Effect of extended solid solution of Hf on the microstructure of the laser clad Ni-Fe-Cr-Al-Hf alloys

Alloys and coatings for alloys for improved high temperature service life under aggressive atmosphere are of great contemporary interest. There is a general consensus that addition of reactive elements such as Hf will provide many beneficial effects for such alloys. The laser cladding technique was used to produce Ni-Fe-Cr-Al-Hf alloys with extended solid solution of Hf. A 10 kW CO₂ laser with mixed powder feed was used for laser cladding. Optical, scanning electron (SEM) and scanning transmission electron (STEM) microscopy were employed for microstructural evolution of alloys produced during laser cladding processes. The electron probe microanalysis and the auger electron spectroscopy were also used for micro-chemical analysis of different phases. Microstructural studies revealed a high degree of grain refinement, considerable increase in solubility of Hf in matrix and Hf rich precipitates and new metastable phases. This paper will report the microstructural development in this laser clad Ni-Fe-Cr-Al-Hf alloy.     

Microstructure of laser clad Ni- Cr- Al- Hf alloy on a γ′ strengthened ni- base superalloy

Alloys and coatings for alloys for improved high temperature service life under aggressive atmo-spheres are of great contemporary interest. There is a general consensus that the addition of rare earths such as Hf will provide many beneficial effects for such alloys. The laser cladding technique was used to produce Ni-Cr-AI-Hf alloys with extended solid solution of Hf. A 10 kW CO₂ laser with mixed powder feed was used for laser cladding. Optical, scanning electron (SEM) and scanning transmission electron (STEM) microscopy were employed to characterize the microstructure of alloys produced during laser cladding processes. Microstructural studies revealed grain refinement, considerable in-crease in solubility of Hf in the matrix, Hf-rich precipitates, and new metastable phases. The size and morphology of γ′ (Ni₃Al) phase were discussed in relation to its microchemistry and the laser processing conditions. This paper will report the microstructural development in this laser clad Ni-Cr-AI-Hf alloy. Formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

Laser-Clad NiAICrHf alloys with improved alumina scale retention

A new mechanism for the improved retention of alumina scales formed on laser-clad NiAICrHf alloys has been observed. Laser cladding is the process where fine metal powders are rapidly melted and fused to a solid substrate using a CO2 laser. The effects of laser cladding upon scale retention on NiAICrHf alloys after cyclic and isothermal exposure to air were investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The calculated compressive stress in the scale due to constrained cooling exceeded the probable compressive strength of alumina. Additions of up to ≈ 2.5 wt pct Hf increasingly promote retention of scales grown at 1200 °C, with laser-clad samples of ≈ 2.5 wt pct Hf alloy retaining almost completely intact scales. The improvement in scale retention is due to improved toughness in scales containing hafnia polycrystallites, possibly_via microcracking initiated by anisotropic thermal contraction of the hafnia. Laser cladding the 2.5 wt pct Hf alloy provides a large concentration of ~ 1 μm Hf-rich particles that are precursors of the hafnia in the scale as well as a finer dendrite spacing that reduces the mean free distance between particles. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign     

Microstructure evolution and nonequilibrium phase diagram for Ni-Hf binary alloy produced by laser cladding

Synthesis of nonequilibrium Ni-Hf binary alloys were carried out using laser cladding technique. In this process mixed powder in the ratio of Ni-26 wt% Hf was delivered using a screw feeder into a melt pool of the substrate, generated by a high power continuous wave CO₂ laser beam. The microstructure of the claddings thus produced were investigated using optical, scanning and transmission electron microscopy and X-ray microanalysis techniques. Due to the inherent rapid cooling rate associated with the process of laser cladding process, some nonequilibrium hereto unreported phases formed in the claddings. There is also an extension in the solid solubility of Hf in the terminal α phase as compared to the equilibrium Ni-Hf binary phase diagram. This paper investigates this solid solubility extension, the evolution of the microstructure in the claddings in the system and also characterizes the metastable phases formed in terms of crystal structure and microchemistry. A nonequilibrium phase diagram for Ni-Hf binary alloy is recommended based on the micro-chemistry and differential thermal analysis data.     

Laser-clad Ni70Al20Cr7Hf3 alloys with extended solid solution of Hf: Part II. Oxidation behavior

Ni-Al-Cr-Hf coatings with an extended solid solution of Hf were fabricated by laser cladding a premixed powder with a Ni-Al-Cr-Hf ratio of 70∶20∶7∶3 onto a substrate of RENé 80. Isothermal oxidation tests in slowly flowing air reveal that the claddings have a lower weight-gain rate than the substrate itselt. Microchemistry and microstructures of the oxidized samples were examined using scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The improvement in oxidation resistance is believed to be at least partially due to the formation of large numbers of hafnium-rich oxide spikes penetrating the unoxidized sample.     

Microstructure of Fe-Based Alloy Composite Coatings Reinforced by Ti (C0.3 N0.7) Particles Through Laser Cladding Technology

Fc-bascd alloy layer reinforced by Ti(C, N) particles was produced on the surface of cast steel. X-ray diffraction (XRD) was used for phase identification in the composite coating. The microstructure of laser cladding layer was analyzed by means of optical microscope (OM), electron probe microscope analyzer (EPMA), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that Ti(C_0.3 N_0.7) particle is introduced by an in-situ metallurgical reaction of TiN particle and graphite powder in the process of laser cladding. The shape of lots of Ti(C_0.3 N_0.7) particle is irregular. The sizes of Ti(C_0.3 N_0.7) particles range from 0.1 to 6.0 μm, and they arc dispersed evenly in the matrix, which is fine dendritic or cellular crystal. A new kind of phase named Ti(C_0.3 N_0.7) particles arc tightly bonded with α-Fe microstructure, and there is a clean and smooth phase interface between ceramic reinforcement phase and the matrix.     

Microstructure and wear properties of laser clad Fe−Cr−Mn−C alloys

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10∶1∶1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Microstructure and wear properties

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10:1:1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Effect of Hf and Zr Contents on Stress-rupture and Fatigue Crack Growth Rate Performances in FGH96PM Superalloy

Four experimental FGH96alloys with various contents of Hf and Zr(0and 0.04%,0.3% and 0.04%,0.6% and 0.04%,0.3%and 0.06%,respectively)were produced using PREP(plasma rotating electrode process)+HIP(hot isostatic pressing)route.The unnotched and notched stress-rupture properties and fatigue crack growth rate(FCGR)of all the experimental alloys were investigated to study the effect of Hf and Zr.Relevant fracture morphology and microstructure were observed by scanning electron microscopy and transmission electron microscopy.The results revealed that appropriate content of Hf could lengthen stress-rupture life,eliminate notch sensitivity and slower FCGR.Microstructure analysis showed that the amount ofγ′phase should be increased or decreased by adjusting Hf and Zr contents,and MC carbide and oxide coupled growth should be increased by adding Hf content,which caused oxycarbide to precipitate along grain boundary and strengthen the alloy.It was found that excessive Zr in Hfcontaining FGH96alloy had certain deleterious effects on stress-rupture property because there was strong Zr segregation at prior particle boundary,leaving a long chain of large-size oxides along the boundary.The optimal content of Hf and Zr in FGH96alloy was 0.6%and 0.04%,respectively.     

The age hardening characteristics of Nb-Base alloys containing carbon and/or silicon as interstitiels: Part II. (Nb-33 At. Pct Hf)

A study was made to investigate the solutioning and aging reactions and their effects on microstructure and microhardness in nine experimental Nb-base alloys involving Nb-33 Hf, Nb-33Hf-C, Nb-33Hf-Si, and Nb-33Hf-C-Si* systems. Alloys were solution treated, quenched, *Alloy compositions are in atomic percent. then aged for 2 h at temperatures ranging from 800 to 1600°C to examine the effect of temperature, and aged at 900 to 1100°C isothermally for times varying from 2 to 20 h. Microstructural changes were determined by optical and electron microscopy, the phases were chemically extracted and identified by X-ray diffraction, and hardness provided the means of evaluating structural effects. The Nb-33Hf-Si and Nb-33Hf-C-Si alloys exhibited age hardening characteristics, which appear to be due to the precipitation of complex monocarbide (Nb, Hf) (C, O, N), while Nb-33Hí-C alloys showed separation of Nb₂C phase from (Nb, Hf) (C, O, N) on aging.     

Ordering transformation and spinodal decomposition in Au-Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L1₀ LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490 °C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au-Ni system, although it appeared during a reversion process. Attempts to find the L1₀ and L1₂ LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature-transformation (TTT) diagrams for the two Au-Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed.     

Microstructure of bonding zones in laser-clad Ni-alloy-based composite coatings reinforced with various ceramic powders

Microstructure of the bonding zones (BZs) between laser-clad Ni-alloy-based composite coatings and steel substrates was studied by means of scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. Observations indicate that for pure Ni-alloy coating the laser parameters selected for good interface fusion have no effect on the microstructure of the BZ except for its thickness. However, the addition of ceramic particles (TiN, SiC, or ZrO₂) to the Ni alloy varies the compositional or constitutional undercooling of the melt near the solid/liquid interface and consequently leads to the observed changes of microstructure of the BZs. For TiN/Ni-alloy coating the morphology of y-Ni solid solution in the BZ changes from dendritic to planar form with increas-ing scanning speed. A colony structure of eutectic is found in the BZ of SiC/Ni-alloy coating in which complete dissolution of SiC particles takes place during laser cladding. The immiscible melting of ZrO₂ and Ni-alloy powders induces the stratification of ZrO₂/Ni-alloy coating which consists of a pure ZrO₂ layer in the upper region and a BZ composed mainly of y-Ni dendrites adjacent to the substrate. All the BZs studied in this investigation have good metallurgical characteristics between the coatings and the substrates.     

Structural Features of Al–Hf–Sc Master Alloys

Microstructural features of new master alloys of the Al–Hf–Sc system with metastable aluminides with a cubic lattice identical to the lattice of a matrix of aluminum alloys are investigated using optical microscopy, scanning electron microscopy, and electron probe microanalysis. Binary and ternary alloys are smelted in a coal resistance furnace in graphite crucibles in argon. Alloys Al–0.96 at % Hf (5.98 wt % Hf) and Al–0.59 at % Hf (3.77 wt % Hf) are prepared with overheating above the liquidus temperature of about 200 and 400 K, respectively. Alloys are poured into a bronze mold, the crystallization rate in which is ~10³ K/s. Metastable Al₃Hf aluminides with a cubic lattice are formed only in the alloy overheated above the liquidus temperature by 400 K. Overheating of ternary alloys, in which metastable aluminides Al _n (Hf_1–xSc _x ) formed, is 240, 270, and 370 K. Depending on the Hf-to-Sc ratio in the alloy, the fraction of hafnium in aluminides Al _n (Hf_1–xSc _x ) varies from 0.46 to 0.71. Master alloys (at %) Al–0.26Hf–0.29Sc and Al–0.11Hf–0.25Sc (wt %: Al–1.70Hf–0.47Sc and Al–0.75Hf–0.42Sc) have a fine grain structure and metastable aluminides of compositions Al _n (Hf_0.58Sc_0.42) and Al _n (Hf_0.46Sc_0.54), respectively. Sizes of aluminides do not exceed 12 and 7 μm. Their lattice mismatch with a matrix of aluminum alloys is smaller than that for Al₃Sc. This makes it possible to assume that experimental Al–Hf–Sc master alloys manifest a high modifying effect with their further use. In addition, the substitution of high-cost scandium with hafnium in master alloys can considerably reduce the consumption of the latter.     

Effects of adding different types of carbon on the structure and magnetic properties of SmCo6.9Hf0.1 alloy

In this paper,SmCo6.9 Hf0.1 as-cast alloys and ribbons with the addition of either graphite（C） or carbon nanotubes（CNTs） were prepared by arc melting and melt-spinning,respectively.The effects of adding carbon on the structure and magnetic properties SmCo6.9 Hf0.1 were investigated by means of X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,magnetic force microscopy（MFM） and vibrating sample magnetometer（VSM）.It was found that the microstructure and magnetic structure of SmCo6.9 Hf0.1 ribbons were changed obviously due to the introduction of C or CNTs,although their crystal structure was characterized as the same Sm（Co,Hf）7 single phase,no matter carbon was added or not.As a result,the magnetic properties of carbon-contained ribbons were enhanced in a certain degree.This was considered to be related to the refined equiaxed grains,small domain size and the pinning effect of C or CNTs-rich regions.The magnetic properties of SmCo6.9 Hf0.1（CNTs）0.05 ribbons reached Hc =12.5 kOe,Mr =57.0 emu/g and Mr/M2 T =0.788.     

Oxidation behavior and transmission electron microscope characterization of Ti-44Al-x Nb-2(Ta,Zr) alloys

Quaternary additions of 2 at. pct of Ta or Zr were made to the ternary Ti-44Al-xNb (X=9 and 11) alloys to study the oxidation behavior at 900 °C, 950 °C, and 1000 °C for a period of 1 week. The Ta addition improves the oxidation resistance, while it is degraded by Zr compared to the ternary alloys. Identification of the oxides formed in the scale has been characterized by energy-dispersive atomic X-ray (EDAX) in a scanning electron microscope (SEM). The transmission electron microscope (TEM) analysis of the microstructures developed during oxidation has been compared with Ti-44Al-xNb alloys in order to determine the influence of quaternary additions of Ta and Zr on the phase transformations taking place during the extended period of heating. The formation of spotty α ₂ in the isolated γ grains appears to be associated with the inferior oxidation resistance of xNb2Zr alloys. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Ir-base refractory superalloys for ultra-high temperatures

The microstructure and compression strengths of Ir-15 at. pct X (X=Ti, Ta, Nb, Hf, Zr, or V) binary alloys at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. The fcc and L1₂ two-phase structures of these alloys were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The strengths of the Ir-Ta, -Nb, -Hf, and -Zr alloys were above 800 MPa at temperatures up to 1200 °C and about 200 MPa at 1800 °C. The strengths of these alloys under 1000 °C are equivalent to or higher than those of the commercially used Ni-base superalloys, MAR-M247 and CMSX-10. The Nb concentration dependence of strength was investigated using a series of Ir-Nb alloys with Nb concentrations from 0 to 25 at. pct. It was found that the Ir-base alloys were strengthened by L1₂ precipitation hardening. The potential of the Ir-base alloys for ultra-high temperature use is discussed.     

Formation and Mechanical Behavior of Body-Centered-Cubic Zr(Hf)-Nb-Ti Medium-Entropy Alloys

The present work investigated the formation and mechanical behavior of body-centered-cubic (BCC) Zr(Hf)-Nb-Ti medium entropy alloys (MEAs), in which three series of alloys, [Zr-Zr₁₄](Zr,Nb)₃, [Ti-Zr₁₄](Ti,Nb)₃, and [Ti-(Hf,Zr)₁₄](Nb)₃, were designed by the cluster formula approach. With increasing the Nb content, the BCC-β structural stability of the [Zr-Zr₁₄](Zr,Nb)₃ alloys would be enhanced, as evidenced by the BCC [Zr-Zr₁₄](Nb)₃ (Zr_83.33Nb_16.67 in atomic percent at. pct) alloy containing a minor amount of ω phase. An appropriate content of Ti addition can further improve the BCC-β stability of [Ti-Zr₁₄](Nb₃) (Zr_77.77Ti_5.56Nb_16.67) alloy without any ω precipitation. The further substitution of Hf/Ti for the Zr could also render the [Ti-Zr₈Hf₄Ti₂](Nb₃) (Zr_44.44 Hf_22.22Ti_16.67Nb_16.67) alloy with a single BCC structure. All these BCC MEAs exhibit prominent mechanical properties, as exemplified by the [Ti-Zr₈Hf₆](Nb₃) (Zr_44.44Hf_33.33Ti_5.56Nb_16.67) with a higher yield strength of 662 MPa, a larger elongation to fraction of 15.2 pct, and a lower Young’s modulus of 71 GPa.

     

Microstructure and tensile properties of Fe-40 At. pct Al alloys with C,Zr, Hf,and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe₆Al₆Zr and Fe₆Al₆Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.     

Ordering transformation and spinodal decomposition in Au−Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L1₀ LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490°C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au−Ni system, although it appeared during a reversion process. Attempts to find the L1₀ and L1₂ LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature—transformation (TTT) diagrams for the two Au−Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed. JI-CHENG ZHAO, formerly Graduate Student, Department of Materials Science and Engineering, Lehigh University     

Precipitation of heusler phase (Ni<Subscript>2</Subscript>TiAl) from B2-TiNi in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf,Zr) alloys

The precipitation of Heusler phase (L2₁: Ni₂TiAl) from a supersaturated B2 (TiNi-based) matrix at 600°C and 800°C is studied using transmission electron microscopy (TEM), analytical electron microscopy (AEM), and three-dimensional atom-probe (3DAP) microscopy in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf and Zr) alloys. The B2/L2₁ two-phase system, with ordered structures based on the bcc lattice, is chosen for its microstructural analogy to the classical γ/γ′ system with an fcc lattice. Knowledge of the temperature-dependent partitioning of alloying elements and their atomic volumes in the B2-TiNi and L2₁ phases is desired to support design of high-performance shape-memory alloys (SMAs) with controlled misfit strain and transformation temperatures. After aging at 600°C for up to 2000 hours, the L2₁ precipitates remain fully coherent at a particle diameter of ∼20 nm. The observed effects of a misfit strain of −1.9 pct on the microstructure of the B2/L2₁ system are similar to those theoretically predicted and experimentally observed for the γ/γ′ system. The similarities are demonstrated in terms of the precipitate shape, spatial distribution, and minimum distance of separation between L2₁ precipitates. However, all these effects disappear after aging the alloys at 800°C for 1000 hours, when the L2₁ precipitates become semicoherent at particle diameters above ∼400 nm. A simple analysis of the size evolution of L2₁ precipitates after an isochronal aging (1000 hours) experiment suggests that they follow coarsening kinetics at 600°C and growth kinetics at 800°C, consistent with the Langer-Schwartz theory of precipitation kinetics, which predicts that a high supersaturation suppresses the growth regime. Microanalysis using AEM and 3DAP microscopy define the TiNi-Ni₂TiAl phase boundaries at 800°C and 600°C. At 800°C, Hf and Zr partition to the B2-TiNi, while at 600°C, they partition slightly to the L2₁ phase, reducing the lattice misfit to −1.7 and −0.011 pct, respectively, and partition strongly to the metastable phase Ti₂Ni₃. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti, and Zr in the B2-TiNi and L2₁ phases are determined. A simple model is proposed to predict the lattice parameters of these phases in multicomponent systems.