduate Assistant, Division of Metallurgy Formerly Graduate Assistant, Division of Metallurgy     

Dislocation in total hip arthroplasties

In a series of 1,443 total hip arthroplasties performed between 1970 and 1975, dislocation was the most frequent complication and its incidence was found to be increasing. Dislocatin was frequently associated with component malposition that the surgeon was not aware of at the time of surgery and was relatively frequent among less experienced surgeons. It was especially frequent if the patient had had previous hip surgery and was related to the difficulty of the surgery rather than the primary hip disease. Increasing incidence was associated with change in operative technique and less stringent patient selection. Dislocation was 2 1/2 times more frequent if trochanteric osteotomy was not performed. The tip of the greater trochanter was moved significantly closer to the center of the prosthetic head in the patients who dislocated. Traction for up to three weeks did not lower the incidence of subsequent dislocation compared with mobilization of the patients as soon as tolerated after dislocation. Surgery was effective in preventing further dislocations in patients with recurrent dislocation and component malposition. The use of an articulated prosthesis is not recommended.     

Effect of the size distribution of alpha particles on microstructure evolution during heat treatment of an alpha/beta titanium alloy

A model was developed to describe the diffusional growth of a size distribution of alpha particles in Ti-6Al-4V during cooldown following alpha/beta solution treatment. Model results for initial microstructures with the same initial volume fraction of alpha but comprising either a distribution of particle sizes or a uniform particle size (equal to the average size of the distribution) yielded almost identical results in terms of final volume fraction and average particle size. The simulations quantified the narrowing of the size distribution associated with the different growth rates of small and large particles.     

Oxygen diffusion in alpha and beta titanium in the temperature range of 932° to 1142°C

An attempt is made to determine the diffusion coefficient of oxygen in α-Ti from oxidation studies of oxygen-unsaturated and oxygen-saturated β-Ti. Knowledge about the rates of movements of the α/β interface permits evaluation of oxygen diffusivity in α-Ti. The results conform to the relationship:D _α=0.778 exp (?48,600/RT), sq cm per sec, for the temperature range of 932° to 1142°C. From microhardness measurements, the diffusion coefficient of oxygen in β-Ti can be expressed as:D _β=3.30×10² exp (?58,800/RT), sq cm per soc, for the same temperature range.