Warm deformation behaviour of UFG CP-Titanium produced by I-ECAP

The objective of the present study is to investigate the deformation behaviour of Ultrafine-grain (UFG) commercial purity Titanium (CP-Ti) at warm temperatures. Firstly, CP-Ti billets were processed through six passes of incremental equal channel angular pressing (I-ECAP) at 300° C using a die channel angle (Φ) of 120°. Uniaxial compression tests were then performed under isothermal conditions on cylindrical samples obtained from the UFG CP-Ti billets. A series of these tests were conducted at different temperatures of 400, 500 and 600 °C and at varying strain rates of 0.01, 0.1 and 1.0 s-1. In each test, the original height of the sample was deformed by ~50% of its original value. The true stress-strain curves obtained, revealed that the flow stress was sensitive to both temperature and strain rate. In general, the flow stress was higher for lower temperatures and higher strain rates. For tests conducted at 400 and 500 °C, the flow stress quickly reaches a peak value, beyond which it exhibits a steady state response where there is no appreciable change in flow stress with increasing strain. The 600 °C tests however shows a strain hardening behaviour. Microstructure of the sample deformed at 600 °C and 0.01 s-1, exhibited significant grain growth

On the evolution of microstructure and texture in commercial purity titanium during multiple passes of incremental equal channel angular pressing (I-ECAP)

This paper presents an investigation on the evolution of microstructure and deformation characteristics of commercial purity Titanium (CP-Ti) during incremental equal channel angular pressing (I-ECAP). CP-Ti grade 2 was subjected to six passes at 300 °C following route BC, using a die with channel angle of 120°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first, second, fourth and sixth pass, in the flow and transverse plane of the samples. Texture development through subsequent processing was also investigated using pole figures in both planes. Following first pass, the grain boundary maps across both flow and transverse plane showed a high degree of heterogeneity in grain morphology with the presence of elongated and fine grains. Also, misorientation peaks associated with {10-12} tensile twins and a small fraction of {11-22} compressive twins were observed in the microstructure. After second pass, microstructure was further refined and the twinning activity was greatly reduced with no noticeable activity after the fourth pass. Remarkable grain refinement was achieved after sixth pass with majority of grains in the ultrafine grain (UFG) range and with a relatively homogenous microstructure. Continuous dynamic recrystallization (CDRX) has been observed during subsequent I-ECAP processing. It was seen that twinning alongside CDRX acted as a dominant grain refinement mechanism during the initial passes of I-ECAP process beyond which slip was dominant deformation behaviour

Influence of incremental ECAP on the microstructure and tensile behaviour of commercial purity titanium

Severe plastic deformation (SPD) is an effective method for producing ultrafine grained (UFG) structures in metals. UFG materials are characterized by an average grain size of <1 µm and mostly high angle grain boundaries. These materials exhibit exceptional improvements in strength, superplastic behaviour and in some cases enhanced biocompatibility. Among various SPD methods available, equal channel angular pressing (ECAP) is the most effective method for obtaining bulk UFG billets. Lately, the interest is towards industrialization of the ECAP technique to enable processing of very long or continuous billets. Incremental ECAP (I-ECAP) developed at University of Strathclyde, offers such possibility. The present work details the processing of commercial purity titanium (CP-Ti), using I-ECAP process, with the objective of improving its strength characteristics. CP-Ti billets were successfully processed for up to four passes at 300 °C using an I-ECAP die with a channel angle of 90°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first and fourth pass of the process. Analysis of the first pass sample revealed heterogeneous structure with a mixture of elongated and refined equi-axed grains. Moreover, existence of {101 ̅2} tensile twinning in the microstructure was also observed. Remarkable refinement was achieved after fourth pass and ultrafine-grain (UFG) structure was successfully achieved. Room temperature tensile tests carried out on unprocessed and UFG material, display the improvement in strength. The yield strength of the processed material was increased from 308 to 671 MPa and the ultimate tensile strength from 549 to 730 MPa. However, strain-hardening ability of the material was greatly reduced because of processing. Consequently, the material suffers loss in ductility, from 31.9% elongation to failure in the unprocessed form to 21.1% in UFG form. Finally, fracture morphology of the unprocessed and processed CP-Ti displays characteristics of ductile failure. It has been shown that I-ECAP is an effective method for improving strength characteristics of CP-Ti

Effect of incremental equal channel angular pressing (I-ECAP) on the microstructural characteristics and mechanical behaviour of commercially pure titanium

Incremental equal channel angular pressing (I-ECAP) is one of the continuous severe plastic deformation (SPD) processes. This paper presents the processing of commercial purity titanium (CP-Ti) using a double billet variant of I-ECAP process. Ultrafine-grain (UFG) structure was successfully achieved after six passes of I-ECAP at 300 °C. Microstructural evolution and texture development were tracked using EBSD. Analysis revealed continuous dynamic recrystallization (CDRX) as one of the grain refinement mechanism during processing. Room temperature tensile tests carried out before and after six passes, shows significant increase in strength with acceptable levels of ductility. The yield strength was increased from 308 to 558 MPa and ultimate tensile strength from 549 to 685 MPa. Compression tests conducted at different strain rates shows considerable increase in strength and enhanced strain rate sensitivity after processing. A distinct three-stage strain hardening was observed during compression. However the processed material displayed a loss in strain hardening ability during tensile as well as in compression tests. Detailed microhardness measurements show the evolution of hardness after subsequent passes with a reasonable level of homogeneity after the sixth pass. It is demonstrated that I-ECAP is an effective method for grain refinement in CP-Ti and subsequently improving its mechanical properties

In situ analysis of the influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy

The influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy was studied in this work by in situ microstructural analysis during tensile testing in a chamber of scanning electron microscope. Three types of samples used in this study were obtained by (1) extrusion (as-supplied), (2) I-ECAP and (3) I-ECAP followed by side upsetting. Microstructures, textures and mechanical properties were examined after each processing step. An analytical equation was used to describe flow stress curves of the samples which exhibited various modes of deformation (1) only by slip, (2) dominated by tensile twinning followed by slip and (3) dominated by contraction twinning followed by slip. It was shown that tensile twinning increases strain hardening rate, while the opposite is observed for contraction twinning. The effective Schmid factors for slip in volumes deformed by tensile and contraction twinning were determined in this work using modelling approach as 0.215 and 0.45, respectively. Contraction twinning was also revealed to be responsible for an earlier fracture of the extruded sample subjected to tension, since microcracking was shown explicitly to be initiated within twins

The origin of fracture in the I-ECAP of AZ31B magnesium alloy

Magnesium alloys are very promising materials for weight-saving structural applications due to their low density, comparing to other metals and alloys currently used. However, they usually suffer from a limited formability at room temperature and low strength. In order to overcome those issues, processes of severe plastic deformation (SPD) can be utilized to improve mechanical properties, but processing parameters need to be selected with care to avoid fracture, very often observed for those alloys during forming. In the current work, the AZ31B magnesium alloy was subjected to SPD by incremental equal-channel angular pressing (I-ECAP) at temperatures varying from 398 K to 525 K (125 °C to 250 °C) to determine the window of allowable processing parameters. The effects of initial grain size and billet rotation scheme on the occurrence of fracture during I-ECAP were investigated. The initial grain size ranged from 1.5 to 40 µm and the I-ECAP routes tested were A, BC, and C. Microstructures of the processed billets were characterized before and after I-ECAP. It was found that a fine-grained and homogenous microstructure was required to avoid fracture at low temperatures. Strain localization arising from a stress relaxation within recrystallized regions, namely twins and fine-grained zones, was shown to be responsible for the generation of microcracks. Based on the I-ECAP experiments and available literature data for ECAP, a power law between the initial grain size and processing conditions, described by a Zener–Hollomon parameter, has been proposed. Finally, processing by various routes at 473 K (200 °C) revealed that route A was less prone to fracture than routes BC and C

The role of microstructure and texture in controlling mechanical properties of AZ31B magnesium alloy processed by I-ECAP

Abstract Mechanical properties of AZ31B magnesium alloy were modified in this work by various processing routes of incremental equal channel angular pressing (I-ECAP) followed by heat treatment. Possible strategies for improving ductility and strength of the alloy were investigated. Processing by routes A and BC showed that texture plays predominant role in controlling mechanical properties at room temperature. Four passes of I-ECAP by route C followed by annealing enhanced ductility up to 0.35 of true strain. It was found that tensile twinning was important in accommodating strain during tensile testing, which resulted in a very good hardening behaviour. The yield strength was improved to 300 MPa by refining grain size to 0.8 µm in I-ECAP at 150 °C. The obtained structure and properties were shown to be stable up to 150 °C. True strain at fracture was increased to 0.2 after annealing at 150 °C without lowering strength

CHARACTERIZATION OF THE GRAINS IN 2014 ALUMINIUM ALLOY AFTER EQUAL CHANNEL ANGULAR EXTRUSION (ECAE) PROCESS

In 2014 alloy deformed by Equal Channel Angular Extrusion process (ECAE) the changes in the size and shape of structural constituents were examined. The samples subjected after deformation to additional annealing at 300°C/10min were characterized by larger grains of nearly-equiaxial shapes. The microstructure after deformation was composed of a large number of the mutually crossing bands and microbands. The intersection of microbands resulted in formation of rectangular and rhombohedral grains. It was noted that the average grain size after ε = 4.6 (4 passes) was 0.2 μm

Microstructure, Mechanical Properties, and Corrosion Resistance of Thermomechanically Processed AlZn6Mg0.8Zr Alloy

The paper presents results of the investigations on the effect of low-temperature thermomechanical treatment (LTTT) on the microstructure of AlZn6Mg0.8Zr alloy (7000 series) and its mechanical properties as well as electrochemical and stress corrosion resistance. For comparison of the LTTT effect, the alloy was subjected to conventional precipitation hardening. Comparative studies were conducted in the fields of metallographic examinations and static tensile tests. It was found that mechanical properties after the LTTT were better in comparison to after conventional heat treatment (CHT). The tested alloy after low-temperature thermomechanical treatment with increasing plastic deformation shows decreased electrochemical corrosion resistance during potentiodynamic tests. The alloy after low-temperature thermomechanical treatment with deformation degree in the range of 10 to 30% is characterized by a high resistance to stress corrosion specified by the level of PSCC indices