Combined Effects Of Grain Refinement Research Articles

Fabrication of Strong and Ductile AZ31 Magnesium Alloy Using High Strain Rate Multiple Forging in a Wide Temperature Range

High strain rate multiple forging (HSRMF) was successfully carried out on AZ31 magnesium alloy at a temperature range of 250–400 °C, and the microstructure, texture and mechanical properties were examined. Full recrystallized structure developed at a relatively lower strain due to the twining induced dynamic recrystallization (TDRX) mechanism, which is also responsible for the feasibility of HSRMF deformation at relative low temperature. The average grain sizes of the alloys high strain rate multiple forged (HSRMFed) to the accumulated strain of ∑Δε = 1.32 increased from 7.07 to 9.99 μm as the temperature ranged from 250 to 400 °C, i.e., the grain sizes of the HSRMFed alloy were less sensitive to temperature. The weakened basal texture characteristic of titled or double peak achieved was ascribed to the alteration of forging direction. The HSRMFed alloys demonstrated both excellent strength (UTS > 300 MPa) and good ductility (δ > 20%), which resulted from the combined effects of grain refinement and weakened basal texture. Therefore, HSRMF was an efficient technique to produce strong and ductile wrought AZ31 alloy.

Microstructure, solidification behavior and mechanical properties of Al-Si-Mg-Ti/TiC fabricated by selective laser melting

Ball-milled Ti/TiC composite particles (TiC nanoparticles assembled on Ti microparticles) were designed, prepared, and mixed with Al-Si-Mg powder to fabricate an Al-Si-Mg-Ti alloy with TiC nanoparticles (Al-Si-Mg-Ti/TiC) by selective laser melting (SLM). Microstructure features, solidification behavior, and mechanical properties were investigated, and the relationship among them was established. The SLM-manufactured Al-Si-Mg-Ti/TiC material exhibited fine equiaxed-shaped α(Al) grains with nanoscale Si4Ti5 phases and Mg segregation along the grain boundaries. This structure benefited from heterogeneous nucleation as well as the grain growth restriction capabilities of TiC nanoparticles on α(Al), fast diffusion of Ti in the superheated Al liquid, and high chemical activity of Ti to Si during solidification. Furthermore, Ti enrichment in some local areas of the high-temperature pool and the consequently intense Marangoni convection improved the wettability between TiC nanoparticles and liquid Al without the interfacial reaction. Consequently, the SLM-manufactured Al-Si-Mg-Ti/TiC showed a high ultimate tensile strength of up to 562 ± 7 MPa and an elongation of up to 8.8 % ± 1.3 % before fracture. These increased mechanical properties are attributed to the combined effect of grain refinement and Orowan and load-bearing strengthening mechanisms.

Microstructure characterization and tensile properties of processed TC17 via high energy shot peening

The microstructure and tensile properties of the processed TC17 via high energy shot peening (HESP) were investigated, in which special attention was paid to reveal the fracture behaviour of the HESP processed surface layer. The results indicated that the severe plastic deformation (SPD) zone with the nanograins appeared in the HESP processed surface layer of TC17, in which the SPD thickness increased with the increasing of air pressure while it increased firstly and then decreased with the prolonging of peening duration, and the finer nanograins could be obtained at the higher air pressure and longer peening duration. Due to the generation of the gradient structure layer, the HESP processed TC17 had ruptured in a hybrid fracture. The fracture features corresponding to nanograined structure, ultrafine-grained structure and coarse-grained structure were the brittle fracture, the mixed fracture and the ductile fracture respectively. Comparing with the properties of the annealed TC17, the HESP processed TC17 possessed a better strength-ductility balance owing to combined effects of grain refinement, high-density dislocations, work hardening and compressive residual stress (CRS), in which the yield strength (YS) and the ultimate tensile strength (UTS) were increased by 10.1% and 13.9%, respectively.

Consequence of reinforced SiC particles and post process artificial ageing on microstructure and mechanical properties of friction stir processed AA7075

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material. Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy, electron backscattered diffraction and X-Ray diffraction. Hardness, impact and wear tests were carried out to investigate mechanical behaviour before and after processing. Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement, Zener pinning, dispersion strengthening and precipitation hardening. Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6. Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test.

Enhancement of strength and ductility of SiCp/AZ91 composites by RD-ECAP processing

In this research, as-cast SiCp/AZ91 composites were pre-homogenized at 420 °C for 20 h and then processed by rotary-die equal channel angular pressing (RD-ECAP) at 250 °C for three different passes (4P, 8P, 16P). The effects of ECAP on the microstructure and mechanical properties were investigated. Results showed that SiC particles were more uniformly distributed, matrix grains were significantly refined, β-Al12Mg17 phases were precipitated and homogenized, and texture was reformed to preferred crystal orientation after ECAP processing. The mechanical tensile tests showed an effective increase in mechanical properties after ECAP process. After 4P, 8P, 16P ECAP, the strength of the composites increased to 306 MPa, 288 MPa and 285 MPa, respectively; and the ductility increased to 6.4%, 7.3% and 8.2%, respectively. The strength of the composite was slightly decreased and the elongation was gradually increased with the pass number increasing from 4–16P. The enhanced mechanical properties of the composites in this study were ascribed to a combined effect of grain refinement, second phase precipitation, dislocations and texture reforming.

Effect of Tool Shoulder Diameter on the Surface Hardness of Aluminum-Molybdenum Surface Composites Developed by Single and Double Groove Friction Stir Processing

Molybdenum (Mo), possessing higher hardness compared to aluminum (Al), is used for the surface hardness improvement in Al 1050 alloy. Al-Mo surface composites are developed using friction stir processing with different tool shoulder diameters. Single groove composite (SGC) and double groove composite (DGC) samples are processed with 22-, 20-, and 18-mm shoulder diameter tools to study the effect of tool shoulder diameter and Mo content on the surface hardness of Al-Mo surface composites. Microhardness increased from 26 to 51 Hv on the top surface and 52 Hv on the cross section surface, in 18 DGC. The surface hardness of the composite samples increased with decrease in tool shoulder diameter due to the combined effect of grain refinement and Mo reinforcement particles. Unprocessed Al 1050 base alloy possessed an average grain size of ~ 99 µm. The grain size in the processed samples reduced with decrease in tool shoulder diameter. Surface composites processed with 22-, 20-, and 18-mm tools exhibited an average grain size of ~ 23, ~ 22, and ~ 21 µm, respectively. The amount of Mo particles incorporated in the composite material increased with decrease in tool shoulder diameter. Microstructural analysis confirmed that the FSP groove method incorporated about ~ 8 wt pct Mo in 22 SGC and ~ 12 wt pct in 22 DGC, respectively. The amount of Mo particles in 20 SGC and 20 DGC is ~ 17 wt pct and ~ 22 wt pct, respectively, whereas 18 SGC and 18 DGC possessed ~ 27 wt pct and ~ 31 wt pct Mo particles. Mechanical shearing through severe plastic deformation during the friction stir processing reduced the average Mo particle size in the surface composites. Molybdenum is distributed in its elemental form in the Al-Mo surface composites without any intermetallic formation.

Influence of over-aging and sub-zero temperature rolling on strength-ductility balance in AA6061 alloy

The present investigation aims to achieve a balance between strength and ductility in AA6061 alloy by tailoring the microstructure through sequential combinations of multi-pass rolling near sub-zero temperature and age hardening. At the initial stages of the investigation, it was observed that controlled over-aging can induce a substantial amount of ductility (elongation at failure, TE ∼17%) in the alloy with moderate strength (yield strength ∼143 MPa and ultimate tensile strength ∼272 MPa). Therefore, in order to attain a considerable balance between strength, ductility and strain hardening, the alloy has been subjected to different sequences of sub-zero temperature rolling and over-aging. The enhancement of ductility and work-hardening induced by the thermomechanical processing can be attributed to the combined effect of grain refinement, precipitation, and dynamic recovery. The sub-zero temperature rolling produces fine and elongated grains; whereas, the over-aging process helps in coarsening of the precipitate phase Mg2Si as well as recovery. The thermomechanical treatments also result in proliferation of dislocations in the microstructure. Their interactions with the fine grains result in higher strength (maximum yield strength ∼318 MPa and maximum ultimate tensile strength ∼322 MPa), whereas the coarse grains aid their flow to maintain sufficient ductility.

Manufacture of biodegradable magnesium alloy by high speed friction stir processing

Friction stir processing (FSP) with a high rotational speed (6000 rpm) was employed to modify surface properties of magnesium alloy through grain refinement and incorporated nano-hydroxyapatite (nHA) particles. Hardness testing indicated that the distributed nanoparticles have largely influenced the hardness of the surface layer and measurements of average hardness varied with different amounts of nHA particles applied. Corrosion tests revealed that FSP had reduced the corrosion rate of ZK60 magnesium alloy by more than one order of magnitude due to the combined effects of grain refinement and low surface energy. The corrosion rates of modified layers could be manipulated by nHA content of selection since nHA plays an important role in destroying the continuity of magnesium matrix and producing a more protective biomineralized film on composites when immersed in simulated body fluid (SBF). Two kinds of equivalent circuits were proposed to explain the corrosion mechanism. The simulation values fitted the experimental data very well by using the equivalent circuits, regardless of differences in samples.

Effects of Ti addition on the formation of LPSO phase and yield asymmetry of Mg-Zn-Y-Mn alloy

The microstructure and mechanical properties of Mg94Zn2.5Y2.5Mn1 (at%) with different Ti contents were investigated. The addition of 0.3 at% Ti yielded the best strength. However, Mg93.5Zn2.5Y2.5Mn1Ti0.5 exhibited the lowest yield asymmetry. These changes were attributed to the combined effects of grain refinement, increased LPSO phase, and reduced axial ration (c/a).

Development and Characterization of a Novel Ti-Modified High-Si Medium-Mn Steel Possessing Ultra-High Strength and Reasonable Ductility After Hot Rolling

The present work systematically correlates the strength–ductility relationship of a Ti-Si-modified Fe-Mn-Al-C steel with its microstructural characteristics such as grain size, type and morphology of precipitates, and the transformation-induced plasticity (TRIP) effect during hot deformation. The Ti-Si-modified multicomponent steel developed through melting and casting route is subjected to thermo-mechanical processing such as hot forging (at 1373 K) and subsequent hot rolling (at 1173 K). An excellent combination of ultra-high tensile strength (UTS ~ 1700 ± 20 MPa), reasonable ductility (elongation ~ 11%) and high work hardening behavior (n ~ 0.89) is achieved in the hot-rolled specimen as compared to the hot-forged one (UTS ~ 824 ± 9 MPa, n ~ 0.07) with negligible change in the elongation. The better tensile properties of the hot-rolled specimen in contrast to the hot-forged one are due to the combined effects of grain refinement during rolling, twin–twin interactions, precipitation strengthening by mixture of hexagonal structured Ti3(Al,Si)C2 and Mn-Al-Si-rich carbide precipitates and most significantly the enhanced TRIP effect. The lower critical resolved shear stress value of martensitic transformation instigates the TRIP effect during tensile testing which resulted in the increased hardness and strength of the hot-rolled specimen. The above observation offers a strong support to the proposition that TRIP effect is the dominant plasticity-enhancing mechanism activated during the deformation of the low-stacking fault energy (~ 12.2 mJ/m2) Si-Ti-modified, medium-Mn multicomponent steel employed in the present investigation.

Mechanical Behaviour of 304 Austenitic Stainless Steel Processed by Room Temperature Rolling

To study the effect of room temperature rolling on mechanical properties of 304 Austenitic Stainless Steel, the as received 304 ASS was rolled at room temperature for different percentage of plastic deformation (i.e. 30, 50, 70 and 90 %). Microstructural study, tensile and hardness tests were performed in accordance with ASTM standards to study the effect of rolling. The ultimate tensile strength (UTS) and hardness of a rolled specimen have enhanced with rolling. The UTS has increased from 693 MPa (as received) to 1700 MPa (after 90% deformation). The improvement in UTS of processed samples is due to combined effect of grain refinement and stress induced martensitic phase transformation. The hardness values also increases from 206 VHN (as received) to 499 VHN (after 90% deformation). Magnetic measurements were also conducted to confirm the formation of martensitic phase.

Studies on tensile behaviour and microstructural evolution of UFG Mg-4Zn-4Gd alloy processed through hot rolling

The effect of rolling at elevated temperature on mechanical behaviour of bulk Mg-4Zn-4Gd alloy was investigated. The alloy was subjected to solution treatment at 673K for 24h, followed by flat and cross rolling at elevated temperature (723K). The microstructure of ultrafine grains (less than ~1µm size) was successfully achieved from solutionized sample with grain size ~78µm after the rolling process, which has significantly influenced the mechanical behaviour of the deformed Mg alloy. The yield strength (YS), ultimate tensile strength (UTS), elongation-to-failure (%E) and hardness of a rolled specimen have enhanced maximum up to the value of 276.3 ± 13MPa, 341.3 ± 7MPa, ~26.1% and 74.2 ± 2HV, respectively. The improvement in mechanical properties of processed Mg alloys due to the combined effect of grain refinement and secondary phase (W-phase) distribution into the Mg matrix upon the repetitive rolling process. In addition, the improvement in tensile properties is explained by correlating the dislocation density (ρ), scanning electron (SEM) and transmission electron (TEM) micrographs. The developed intrinsic property of rolled specimens is substantiated by tensile fractography obtained from SEM equipped with energy dispersive X-ray spectroscopy (EDS). Various other microstructural features were also revealed by TEM and X-ray diffraction (XRD) analysis. The anisotropy of various rolled Mg-4Zn-4Gd specimens was also elucidated on the basis of tensile results.

Tailoring the Microstructure and Mechanical Property of AZ80 Alloys by Multiple Twinning and Aging Precipitation

AZ80 alloy is a relatively inexpensive age‐hardenable Mg alloy. To improve its precipitation strengthening effect, the discontinuous precipitates generally nucleating from grain boundaries should be reduced. In this study, the authors applied strain‐path change compression on a wrought AZ80 alloy to generate multiple {10‐12} twins and then the pre‐deformed alloy is subjected to aging at 180 °C for different period. The effects of the pre‐generated twins on precipitation behavior and hence the mechanical properties are investigated. It is found that multiple {10‐12} twins are formed in the AZ80 alloy after two paths of compressions along the transverse and rolling directions. Consequently, continuous precipitation is largely promoted during the subsequent aging process. Moreover, the continuous precipitates preferred to form inside the {10‐12} twin lamellae. Discontinuous precipitation is almost inhibited during aging at 180 °C for 4–96 h in the sample containing many multiple twins. Compare to the directly peak‐aged sample, the pre‐twinned and subsequently peak‐aged alloys generally exhibit greatly superior mechanical properties. The yield strength, ultimate strength, and elongation for tension along RD are 242 MPa, 476 MPa, and 19.4%, respectively. These excellent properties are attributed to the combined effects of grain refinement, texture weakening, and enhanced precipitate strengthening.

Microstructure and Mechanical Properties of ZK60 Magnesium Alloy Prepared by Multi-Pass Friction Stir Processing

Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. In this paper, the microstructure and mechanical properties of ZK60 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The first-pass was subjected with 800r/min-100mm/min and the second-pass was subjected with 600r/min-100mm/min. The results show that the coarse second phase was dissolved into magnesium matrix and the α-Mg grains were remarkably refined in stir zone after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 6.8μm and 6.0μm respectively. The microhardnesses of the specimens were 70HV and 65HV, the ultimate tensile strengths were 276MPa and 272MPa, and the elongations were 31.6% and 42.5%, respectively. Through increasing the processing pass, the microstructure became finer and the second phase is dissolved more thoroughly. The combined effect of grain refinement and second phase dissolved was responsible for the change of mechanical properties.

Microstructure and mechanical properties of an extruded Mg-8Bi-1Al-1Zn (wt%) alloy

In this study, the microstructural evolution and mechanical properties of a newly developed rare earth free Mg-8Bi-1Al-1Zn (BAZ811, in wt%) alloy were investigated and compared with those of a commercial AZ31 alloy. The as-extruded BAZ811 alloy with much finer grain size shows more homogeneous dynamical recrystallized (DRXed) microstructure and weaker basal texture than those of AZ31 alloy. In addition, compared with bimodal structure AZ31 alloy containing only relatively coarse and sparse Al8Mn5 phases, the coexistence of strip-like fragmented Mg3Bi2 precipitate and nano-size Mg3Bi2 particles in the microstructure was observed in BAZ811 alloy. Moreover, the BAZ811 alloy exhibits a tensile yield stress of 291MPa, an ultimate tensile strength of 331MPa, an elongation to failure of 14.6% as well as a reduction in yield asymmetry, which is mainly attributed to the combined effects of grain refinement and micro-scale broken Mg3Bi2 particles together with nano-scale spherical Mg3Bi2 precipitates. The strain hardening behavior of both BAZ811 and AZ31 alloys were also discussed in terms of microstructure variation.

Combined effect of grain refinement and surface modification of pure titanium on the attachment of mesenchymal stem cells and osteoblast-like SaOS-2 cells

Surface modification is an important step in production of medical implants. Surface roughening creates additional surface area to enhance the bonding between the implant and the bone. Recent research provided a means to alter the microstructure of titanium by severe plastic deformation (SPD) in order to increase its strength, and thereby reduce the size of the implants (specifically, their diameter). The purpose of the present study was to examine the effect of bulk microstructure of commercially pure titanium with coarse-grained (CG) and ultrafine-grained (UFG) bulk structure on the surface state of these materials after surface modification by sand blasting and acid etching (SLA). It was shown that SLA-modified surface characteristics, in particular, roughness, chemistry, and wettability, were affected by prior SPD processing.Additionally, biocompatibility of UFG titanium was examined using osteosarcoma cell line SaOS-2 and primary human adipose-derived mesenchymal stem cell (adMSC) cultures. Enhanced cell viability as well as increased matrix mineralization during osteogenic differentiation of MSCs on the surface of ultrafine-grained titanium was shown.

Influence of particulate reinforcement on microstructure evolution and tensile properties of in-situ polymer derived MMC by friction stir processing

The authors recently reported a novel in-situ method of fabricating nano-polymer derived metal matrix composite (PD-MMC) by friction stir processing (FSP) and addressed the issues of tool wear and particle agglomeration. In the present work, the microstructural evolution and tensile properties of the processed composite are reported. The microstructure during FSP evolved by discontinuous dynamic recrystallization. In the composite, fine ceramic particles pin the grain boundaries, preventing grain growth resulting in a fine grain (2μm) structure being retained. FSPed Cu (processed with the same process parameters as that of the composite) exhibited a grain size of 100μm compared to 400μm in the base Cu. The composite microstructure was characterized by equiaxed grains with narrow grain size distribution and a high fraction (>80%) of high angle grain boundaries. The combined effect of grain refinement and ceramic particle incorporation lead to a twofold improvement in the proof stress of the composite (201MPa compared to 98MPa of base copper). The ultimate tensile strength improved by 33% and there was small drop in the ductility of the composite when compared to base Cu. Kocks-Mecking plot of the composite showed stage III of work hardening.

Effects of TMCP Schedule on Precipitation, Microstructure and Properties of Ti-microalloyed High Strength Steel

Using the similar compositions of the Ti-microalloyed high-strength steels produced by the thin-slab casting process of compact strip production (CSP), four thermo-mechanical control processes (TMCP) after the simulated thick-slab casting, i. e. the two hot rolling routes and the two cooling processes, were designed, aiming at achieving the same mechanical properties as the thin strip products. The final microstructures after the four TMCP processes were examined by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile properties and Charpy impact energy were measured correspondingly. Strain-induced TiC precipitation was found in the two-stage rolling route with the finish rolling temperature at low levels, leading to grain refinement due to the pinning effect during austenite recrystallization. Precipitation hardening in ferrite was observed when a period of isothermal holding was applied after hot rolling. It could be concluded that both finish rolling temperature and the subsequent isothermal holding temperature were crucial for the achieved strength level due to the combined effect of grain refinement and precipitation hardening. At the same time, it was found that the isothermal holding led to poor impact toughness because of remarkable precipitation hardening. Therefore, it was suggested that the precipitation kinetics of titanium carbides in both austenite and ferrite should be investigated in future.

Selective laser melting of TiC reinforced 316L stainless steel matrix nanocomposites: Influence of starting TiC particle size and volume content

Selective laser melting (SLM), an additive manufacturing technology, was utilized to process TiC/316L nanocomposite systems with different starting TiC particle sizes and volume contents. The influence of the starting TiC particle size and volume content on the constitutional phases, microstructural features, and mechanical properties of the SLM-processed nanocomposite parts was investigated. The densification behavior was controlled by both the starting TiC content and the particle size; the densification level was enhanced with the use of fine starting TiC particles owing to the improvement of the reinforcement–matrix wettability. In general, by increasing the volume content of the TiC, the hardness increased and the coefficient of friction (COF) and wear rate deceased owing to the combined effects of grain refinement and grain-boundary strengthening. However, in contrast to the starting coarse TiC particles, the SLM-part processed with the starting fine TiC particles shows better wear resistance, in particular at a 10–15% TiC content, owing to improved TiC dispersion throughout the matrix and increased density.

Rapid fabrication of bulk-form TiB2/316L stainless steel nanocomposites with novel reinforcement architecture and improved performance by selective laser melting

Selective laser melting (SLM), a novel additive manufacturing technology, was employed to fabricate nanostructured TiB2/316L stainless steel nanocomposites with varying nanoscale TiB2 content (vol.%). The resulting microstructure of the SLM-processed nanocomposites was characterized by SEM, EBSD, and TEM, while the mechanical properties were characterized using microhardness, compression tests, and wear tests. It was found that the microstructure and mechanical properties of the SLM-processed nanocomposites were sensitive to the TiB2 content. The optimal TiB2 content was determined to be 10 vol.%. Further, the TiB2 particles were homogeneously dispersed and they formed nanoscale ring-like structures along the grain boundaries. In contrast to the SLM-processed unreinforced 316L stainless steel sample, the TiB2/316L nanocomposites exhibited higher microhardnesses and yield strengths while showing low coefficients of friction and wear rates; this was owing to the combined effects of grain refinement and grain-boundary strengthening. The nanocomposites showed good combination of compression yield strength and ductility during microcompression tests.