Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures （850- 1200 C） and strain rates （0. 001- 10 s^- 1 ） on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By re gression analysis, the activation energy （Q） in the whole range of deformation temperature is determined to be 367. 562 kJ/mol. The effects of the temperature and the strain rate on mierostructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstrueture.     

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.     

The brittle compressive fracture of ice

The brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied. Measurements are reported of the fracture stress at temperatures from −10 to −50°C at strain rates of 10⁻³ and 10⁻¹ s⁻¹ for grain sizes from approximately 1 to 10 mm. Also a summary is reported of measurements by Jones et al. (unpublished) of the kinetic coefficient of friction for ice on ice at temperatures from −10 to −40°C at sliding velocities from 5 × 10⁻⁷ m s⁻¹ to 5 × 10⁻² ms⁻¹. Observations via high speed photography of internal cracking during loading are included. The strength, albeit scattered, increases with decreasing grain size, with decreasing temperature and at −10°C with decreasing strain rate. Similarly, the coefficient of friction increases with decreasing temperature and at −10°C with decreasing sliding velocity. Wing cracks were observed on some inclined cracks nucleated during loading. The results are explained in terms of the frictional crack sliding-wing crack model [as developed by Ashby and Hallam, Acta metall.34, 497 (1986)] of compressive fracture. Finally, a simple model is presented for the transition from ductile to brittle behavior. It is based upon the competition between the building up and the relaxation of internal stresses within the vicinity of the internal cracks, and it leads to a transition strain rate which can be expressed in terms of the fracture toughness, the creep rate, the kinetic coefficient of friction and the microstructural scale of the material.     

Microstructural Evolution and Flow Analysis during Hot Working of a Fe-Ni-Cr Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds⁻¹ to 1 second⁻¹ to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds⁻¹ to 0.1 seconds⁻¹, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore, at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16 for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels. However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental data.     

Interdependence of Distortion and Residual Stress Relaxation of Cold-Rolled Bearing Rings During Heating

This article discusses the distortion behavior during heating of bearing rings produced by cold rolling. The residual stress relaxation was characterized intensively and correlated to the distortion behavior. In the initial state, the rings show compressive residual stresses in tangential and axial direction with almost no variations along the circumference. Because of the cold-rolling process, the entire cross section is affected by residual stresses. The rings present a characteristic size change between the soaking temperatures 773 K and 823 K (500 °C and 550 °C), which can be correlated with the macroresidual stress relaxation in the core of the rings. Changes in roundness deviation were found, but the amplitude of oval and triangular shape increases continuously until austenitizing temperature is reached. As the macrostress relaxation is already completed at 873 K (600 °C), another mechanism should be responsible for these distortions. A correlation between amplitude of the oval shape and decrease of full width at half maximum seems to be present. This may indicate that inhomogeneous recrystallization happens along the circumference of the rings. A triangular shape may result from the influence of the loading tool used as rings are positioned on three contact points during the stress relief treatment.     

Elevated temperature wear behaviour of CeO2 modified WC-12Co coating

With the service environment becoming more and more severe, WC-Co coatings are required to apply in high temperature wear condition. In the present study, the sliding wear tests of CeO₂ modified WC-12Co coatings were conducted at temperature of 450, 550 and 650 °C. The wear loss and friction coefficient were recorded. The morphologies of wear tracks were observed every 1 h to investigate the dynamic wear mechanisms. The results show that the volume wear loss decreases with temperature increasing. The lowest volume wear loss is obtained at the temperature of 650 °C due to oxide films generated in the process of wearing. The wear mechanism is different at the temperature of 450, 550 and 650 °C. Micro cutting wear, abrasive wear and oxidation wear dominate the wear mechanism at 450, 550 and 650 °C, respectively. Abrasive wear and oxidation wear are the wear mechanisms at various temperatures.     

The effect of environment on high-temperature hold time fatigue behavior of annealed 2.25 pct Cr 1 pct Mo steel

Total strain control fatigue tests with a 120-second hold period at either peak compressive or tensile strain were conducted on annealed 2.25 pct Cr 1 pct Mo steel. Tests were performed at the total strain range of 1.0 pct at 500 °C or 600 °C in air, 1.3 Pa (10⁻² torr) or 1.3 × 10⁻³ Pa (10⁻⁵ torr) vacuum. The nature of the hold and the environment affect fatigue life and surface crack patterns. A compressive hold is more deleterious than a tensile hold in high-temperature air, while the reverse is true in environments in which oxidation is limited. Observations of cracks at the surface and in cross section indicate that an oxidation-fatigue interaction accounts for the damaging effect of a compressive hold in air tests. In vacuum tests, creep damage has the opportunity to accumulate and causes the tension hold to exhibit the shortest fatigue lifetime.     

Fatigue-crack growth behavior in the superelastic and shape-memory alloy nitinol

This article presents a study of fatigue-crack propagation behavior in Nitinol, a 50Ni-50Ti (at. pct) superelastic/shape-memory alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance. Specifically, fatigue-crack growth was characterized in stable austenite (at 120 °C), superelastic austenite (at 37 °C), and martensite (at −65 °C and − 196 °C). In general, fatigue-crack growth resistance was found to increase with decreasing temperature, such that fatigue thresholds were higher and crack-growth rates slower in martensite compared to stable austenite and superelastic austenite. Of note was the observation that the stress-induced transformation of the superelastic austenite structure, which occurs readily at 37 °C during uniaxial tensile testing, could be suppressed during fatigue-crack propagation by the tensile hydrostatic stress state ahead of a crack tip in plane strain; this effect, however, was not seen in thinner specimens, where the constraint was relaxed due to prevailing plane-stress conditions.     

Dynamic Recrystallization in Sintered Cobalt during High-Temperature Deformation

The constitutive flow behavior of sintered cobalt in the temperature range 873 to 1473 K (600 to 1200 °C) and at strain rates from 0.001 to 10 s⁻¹ has been studied using constant true strain rate hot compression tests. On the basis of these data, a processing map has been generated that depicts the variation of strain rate sensitivity with temperature and strain rate. The processing map reveals a domain of dynamic recrystallization (DRX) with an optimum condition of processing at 1273 K (1000 °C) and at 10⁻³ s⁻¹. When deformed within the domain, the stress-strain curves exhibit a single peak followed by flow softening, leading to steady-state behavior. In addition to this, a recently developed approach based on flow curve analysis is used to study the DRX kinetics, which is found to follow an Avrami-type relation.     

The influence of substructure on the elevated and room temperature strength of a 26 Cr-1 Mo ferritic stainless steel

The high temperature flow stress behavior of an electron beam melted 26 Cr-1 Mo ferritic stainless steel was determined for large torsion strains (e ~ 15) over a temperature range from 400 °C (750 °F) to 1000 °C (1830 °F) and a strain rate range from 6 × 10^-3 to 6.3 s^-1. The room temperature compressive yield strength measured after torsional warm working was also investigated. It was found that the high temperature flow strength and the room temperature compressive yield strength were strong functions of the subgrain size. Strain softening was observed during warm working while the room temperature compressive yield strength was found to increase with prior torsional strain. The increase in the subgrain misorientation angle and, to a lesser extent, the subgrain shape change that occurs with increased warm working strain appear to be responsible for the strain dependence of the flow stress at both elevated and low temperatures. At the time this investigation was performed, all authors were affiliated with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305.     

Mechanical behavior of thein situ composite alloys in the al-ni-ti system near the l12 phase field

The Vickers microhardness (VHN) test at room temperature and compressive tests at temperatures up to 1000 °C were carried out on the three-phase composite alloy, consisting of the Ll₂, facecentered cubic (fcc) Al₂TiNi, and Al₂Ti intermetallic phases, in the Al-Ti-Ni system. The microhardness tests indicated that the fcc Al₂TiNi phase was very hard and brittle. Comparatively, the L1₂ phase was softer and more crack resistant. A considerable hardening was noticed due to the precipitation of Al₂Ti within L1₂. In addition, the VHN of the L1₂ phase was found to increase with the combined content of nickel and titanium without the presence of any observable precipitates. Under compressive loading at room temperature, microcracks nucleated in the fcc Al₂TiNi phase. These cracks propagated catastrophically at a stress barely approaching yield stress, resulting in nil ductility. This behavior was observed up to 800 °C. Between 900 °C and 950 °C, brittle-to-ductile transition in compressive behavior was observed for the three-phase alloy. Compressive ductility of the order of 80 pct was observed at 1000 °C. The mechanism of dynamic recrystallization was found to be operative at 1000 °C. Metallographic investigation revealed new recrystallized grains in the primary L1₂ matrix. However, the oscillatory nature of the true stress-true strain curve could not be explained with the help of the existing model of dynamic recrystallization. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys

Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3 joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms (dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a homogeneous subgrain structure.     

Interfacial Microstructural Evolution and Metallurgical Bonding Mechanisms for IN718 Superalloy Joint Produced by Hot Compressive Bonding

The novel metallurgical joining process for bonding IN718 superalloy was investigated by hot compressive bonding (HCB) process under the deformation temperature range of 1000 °C to 1150 °C and true strains ranging from 0 to 0.5 at a strain rate of 0.001 s^?1. The effect of HCB process parameters on the tensile strength was analyzed. Both the as-deformed and the interfacial microstructures were characterized using the optical microscope, electron backscattered diffraction and transmission electron microscope (TEM) analysis. The results of tensile property revealed that the degree of metallurgical bonding is promoted by increasing deformation temperature and strain. The evolution of the interfacial microstructure showed that the migration of interfacial grain boundary (IGB), which is characterized by discontinuous dynamic recrystallization, is the dominant metallurgical bonding mechanism in the early stages of bonding. TEM analysis indicated that the dislocation density is distributed heterogeneously over both sides of IGB, which is the significant reason for the migration of IGB, during the initial stage of HCB process.     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.