Temperature-strain Rate Regimes Research Articles

Serrated yielding in austenitic stainless steels

ABSTRACT Austenitic stainless steels are widely employed in various applications due to many desirable properties such as excellent mechanical properties and corrosion/oxidation resistance. Serrated yielding phenomenon that occurs in austenitic stainless steels during deformation can cause significant changes in microstructures and mechanical properties. Thus, understanding the serrated yielding mechanisms and their influence on the mechanical behaviour on austenitic stainless steels is essential for developing and designing an advanced steel for harsher environment applications. In this article, a concise review of serrated yielding behaviour is examined and discussed with special attention to serration types, models describing the origin of the serrated flow, temperature-strain rate regimes for the serration occurrence, recent findings of the serration behaviour in a variety of austenitic steels and the role of dynamic strain ageing. Contradiction about the negative/beneficial impact of serrated flow on mechanical properties of newly developed austenitic stainless steels also is presented.

Primary hot working characteristics of an as-cast and homogenized nickel base superalloy DMR-742 in the sub and super-solvus temperature regime

Primary deformation behaviour of a medium alloyed nickel base superalloy DMR-742 in the as-cast and homogenized condition (furnace cooled) was evaluated in the sub-solvus and super-solvus temperature regimes. The flow stress in the sub-solvus regime was found to be highly sensitive to temperature. The contribution of back stress to the flow stress sensitivity was estimated using the microstructure dependent analytical equations and compared with the back stress estimated using experimental steady state flow stress. It was established that Orowan looping or by-passing mechanism contributes to the back stress resulting in high flow stress sensitivity in the sub-solvus regime. In the super-solvus regime, the material was relatively temperature insensitive and exhibited discontinuous dynamic recrystallization (DDRX) with a characteristic power dissipation value of 38–44% in three different temperature-strain rate regimes. Disregarding the industrially non-viable low strain rate regime, the recrystallization kinetics in the moderate and high strain rate regime was estimated using grain size factor compensated grain orientation spread maps. A higher rate of recrystallization observed in the material deformed at high temperature-strain rate was attributed to relatively strong dislocation interactions and weak recovery. The true stress exponent, activation volume and energy estimated using back stress and elastic modulus compensated flow stress revealed that diffusion controlled climb and cross-slip as the rate controlling deformation mechanism. Side-pressing under high secondary tensile stress conditions was used to evaluate the workability and validate the experimental findings.

Formation of the Structural State of a High-Strength Low-Alloy Steel upon Hot Rolling and Controlled Cooling

This article presents a review of current results of theoretical and experimental studies of the specific features of the structure that is formed in high-strength low-alloy steel in the process of hot rolling and which determines the properties of the steel. The current concepts of the physical processes that are developed at different stages of a thermomechanical treatment depending on the temperature–strain-rate regimes and chemical composition of the steel are considered. Particular attention is paid to the problems of the formation of the structural state that continue to be debated. The simulation methods of different scale level used to solve the problems of controlling structure formation at all stages of thermomechanical processing process are discussed.

Effect of temperature and strain rate on hot deformation behavior and microstructure of Al-Cu-Li alloy

Al-Cu-Li alloy (AA2195) was produced using vacuum induction melting (VIM) furnace under dynamic argon atmosphere. The as-cast billets were homogenized using a two-step homogenization cycle. The hot deformation behavior of homogenized and forged AA2195 alloy was studied by hot isothermal compression in a thermo-mechanical simulator. The contour maps of efficiency of power dissipation and instability maps have been generated within the temperature range of 250–450 °C and strain rate range of 10−3–102 s−1. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of these maps and complementary microstructural analysis of the deformed specimens was carried out. Results indicate four distinct deformation domains within the range of experimental conditions examined. Out of these four domains, the optimum temperature and strain rate range for obtaining a completely reconstituted microstructure is T: 400 °C-450 °C andε˙: 10−2-10−1.5 s−1 andε˙: 10−0.5-101 s−1. Instability mechanisms in the material are attributed to localized plastic flow and cracking. A constitutive equation that describes the flow stress of AA2195 alloy as a function of strain rate and deformation temperature was also established.

High-Temperature Deformation Processing Map Approach for Obtaining the Desired Microstructure in a Multi-component (Ni-Ti-Cu-Fe) Alloy

An equiatomic NiTiCuFe multi-component alloy with simple body-centered cubic (bcc) and face-centered cubic solid-solution phases in the microstructure was processed by vacuum induction melting furnace under dynamic Ar atmosphere. High-temperature uniaxial compression experiments were conducted on it in the temperature range of 1073 K to 1303 K (800 degrees C to 1030 degrees C) and strain rate range of 10(-3) to 10(-1) s(-1). The data generated were analyzed with the aid of the dynamic materials model through which power dissipation efficiency and instability maps were generated so as to identify the governing deformation mechanisms that are operative in different temperature-strain rate regimes with the aid of complementary microstructural analysis of the deformed specimens. Results indicate that the stable domain for the high temperature deformation of the multi-component alloy occurs in the temperature range of 1173 K to 1303 K (900 degrees C to 1030 degrees C) and (epsilon) over dot range of 10(-3) to 10(-1.2) s(-1), and the deformation is unstable at T = 1073 K to 1153 K (800 degrees C to 880 degrees C) and (epsilon) over dot = 10(-3) to 10(-1.4) s(-1) as well as T = 1223 K to 1293 K (950 degrees C to 1020 degrees C) and (epsilon) over dot = 10(-1.4) to 10(-1) s(-1), with adiabatic shear banding, localized plastic flow, or cracking being the unstable mechanisms. A constitutive equation that describes the flow stress of NiTiCuFe multi-component alloy as a function of strain rate and deformation temperature was also determined. (C) The Minerals, Metals & Materials Society and ASM International 2015

Hot deformation behaviour and microstructure control in AlCrCuNiFeCo high entropy alloy

An AlCrCuNiFeCo high entropy alloy (HEA), which has simple face centered cubic (FCC) and body centered cubic (BCC) solid solution phases as the microstructural constituents, was processed and its high temperature deformation behaviour was examined as a function of temperature (700–1030 °C) and strain rate (10−3–10−1 s−1), so as to identify the optimum thermo-mechanical processing (TMP) conditions for hot working of this alloy. For this purpose, power dissipation efficiency and deformation instability maps utilizing that the dynamic materials model pioneered by Prasad and co-workers have been generated and examined. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate two distinct deformation domains within the range of experimental conditions examined, with the combination of 1000 °C/10−3 s−1 and 1030 °C/10−2 s−1 being the optimum for hot working. Flow instabilities associated with adiabatic shear banding, or localized plastic flow, and or cracking were found for 700–730 °C/10−3–10−1 s−1 and 750–860 °C/10−1.4–10−1 s−1 combinations. A constitutive equation that describes the flow stress of AlCrCuNiFeCo alloy as a function of strain rate and deformation temperature was also determined.

High temperature deformation processing maps for boron modified Ti–6Al–4V alloys

The alloy, Ti–6Al–4V is an α + β Ti alloy that has large prior β grain size (∼2 mm) in the as cast state. Minor addition of B (about 0.1 wt.%) to it refines the grain size significantly as well as produces in-situ TiB needles. The role played by these microstructural modifications on high temperature deformation processing maps of B-modified Ti64 alloys is examined in this paper. Power dissipation efficiency and instability maps have been generated within the temperature range of 750–1000 °C and strain rate range of 10 −3–10 +1 s −1. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate four distinct deformation domains within the range of experimental conditions examined, with the combination of 900–1000 °C and 10 −3–10 −2 s −1 being the optimum for hot working. In that zone, dynamic globularization of α laths is the principle deformation mechanism. The marked reduction in the prior β grain size, achieved with the addition of B, does not appear to alter this domain markedly. The other domains, with negative values of instability parameter, show undesirable microstructural features such as extensive kinking/bending of α laths and breaking of β laths for Ti64–0.0B as well as generation of voids and cracks in the matrix and TiB needles in the B-modified alloys.

Effect of Hot Deformation under Complex Loading on the Transformation of Lamellar Type Microstructure in Ti-6.5Al-3.5Mo-1.6Zr-0.27Si Alloy

The transformation of the lamellar type microstructure to an equiaxed one in two-phase titanium alloys is determined not only by temperature-strain rate regimes but also by the modes of loading. The experimental results of investigations relating to the influence of one- and two-component monotonous loading on microstructure and mechanical behavior of the two-phase Ti-6.5Al-3.5Mo-1.6Zr-0.27Si titanium alloy are presented in this work. It is shown that under conditions of proportional torsion + tension loading the transformation of the lamellar microstructure occurs more uniformly in the transverse section of a sample and over its length than under conditions of simple torsion or uniaxial tension. The transformation is accompanied by formation of a typical texture based on the strain state. The following uniaxial tension in optimal temperature-strain rate regimes of superplasticity is characterized by a lower value of flow stress, uniform distribution of deformation and further microstructure transformation within the whole sample volume.

Deformation mechanisms in the O phase

The intermetallic Ti2AlNb has a ternary ordered, orthorhombic structure (which is a slightly distorted form of the hexagonal D019, Ti3Al phase) and is the major constituent of the ‘orthorhombic’ alloys of the Ti–Al–Nb system. We explore in this paper the mechanical behaviour of this intermetallic over a wide range of strain rates and temperatures. Thermal activation analysis of the flow behaviour indicates that at low temperatures the flow behaviour is controlled by a strong thermally activated process with small activation volumes typical of a Peierls-type barrier. Dynamic recovery occurs at higher temperatures, but a specific rate-controlling mechanism for flow remains to be identified. The creep of the intermetallic has also been examined in a temperature–strain-rate regime well removed from the DSA regime. Stress exponents decrease slowly from 7 to 5 with increasing temperature and the dislocation structure consists of a three dimensional network linked by attractive junctions. A network model of creep, as proposed for example by McLean, may be appropriate to describe the creep of this intermetallic. The effect of Nb content and a variety of quaternary additions on the mechanical behaviour of the intermetallic has also been evaluated. Nb is shown to strengthen the intermetallic through the thermal component of the flow stress, while Si is found to have the strongest solid solution strengthening effect at low temperatures. Zr and oxygen additions enhance the dynamic strain ageing effect at intermediate temperatures. The Nb content does not affect the creep strength of the intermetallic.

The disappearance of serrated flow in two copper alloys

The disappearance of serrated flow was investigated for a commercial Cu2Be and for Cu3AgZr alloy (where the compositions are in approximate weight per cent.) In both of these alloys the serrated flow disappears at higher temperatures and slower strain rates by a progressive delay in the onset of serrated flow. The activation energy for the disappearance was found to be 1.16 eV for the Cu2Be alloy and 1.68 eV for the Cu3Ag0.38Zr alloy. The disappearance of serrated flow in both alloys is suggested to be due to the presence of solute clusters in the matrix which act as a second-phase sink which depletes the dislocation lines of their atmospheres during the dislocation arrest time at the clusters. In the temperature-strain rate regime where the disappearance occurs, serrations appear on the flow curve only after the matrix solute cluster sinks approach saturation (i.e. the clusters are approaching their stable equilibrium size) or when the atmosphere species is arriving at a rate greater than the cluster growth rate. Serrated flow then progresses with the stress decrement increasing progressively until failure of the specimen. The second-phase matrix sinks responsible for the disappearance in Cu2Be are suggested to be beryllium-rich solute clusters. The sinks responsible for the disappearance in the Cu3Ag0.38Zr alloy are suggested to be silver-rich clusters. In the higher temperature regime of the Cu3Ag0.38Zr alloy, serrated flow also disappears off the outer end of the flow curve. In this regime, after saturation of the silver cluster sinks, serrated flow initiates and persists for a small strain and then disappears. This disappearance near the end of the flow curve in the higher temperature regime is suggested to occur by a precipitation reaction between the retained silver atmosphere on the dislocations and substitutional zirconium atoms. In this high temperature regime, increasing temperature or strain and decreasing strain rate allows the zirconium atoms to diffuse through the copper matrix to the dislocation lines containing the silver atmospheres. A precipitation reaction then occurs between the silver and zirconium, resulting in an Ag nZr intermetallic directly on the dislocation lines, which thus depletes the dislocation line of its atmosphere.

Processing-microstructure relationships for Ti-6Al-2Sn-4Zr-2Mo-0.1Si

The detailed relationships between thermal-mechanical processing parameters and resulting microstructures for Ti-6Al-2Sn-4Zr-2Mo-0.1 Si (Ti-6242) have been established through compression testing and heat treatment. Beginning with either an equiaxed alpha or Widmanstatten alpha preform microstructure, isothermal compression tests were run at strain rates typical of isothermal forging (10−3 to 10−1 s−1) and conventional forging (1 to 100 s−1). Metallographic investigation of these test specimens in as-deformed and heat treated conditions was used to characterize deformation-induced microstructures and transformations. For the equiaxed alpha microstructure, it was shown that deformation, as well as post-deformation heat treatment, were more effective in promoting microstructures close to the expected equilibrium ones than heat treatment alone, a finding similar to that for other alloy systems. For the metastable Widmanstatten alpha microstructure, the deformation and heat treatment parameters that promote the development of an equilibrium, equiaxed alpha microstructure have been determined. For this microstructure, two separate temperature-strain rate regimes have been identified, and the resulting microstructures correlated with the measured flow stress behavior. For the low temperature regime, deformation is highly nonuniform, and the microstructural features are shown to be similar to those in pearlitic steels and other lamellar alloys. In the higher temperature regime, on the other hand, deformation is much more uniform. The results presented can be applied to select hot forging parameters for the control of final microstructure and properties in Ti-6242 and similarα/β titanium alloys.

Deformation processing of an aluminum alloy containing particles: Studies on AI-5 pct Si alloy 4043

Al-5 wt pct Si alloy is processed by upset forging in the temperature range 300 K to 800 K and in the strain rate range 0.02 to 200 s−1. The hardness and tensile properties of the product have been studied. A “safe” window in the strain rate-temperature field has been identified for processing of this alloy to obtain maximum tensile ductility in the product. For the above strain rate range, the temperature range of processing is 550 K to 700 K for obtaining high ductility in the product. On the basis of microstructure and the ductility of the product, the temperature-strain rate regimes of damage due to cavity formation at particles and wedge cracking have been isolated for this alloy. The tensile fracture features recorded on the product specimens are in conformity with the above damage mechanisms. A high temperature treatment above ≈600 K followed by fairly fast cooling gives solid solution strengthening in the alloy at room temperature.

Temperature-strain rate dependance of compressive strength and damage mechanisms in aluminium oxide

The results of compression tests of Al3O3 performed over a wide range of temperatures and strain rates are interpreted in terms of dominant damage mechanisms. It is shown that compressive failure in Al2O3 is caused by one of three different mechanisms, each based on tensile (Mode I) growth of predominantly axial microcracks, and each characteristic of a specific temperature-strain rate regime. The concepts developed should be applicable to other strong ceramics.

A model of hydrolytic weakening in quartz

Hobbs et al. (1972) suggest that the Haasen theory for diamond structure crystals may explain the observed behavior of hydrolytically weakened single crystals of quartz. There is no provision for the effect of varying OH content in the theory, of course, nor does it explain the great work hardening observed in quartz at low temperature or high strain rate. This paper is concerned with extensions to the Haasen theory required to account for the varying shapes of the observed stress-strain curves, the effect of varying water content, and the behavior observed in stress relaxation experiments. The rate-controlled process in hydrolytically facilitated slip is believed to be dislocation growth, which requires either ‘radial’ diffusion of HOH to the newly growing segment or ‘core’ diffusion, in which HOH diffuses parallel to the dislocation line to permit creation of a new segment. Radial diffusion is limited by exhaustion of free HOH as the dislocation density increases. Dislocation velocity proportional to HOH concentration is assumed, as suggested by Blacic. A Haasen-type model incorporating these processes plus recovery fits the observations of Hobbs et al., Balderman and Blacic, and Griggs on crystals of the O+ orientation. The Hobbs et al. observations on ⊥r crystals of varying water content are in conflict with this model. The model predicts the existence of a ‘fundamental strength’ in a temperature-strain rate regime that is easily accessible for experimental verification or disproof.