High-temperature Deformation Process Research Articles

Control of beta phase in an EPM-processed intermetallic compound based on Ti[sbnd]Al[sbnd]Mn[sbnd]Mo

Gamma TiAl alloys have been under considerable research in recent years because of their good specific strength and the high resistance to creep and oxidation at elevated temperatures. Since the first report on their promising high temperature properties the gamma alloys have been continuously improved and their merits are highly valued now. Among the methods to produce the gamma alloys, EPM (Elemental Powder Metallurgy) method drew attention due to its amenability to the cost-effective near-net shape processing. In the EPM method alloying reaction is basically the exothermic reactions between elemental powders. A practical drawback of using this reaction as a sole method of alloying is the Kirkendall pores generated by the difference in the diffusivities of each alloying element. Therefore high temperature deformation process is added to seal the pores and consolidate the alloying reaction. In the previous attempt, the authors used hot extrusion to consolidate the TiAl compound containing Mn and Mo for improved mechanical properties. The Mo addition and the inherent characteristics of EPM, however, raised a question about the presence of {beta} phase that may cause undesirable effect on high temperature mechanical properties. In the present work, the authors attempted to answer this question from the standmore » point of finding a practical control method of {beta} phase and optimizing the microstructure.« less

High-temperature deformation and fracture processes in sintered reaction-bonded silicon nitride

Studies of the high-temperature deformation behaviour of sintered reaction-bonded silicon nitride (SRBSN) materials were conducted at 1200 °C in air under selected stress levels, which were applied at a single stress or as a sequence of stepwise increasing stresses. The objective was to evaluate the effects of the fabrication methods (conventional versus microwave heating process), microstructure, and precursor silicon powder purity on the deformation and fracture processes during creep loading of SRBSN materials containing a mixture of 3 wt% Al2O3 and 9 wt% Y2O3 sintering additives. Results indicated that all of the SRBSN materials exhibited a threshold stress above which the dominant process underwent transition from creep to extensive creep-assisted crack growth (CACG) from existing pores. In addition, the microwave SRBSN materials exhibited a better resistance (higher threshold stress) to CACG process, compared with those fabricated by conventional heating with the same metallurgical grade of silicon powder. The higher threshold stress observed in microwave SRBSN is mainly associated with the increased number density of elongated grains and the related higher fracture toughness. However, the minimum creep rates and stress exponents obtained in the creep regime were independent of the heating method. The microwave SRBSN material fabricated with lower purity silicon also exhibited a higher threshold stress for multiple crack formation and growth as compared with that processed with higher purity silicon. Conversely, the creep rate of microwave SRBSN materials was decreased by decreasing the impurity level (i.e. iron) in silicon powder.

Investigation of the α- and β- texture evolution of hot rolled Ti-64 products

The α textures and their parent high temperature β textures of different Ti-64 samples rolled in the β field were investigated. The parent high temperature β texture was modified by different rolling rates. Experimentally, for 30% deformation, the parent high temperature β texture was a partial fibre type texture, characterized by the 〈100〉//DN and 〈111〉//DN fibres. For 75% deformation, the 〈111〉//DN fibre was lowered and the orientations characterized by the Euler angles (45°, 0°, 0°) were promoted. In parallel, the deformation β texture was simulated owing to a rate sensitive Taylor model in order to draw the main crystallographic features involved in the high temperature deformation process. It can be seen that the high temperature deformation influenced the inherited α texture which evolved to a major component described by the Euler angles (0°, 90°, 0°). This evolution can be explained using the corresponding evolution of the β texture and the phase transformation. As a result, it can be concluded that the deformation in the β field induced a variant selection during cooling down.

High-temperature wear and deformation processes in metal matrix composites

Dry-sliding wear behaviors of a particulate-reinforced aluminum matrix composite 6061 Al-20 pet A12O3 and an unreinforced 6061 Al alloy were investigated in the temperature range 25 °C to 500 °C against a SAE 52100 bearing steel counterface. Experiments were carried out at a constant sliding speed of 0.2 m·s- at different test loads. The deformation behavior of the materials was studied by performing uniaxial compression tests in the same temperature range as the wear tests. Both alloys showed a mild-to-severe wear transition above a certain test temperature. In the mild wear regime, the wear rate and the coefficient of friction of the unreinforced 6061 Al decreased slightly with temperature, but the temperature had almost no effect on the wear rate and the coefficient of friction of the 6061 Al-20 pet Al2O3 in the same regime. Particulate reinforcement led to an increase in the transition temperature and a 50 to 70 pet improvement in the wear resistance in the severe wear regime. This was attributed to the formation of tribological layers consisting of comminuted A12O3 particles at the contact surface. High-temperature compression tests showed that the flow strength of 6061 Al-20 pet A12O3 and 6061 Al decreased monotonically with temperature and both alloys exhibited a work-softening behavior at temperatures higher than the inflection point on the flow stressvs temperature curves. The logarithmic maximum stressvs reciprocal temperature relationship was not linear, indicating that the deformation processes were too complicated to be characterized by a single activation energy over the whole temperature range. For the range of 250 °C to 450 °C, the activation energy for deformation was estimated to be 311 kJ·mol-1; for both the matrix alloy and the composite. Severe wear proceeded by thermally activated deformation processes involving dynamic recrystallization along a subsurface strain gradient. A power-Arrhenius type relationship was found to describe well the observed dependence of severe wear rates on the applied load and temperature. This relationship was used to calculate an apparent activation energy for wear of 87 kJ·mol-1 for the particulate-reinforced composite and 33 kJ·mol-1 for the matrix alloy. The wear regimes at elevated temperatures are represented in a deformation mechanism map and the relationship between high-strain deformation processes and severe wear are discussed.

Deformation attending internal nitridation of Ni-Ti alloys

Net volume increase attending precipitation is the source of a number of interesting deformation processes including creep and grain boundary sliding. Grain boundary sliding, a common high temperature deformation process, was observed in Ni-Al but was absent in Ni-Ti and Ag-In. It appears that in Ni-Ti, Ni is preferentially transported outwards along dislocation pipes and some twin boundaries countercurrent to a bulk stress driven vacancy flow (similar to the Nabarro-Herring creep attributed to Ni-Al). Some Ti may be preferentially transported outward by grain boundaries since at compositions near the transition to a superficial scale of TiN outward solute diffusion is significant. A strong element of Ni surface diffusion is involved in redistributing extruded Ni and producing facetted surface features. In respect to Ti diffusion towards the surface the authors have to record a distinction between the process and the classical Wagner models of frontal internal oxidation which applies to Ag-In and Ni-Al.

Dynamic Strain Aging in a New Single Crystal Nickel-Based Superalloy (CMSX-4)

Abstract Tensile behavior of a new single crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures in two different heat-treated conditions. Detailed fractographic studies were carried out to determine the micromechanism of crack growth in tensile loading. Round cylindrical tension specimens were prepared from the single crystal nickel-based superalloy CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. The specimens were given two different heat treatments to produce two different γ′ precipitate sizes, i.e., 0.3 μm and 0.5 μm, respectively. The tensile properties of these materials were evaluated at room and elevated temperatures in ambient atmosphere as per ASTM E 21, Test Methods for Elevated Temperature Tension Tests of Metallic Materials. The results of this investigation indicate that CMSX-4 undergoes dynamic strain aging in the temperature range of 260° to 875°C. This dynamic strain aging appears to take place by the trapping of moving dislocations by the substitutional solute atoms (like rhenium, tantalum, tungsten, cobalt, and molybdenum) during the high-temperature deformation process. Fractographs at lower temperatures (T ≤ 800°C) exhibit an unusual type of cleavage fracture with several operative slip systems, while at higher temperatures (T ≥ 1094°C) the crack growth process is characterized by a relatively smooth fracture surface, with the presence of dimples created by ductile failure (typical of many nickel-based superalloys).

High temperature deformation and fracture processes in [formula omitted] weldments

In-service weldments, which operate under high temperature creep conditions, are often manufactured with 2 1 4 Cr1Mo weld metal in 1 2 Cr 1 2 Mo 1 4 V pipe. The creep resistance of the weld metal is considerably less than the base metal. Therefore, during cross-weld uniaxial creep testing, deformation would be expected to occur predominantly in the weld metal. In the present programme, cross-weld uniaxial tests have been conducted on a typical ex-service weldment over a range of temperatures and stresses. Displacement during the test was monitored, and the specimens were examined both macroscopically and microscopically after failure. Results show that deformation is concentrated in the fine grained intercritical region of the weldment heat affected zone. The reasons for this seemingly anomalous behaviour are discussed.

High-temperature dislocation configurations and their effect on ductilization of Ni—Al single crystals

Abstract The effect of high-temperature pre-deformation on the ductility achieved during room-temperature bending has been compared between 〈100〉-oriented single crystals of Ni—Al and those containing iron and hafnium additions. Detailed dislocation analysis carried out on the high-temperature-pre-deformed samples and after hardness indentation at room temperature has been carried out. The improved ductility obtained from the pre-deformed samples of the binary and iron-containing alloys has been related to the increased production of 〈100〉 mobile dislocations during the high-temperature deformation process in these alloys.

Flux pinning by dislocations in deformed melt-textured YBa 2Cu 3O x superconductors

The potential of flux pinning by dislocations to increase the critical current density ( J c) of melt-textured YBa 2Cu 3O x superconductors in a variety of orientations of magnetic field ( H) with respect to the a-b plane was studied. The melt-textured samples were subjected to high temperature uniaxial and isostatic deformation to increase the dislocation density. A broad peak in J c is observed in these samples at H‖ c-axis and is attributed to point pinning by dislocations generated in the a-b plane by the deformation process. A sharp peak in J c is seen at 45° to the a-b plane in some isostatically deformed samples. Edge type 1 6 [031] partial dislocations located at the boundary of the stacking faults observed along the [ 1 10] zone axis of these samples are proposed to contribute to this peak. The results indicate that the J c of melt-textured samples can be enhanced in magnetic field orientations other than H‖ a-b by introduction of dislocations through high-temperature deformation processing.

Creep and high temperature deformation of titanium aluminide alloys: influence of α2 phase distribution and B2 phase decomposition

The high temperature deformation properties of a titanium aluminide alloy Ti-24at.%Al-11at.%Nb-2at.%V-1at.%Mo have been studied as a function of the microstructural distribution of α 2 and B2 phases. Controlled annealing treatments were given as initial starting conditions in order to obtain different volume fractions of the α 2 phase. These materials were used to study creep and compression properties at 700°C. Decomposition of the B2 phase was observed to occur during the high temperature exposure of the deformation tests. The stress and temperature-dependent parameters determining the high temperature deformation process have been confirmed to depend on the microstructural distribution.

J.‐P. Poirier. Creep of crystals. High‐temperature deformation processes in metals, ceramics and minerals. Cambridge University Press. Cambridge – London – New York – New Rochelle – Melbourne – Sydney 1985. 145 figs., XII + 260 p., price £ 10.95 (paperback). ISBN 0‐521‐27851‐1

Crystal Research and TechnologyVolume 21, Issue 10 p. 1338-1338 Book Review J.-P. Poirier. Creep of crystals. High-temperature deformation processes in metals, ceramics and minerals. Cambridge University Press. Cambridge – London – New York – New Rochelle – Melbourne – Sydney 1985. 145 figs., XII + 260 p., price £ 10.95 (paperback). ISBN 0-521-27851-1 P. Paufler, Search for more papers by this author P. Paufler, Search for more papers by this author First published: October 1986 https://doi.org/10.1002/crat.2170211021AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue10October 1986Pages 1338-1338 RelatedInformation

Weak-Beam Imaging of Coherent Twin Boundaries After High-Temperature Deformation

Weak beam dark field microscopy has been used extensively to study deformation induced intragranular defects in materials. However as noted by Tones et al weak beam imaging is also an asset in examining intergranular defects. The localisation of strain contrast close to the defect core by weak beam imaging allows the interaction of matrix dislocations with the boundary (to form extrinsic dislocations) to be studied more easily. Also, the reduction in extinction distance with weak beam microscopy produces a higher density of extinction contours, which allows topographical discontinuities associated with long range strain fields to be more readily observed.Thus in the present study, where we were interested in interfacial phenomena during high temperature deformation processing of a cobalt base alloy (Co-Cr-Mo), weak beam imaging was used to obtain a better understanding of the role of intergranular defects.

Effects of combined high and low temperature deformation processing of β III titanium

The structure and properties of β III titanium alloy (nominal composition: 11.5 pct Mo, 6 pct Zr, 4.5 pct Sn, bal Ti) were studied as a function of combined high and low temperature ther-momechanical processing. A water quenched extrusion was deformed various amounts by swaging at room temperature prior to the aging treatment. No re-solution heat treatment was employed. The swaging introduced mechanical twinning and a small amount of stress induced orthorhombic martensite. Following a 900°F, 8 hr aging treatment, substantial increases in yield and tensile strength were observed, combined with a severe decrease in tensile ductility in samples with small amounts of swaging. The orientation and morphology of the deformation products have a critical influence on tensile ductility. A decrease in the plane strain fracture toughness accompanied the large increase in tensile strength.