Effect of commercial purity levels on the mechanical properties of ultrafine-grained titanium

Two grades of commercial purity (CP) titanium (grades 2 and 4) were processed using equal-channel angular extrusion (ECAE) at 300 °C and 450 °C, respectively. The processing temperatures were the minimum temperatures at which eight pass ECAE could be performed without any shear-localization. The coarse-grained (CG) microstructures of as-received grade-2 and grade-4 CP-Ti, with average grain sizes of 110 μm and 70 μm, respectively, were refined down to sub-micron levels with a mean grain size of about 300 nm for both grades after 8 ECAE passes. The ultrafine-grained (UFG) microstructures led to substantial enhancement in strength for both grades. The grade-2 sample showed a more than two fold increase in yield strength (σ_y), from 307 MPa for the as-received one to about 620 MPa for the processed samples. The grade-4 CP-Ti exhibited a relatively smaller increase in strength due to the higher processing temperature, and it showed about 50% increase in σ_y after eight pass ECAE, from 531 to 758 MPa. These strength levels were obtained with high ductility levels of 21% and 25% for UFG grade-2 and grade-4 Ti, respectively. These improvements in mechanical properties are attributed to the substantially refined grain size and increased dislocation density. Grade-4 Ti is stronger than grade-2 because of the higher oxygen content. The higher ductility and significantly higher strain hardening capability of UFG grade-4 Ti, in spite of the similar grain size and microstructure with UFG grade-2 Ti, is also due to the higher impurity content, probably resulting in a higher dislocation storage capability during room temperature deformation, and thus, higher strain hardening capacity. Such properties make UFG grade-4 Ti comparable to the commercial Ti-6Al-4V alloy for biomedical applications without negative effects of the alloying elements on biocompatibility.     

Microstructure and mechanical properties of Mg–Al–Zn alloy sheets severely deformed by accumulative roll-bonding

The as-rolled AZ31 Mg alloy sheets were subjected to accumulative roll-bonding (ARB) at 300 °C up to three cycles. The microstructure and macro- texture are investigated by means of optical microscopy and X-ray analysis. The mechanical properties are evaluated by micro-hardness and tensile tests. Very fine grain size of 2.4 μm could be achieved after three passes of 50% thickness reduction. The recrystallized structure was already formed after one cycle of ARB. ARB processing resulted in a significant increase of ductility and slight decrease of tensile strength of the AZ31 alloy sheet. Basal texture was notably weakened after ARB processing.     

Workability characteristics and mechanical behavior modeling of severely deformed pure titanium at high temperatures

In the present study, compression tests were performed at temperatures of 600–900 °C and at strain rates of 0.001–0.1 s⁻¹ to study the deformation and workability characteristics of commercially pure titanium after severe plastic deformation (SPD). It was found that the effects of temperature and strain rate are significant in dictating the steady state flow stress levels and the strain values corresponding to peak flow stress. The strain rate sensitivity (m) during hot compression of severely deformed Ti was shown to be strongly temperature dependent, where m increased with the increase in deformation temperature up to 800 °C. High temperature workability was analyzed based on the flow localization parameter (FLP). According to the FLP values, deformation at and below 700 °C is prone to flow localization. The flow behavior was predicted using Arrhenius type and dislocation density based models. The validities of the models were demonstrated with reasonable agreement in comparison to the experimental stress–strain responses.     

Influence of mechanical surface treatments on fatigue strength of commercial purity titanium

A comparative study of two mechanical surface treatments of shot peening (SP) and cold rolling (CR) on fatigue strength of commercial purity titanium has been conducted. The treated surface was characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and surface roughness measurements. Experimental result shows that SP and CR increased the fatigue strength of commercial purity titanium, and moreover, the following results were obtained: (1) The improvement of fatigue strength is related to the formation of deformation twins in strengthened layer. Before or after fatigue, the samples strengthened by SP or CR not only have twin shape and number change, but also have twin interactions in the SP and twin-grain boundary interactions in the CR. (2) XRD measurement demonstrated that SP leads to surface compressive residual stress are much higher than those after CR. Surface compressive residual stress has higher relaxation in the SP than in the CR condition during cyclic loading, then the surface compressive residual stress has the same values after fatigue deformation. (3) Surface roughness resulting from SP is ten times of CR.     

Effects of Si addition on mechanical properties of copper severely deformed by accumulative roll-bonding

Effects of Si addition on mechanical properties of severely deformed copper by accumulative roll-bonding (ARB) have been investigated. Tensile tests and strain-rate jump tests have been carried out at room temperature. For annealed coarse-grained polycrystals, the difference in yield stress σ _0.2 between pure copper and a Cu–1.64at.%Si alloy was only about 15 MPa while the difference became 170 MPa after the ARB process by six cycles. The strain-rate sensitivity m of pure copper increased with increasing the number N of the ARB cycles for N ≥ 5. However, the increase in m becomes less significant for Cu–Si alloys. These findings have been discussed in terms of thermally activated dislocation processes.     

Assessment of mechanical behaviour,microstructure and critical conditions of welded commercial purity titanium

An assessment of microstructure and mechanical properties of weldments in rolled and forged versions of commercially pure titanium has been undertaken. A comparison is also made between tungsten inert gas (TIG) and electron beam (EB) weldment properties.Fracture and fatigue behaviour of weldments have been investigated experimentally by fracture toughness and fatigue crack growth studies employing notched three point bend specimens. The implications of the data produced are expressed in a design philosophy which enables an estimate of critical defect size to be made based on either stable or unstable crack growth or plastic collapse. The conjoint action of fatigue and stable crack growth is also considered.     

Improvement of mechanical properties in severely plastically deformed Ni–Cr alloy

This study was carried out to evaluate the grain refining and mechanical properties in alloys that undergo severe plastic deformation (SPD). Conventional rolling (CR) and cross-roll rolling (CRR) were introduced as methods for SPD, and a Ni–20Cr alloy was selected as the experimental material. The materials were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. The annealed materials after cold rolling were assessed through electron backscattered diffraction (EBSD) analysis to investigate the grain boundary characteristic distributions (GBCDs). The CRR process was more effective than the CR process in developing grain refinement; the grain size decreased from 70 μm in the initial material to 4.2 μm (CR) and 2.4 μm (CRR), respectively. The grain refinement affected mechanical properties such as microhardness, yield, and tensile strength, which were significantly increased relative to the initial material.     

Evolution of ultrafine microstructures in commercial purity aluminum heavily deformed by torsion

Commercial purity aluminum (1100Al) bars were severely plastic deformed by torsion deformation at room temperature. The specimens were deformed to ultrahigh equivalent strain of 5.85 in maximum. Microstructure evolution during the torsion deformation was characterized using electron back scatter diffraction analysis on two different sections: the longitudinal section parallel to the torsion axis and transverse section perpendicular to the torsion axis. The grain size decreased and the fraction of high angle grain boundary increased with increasing equivalent strain. Elongated ultrafine grained structure was obtained after an equivalent strain of 3.27. We have found that the microstructure evolution in the specimen deformed by torsion exhibited similar behavior to those in the same material heavily deformed by accumulative roll bonding. The average grain size of 0.32 μm with the high angle boundary fraction of 0.76 was achieved in the specimen deformed to an equivalent strain of 5.27. Though the microstructure and hardness on the transverse section varied depending on the radial positions, they could be arranged as a simple function of equivalent strain. The present work confirmed that the torsion deformation worked as a kind of severe plastic deformation.     

Significant improvement of semi-solid microstructure and mechanical properties of A356 alloy by ARB process

Accumulative roll bonding (ARB) process was used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform A356 alloy. It was found that when the number of ARB cycles was increased, the uniformity of silicon particles in the aluminum matrix improved, the particles became finer and spheroider and therefore, the tensile strength (TS) and ductility of the samples improved. The microstructure of the manufactured A356 alloy after five ARB cycles indicated a totally modified structure such that it's TS and elongation values reached 269 MPa and 5.3% which were 2.6 and 2.5 times greater than those of the as-cast material, respectively. Also, the hardness value increased from 55.4 (for as-cast sample) to 100.2 HV (after the fifth cycle of ARB), and registered 81% increase.     

Microstructure and mechanical properties of a high nitrogen titanium alloy

High nitrogen titanium alloy with the chemical composition of Ti–4%Cr–0.6%N was produced by solution nitriding to nitrogen-free Ti–4%Cr alloy, and then its microstructure was controlled to have fine (α + β) dual phase structure by aging treatment to improve the ductility. As solution-nitrided specimen has a platelet hcp-martensitic structure (α′) and is characterized by hard but brittle nature that has been produced by solid solution of 0.6% of nitrogen. On the following aging treatment, fine β phase formed along the original plate boundaries, resulting in a fine (α + β) dual phase microstructure. X-ray and EELS analyses revealed that nitrogen is greatly concentrated in the tempered α′ phase. Although the hardness of as-quenched material gradually decreases during the aging treatment with increasing volume fraction of β, the hardness can be kept much higher than that of the aged Ti–4%Cr alloy without nitrogen. As a result of tensile testing, it was found that the aged Ti–4%Cr–0.6%N alloy has high tensile strength over 1 GPa with moderate ductility.     

Microstructure and mechanical properties of brazed titanium/steel joints

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler.     

The corrosion behaviour of commercial purity titanium processed by high-pressure torsion

Commercial purity (CP) titanium was processed by high-pressure torsion (HPT) under an applied pressure of 6.0 GPa for different numbers of torsional revolutions and then exposed to a 3.5 % NaCl solution for open-circuit potential measurements followed by electrochemical impedance spectroscopy and potentiodynamic polarization tests. The electrochemical results exhibit a complicated relationship between the corrosion resistance and grain refinement. Thus, microhardness measurements reveal an improvement in hardness for CP titanium after processing by HPT but the corrosion resistance is lower in the NaCl solution than for the annealed coarse-grained Ti. It is shown that the corrosion susceptibility of the HPT-processed samples decreases with increasing torsional strain. The effect of grain size and microstructure on the corrosion properties of ultrafine-grained CP Ti is also examined.     

Microstructure and mechanical properties of SiCp/AZ91 composite deformed through a combination of forging and extrusion process

In this paper, 10 vol.% SiCp/AZ91 magnesium matrix composites were fabricated by stir casting technology. The as-cast ingots were forged at 420 °C with 50% reduction, and then extruded at 370 °C with the ratio of 16 at a constant ram speed of 15 mm/s. The results showed that the grains were refined during forging. A much finer grain size (∼2.7 μm) of composite matrix was obtained by subjecting the as-forged composite to hot extrusion. The fine SiC particulates restricted the dynamic recrystallized grain growth during the hot extrusion processing, resulting in a remarkable grain refinement. The yield stress and ultimate tensile stress were increased in the as-extruded composite, with the reasons of eliminated casting flaws, the uniform particle distribution and grains refinement. The grain refinement and uniform particle distribution caused an obvious increase in work hardening rate in the as-extruded composite during tensile deformation at room temperature.     

室温ECAP和冷轧复合变形工业纯钛的组织和性能

采用ECAP技术和常规冷轧复合变形工艺制备了高强度工业纯钛,研究了复合变形后试样的力学性能与显微组织的关系.结果表明,工业纯钛经室温单道次ECAP和冷轧复合变形后,晶粒被严重拉长,形成了明显的纤维状组织,试样的抗拉强度高达805MPa;随着冷轧变形量的增大,变形组织的细化程度和均匀性提高,使试样的强度和塑性进一步提高.位错滑移和孪生是工业纯钛室温ECAP和冷轧复合变形的主要变形机制.     

Enhanced grain refinement and mechanical properties in a severely plastic deformed Ni-30Cr alloy

The present study was carried out to evaluate the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) was introduced as a severe plastic deformation (SPD) process. Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the recrystallized microstructure. Electron back-scattered diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy effectively enhanced grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33 μm in the initial material to 0.6 μm. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement in mechanical properties in terms of the effective strain imposed by CRR relative to conventional rolling (CR).     

Ductile fracture of commercial purity titanium at room temperature

An experimental and numerical program was carried out to examine and assess the deformation and fracture behaviour of alloys of commercial purity (CP) titanium. The material rate-dependent deformation under constant displacement rates and under sustained loads was directly simulated in finite element analyses using an implemented unified material model. The simulations predicted the fracture of compact tension specimens subjected to J – R tests and sustained load tests employing a dimensional analysis and strain-hardening approach. Differences between two batches with different oxygen contents were examined and the limitation of the material model was investigated.     

Mechanism of cube grain nucleation during recrystallization of deformed commercial purity aluminium

Cube texture is a sharp recrystallization texture component infcc metals like aluminium, copper, etc. It is described by an ideal orientation i.e. (100) (100). The subject of cube texture nucleation i.e. cube grain nucleation, from the deformed state of aluminium and copper is of scientific curiosity with concurrent technological implications. There are essentially two models currently in dispute over the mechanism of cube grain nucleation i.e. the differential stored energy model founded on the hypothesis proposed by Ridha and Hutchinson and the micro-growth selection model of Dugganet al. In this paper, calculations are made on the proposal of Ridha and Hutchinson model and the results are obtained in favour of the differential stored energy model. It is also shown that there is no need for the micro-growth model.     

Microstructure and mechanical properties of commercial, bronze-bond, diamond-abrasive tool materials

Bronze-matrix, diamond-particle composites are commonly employed as tool materials in grinding applications, in particular for precision grinding of optical materials. However, while tool selection and performance is often rationalized in terms of changes in tool “modulus/stiffness” or “hardness,” neither the range and variability of the mechanical properties nor the fundamental microstructural parameters controlling them are well understood. This has hindered quality control and prevented the accumulation of the industrial property-performance data essential to the development of improved tool materials and processes. In this study, bronze-bond diamond-abrasive composite tool materials, with systematic differences in their diamond sizes and concentrations, were obtained from different commercial vendors, characterized mechanically and microstructurally, and the results statistically analyzed. Microstructurally, a size dependent diamond distribution and relatively high levels of porosity (~10 vol%) were observed. The mechanical properties exhibited a high degree of variability, with statistically significant differences occurring based on the vendor and diamond concentration, but not diamond size. Porosity was shown to be the key microstructural parameter controlling mechanical properties. Porosity depended on vendor, diamond size, and diamond concentration, but large, nonsystematic variations were also observed, indicating that it is not consistently controlled in current commercial materials. Finally, the porosity and mechanical properties were shown to correlate strongly with ultrasonic wave speed over a wide range of tool materials, demonstrating a practical nondestructive method for tool characterization.