As-extruded Bar Research Articles

The Effect of Heat Treatments on Microstructure and Mechanical Properties of As-Extruded Ti-6Al-4V Alloy Rod from Blended Elemental Powders

In this study, Ti-6Al-4V bars were first prepared by extrusion of powder compacts from blended powder mixtures in the beta phase region, then the as-extruded bars were heat-treated following four different conditions: beta quenching and aging (βQA), broken up structure (BUS) treatment, solution treatment and aging (STA) and recrystallization annealing (RA). The effect of the heat treatments on microstructure and mechanical properties was studied using optical microscopy, scanning electron microscopy, and mechanical test to determine which heat-treatment condition has the greatest impact on the mechanical properties of the as-extruded Ti-6Al-4V alloy. The results show that the as-extruded condition has the best balance of strength (1120 MPa of UTS) and ductility (11% of elongation to failure). βQA and STA lead to a slight increase in strength but ductility decreases considerably. After BUS and RA treatments, both strength and ductility are reduced. The relationship between processing, microstructure and properties was studied, and their implications towards fatigue behaviour and fracture toughness were discussed.

Texture evolution and microcracking mechanisms in as-extruded and cross-rolled conditions of a 14YWT nanostructured ferritic alloy

Cr-stabilized nanostructured ferritic alloys (NFAs), dispersion strengthened by an ultra-high density of nanooxides, are attractive candidates for many nuclear energy applications due to their high-temperature strength, in-service stability and remarkable irradiation tolerance. However, typical NFA deformation processing paths lead to crystallographic texturing, formation of brittle microstructures and low toughness orientations, making fabricating components very difficult. Here, we characterize the dislocation-mediated deformation mechanisms that lead to the brittle texture component. The as-extruded bar is less brittle than the cross-rolled plate, which contains a large population of pre-existing cleavage microcracks. More generally, deformed ODS/NFAs are most often textured and have anisotropic low toughness orientations, even absent microcracks. However, cross-rolling produces a very high volume fraction of a plate normal {001}<110>-texture component, which constitutes the brittle cleavage system in iron. Microcracks propagate along {001} low angle deformation induced subgrain boundaries in <110> directions after nucleating by the Cottrell mechanism.

Study on the mutual effect of La and Gd on microstructure and mechanical properties of Mg-Al-Zn extruded alloy

The effects of rare earth elements (La, Gd) addition on the microstructure and the mechanical properties of as-extruded Mg-Al-Zn magnesium alloy under conditions of a medium temperature (230 °C) and a slow ram speed (0.1 mm/s) have been throughly investigated. The results show that the as-extruded bar presents smooth surface from the macroscopic view due to the fact that many non-basal <a> dislocations and few pyramidal 〈c + a〉 dislocations could be activated to enhance the formability in the extrusion process. A bimodal grain structure composed of fine dynamic recrystallized (DRXed) grains with globular Mg17Al12 precipitates located at grain boundaries and non-recrystallized (non-DRXed) coarse grains with thin continuous precipitation laths inside is generated in the alloy. More dislocation pile-ups and subgrain boundaries are exhibited in the DRXed region which shows a typical texture of extruded alloys containing rare earth elements. The modified extruded alloy shows much higher strength than that of as-extruded AZ80 under either tension or compression condition owing to grain refinement strengthening, precipitation strengthening, solid-solution strengthening, dislocation strengthening and subgrain strengthening. The finer DRXed grain size and the fine precipitates in the modified extruded alloy bring about its lower yield anisotropy. In addition, large and fragile Al2Gd and Al8Mn4Gd phases mainly give rise to the decrease in ductility of the modified extruded alloy.

Recycling of AZ40 Magnesium Alloy Scraps by Hydriding-Dehydriding and Subsequent Consolidation Processing

The hydriding-dehydriding process was used to recycle AZ40 magnesium (Mg) alloy scraps, and the microstructure nanocrystallization was realized. X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were carried out to characterize the microstructure. After mechanically milling in hydrogen for 72 h, matrix Mg was completely turned into nanocrystalline MgH2, with an average crystallite size of about 10 nm. And then, the MgH2 phase was completely transformed into Mg again through vacuum dehydriding treatment at 300 °C for 192 min, with an average crystallite size of about 20 nm. In addition, the nanocrystalline alloy powders were hot-pressed and extruded into bars. The average grain size of the bars was about 500 nm, which had reached the size of ultrafine-grain. Meanwhile, the yield strength and ultimate tensile strength of the as-extruded bars reached about 312 and 497 MPa, respectively. The results indicate that hydriding-dehydriding process is a feasible method for recycling of Mg alloy scraps, and it is expected to have a good application prospect in preparing ultrafine-grain Mg alloys.

Effect of the extrusion ratio on the mechanical properties of as-forged Mg-8Al-0.5Zn alloy

This paper demonstrates the effect of the extrusion ratio on the mechanical properties of the as-forged Mg-8Al-0.5Zn alloy. Three extruded Mg bars with the extrusion ratio of 9:1, 16:1, and 25:1 are prepared in the same extrusion condition, which are applied to the warm forging test to obtain the different microstructure with respect to the degree of extrusion effect in Mg alloys. The extrusion ratio over 16:1 has little effect on the mechanical properties of the as-extruded bars since the average grain size and amount of precipitates hardly change. The forgeability of the highly extruded bar is improved around flange area while the mechanical properties are slightly enhanced. The rod-type and particle-type precipitates tend to be favored in the as-forged microstructure with the extrusion ratio over 16:1.

Origin of the reversed yield asymmetry in Mg-rare earth alloys at high temperature

The mechanical behaviour in tension and compression of an extruded Mg–1wt.% Mn–1wt.% Nd (MN11) alloy was studied along the extrusion direction in the temperature range −175°C to 300°C at both quasi-static and dynamic strain rates. Microstructural analysis revealed that the as-extruded bar presents a recrystallized microstructure and a weak texture that remain stable in the whole temperature range. A remarkable reversed yield stress asymmetry was observed above 150°C, with the compressive yield stress being significantly higher than the tensile yield stress. The origin of this anomalous reversed yield stress asymmetry, which to date remains unknown, was investigated through the analysis of the macro and microtexture development during deformation, as well as by means of crystal plasticity finite element simulations of a representative volume element of the polycrystal. The critical resolved shear stresses of slip and twining for simulated single crystals were obtained as a function of the temperature by means of an inverse optimisation strategy. Experimental and simulation results suggest that the reversed yield asymmetry may be primarily attributed to the non-Schmid behaviour of pyramidal 〈c+a〉 slip, which is the dominant deformation mechanism at high temperatures. It is proposed, furthermore, that the asymmetry is enhanced at quasi-static strain rates by the stronger interaction of 〈c+a〉 dislocations with the diffusing solute atoms and particles in compression than in tension.

Control of the Mechanical Asymmetry in an Extruded MN11 Alloy by Static Annealing

An extruded Mg-1 wt pct Mn-1 wt pct Nd (MN11) alloy with a recrystallized microstructure and a weak texture was subjected to different thermal treatments at temperatures ranging from 548 K to 673 K (275 °C to 400 °C) for time intervals between 1 and 45 hours. Room-temperature mechanical tests were carried out in tension and compression at 10−3 s−1 in order to investigate the effect of annealing on the mechanical behavior. Microstructural examinations revealed that both the annealing temperature and time have little effect on the grain size and on the texture, which are mainly controlled by the presence of thermally stable Mn-containing particles and by the segregation of Nd to the grain boundaries. However, the composition and distribution of the Nd-containing particles vary significantly for the different annealing conditions. The annealed bars exhibit a subtle change in the tensile and compressive yield stress relative to the as-extruded bar and a somewhat larger mechanical asymmetry. The present results suggest that the Nd-containing phases, as well as Nd solute atoms, play a significant role in the mechanical behavior of the MN11 alloy by changing the relative critical resolved shear stress of the different deformation modes.

Effect of Extrusion Temperatures on Microstructures and Mechanical Properties of Mg-3Zn-0.2Ca-0.5Y Alloy

After solution treatment, the Mg-3Zn-0.2Ca-0.5Y alloy was subjected to extrusion at various temperatures and extrusion defects, microstructures and mechanical properties of as-extruded samples were studied in detail. The results show the extrusion temperature has a slight effect on DRXed grain size due to the presence of Y-containing phase with a good thermal-stability. On the contrary, the mechanical properties of as-extruded alloys are remarkably influenced by dynamic recrystallization degrees promoted by extrusion temperatures. A good formability of as-extruded bars with an optimal mechanical properties and fine-grained microstructure was obtained by extrusion at 250°C and the mechanical performances are significantly superior to those of hot-rolled ZK60 obtained at the same temperature. In this paper, the magnesium alloy with fine-grained (<5μm), high ductility (>30%) and high strength (>250MPa) was fabricated easily by a simple extrusion process.

Microstructures and mechanical properties of pure magnesium bars by high ratio extrusion and its subsequent annealing treatment

Pure magnesium bars were prepared by two-pass cumulative high ratio extrusion with as-cast Mg as the original material and then the final as-extruded bars were annealed. The effect of extrusion deformation and annealing treatment on the microstructure, mechanical properties and fracture behaviour of the Mg were investigated by optical microscopy (OM), mechanical properties test and scanning electron microscopy (SEM), respectively. The results show that the grain size is obviously refined by the effect of dynamic recrystallization during the extrusion deformation. Thus the room-temperature mechanical properties and fracture behaviour of the material were significantly improved. After the first extrusion, the coarse as-cast grain size was reduced to 35 μm, and the yield strength (YS), ultimate tensile strength (UTS) and elongation of the bar achieved 84 MPa, 189 MPa, and 12%, respectively. After the further extrusion, the YS of as-extruded bar was over 120 MPa; however, the elongation decreased due to work hardening. Finally the grain size of the as-extruded bar was 9-10 μm after annealing treatment, and its YS, UTS and elongation of the bar achieved 124 MPa, 199 MPa, and 10.7%, respectively. The microstructures and mechanical performance of the material were enhanced obviously.

Elevated-temperature plasticity and mechanical properties of a rare earth-modified Mg–Zn–Al alloy

The elevated-temperature plasticity and flow behavior of an Er-modified, heat-resistant ZA73 alloy was evaluated by thermal simulation. The results showed that the addition of Er to ZA73 alloy notably improves the deformability and higher strain rate and temperature favors hot deformation. Bars with sound surface quality were successfully extruded at 350 °C and a strain rate of ~ 0.1 s − 1 . Furthermore, dynamic precipitation of nano-sized spherical τ phase was found to occur uniformly in the α-Mg matrix during hot extrusion, which is considered helpful to both strength and plasticity enhancement. The yield strength and ultimate tensile strength of the as-extruded bars reached 240–265 MPa and 355–360 MPa, respectively, while maintaining a large elongation rate of 18–19.5%.

Microstructural evolution during the annealing of an extruded AZ31 magnesium alloy

Microstructural evolution during the annealing of AZ31 extruded rod at 400 °C has been examined by employing neutron diffraction and electron backscatter diffraction (EBSD). In the as-extruded bar, equiaxed grains smaller than 5 μm and large elongated grains having significant degrees of internal misorientation are oriented mainly with < 1 0 1 ¯ 0 > parallel to the extrusion direction. Rapid grain growth occurs within the 180 s annealing period at 400 °C at the expense of the small grains with the internal orientation gradient as driving force. After short time annealing, small equiaxed grains are formed inside the large elongated grains, and grains having < 1 1 2 ¯ 0 > parallel to the extrusion direction show preferred growth. As a result, a transition of the main texture component to the < 1 1 2 ¯ 0 > component occurs after annealing for 1800 s at 400 °C.

Effect of annealing temperatures on the secondary re‐crystallization of extruded PM2000 steel bar

The ferritic oxide dispersion-strengthened alloy PM2000 is an ideal candidate for high-temperature applications as it contains uniform nano-oxide dispersoids, which act as pinning points to obstruct dislocation and grain boundary motion and therefore impart excellent creep resistance. The development of the microstructure during re-crystallization of oxide dispersion-strengthened alloys has been discussed by a number of authors, but the precise mechanism of secondary re-crystallization still remains uncertain. Hence, this work is aimed at investigating the re-crystallization behaviour of extruded PM2000 bar for different annealing temperatures, using electron backscatter diffraction, in particular, to determine grain orientations, grain boundary misorientation angles, etc. The results show that the as-extruded bar microstructure comprises both low-angle grain boundaries pinned by oxide particles and high-angle boundaries that will have inherent boundary mobility to allow boundary migration. In addition, dynamical re-crystallization was found in the outer region of the non-heat-treated PM2000 bar, which suggested that deformation heterogeneities can be introduced during thermo-mechanical processing that enhance the nucleation of re-crystallization. Subsequent heat treatments promote and stimulate secondary re-crystallization, giving rise to large grains with few sub-grain boundaries.

Microstructural evolution of Al–Zn–Mg–Cu–(Sc) alloy during hot extrusion and heat treatments

The microstructural evolution during hot extrusion and post heat treatment was investigated for two kind of Al–Zn–Mg–Cu–(Sc) alloy and AA7075. The microstructure of as-extruded bar is mainly comprised of recovered structure for all alloys, however, different restoration processes are observed during post heat treatment. For AA7075 and S1, which contains 0.1% Sc with relatively higher Zn and Cu content than S2, recovery still proceeds during the heat treatment, while the recrystallization becomes main restoration process during the heat treatment for S2.The differences in abnormal grain growth and hardening behavior between S1, AA7075 and S2 during the heat treatment are discussed in connection with the restoration processes and resultant microstructures.

Microstructure-property relations in as-extruded ultrahigh-carbon steels

The microstructure and mechanical properties of as-extruded ultrahigh-carbon steels (UHCSs) are described. Extrusion ratios of 16:1 reduction were used at extrusion temperatures of 900 °C, 1025 °C, and 1150 °C. Air cooling followed the extrusion process. The compositions of the four UHCSs investigated ranged from 1.2 to 1.8 pct C and contained either aluminum or silicon and/or chromium as the principal alloying elements. The as-extruded microstructures were predominantly pearlitic. Detailed examination of the microstructures allowed determination of the composition and extrusion temperature at which grain-boundary carbide networks and graphite can be avoided. It is shown that the yield strength of the as-extruded bars, for a given UHCS alloy composition, correlates well with the interlamellar spacing, independent of the extrusion temperature. Ultimate tensile strengths ranged from 1000 to 1690 MPa, and elongations to failure ranged from 3 to 12.5 pct. The present results indicate that single-step mechanical working of UHCSs to achieve fully pearlitic structures, with no postworking heat treatment, can lead to a new class of ultrahigh-strength steels.