Acicular Martensitic Structure Research Articles

Effect of different hydrogen Fugacities on the microstructure of additively manufactured Ti-6Al-4V

In this study we compared the microstructural changes in the presence of hydrogen of Ti-6Al-4V alloy prepared by two different additive manufacturing (AM) methods: electron beam melting (EBM) and selective laser melting (SLM). Cathodic hydrogenation of AM Ti-6Al-4V resulted in significant expansion of β Ti phases due to the solute hydrogen, increasing of micro-strains, and α → titanium hydride/βH transformation. It was shown that SLM Ti-6Al-4V with acicular martensitic structure is more prone to hydrogen-induced phase transformation and hydrogen-assisted damage, compared to EBM. Moreover, it was revealed that different hydrogen charging environments cause different microstructural changes in EBM Ti-6Al-4V. Unlike cathodic hydrogen charging, gaseous hydrogen charging at elevated temperature reduced the process-induced micro-strains and promoted Al redistribution within the EBM Ti-6Al-4V. This resulted in massive precipitation of brittle α2-Ti3Al intermetallic compound, which act as a strong hydrogen trapping site aside from the Ti hydride phase.

Excellent mechanical properties of a ZrTi-based alloy via laser surface remelting

In recent years, more and more attention has been paid to the research and development of high strength Zirconium-Titanium (ZrTi) based alloy system. However, like almost all metallic materials, increasing the strength of ZrTi alloy leads to a significant decrease in plasticity. In this paper, high strength and toughness ZrTi alloy with gradient structure was prepared by laser surface remelting. The metastable β phase with high dislocation density was obtained by laser local heating and rapid cooling, and a large amount of acicular martensite structure was formed in the heat affected zone, combined with the dual phase of the matrix, a gradient structure was formed from the surface to the matrix. The tensile strength was increased to 1550 MPa and the tensile elongation was kept above 10%. The composite deformation mechanisms of dislocation slip, twinning and stress-induced martensitic transformation of gradient structure have also been discussed.

Geometrical Influence on Material Properties for Ti6Al4V Parts in Powder Bed Fusion

One major advantage of additive manufacturing is the high freedom of design, which supports the fabrication of complex structures. However, geometrical features such as combined massive volumes and cellular structures in such parts can lead to an uneven heat distribution during processing, resulting in different material properties throughout the part. In this study, we demonstrate these effects, using a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux. We manufacture the parts via powder bed fusion of Ti6Al4V by applying a laser beam (PBF-LB/M) as well as an electron beam (PBF-EB). We investigate the impact of the different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements. Both simulations and thermography show an increase in inter-layer temperature with increasing part radius, subsequently leading to heat accumulation along the build direction. While the geometry and thermal history have a minor influence on the relative density of the parts, the microstructure is greatly affected by the thermal history in PBF-LB/M. The acicular martensitic structure in the narrow parts is decomposed into a mix of tempered lath-like martensite and an ultrafine α + β microstructure with increasing part radius. The EBM part exhibits a lamellar α + β microstructure for both the cylindric and conic structures. The different microstructures directly influence the hardness of the parts. For the PBF-LB part, the hardness ranges between 400 HV0.5 in the narrow sections and a maximum hardness of 450 HV0.5 in the broader sections, while the PBF-EB part exhibits hardness values between 280 and 380 HV0.5.

Grain structure orientational change in Ti6Al4V alloys induced by sea water quenching and novel stress relief annealing process

We report on the microstructures and properties of Ti6Al4V alloys, which were achieved upon quenching in sea water medium with potential high cooling rate. The Ti6Al4V alloys were quenched at 1000 and 1100 °C, respectively. Moreover, the effect of post-quenching annealing performed at 900 °C was analyzed. As a result, the quenched alloy experienced surface thermal stress, due to rapid cooling and thermal shock, due to exposure to high temperature annealing. The alloy quenched at 1000 °C developed equiaxed grain structure after annealing, while the 1100 °C-quenched alloy generated irregular shaped lamellae structures. TEM analysis for the 1000 °C-quenched samples revealed the α′-martensite decomposed into the equilibrium α + β phases. Moreover, the 1100 °C-quenched Ti6Al4V alloy revealed an α′-acicular martensitic structure.

Role of martensitic interface on the discontinuous precipitation of U-Ti alloys at 400 °C

The microstructure evolution of U-0.8 wt%Ti (U-0.8Ti) during aging at 400 ºC was investigated through SEM and TEM techniques. Three kinds of decomposition routes were identified. One route is through continuous precipitation (CP) process while the other two are through discontinuous precipitation (DP) process. In the early stage of aging, CP reactions of U2Ti occurred at the boundaries of martensitic plates. DP reactions were discovered both around grain boundaries and inside large-sized martensitic plates. The colonies of the former one were found at prior grain boundaries but were soon suppressed in the later stage of aging. DP reactions inside large-sized martensitic plates did not take place until the final stage of aging. By applying the Hatt martensitic transformation theory to the electron backscatter diffraction results, it was identified that most of the boundaries of martensitic plates were incoherent high-angle boundaries. Such boundaries compensate the opposing energy of the nucleation of the U2Ti during CP process and offer extra surface energy to promote the DP process around grain boundaries. This paper provides an overview of the decomposition process and an undocumented understanding of the role of martensitic plate interfaces on the DP and CP processes during aging at 400 ºC in U-0.8Ti alloy, which is also worth considering in other systems with acicular martensitic structure, such as U-0.8 wt%Mo and U-1 wt%Nb alloys.

Effect of Yb2O3 on superplastic behavior of laser welded joint of TC4 titanium alloy

The inhomogeneity of joints during superplastic deformation greatly limits the application of laser beam welding/superplastic forming (LBW/SPF). In order to adjust the deformation uniformity of the joint, Yb2O3 with different mass fractions was added to the laser welded joint of TC4 titanium alloy, and the effect of Yb2O3 on the superplastic deformation behavior of the laser welded joint of TC4 titanium alloy was studied. The results showed that the addition of Yb2O3 can effectively refine β grains. At Yb2O3 content of 6 wt%, the refinement effect of Yb2O3 on the weld grains is the most obvious. At this time, the peak flow stress of the longitudinal weld sample reaches a minimum value of 25.6 MPa, and the superplasticity of the joint is significantly improved. The elongation of the transverse welded sample reached 29% of that of the base metal and the joint deformation uniformity was significantly improved. The prior β grain of weld broke during superplastic deformation, and the acicular martensite structure in the weld was gradually transformed into a lamellar structure. After the addition of Yb2O3, the lamellar structure inside the weld becomes short and disordered, which promotes occurrence of superplastic deformation.

Effect of induction post-heating temperature on the morphology, microstructure and mechanical performance of the heat affected zone in laser-induction hybrid cladding of full-scale rail

Abstract In the present work, the laser-induction hybrid cladding technology with induction post-heating (post-LIHC) was utilized to deposit coatings on the full-scale rail surface, and the effects of different induction post-heating temperature on the morphology, microstructure and mechanical performance of the heat affected zone (HAZ) were first in-depth analyzed. Results indicate that with the increase of the average induction post-heating temperature (Ta) in post-LIHC, the microstructure of the HAZ first transforms from the fine acicular martensite structure to the composite structure of martensite and pearlite, then changes to the pure pearlite structure, and finally retransforms to the coarse martensite structure. Especially, the morphology of the “Martensite Zone” (MZ) in HAZ undergoes three remarkable change with Ta increasing: “flat-U-shaped” → “flat-W-shaped” → “vanished” → “flat-V-shaped”. Under the induction post-heating window of Ta = 508 °C ~ 565 °C, not only the MZ in HAZ can be inhibited absolutely, but also fine pearlite with much lower interlamellar spacing, smaller grain size and higher grain misorientation forms instead, making the strength and toughness are both enhanced significantly than the other three type HAZs and the rail substrate. In addition, increasing the induction post-heating temperature within the window of Ta = 508 °C ~ 565 °C can increase the interlamellar spacing and reduce the ferrite grain misorientation of the pearlite structure in HAZ, but has no obvious effect on the grain size therein.

Microstructural Evolution in the TC17 Titanium Alloy Processed During Laser Stereo Forming

In this paper, the microstructure evolution of TC17 titanium alloy prepared by laser stereo forming (LSF) was investigated. Forged TC17 plate was selected as the base material, and TC17 titanium alloy spherical powder with the size of 50-150 μm was the candidate cladding material. The microstructure in deposited TC17 titanium alloy was consisted of three parts, namely fine grains at the bottom of the deposition area, coarse β columnar crystals at the middle, while coarse β columnar crystals or equiaxed crystals at the top. The microstructure is significantly affected by the thermal cycle history during the LSF process. At different locations of the deposited samples, the composition, morphology, size and volume fraction of the phases in the grains are discrepant, as the metastable β phase and the acicular martensite (α′) structure were obtained at the bottom and in the middle, respectively. Due to thermal cycle during multi-layer deposition, the metastable β phase will gradually evolved into α + β phases. As the subsequent deposition process continued, the temperature in the middle of the deposition area decreased below the transformation point, and α′ is decomposed into α + β. The precipitated α phase gradually grew up under the action of thermal cycle. Meanwhile, the fine secondary α phase precipitated from the metastable β phase, and finally the coarser α lath and inter lath were obtained β structure.

Effects of Welding Speed on Microstructure and Mechanical Properties of CO2 Laser Welded Dissimilar Butt Joints between Low Carbon Steel and Austenitic Stainless Steel

ABSTRACT Austenitic stainless steel (AISI316) and low carbon steel (AISI1018) sheet joints are a popular dissimilar metal combination used in power generation applications. The 4 kW CO2 laser welding has recently been used for the production of dissimilar metal joints with appropriate mechanical and metallurgical properties. In this study, the influence of laser welding process parameters of dissimilar weld joints was investigated with the varying laser powers, welding speeds and constant shielding gas flow rate. The variation of mechanical properties and microstructures of the dissimilar weld zone and heat-affected zones were investigated and resulted. The microstructure of the weld zone was predominantly fine columnar dendrites along with carbides were observed. The stainless steel heat-affected zone consists of coarser austenite grains with twin structure and little carbide precipitation. The low carbon steel heat-affected zone results, the mixed structure of acicular ferrite, pearlite, bainite, and martensite structure along with enriched iron. The chemical composition of the weld zone and phase structure of the dissimilar weld zone was examined through chromium and nickel equivalents. The higher hardness value was obtained in the weld zone and heat-affected zone of low carbon steel.

Experimental Investigation of Deformation and Spring-Back and Spring-Go Amounts of 1.5415 (16MO3) Sheet Material

The spring-go and spring-back behavior of 16Mo3 sheet metal as a result of the V-deep bending process was examined. In the bending process, the effect of normalization and tempering heat treatment on 16Mo3 sheet metals was also microstructurally examined. The experimental study was performed in two stages. In the first stage, the effects of the thickness of the sheet metal on spring-go and spring-back behavior were inspected. Thus, 3-, 4-, 5-, and 6-mm-thick sheet metals to which unheated treatment was applied were subject to normalization and tempering, then bending processes were applied at a 150o bending angle in a punch tip radius of R 4.5 mm, and their spring-back and spring-go behaviors were examined. Secondly, the bending processes were applied. By removing small samples from the middle deformation area of the unheated treatment, normalization, and tempered sheet metals, their ferrite, pearlite, and martensite structures were microstructurally characterized, as a result of which, it was found that the material used in the heating process had a ferrite phase and pearlite structure, the normalization materials had fine pearlite structure and ferrite phase, and the annealed materials had a random and acicular martensite structure.

Low-Temperature Induced Martensitic Transformation Enhancing Mechanical Properties of Metastable Fe-Ni-P Alloy

The metastable Fe-Ni-P alloy with phosphorus (P) solid-solution structure has been fabricated by spark plasma sintering. Its face-centered cubic (FCC) matrix without the precipitation of phosphide attains a high plasticity and an excellent strain hardening ability at room temperature. This Fe-Ni-P alloy is subjected to cryogenic treatment at various temperatures (−20 °C and −50 °C), to investigate the role of phosphorus on the microstructural evolution and mechanical properties of γ-(Fe-Ni) alloy at low temperatures. The results indicate that the addition of phosphorus can destabilize the Fe-Ni-P alloy and facilitate its martensitic transformation during cryogenic treatment. P-doping does not lead to obvious embrittlement of Fe-Ni-P alloy at low temperatures, but strengthens the alloy by promoting microstructure evolution. The Fe-Ni-P alloy has high plasticity and good strain hardening ability after treated at −20 °C, and is converted to acicular martensite structure after being treated at −50 °C, resulting in a significant increase in its hardness (433 HV) and compressive yield strength (1271 MPa). Developing this Fe-Ni-P alloy as a load-bearing component for low-temperature conditions shows great promise.

Effect of Manganese Addition on the Microstructure and Mechanical Properties of Ti-Nb Biomedical Alloys

A series of Ti-24Nb-xMn alloys were fabricated by no consumable electrode vacuum arc furnace, with the focus on the effect of Mn addition on the structure and mechanical properties of the alloys. Experimental results indicated that α” phase-dominated binary Ti-24Nb alloy exhibits a fine, acicular martensitic structure. When 1wt% Mn was added, equated β phase structure was retained. The M1 alloy with instability β phase exhibits the two-stage yielding from stress-strain curves due to the stress-induced martensite transformation from β to α” phase during tensile deformation. Addition of a small amount of Mn (1 and 3 wt %) improved the plasticity of alloys and the elongation increases from 33% to 41%. The strength of the alloy with Mn content of 3% is the highest, and the tensile strength is 893MPa. All the alloys with Mn exhibit the high micro hardness, the highest is 345HV, which is 1.13 times than binary Ti-24Nb alloy. The elastic modulus of M1 is the highest of all alloys-172GPa. The elastic modulus of the other alloys with Mn are about 82-87GPa, close to those of human skeletons. These alloys seem to have a great potential for use as an implant material.

Tribological Properties of Three-Dimensionally Printed Ti–6Al–4V Material Via Electron Beam Melting Process Tested Against 100Cr6 Steel Without and With Hank's Solution

Three-dimensionally (3D) printed Ti–6Al–4V (Ti64) samples via an electron beam melting (EBM) process were developed to investigate their microstructure and mechanical and tribological properties in comparison with those of commercial Ti64 samples. The 3D-printed Ti64 samples had a heavily twinned and acicular martensitic structure that was responsible for their higher surface hardness than that of the commercial Ti64 samples. The 3D-printed Ti64 samples tested against a 100Cr6 steel counter ball without and with Hank's solution had a higher wear resistance associated with their higher surface hardness than the commercial Ti64 samples. The use of Hank's solution during sliding reduced the wear of the both Ti64 samples as a result of the lubricating effect of the solution. It could be concluded that the 3D-printed Ti64 samples in this study had comparable mechanical and tribological properties to those of the commercial Ti64 samples.

Modification of the Primary and Peritectic Phases in Directionally Solidified Cu-20 wt.% Sn Alloy by Magnetic Field

Directional solidification experiments on Cu-20 wt.% Sn peritectic alloy were carried out in a direct current magnetic field device to investigate the modification of the primary and peritectic phases. A precipitation of the acicular martensitic structure of peritectic phase was observed below the solid-liquid interface. The magnetic field increased the length of precipitation zone and moved the solid-liquid interface to high temperature side. Solid-state transformation of the peritectic phase was observed below the eutectoid temperature. The magnetic field increased first and then decreased the transformation. A transition of the primary dendrites from arrayed to nonaligned growth under the magnetic field was characterized and analyzed. 3D numerical simulations of the thermoelectric magnetic flows and the thermoelectric magnetic forces were performed. The modification of the primary and peritectic phases under the magnetic field should be attributed to the fluid flows in the liquid and the forces on the solid.

Antibacterial Properties and Corrosion Resistance of the Newly Developed Biomaterial, Ti–12Nb–1Ag Alloy

Currently, the development of biomaterials has focused on having a low Young’s modulus, biocompatibility, corrosion resistance, and antibacterial properties. Ti–Nb alloys have higher research value due to their excellent corrosion resistance and low Young’s modulus. In recent years, the antibacterial properties of materials have been enhanced by the addition of Ag and Cu. Therefore, the corrosion resistance and antibacterial properties of the Ti–12Nb–1Ag alloy formulated in the current study were investigated and compared to those of commonly used Ti alloys, G2 pure Ti (ASTM B348 CP Grade 2), and Ti–6Al–4V, via electrochemical and E. coli antibacterial tests. Meanwhile, we also carried out a microstructural analysis to investigate the composition of the alloy. The results were as follows: (1) The electrochemical test demonstrated that Ti–12Nb–1Ag had a higher corrosion resistance than Ti–6Al–4V, which is similar to the properties of pure Ti. (2) The E. coli antibacterial test demonstrated that the sterilization rate of Ti–12Nb–1Ag was higher than that of the Ti–6Al–4V alloy and pure Ti. (3) The microstructural analysis revealed that Ti–12Nb–1Ag had an acicular martensite structure, with nano-Ag precipitates observed. Based on the results of the E. coli antibacterial test and the principles of sterilization of nano-precipitates and Ag, we inferred that the nano-Ag precipitates of Ti–12Nb–1Ag enhanced the antibacterial properties of the newly developed biomaterial, which is, namely, the Ti–12Nb–1Ag alloy.

Structural Evolution and Mechanical Properties of a VT22 Titanium Alloy Under High-Temperature Deformation

The special features inherent in the development of high-temperature deformation and structural evolution in materials are investigated, using a VT22 titanium alloy of the transition class (Ti – 4.74 mass% Al – 5.57 mass% Mo – 5.04 mass% V) subjected to helical rolling + aging as an example. This treatment is found to give rise to an intragrain fine acicular martensite structure with fine inclusions of α-phase particles of size ~1 μm. It is shown that in the alloy undergoing plastic deformation at temperatures approaching the polymorphic transformation temperature, the elongation to failure is in excess of 300%. The high plasticity of the alloy in the conditions considered is likely to be due to vigorous development of phase transformations and intensification of diffusion-controlled processes, including the effects of the evolution of the dislocation structure, growth of subgrains, and formation of new grains in the bulk of the pre-existing ones during plastic deformation.

Microstructures and Electrochemical Behavior of Ti-Mo Alloys for Biomaterials

The Ti alloy with 7 wt% Mo revealed a microstructure that contained only the orthorhombicα′′phase of a fine acicular martensitic structure. The corrosion resistance of the Ti-Mo alloys increased as the Mo content increased. Based on the results obtained from the polarization curve and electrochemical impedance, the Ti-Mo alloys were shown to be corrosion resistant because of the passive films formed on their surfaces. No ion release was detected in SBF (simulated body fluid) solution, while Ti ions were released in 0.1% lactic acid ranging from 0.05 to 0.12 μg/mL for the Ti-Mo alloys. In vitro tests showed that MC3T3-E1 cell proliferation on Ti-7 wt% Mo alloy was rather active compared to other Ti-Mo alloys and commercial-grade pure Ti.

Thermodynamic Basis of Non-equilibrium Phase Transformations of bcc β-Phase in Ti–Mo System

This study reports the experimental identification of transformation products of high temperature bcc β phase in Ti–xMo (x = 1, 7, 15, 25 wt%) alloys and aims to understand the transformations by thermodynamic modelling. The high temperature bcc β phase had undergone martensitic transformation in Ti–1Mo and Ti–7Mo alloys, resulting in acicular martensitic structure within large β grains. X-ray diffraction (XRD) analysis confirmed the martensite to be hcp (α′) and orthorhombic (α″) in Ti–1Mo and Ti–7Mo alloys respectively. Combined analysis of XRD and transmission electron microscopy (TEM) suggested the formation of fine plates of α″, omega (ω) and bcc β phases in Ti-15 and Ti-25 Mo alloys. Calculation of enthalpy of formation supported the stability of solid solution phase over the amorphous phase in the entire concentration range of Mo.

MODIFICATION OF CARBON STEEL BY LASER SURFACE MELTING: PART I: EFFECT OF LASER BEAM TRAVELLING SPEED ON MICROSTRUCTURAL FEATURES AND SURFACE HARDNESS

The present study aims to improve the surface hardness of carbon steel by application of laser surface melting of effective conditions. The travelling speed of laser beam during this treatment is one of the important treatment conditions. This study aims to investigate the effect of laser surface melting with different beam speeds on macro and microstructure as well as the hardness distribution through the thickness of carbon steel. To achieve this target, three different travelling speeds (1500, 1000 and 500 mm min-1) at a constant beam power of 800 W were chosen in this study. The resulted laser treated specimens were investigated in macro and microscopically scale using optical and scanning electron microscope. Hardness measurements were also carried out through the thickness of the laser treated specimens. The laser treated areas with all used travelling speeds results in melted and solidified zone on the surface of the steel. In the same time, Plates of acicular martensite structure were observed within the upper part of the melted and solidified zone in almost all experimental conditions, while some bainite structure in ferrite grains are detected in its lower part. By increasing the travelling speed, the depth of the laser treated zone was decreases, while travelling speed has much less significant effect on the laser treated zone width. The size of the formed martensite plates was increased by decreasing the travelling speed from 1500 to 500 mm min-1. On the other hand, the travelling speed has a straight effect on the length of the acicular martensite; as the travelling speed increases, the acicular martensite became longer, while it shows fine acicular martensite at lower travelling speeds. The depth that full martensite structure can be reached is increased by increasing travelling speed. At lower travelling speed (500 mm min-1), large amount of bainite structure is observed at the center of the treated zone up to its lower end. The fast travelling speed (1500 mm min-1) show higher hardness on the free surface than that of slow travelling speed (500 mm min-1). On the other hand, the travelling speed has a reverse effect on the depth of this hardness increment; the slower travelling speed give deeper areas of high hardness than that of fast speed. The Heat Affect Zone (HAZ) areas were increased by decreasing the travelling speed. In all conditions, the heat affected zone areas were composed of partially decomposed pearlite in ferrite grains. Finally, the microstructure of the base metal far from the laser treated areas show normal ferrite-sound pearlite microstructure.

Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints

Direct welding between spheroidal graphite cast iron (FCD700) and mild steel (SS400) was conducted using electron beam welding to study the microstructure and the mechanical properties of the welds.Results showed that one-pass welding yielded an over-hardened fusion zone exhibiting acicular martensite structure (815 HV) with cracks and porosities. Two-pass welding contained fewer porosities. Both one-pass and two-pass welding displayed acicular martensite and ledeburite within the microstructure of spheroidal graphite cast iron in heat-affected zone. The tensile strength of both one-pass and two-pass welded joints was lower than that of mild steel base metals. These joints ruptured at the fusion zone or the mild steel bond. However, two-pass welded joints demonstrated tensile yield stress values greater than or equal to those of mild steel base metals. Hardening of the fusion zone and the heat-affected zone of spheroidal graphite cast iron made the impact strength values of two-pass welded joints conspicuously lower than those of the spheroidal graphite cast iron base metal. The fatigue strength of two-pass welded joints almost equaled that of mild steel base metal, with a fatigue limit of 209 MPa.