Martensite Plates Research Articles

Effect of plasma beam irradiation on the microstructure and phase composition of high-speed steel R6M5

Abstract The present study is devoted to research into the morphology and structural-phase state of an R6M5 high speed steel surface layer after it was affected by a plasma beam. It was discovered, that R6M5 steel is a multi-phase material with an α-phase, γ-phase (retained austenite) and carbide phase just as in the initial state. The main phase component of the R6M5 steel matrix before and after plasma beam processing was an α-phase (γ → α’-martensite): lamellar and lath martensite; but lamellar martensite is dominated by a volume ratio ~90 % gross share α’-martensite. The γ-phase as the second morphological component of the steel matrix (retained austenite) yields within the martensite plates the form of twintype colonies with a volume ratio of ~6 %. It was established that there were М6С -type carbide particles of complex composition (Fe,W,Mo)6C in the material just as in the initial state. It should be said that in its initial state, the dislocation structure, formed under the effect of the plasma beam, is characterized by fairly high value of excess density of dislocations with an average value of ρ± = 2.0 × 1010 cm-2 (in the initial state ρ± = 1.7 × 1010 cm-2) along with scalar density. The depth of torsion curve of the face-centered space lattice in the α’-martensite is χ = 500 cm-1 (in the initial state it is equal to χ ~436 cm-1), the amplitude of the inner long-distance voltage is σ∂ = 280 МPа (in the initial state it is equal to σ∂ = 260 МPа). Thus, the amplitude of shift voltage is equal to σL = 420 МPа (in the initial state it is σL = 350 МPа); nevertheless, σL > σ∂ remains the mean torsion-curve of the fcs-lattice of α’-martensite maintaining its lamellar nature after the effect of the electron beam much as in the initial state.

The Effect of the addition of Ta, Zr and in on the corrosion behaviour of CuAlMn alloys prepared via powder metallurgy

CuAlMn-X (X= Ta, Zr and In) shape memory alloy was fabricated using the powder metallurgy (PM) technique. After sintering at 950 °C, the products were processed using two heat treatments (solution heat treatment at 900 °C and aging at 500 °C). The influence of Tantalum (Ta), Zirconium (Zr) and Indium (In) with three wt.% on the mechanicals properties (hardness), corrosion behaviour and microstructure of CuAlMn shape memory alloys was investigated in this paper. The effect of the addition of Ta, Zr and In alloys was investigated using X-rays diffraction (XRD), open circuit potential, electrochemical tests (Tafel extrapolation method) and Vickers micro-hardness. For the corrosion tests, a 3.5 % NaCl (chloride solution) was used as corrosion solution. The results showed that there is an increase in hardness with addition of the alloy element (Ta, Zr and In) and decrease in the size of martensite plate form, as shown in the microstructures. Furthermore, it was found that the corrosion resistance of Cu-Al-Mn after the addition improved, as shown by the increase in the corrosion potential and decrease in the corrosion current densities. The improvement percentage for CuAlMnTa, CuAlMnIn and CuAlMnZr in the corrosion rate was 97 %, 91 % and 77 %, respectively.

Effect of plasma beam irradiation on the microstructure and phase composition of high-speed steel R6M5

Abstract The present study is devoted to research into the morphology and structural-phase state of an R6M5 high speed steel surface layer after it was affected by a plasma beam. It was discovered, that R6M5 steel is a multi-phase material with an α-phase, γ-phase (retained austenite) and carbide phase just as in the initial state. The main phase component of the R6M5 steel matrix before and after plasma beam processing was an α-phase (γ → α’-martensite): lamellar and lath martensite; but lamellar martensite is dominated by a volume ratio ~90 % gross share α’-martensite. The γ-phase as the second morphological component of the steel matrix (retained austenite) yields within the martensite plates the form of twintype colonies with a volume ratio of ~6 %. It was established that there were М6С -type carbide particles of complex composition (Fe,W,Mo)6C in the material just as in the initial state. It should be said that in its initial state, the dislocation structure, formed under the effect of the plasma beam, is characterized by fairly high value of excess density of dislocations with an average value of ρ± = 2.0 × 1010 cm-2 (in the initial state ρ± = 1.7 × 1010 cm-2) along with scalar density. The depth of torsion curve of the face-centered space lattice in the α’-martensite is χ = 500 cm-1 (in the initial state it is equal to χ ~436 cm-1), the amplitude of the inner long-distance voltage is σ∂ = 280 МPа (in the initial state it is equal to σ∂ = 260 МPа). Thus, the amplitude of shift voltage is equal to σL = 420 МPа (in the initial state it is σL = 350 МPа); nevertheless, σL > σ∂ remains the mean torsion-curve of the fcs-lattice of α’-martensite maintaining its lamellar nature after the effect of the electron beam much as in the initial state.

Identification of nano-width variants in a fully monoclinic martensitic Ni50Ti50 alloy by scanning electron microscope-based transmission Kikuchi diffraction and improved groupoid structure approach

Nano-width martensite plates in a fully martensitic Ni50Ti50 alloy are indexed successfully by using the off-axis transmission Kikuchi diffraction in scanning electron microscope (i.e., SEM-based TKD). The data obtained by SEM-TKD are effectively interpreted using an improved approach based on the framework of the theoretical groupoid structure method, where the equivalent variants transformed from the monoclinic variants are introduced to calculate all theoretical axis/angle pairs of rotation, and to formulate a complete list of source martensite to target martensite pairs. Consequently, B19′ monoclinic martensite variants in NiTi alloys are identified unambiguously, by using numerical comparison between the experimental and theoretical rotation components, without the reference of retained parent phase.

On non stress-free junctions between martensitic plates

The analytical understanding of microstructures arising in martensitic phase transitions relies usually on the study of stress-free interfaces between different variants of martensite. However, in the literature there are experimental observations of non stress-free junctions between martensitic plates, where the compatibility theory fails to be predictive. In this work, we focus on V_{II} junctions, which are non stress-free interfaces between different martensitic variants experimentally observed in Ti _{74} Nb _{23} Al _3 . We first motivate the formation of some non stress-free junctions by studying the two well problem under suitable boundary conditions. We then give a mathematical characterisation of V_{II} junctions within the theory of elasto-plasticity, and show that for deformation gradients as in Ti _{74} Nb _{23} Al _3 our characterisation agrees with experimental results. Furthermore, we are able to prove that, under suitable hypotheses that are verified in the study of Ti _{74} Nb _{23} Al _3 , V_{II} junctions are strict weak local minimisers of a simplified energy functional for martensitic transformations in the context of elasto-plasticity.

Influence of tempering temperatures on the microstructure, secondary carbides and mechanical properties of spray-deposited AISI M3:2 high-speed steel

The evolution of microstructure and mechanical properties of a spray-deposited AISI M3:2 high-speed steel (HSS) are investigated using a series of tempering temperatures. The results demonstrate a strong correlation between secondary precipitates and tempering. With increasing tempering temperature, secondary precipitates exhibit a transformation from coarse strips of M3C type to a refined and dense coherent precipitates of types MC and M2C. Highly dispersed fine secondary precipitates contribute to an enhanced bending strength and the secondary hardening peak without reduction of its impact toughness. The dimension of MC and M2C precipitates increased while the volume fraction decreased with the increasing temperature. Tempering at 650 °C can result in the generation of ellipsoidal M3C carbides at the cost of the fine MC and M2C precipitates. Overaging at 700 °C lead to the transformation of the fine precipitates to coarse MC, M3C and M7C3, and finally to M23C6 and M6C, along with the matrix structure change from plate martensite to recrystallized ferrite, which enables the toughness to increase significantly. The strengthening effects of secondary precipitates are also quantitatively analyzed and assessed.

Mechanical properties and microstructures data of AISI 1070 steel quenched in epoxidized transesterified cottonseed oil.

The quenching ability of modified and unmodified cottonseed oils was investigated using AISI 1070 steel. In the event of the quenching, the steel samples immersed in each of five distinct quench media, namely epoxidized cottonseed oil (EC), epoxidized-transesterified cottonseed oil (ETC), transesterified cottonseed oil (TC) and fresh cottonseed oil (FC). Tests and analysis conducted determined mechanical properties and microstructures of the quenched samples. The data obtained showed that ETC outperformed other quench media with hardness value of the quenched sample; 407 HVN (hardness value from the FC-quenched sample) increased to 746 HVN indicating an 83.29% improvement. Notably, in the microstructure of ETC-quenched sample, a unique homogeneous microstructure containing a mixture of lath and plate martensite observed with largest martensite per cent of 95.

Influence of Ta Additive into Cu84−xAl13Ni3 (wt%) Shape Memory Alloy Produced by Induction Melting

Thermal and morphological analysis of shape memory alloys is one of the important branches of research to obtain new characteristics. In this study, low amounts of tantalum were added into CuAlNi alloy. The alloy was produced with the induction melting method. The phase transformation of the alloys was characterized by DSC and electrical resistivity for an equal range of temperatures. The alloys showed high-temperature phase transformations. Although the α phase existed in all cases, its peak intensity in the XRD result was significantly decreased by adding Ta. The matrixes showed fine martensite plates with β1′, and there were also coarse martensite plates with γ′. Crystallite sizes and Vickers microhardness of the alloys were decreased with increasing tantalum additive. The Cu84Al12Ni3Ta wt% alloy recorded maximum elastic energy and Gibbs free energy.

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

The complex and intricate microstructure of B19′ martensite in shape memory nickel titanium alloys is generally explained with the Phenomenological Theory of Martensitic Crystallography (PTMC). Over the last decade, we have developed an alternative approach that supposes the existence of a “natural” parent–daughter orientation relationship (OR). As the previous TEM studies could not capture the global crystallographic characteristics of the B2→B19′ transformation required to discriminate the models, we used Electron BackScatter Diffraction (EBSD) and Transmission Kikuchi Diffraction (TKD) to investigate a polycrystalline NiTi alloy composed of B19′ martensite. The EBSD maps show the large martensite plates and reveal the coexistence of different ORs. The TKD maps permit us to image the “twins” and confirm the continuum of orientations suspected from EBSD. The results are interpreted with the alternative approach. The predominant OR in EBSD is the “natural” OR for which the dense directions and dense planes of B2 and B19′ phases are parallel—i.e., (010)B19′//(110)B2 and [101]B19′//[ 1 ¯ 11]B2. The natural OR was used to automatically reconstruct the prior parent B2 grains in the EBSD and TKD maps. From the distortion matrix associated with this OR, we calculated that the habit plane could be (1 1 ¯ 2)B2//(10 1 ¯ )B19′. The traces of these planes are in good agreement with the EBSD maps. We interpret the other ORs as “closing-gap” ORs derived from the natural OR to allow the compatibility between the distortion variants. Each of them restores a parent symmetry element between the variants that was lost by distortion but preserved by correspondence.

Hydrogen Enhances Shape Memory Effect of a Ferrous Face-Centered Cubic Alloy

This paper marks the first report on the fact that interstitial hydrogen improves the shape recovery property of an Fe-based face-centered cubic alloy, regardless of pre-strain level. The maximum shape recovery strain when using hydrogen was increased from 2.8 to 3.4 pct at an approximately 7 pct pre-strain. The enhancement of the shape memory effect by hydrogen stems from (1) increasing deformation-induced martensite fraction, (2) thinning of martensite plates, and (3) increasing the number of martensite plates.

Martensite Stabilization Effect in the Ni50Ti50 Alloy After Preliminary Deformation by Cooling Under Constant Stress

The martensite stabilization effect was studied in Ni − 50.0 at% Ti alloy after cooling under stress and compared to the same observed effect after deformation in the martensite state. After cooling under stress, three peaks of heat release were observed during the first heating, and an increase in the residual strain significantly increased the temperature of the third peak. The existence of three peaks was attributed to the formation of both non-oriented and oriented martensite upon cooling under stress. The way of the preliminary deformation hardly affected the shift in the finish temperature of the reverse transformation. Thus, the reason for the martensite stabilization effect did not depend on the type of deformation. It was assumed that the damaged intermartensite interfaces, which formed when the oriented martensite plates touched each other during growth, lost their mobility and overheating was needed to provide their movement upon subsequent heating. It was found that the way of the preliminary deformation influenced the martensite stabilization effect for the start temperature of the reverse transformation. It was assumed that this effect might be caused by different variations in the stored elastic energy during the active deformation in martensite or on cooling under stress.

Internal Stresses and their Sources in BCC and FCC Steels

The present work summarizes and presents separate results obtained by the authors when investigating mesoscopic and microscopic internal stresses formed under the conditions of thermal and mechanical treatment of martensitic, pearlitic and austenitic steels. Internal stresses were investigated using the method based on the analysis of bend extinction contours. The results obtained on industrial steels were presented. The sources were described and examples of internal stresses induced by these sources were given. The nature of bending-torsion of the crystal lattice depending on the averaging volume was determined. It has been shown that in martensitic steels along with the increase in the averaging volume (carbide particle → separate martensitic lath → martensite packet→ martensitic plate → grain) the amplitude of bending-torsion of the crystal lattice decreases. The nature of distortions also changes. At large amplitudes and low volumes of averaging they are completely or partly elastic, at large volumes of averaging they are completely plastic. Thereby, distortions are fully driven by the excess dislocation density.

Shape Memory Effect of Polycrystalline Ni<sub>54</sub>Mn<sub>25</sub>Ga<sub>17.5</sub>Ta<sub>0.5</sub> High Temperature Shape Memory Alloy with Wide Hysteresis Loop

Effect of Ta-alloying on microstructure, martensitic transformation, mechanical property and shape memory effect of Ni54Mn25Ga17.5Ta0.5 alloy has been systematically investigated. The results show that the substructure of Ni-Mn-Ga alloy significantly changed, which was converted from the plate martensite to the lath martensite. Compression tests show that a compressive strength of 1380 MPa with a fracture strain up to 21.92% can be achieved in the Ni54Mn25Ga17.5Ta0.5 alloy at room temperature. This is no changed martensite structure with non-modulated T martensite. In addition, the martensitic transition temperature obviously decreases from 350 °C to 208 °Cand hysteresis loop increases about 20 °Cwhen Ta substituted of Ni. The shape memory effect increased with the increase of pre-deformation, nevertheless, the shape recovery ratio appeared firstly increases and then decreases. When the pre-deformation is 10%, 15%, 20%, the shape memory effect of the alloy is 5.1%, 6.8% and 10%, respectively.

Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel

This comprehensive study gives evidence of reversible and irreversible embrittlement in ultra-high-strength steel through destructive testing. Several tempering temperatures ranging from 350 °C to 700 °C were designated to observe temper embrittlement in the steel. Charpy impact, hardness, and tensile tests were performed for assessing the mechanical behavior of the steel. Stereo microscopy was utilized to examine the fractured surfaces and microstructural analysis was performed with the help of an optical microscope. During tempering, reversible temper embrittlement occurs due to the formation of alloy carbides between martensite plates while irreversible temper embrittlement is preferably due to the segregation of impurity elements (i.e. P and S) on grain boundaries as well as due to the coarsening of alloy carbides. It was found that irreversible temper embrittlement transpired when tempering was done in the range of 400–450 °C and reversible temper embrittlement occurred in 650–700 °C temperature range. Results of the notched-bar impact test clearly showed a reduction in impact toughness in susceptible temperature ranges; moreover, the revealed fracture surface and micrograph also validate these findings. Experimental results also validate that both types of embrittlement are detrimental to the mechanical properties owing to the decrease in hardness and tensile strength.

Quantitative Analysis of Microstructural Refinement in Simulated Carburized Microstructures

Microstructure refinement strategies in simulated carburized microstructures were evaluated because of their potential for improving the fatigue performance of case-carburized components. Commercial 52100 steel was used to simulate the high-carbon content in the case. Specimens were subjected to various thermal treatments in a quenching dilatometer. Reheating cycles to austenitizing temperatures were evaluated with respect to both prior austenite grain size (PAGS) and associated martensite and retained austenite (RA) refinement. Quantitative stereological measurements were performed to evaluate the microgeometry of plate martensite and the size distribution of RA regions. Decreasing the reheating temperature resulted in finer PAGS, and multiple reheating cycles resulted in a narrower PAGS distribution. Refinement in PAGS led to a reduction in martensite plate size and finer distribution of RA. Additionally, interrupted quenching below martensite start (MS) temperature was evaluated. This processing route results in a refinement of martensite plates and more stable RA. The stabilization of austenite may be mechanical or chemical in nature, owing to the deformation of austenite during primary transformation, or due to partitioning of carbon into austenite similar to quenching and partitioning steels.

The Effect of Quenching Methods on Transformation Characteristics and Microstructure of an NiTiCu Shape Memory Alloy

Shape memory alloys are used in diverse fields. It is Ni–Ti alloys that are predominantly used for applications capitalizing on the functional properties of shape memory and superelastic effect. These properties are influenced by factors, such as alloy composition, alloying addition, thermal and mechanical treatment, etc. In any application, the transformation temperatures of the alloy/device are very critical. Moreover, the stability of transformation temperatures and a smaller hysteresis are preferred. Ni–Ti alloys have their own limitations, such as low transformation temperatures and lack of ductility. In order to overcome these limitations, researchers have used ternary additions, such as Zr, Hf, Cu, Pd and Au. The use of copper is practicable because of its lower cost as compared to Ti and Ni. Substitution of Cu for Ni up to ~ 25% in an equiatomic NiTi alloy causes it to exhibit SME. The transformation temperatures of NiTiCu exhibit less sensitivity to composition and a smaller thermal hysteresis. In this work, therefore, the effect of methods of quenching on transformation temperatures and microstructure of an Ni–Ti–4Cu (at.%) alloy has been explored. The material was cast using vacuum induction melting. It was solutionized at 950 °C for 2 h and cooled using different methods/quenchants: liquid nitrogen, water at room temperature, air, annealing (furnace cooling). The samples quenched by different techniques were characterized by XRD, DSC, OM, SEM and hardness test. The results indicated that the martensite plate size varied in samples quenched using different quenchants even though the morphology was the same in all specimens. In addition, the transformation temperatures and hardness values of the alloy specimens varied as the intensity of quenching varied. The results are presented and discussed in detail in the paper.

Stochastic multiscale modeling of the thermomechanical behavior of polycrystalline shape memory alloys

A multi-scale model simulating hysteretic thermomechanical behavior of polycrystalline shape memory alloys (SMA) is presented. Using a kinetic Monte-Carlo approach, the energy-based model estimates the stochastic average in terms of volume fraction for a phase or phase variant deriving from the Gibbs free energy density as a selection inside a given population. Indeed pseudo-elastic behavior for example is well-known to be associated with the nucleation of martensite plates inside the austenite parent phase. Associated variants are similar to sub-domains inside a thermodynamic system following the statistical definition of Sommerfeld et al. (1964). The germination process of a variant is on the other hand dictated by a germination potential barrier identified from a differential scanning calorimetry (DSC) measurement. The stochastic average for each type of variants inside a grain leads then to the numerical simulation of hysteresis phenomena at the single crystal scale. Homogenization operations allow finally macroscopic quantities (phases fraction, deformation and temperature) to be calculated at the polycrystalline scale. This procedure is applied to model the whole thermomechanical behavior of an equiatomic Ni-Ti SMA polycrystalline alloy considering the phase transformation between austenite, R-phase and martensite.

Characterization and correlation of microstructure and hardness of Ti–6Al–4V sheet surface-treated by pulsed laser

A hot-rolled Ti–6Al–4V (Ti-64) sheet was surface-treated by pulsed laser at two different powers (100 and 200 W), with microstructural features and hardness before and after the laser surface treatment (LST) systematically investigated. Results show that after the LST at both powers there are two modification zones with distinct microstructural characteristics: melted zone (MZ) near the laser beam center, completely composed of fine martensitic plates with dense {10–11} nanotwins inside them; heat-affected zone (HAZ) far away from the laser beam center, comprised of mixed structures of short-rod β particles, martensitic plates and untransformed bulk α grains. Hardness measurements reveal that the hardness of the Ti-64 sheet can be markedly increased (especially in the MZ) after the LST. In-depth analyses suggest that the hardness increase in the MZ can be ascribed to combined contributions from grain refinement, presence of nanotwins and solid solution of alloying elements, while only the structural refinement by fine plate structures contributes to hardening in the HAZ. Comparisons between both the LSTed specimens reveal that increasing the laser power from 100 W to 200 W can effectively enlarge the laser-modified zones (both the MZ and the HAZ) and simultaneously refine plate structures, leading to further hardness increase.

Aging Effect on Microstructure and Machinability of Corrax Steel

In this paper, the influence of the aging process on the microstructure and machinability of Corrax Steel was investigated for four samples: a solution heat-treated (A0) and three samples aged at 400ºC (A4), 525ºC (A5.25) and 600ºC (A6) for four hours. The effect of aging temperature on hardness was examined. Machining tests were carried out using a CNC lathe with a multi-layer coated PVD (AlTiN) cutting tool, at various cutting speeds (50, 100, 150, 200, 250, 300, 350m/min) with constant feed rate (0.1mm/rev) and 1mm constant cutting depth. The microstructure was investigated using an optical microscope and EDS attached SEM. The effect of aging on reverted austenite formation was also evaluated. In order to understand the changes in surface topology, cutting forces and vibrations were measured. With increasing aging temperature, the lath martensite was transformed to plate martensite because of the formation of precipitates and reverted austenite. Aging at different temperatures increased hardness up to 58%, cutting forces up to 117% and surface roughness up to 450%. The results describe the effect of the aging treatment on cutting forces, surface topology, tool wear and vibrations.

Surface microstructural characteristics and hardness of Cr-coated Zr702 sheet processed by pulsed laser

Microstructural characteristics and hardness of Cr-coated Zr702 sheet after the laser surface treatment (LST) were characterized and investigated by combined use of electron backscatter diffraction, electron channeling contrast imaging, energy dispersive spectrometry and hardness measurement. Results show that after the LST processing at 100 W, three zones with different microstructural characteristics are presented from the surface to the matrix: melted zone (MZ), solid-state phase transformation zone (SSPTZ) and matrix. Surface alloying with Cr occurs in the MZ, leading to the formation of a large number of discontinuous netlike ZrCr2 Laves phases. The SSPTZ is comprised of untransformed bulk α grains with internal substructures and fine martensitic plates. The appearance of such substructures can be attributed to dislocation movements induced by thermal stresses while the martensitic plates result from rapid β→α transformation induced by the pulsed laser. Hardness measurements show that the LST can remarkably increase surface hardness (by ~110%) of the Cr-coated Zr702 sheet, which can be ascribed to strengthening/hardening effects by solid solution of Cr, the formation of dense second phases and significant grain refinement.